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Intermolecular forces

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Intros
Lessons
  1. Intermolecular forces
  2. Definition of 'intermolecular force'
  3. Where intermolecular forces come from.
  4. Van der Waals: permanent dipole-dipole interactions.
  5. Van der Waals: London dispersion forces.
  6. Hydrogen bonding.
  7. Water and hydrogen bonding.
  8. Solubility and intermolecular forces.
  9. What influence do intermolecular forces have?
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Examples
Lessons
  1. Identify the intermolecular forces present in molecules.
    Look at the formula of the chemical substances listed below. Which type of intermolecular forces would you expect to see between the molecules?
    1. i) CH4_4
      ii) H2_2 O
    2. i) Br2_2
      ii) HBr
  2. Recall the order of intermolecular force strength
    Look at each group of chemical substances below. Order them in how relatively strong their intermolecular forces are. Hint: Identify the intermolecular forces present first.
    1. F2_2 , I2_2 , Br2_2 , Cl2_2
    2. HCl, HF, HBr, HI
    3. NH3_3 , H2_2 O, CH4_4
Topic Notes
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Introduction to Intermolecular Forces

Welcome to the fascinating world of intermolecular forces! These are the subtle yet powerful attractions between molecules that play a crucial role in shaping the physical properties of substances. To kick off our exploration, let's start with a simple definition: intermolecular forces are the interactions that occur between separate molecules. They're like the social glue that keeps molecules together in various states of matter. Our introduction video provides an excellent visual representation of these forces in action, making it easier to grasp this abstract concept. As we delve deeper, you'll discover how these forces influence everything from boiling points to solubility. Understanding intermolecular forces is key to unlocking the mysteries of chemistry and physics. So, let's embark on this journey together, unraveling the invisible bonds that shape our physical properties of substances. Remember, mastering this topic will give you a solid foundation for more advanced chemistry concepts!

Types of Intermolecular Forces

Intermolecular forces play a crucial role in determining the properties of substances and their behavior in various chemical and physical processes. These forces are responsible for the attraction between molecules and can be categorized into different types, each with its own characteristics and strengths. The main types of intermolecular forces include van der Waals forces and hydrogen bonding.

Van der Waals forces are a group of weak intermolecular interactions that encompass dipole-dipole interactions and London dispersion forces. These forces are named after Dutch physicist Johannes van der Waals, who first described them in the late 19th century.

Dipole-dipole interactions occur between polar molecules, where one end of the molecule has a slight positive charge and the other end has a slight negative charge. These partial charges arise from the uneven distribution of electrons within the molecule. For example, in a water molecule (H2O), the oxygen atom is slightly negatively charged, while the hydrogen atoms are slightly positively charged. This polarity allows water molecules to align and attract each other, contributing to water's unique properties.

London dispersion forces, also known as induced dipole-induced dipole interactions, are the weakest type of intermolecular force. These forces occur between all molecules, even non-polar ones, due to temporary fluctuations in electron distribution. As electrons move around atoms, they can create momentary dipoles that induce dipoles in neighboring molecules. Although individually weak, London dispersion forces can become significant in larger molecules with more electrons. For instance, these forces are responsible for the liquid state of non-polar substances like hexane at room temperature.

Hydrogen bonding is a special type of dipole-dipole interaction that occurs between a hydrogen atom bonded to a highly electronegative atom (such as nitrogen, oxygen, or fluorine) and another highly electronegative atom. This type of bonding is stronger than typical dipole-dipole interactions due to the small size of the hydrogen atom and the large electronegativity difference. Hydrogen bonding is particularly important in biological systems and plays a crucial role in determining the properties of water. For example, the hydrogen bonds between water molecules contribute to its high boiling point, surface tension, and ability to dissolve many substances.

When comparing the relative strengths of these intermolecular forces, hydrogen bonding is generally the strongest, followed by dipole-dipole interactions, and then London dispersion forces. However, it's important to note that the overall strength of intermolecular forces in a substance depends on various factors, including molecular size, shape, and polarity.

To illustrate the differences in strength, consider the boiling points of similarly sized molecules. Water (H2O), which experiences strong hydrogen bonding, has a much higher boiling point (100°C) compared to hydrogen sulfide (H2S), which has weaker dipole-dipole interactions and a boiling point of -60°C. Similarly, methane (CH4), which only experiences London dispersion forces, has an even lower boiling point of -161.5°C.

Understanding these different types of intermolecular forces is essential in various fields, including chemistry, biology, and materials science. They help explain phenomena such as surface tension, capillary action, and the behavior of gases, liquids, and solids. In biochemistry, hydrogen bonding is crucial for maintaining the structure of DNA and proteins. In materials science, van der Waals forces play a significant role in the development of new adhesives and nanomaterials.

In conclusion, the various types of intermolecular forces - van der Waals forces (including dipole-dipole interactions and London dispersion forces) and hydrogen bonding - each contribute uniquely to the properties and behavior of substances. Their relative strengths and the specific combinations present in different materials determine a wide range of physical and chemical characteristics, making them a fundamental concept in understanding molecular interactions and their macroscopic effects.

Influence of Intermolecular Forces on Molecular Arrangement

Intermolecular forces play a crucial role in determining the molecular arrangement and properties of substances across different states of matter. These forces, which include hydrogen bonding, dipole-dipole interactions, and van der Waals forces, significantly impact how molecules interact with one another, ultimately affecting properties such as boiling point, melting point, and solubility.

In the solid state, intermolecular forces are at their strongest, resulting in a tightly packed and ordered molecular arrangement. This rigid structure gives solids their definite shape and volume. As we transition to the liquid state, these forces weaken slightly, allowing molecules to move more freely while still maintaining some cohesion. In the gas state, intermolecular forces are at their weakest, leading to a random molecular arrangement with molecules spread far apart.

Water serves as an excellent example to illustrate how intermolecular forces influence molecular behavior. The strong hydrogen bonds between water molecules contribute to its unique properties. In ice, water molecules form a crystalline structure with hexagonal symmetry, creating more space between molecules than in liquid water. This unusual arrangement explains why ice floats on water, a property crucial for aquatic life.

The boiling point of a substance is directly related to the strength of its intermolecular forces. Water's relatively high boiling point (100°C at sea level) is due to the energy required to overcome the strong hydrogen bonds between molecules. Substances with weaker intermolecular forces, such as methane, have much lower boiling points as less energy is needed to separate the molecules into the gas phase.

Similarly, the melting point of a substance is influenced by intermolecular forces. The energy required to break the ordered structure of a solid and transition to the more fluid liquid state depends on the strength of these forces. Water's melting point of 0°C is again a result of its strong hydrogen bonds, which require more energy to disrupt compared to substances with weaker intermolecular forces.

Solubility, another critical property affected by intermolecular forces, determines how well a substance dissolves in a solvent. Water's polar nature, due to its bent molecular shape and uneven electron distribution, makes it an excellent solvent for polar and ionic compounds. The phrase "like dissolves like" encapsulates this concept, as substances with similar intermolecular forces tend to be soluble in one another.

The impact of intermolecular forces on molecular arrangement extends beyond just water. In organic compounds, for instance, the length of hydrocarbon chains influences boiling points. Longer chains have stronger van der Waals forces between molecules, resulting in higher boiling points. This principle explains why butane (C4H10) has a higher boiling point than methane (CH4).

Understanding the relationship between intermolecular forces and molecular arrangement is crucial in various fields, from materials science to environmental studies. For example, this knowledge helps in designing new materials with specific properties, predicting chemical reactions, and understanding climate phenomena like the water cycle.

In conclusion, intermolecular forces significantly influence the molecular arrangement of substances in different states of matter. These forces directly impact properties such as boiling point, melting point, and solubility, with water serving as a prime example of how these interactions shape a substance's behavior. By comprehending these fundamental concepts, we gain valuable insights into the behavior of matter at the molecular level, enabling advancements in numerous scientific and technological fields.

Intermolecular Forces in Specific Molecules

Let's dive into the fascinating world of intermolecular forces, focusing on specific molecules like hydrogen iodide (HI). Understanding these forces is crucial in chemistry as they influence a substance's physical properties of HI and behavior. To begin, let's explore how to identify and predict intermolecular forces in different molecules.

In the case of the HI molecule, we're dealing with a polar covalent bond between hydrogen and iodine. The electronegativity difference between these atoms creates a dipole moment, with the iodine atom carrying a slight negative charge and the hydrogen atom a slight positive charge. This polarity leads to dipole-dipole interactions between HI molecules.

When identifying intermolecular forces, we need to consider the molecule's structure and composition. For HI, we observe:

  • Dipole-dipole interactions: Due to the molecule's polarity
  • London dispersion forces: Present in all molecules, arising from temporary electron fluctuations

Interestingly, despite having a hydrogen atom, HI doesn't exhibit hydrogen bonding. This is because iodine's electronegativity isn't high enough to create the strong partial charges necessary for hydrogen bonding.

To predict intermolecular forces in other molecules, follow these steps:

  1. Determine the molecule's polarity by analyzing its structure and the electronegativity of its atoms
  2. Check for the presence of highly electronegative atoms (N, O, F) bonded to hydrogen, which could indicate hydrogen bonding
  3. Consider the molecule's size and shape, which influence London dispersion forces

Let's compare HI with other molecules:

  • H2O (water): Exhibits strong hydrogen bonding, dipole-dipole interactions, and London dispersion forces
  • CH4 (methane): Only has London dispersion forces due to its non-polar nature
  • HCl (hydrogen chloride): Similar to HI, with dipole-dipole interactions and London dispersion forces, but stronger polarity

Understanding these forces helps explain various phenomena. For instance, the relatively low boiling point of HI compared to H2O is due to the absence of hydrogen bonding in HI. Similarly, the increasing boiling points in the series CH4 < HCl < HI < H2O reflect the strengthening of intermolecular forces.

When studying molecules, remember that multiple types of intermolecular forces can coexist. The overall behavior of a substance is determined by the combined effect of all these forces. For example, in larger molecules like proteins, we see a complex interplay of various intermolecular forces that contribute to their unique 3D structures.

As you continue your chemistry journey, you'll encounter more complex molecules. The principles we've discussed here will serve as a foundation for understanding their behavior. Always start by analyzing the molecule's structure, considering its polarity, and identifying potential sites for specific interactions. With practice, predicting intermolecular forces will become second nature, allowing you to explain and anticipate the physical properties of HI of various substances.

Applications of Intermolecular Forces

Understanding intermolecular forces is crucial in various scientific fields and has numerous real-world applications. These forces play a significant role in chemistry, biology, and materials science, influencing the behavior of substances and the development of new technologies. In chemistry, intermolecular forces are essential for predicting and explaining the properties of compounds, such as boiling points, solubility, and reactivity. For instance, the high boiling point of water is due to hydrogen bonding, an intermolecular force that allows it to remain liquid at room temperature, which is vital for life on Earth.

In biology, intermolecular forces are fundamental to the structure and function of biological molecules. DNA's double helix structure is maintained by hydrogen bonding between base pairs, while protein folding is influenced by various intermolecular interactions. These forces also play a crucial role in enzyme-substrate binding, cell membrane formation, and the transport of molecules across membranes. Understanding these interactions is essential for developing new drugs and therapies, as well as for comprehending biological processes at the molecular level.

Materials science heavily relies on the knowledge of intermolecular forces to design and create new materials with specific properties. For example, the development of self-healing materials is based on the manipulation of intermolecular forces to allow molecules to reconnect after damage. In the field of nanotechnology, intermolecular forces are exploited to create self-assembling structures and to control the properties of nanomaterials. The strength and flexibility of polymers, widely used in everyday products, are determined by the intermolecular forces between polymer chains.

In everyday life, we encounter applications of intermolecular forces without realizing it. The adhesive properties of glues and tapes are based on intermolecular interactions between the adhesive and the surfaces being joined. The ability of geckos to climb walls is due to van der Waals forces between their foot pads and surfaces. Non-stick cookware utilizes materials with weak intermolecular forces to prevent food from sticking. Even the process of cleaning with soap relies on intermolecular forces, as soap molecules interact with both water and oil, allowing for the removal of grease and dirt.

In scientific research, the study of intermolecular forces has led to advancements in various areas. In environmental science, understanding these forces helps in developing more efficient methods for water purification and air filtration. In the energy sector, research on intermolecular forces contributes to the development of better batteries, fuel cells, and energy storage materials. The field of supramolecular chemistry, which deals with the interactions between molecules, has led to the creation of molecular machines and sensors with applications in medicine and technology.

The importance of intermolecular forces extends to the pharmaceutical industry, where they are crucial in drug design and delivery. The solubility, stability, and bioavailability of drugs are all influenced by intermolecular interactions. In crystal engineering, knowledge of these forces is used to design materials with specific properties for applications in electronics, optics, and catalysis. The development of smart materials that respond to environmental stimuli is also based on the manipulation of intermolecular forces.

In conclusion, the applications of intermolecular forces are vast and diverse, spanning from fundamental scientific research to practical everyday technologies. As our understanding of these forces continues to grow, so does our ability to harness them for innovative solutions in various fields, ultimately improving our quality of life and advancing scientific knowledge.

Experimental Techniques for Studying Intermolecular Forces

Intermolecular forces play a crucial role in determining the properties and behavior of materials at the molecular level. To understand and quantify these forces, scientists employ various experimental techniques. This section explores some of the most important methods used to study and measure intermolecular forces, including atomic force microscopy, surface tension measurements, and spectroscopic techniques.

Atomic Force Microscopy (AFM) is a powerful tool for investigating intermolecular forces at the nanoscale. This technique uses a sharp probe attached to a flexible cantilever to scan the surface of a sample. As the probe interacts with the sample, the cantilever deflects, allowing researchers to measure the forces between the probe and the sample. AFM can detect forces as small as piconewtons, making it ideal for studying weak intermolecular interactions. By modifying the probe's surface chemistry, scientists can investigate specific types of intermolecular forces, such as van der Waals forces, hydrogen bonding, and electrostatic interactions.

Surface tension measurements provide valuable insights into the cohesive forces between molecules in liquids. These experiments typically involve measuring the force required to separate a liquid surface or the shape of liquid droplets. Common techniques include the ring method, where a metal ring is pulled from a liquid surface, and the pendant drop method, which analyzes the shape of a hanging droplet. By studying how surface tension changes with temperature or the addition of surfactants, researchers can gain information about the strength and nature of intermolecular forces in liquids.

Spectroscopic techniques offer a non-invasive way to probe intermolecular forces by analyzing how molecules interact with electromagnetic radiation. Infrared (IR) spectroscopy is particularly useful for studying hydrogen bonding, as the frequency and intensity of O-H and N-H stretching vibrations are sensitive to hydrogen bond formation. Raman spectroscopy complements IR spectroscopy by providing information about molecular vibrations that may not be IR-active. Nuclear Magnetic Resonance (NMR) spectroscopy can reveal details about molecular structure and dynamics influenced by intermolecular forces, such as changes in chemical shifts due to hydrogen bonding or π-π stacking interactions.

X-ray diffraction and neutron scattering techniques are invaluable for studying intermolecular forces in crystalline solids. These methods provide detailed information about molecular packing and intermolecular distances, allowing researchers to infer the presence and strength of various intermolecular interactions. Additionally, computational methods, such as molecular dynamics simulations and density functional theory calculations, often complement experimental techniques by providing atomic-level insights into intermolecular forces that may be challenging to observe directly.

Calorimetric methods, including isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC), measure the heat associated with molecular interactions. These techniques are particularly useful for studying the thermodynamics of intermolecular forces in solution, providing information about binding affinities, enthalpy changes, and the number of binding sites in molecular complexes.

By combining these experimental techniques, scientists can build a comprehensive understanding of intermolecular forces across different scales and environments. This knowledge is crucial for advancing fields such as materials science, drug design, and nanotechnology. As technology continues to improve, new and refined methods for studying intermolecular forces are likely to emerge, further enhancing our ability to manipulate and control matter at the molecular level.

Conclusion

In summary, intermolecular forces play a crucial role in determining the properties of substances. These forces include hydrogen bonding, dipole-dipole interactions, and London dispersion forces. The strength of these forces affects physical properties such as boiling point, melting point, and solubility. The introduction video provided a solid foundation for understanding these concepts, highlighting their importance in chemistry and everyday life. To further your knowledge on intermolecular forces, we encourage you to explore additional resources such as textbooks, online tutorials, or scientific journals. Don't hesitate to ask questions or participate in discussions to deepen your understanding. Remember, mastering intermolecular forces is essential for comprehending more advanced chemical concepts and their real-world applications. By continuing to engage with this topic, you'll develop a stronger grasp of molecular interactions and their far-reaching implications in various scientific fields.

Intermolecular Forces

Intermolecular forces Definition of 'intermolecular force'

Step 1: Introduction to Intermolecular Forces

Intermolecular forces are the forces that exist between molecules. These forces are distinct from the forces that hold atoms together within a molecule, which are known as intramolecular forces. Intramolecular forces include covalent bonds and ionic bonds, which are responsible for the formation of molecules and compounds. In contrast, intermolecular forces are responsible for the interactions between molecules, influencing properties such as boiling points, melting points, and solubility.

Step 2: Differentiating Between Intramolecular and Intermolecular Forces

To understand intermolecular forces, it is essential to differentiate them from intramolecular forces. Intramolecular forces, such as covalent and ionic bonds, hold the atoms within a molecule together. For example, in an oxygen molecule (O2), the bond between the two oxygen atoms is an intramolecular force. If this bond were broken, the molecule would no longer exist as O2, but as two separate oxygen atoms.

On the other hand, intermolecular forces act between molecules. For instance, if two O2 molecules come close to each other, they will experience intermolecular forces that influence their behavior and stability. These forces are generally weaker than intramolecular forces but play a crucial role in determining the physical properties of substances.

Step 3: Types of Intermolecular Forces

There are several types of intermolecular forces, each with varying strengths and characteristics. The main types include:

  • Hydrogen Bonding: This is a strong type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms such as nitrogen, oxygen, or fluorine. The hydrogen atom carries a partial positive charge, which attracts the partial negative charge on the electronegative atom of a neighboring molecule.
  • London Dispersion Forces: Also known as van der Waals forces, these are the weakest intermolecular forces. They arise from temporary dipoles that occur due to the momentary distribution of electrons in atoms or molecules. These forces are present in all molecules, whether polar or nonpolar.
  • Dipole-Dipole Interactions: These forces occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another. The strength of dipole-dipole interactions depends on the polarity of the molecules involved.

Step 4: Importance of Intermolecular Forces

Intermolecular forces are crucial in determining the physical properties of substances. For example, the boiling and melting points of a substance are influenced by the strength of its intermolecular forces. Substances with strong intermolecular forces, such as hydrogen bonds, typically have higher boiling and melting points compared to those with weaker forces like London dispersion forces.

Additionally, intermolecular forces play a significant role in solubility. Polar substances tend to dissolve well in polar solvents due to dipole-dipole interactions, while nonpolar substances are more soluble in nonpolar solvents due to London dispersion forces.

Step 5: Application of Electrostatic Principles

Understanding intermolecular forces requires knowledge of electrostatic principles. Electrostatic forces are the forces of attraction or repulsion between charged particles. In the context of intermolecular forces, these principles help explain how molecules interact based on their charge distributions. For example, the partial charges in polar molecules lead to dipole-dipole interactions, while temporary dipoles in nonpolar molecules result in London dispersion forces.

By applying these principles, we can predict and explain the behavior of different substances under various conditions, such as changes in temperature and pressure.

Step 6: Summary and Conclusion

In summary, intermolecular forces are the forces that act between molecules, influencing their physical properties and behavior. These forces include hydrogen bonding, London dispersion forces, and dipole-dipole interactions. Understanding the differences between intramolecular and intermolecular forces, as well as the application of electrostatic principles, is essential for comprehending the nature of molecular interactions.

By studying intermolecular forces, we gain insights into the properties of substances, which is crucial for various scientific and industrial applications, including the design of new materials and the development of pharmaceuticals.

FAQs

Here are some frequently asked questions about intermolecular forces:

  1. What intermolecular forces are present between two molecules of HI?

    The intermolecular forces present between two HI (hydrogen iodide) molecules are dipole-dipole interactions and London dispersion forces. HI is a polar molecule due to the electronegativity difference between hydrogen and iodine, resulting in dipole-dipole interactions. London dispersion forces are present in all molecules due to temporary electron fluctuations.

  2. What are intermolecular forces in simple terms?

    Intermolecular forces are the attractive or repulsive forces between molecules. They are weaker than chemical bonds but play a crucial role in determining the physical properties of substances, such as boiling point, melting point, and solubility.

  3. What is the purpose of intermolecular forces?

    Intermolecular forces serve to hold molecules together in liquids and solids. They determine many physical properties of substances and influence how molecules interact with each other. These forces are essential for processes like dissolution, condensation, and the formation of molecular structures in biology.

  4. What do intermolecular forces determine?

    Intermolecular forces determine various physical properties of substances, including boiling point, melting point, surface tension, viscosity, and solubility. They also influence the state of matter (solid, liquid, or gas) at a given temperature and pressure.

  5. How do intermolecular forces affect the boiling point of a substance?

    Stronger intermolecular forces generally result in higher boiling points. This is because more energy is required to overcome these forces and separate the molecules into the gas phase. For example, water has a relatively high boiling point due to strong hydrogen bonding between molecules.

Prerequisite Topics

Understanding intermolecular forces is a crucial aspect of chemistry that builds upon several fundamental concepts. While there are no specific prerequisite topics provided for this subject, it's important to recognize that a strong foundation in basic chemistry principles is essential for grasping the intricacies of intermolecular forces.

Intermolecular forces are the attractive or repulsive forces between molecules that play a significant role in determining the properties of substances. To fully comprehend these forces, students should have a solid understanding of atomic structure, chemical bonding, and molecular geometry. These foundational concepts provide the necessary context for exploring how molecules interact with one another.

A key aspect of intermolecular forces is the concept of polarity. Students should be familiar with electronegativity and how it affects the distribution of charge within molecules. This knowledge is crucial for understanding dipole-dipole interactions, which are a type of intermolecular force.

Additionally, a basic grasp of thermodynamics can greatly enhance one's understanding of intermolecular forces. Concepts such as energy, enthalpy, and entropy are closely related to the behavior of molecules and the forces between them. These principles help explain why certain intermolecular forces occur and how they influence the physical properties of substances.

Another important prerequisite is an understanding of the different states of matter. Intermolecular forces play a crucial role in determining whether a substance exists as a solid, liquid, or gas under various conditions. Familiarity with phase changes and the factors that influence them can provide valuable insights into the strength and nature of intermolecular forces.

Moreover, a basic knowledge of organic chemistry can be beneficial when studying intermolecular forces. Many examples and applications of these forces are found in organic compounds, so understanding the structure and behavior of carbon-based molecules can provide relevant context.

While specific prerequisite topics were not provided, it's clear that a well-rounded foundation in general chemistry is essential for tackling the subject of intermolecular forces. Students who have a solid grasp of these fundamental concepts will find themselves better equipped to explore the fascinating world of molecular interactions and their impact on the properties of matter.

By building upon these foundational concepts, students can develop a deeper appreciation for how intermolecular forces influence the world around us, from the boiling point of water to the structure of proteins. This understanding is not only crucial for academic success in chemistry but also for applying this knowledge to real-world problems and innovations in fields such as materials science, drug design, and environmental studies.

In this lesson, we will learn:
  • To apply the principles of electrostatic forces to understand intermolecular forces.
  • To explain how hydrogen bonding, London forces and dipole-dipole interactions occur.
  • To recall the relative strength of relative forces between molecules.

Notes:
  • The molecules of chemical compounds are held together by chemical bonds, or forces between the atoms the molecules are made of. As we saw earlier, these forces between atoms are called intramolecular forces and the two major types, like two ends of a left-to-right spectrum, are covalent and ionic bonding.

  • As well as intramolecular forces, there are intermolecular forces that occur between the molecules of any chemical substances. Intermolecular forces are not as strong as intramolecular forces, but they influence a lot of properties in a chemical. For example, the melting point of a substance is greatly influenced by the intermolecular forces holding molecules together. To summarize, we have:
    • Intramolecular forces, which are strong and hold the atoms within a molecule together. These are generally only broken in chemical reactions.
    • The focus of this lesson: intermolecular forces, which are weaker but hold separate molecules together. These can generally be overcome by physical changes such as temperature.

  • We now need to understand what a dipole is, because all intermolecular forces are based on dipole interactions. A dipole (di-pole, ‘two poles’) is a charge difference between atoms in part of a molecule, created by excess charge. Whenever an atom or molecule is more positive than another atom or molecule (which is more negative), you might have a dipole. This can happen in a few ways.
    • Dipoles can form from a covalent bond between two elements of different electronegativity.
      • Example below: carbon and chlorine can make a covalent bond. Because chlorine is more electronegative than carbon, the electron pair they share in the bond is going to be held closer to chlorine (on average) than carbon. This means that this part of the molecule on average is going to be slightly more negative (because the negatively-charged electrons are closer) around the chlorine atom than the carbon atom, where the relative lack of electrons leaves carbon with a slight positive charge. Therefore we give carbon a δ\delta+ (delta positive) and chlorine a δ\delta- (delta negative) charge. These are ‘partial’ positive / negative charges.
        • intermolecular force example 1

      • Because the electronegativity of an atom does not change, this effect will always be present in a carbon-chlorine bond – dipoles like this are called permanent dipoles. Any bond between two atoms with a difference in electronegativity creates a dipole like this. We call the bond a polar bond because it has the positive/negative charged effect of a north/south pole.

  • A major type of intermolecular force are van der Waals forces:
    • One type of van der Waals forces are dipole-dipole interactions.
      • When a molecule of a compound has a permanent dipole (see above), the delta positive ( δ\delta+) and delta negative ( δ\delta-) charge influences other molecules around them too. This includes other identical molecules!
      • Because each molecule has the same permanent dipole, they arrange with the opposite ends of other molecules to maximize their δ\delta+/ δ\delta- attractive forces with as many molecules as possible across 3d space. See below for an example with HCl, where H holds a δ\delta+ and Cl holds a δ\delta- charge in their covalent bond together.
        • dipole-dipole interactions example 1

      • Because this effect is caused by dipoles across molecules interacting, it is called dipole-dipole interactions. This is the stronger of the two van der Waals type forces.

    • Another type of van der Waals forces are London dispersion forces (London forces).
      • Because electrons repel each other and are attracted to positive charge, if two atoms or molecules approach one another, the electrons in each will repel the other. (See below: the solid black is high electron density, spotted is less electrons / lower density).
        • London dispersion forces 1

      • The movement of electrons due to repulsion polarizes both atoms and leads to a dipole being created: see above.
        • London dispersion forces 2

      • This 'forced' polarization of two atoms/molecules that come too close to each other is called an induced dipole. This is the attraction in London dispersion forces.
      • Because this attractive force only exists when atoms/molecules are close together, and vanishes if they are moved apart, it is a temporary dipole, not a permanent one. For this reason we say London forces are temporary induced dipole forces. They are weaker than permanent dipole-dipole interactions (see above) and can exist between any atoms.
      • London forces are stronger on atoms that are more polarizable – meaning the charge (electrons) on the atom can be manipulated and moved around. This occurs much more easily in larger atoms with more electron shells (so the outer shell is further from the nucleus). So, smaller atoms with less electron shells are less polarizable and have weaker London forces. Think of the difference between trying to twist and change the shape of a hard, small marble and a large bean bag – the bean bag is a more ‘polarizable’ larger atom!
      • This explains why the melting point of helium is lowest in the noble gases, and melting point increases going down the group as London forces become stronger, needing more energy to overcome.

  • Another type of intermolecular force is known as hydrogen bonding. These are the strongest intermolecular forces.
    • Hydrogen bonding is a very strong form of dipole-dipole interactions that happens when hydrogen, H, covalently bonds to a very electronegative element ‘X’ (X = F, O or N). Because hydrogen is not very electronegative, these bonds make strong permanent dipoles; the δ\delta+ charge on hydrogen is relatively big, and the δ\delta- on the electronegative atom ‘X’ is equally strong.
    • This causes attractive forces; interactions between the negative lone pair electrons on X in one molecule and the slightly positive hydrogen atom in the other. Chemists say the lone pair donates a hydrogen bond and the hydrogen atom accepts it. Hydrogen bonds are very similar to dipole-dipole interactions but even stronger.
    • The properties affected by hydrogen bonding are cumulative; the more hydrogen bonding there is, the more the properties will are affected. Electronegative atoms with more than one lone pair (like oxygen) can donate more than one hydrogen bond, which affects properties more than if it could donate just one hydrogen bond (e.g. nitrogen).
      • Hydrogen bond

  • Intermolecular forces are cumulative – they have an 'adding up' effect and will occur across all or any fraction of a molecule that makes contact with another. This is why:
    • Longer hydrocarbon chains (with ‘more molecule’ to make contact) have a higher melting/boiling point than shorter ones.
    • Unbranched straight hydrocarbon chains (which can get closer to one another, so they share more van der Waals forces) have a higher melting/boiling point than branched chains which can’t make as much contact.

  • Water is a good example of how the strength and amount of hydrogen bonding in a chemical structure can change properties considerably:
    • Compared to similar compounds with the same number of electrons, water has an unusually high melting/boiling point.
      • With two electronegative lone pairs and two electropositive hydrogen atoms, it can donate two and accept two hydrogen bonds. With four hydrogen bonds per molecule in total, intermolecular forces are much stronger than in ammonia (only one lone pair and N-H being a weaker dipole) and methane (with no lone pairs and therefore no hydrogen bonding).
    • Liquid water has a higher density than solid ice. That’s why icebergs float at the water’s surface!
      • As a solid where hydrogen bonding dictates how the water molecules are ‘packed together’, the molecules are actually less compact than when they are liquid. This makes ice less dense and so it floats on the water’s surface

  • A chemical substance’s solubility depends on the intermolecular forces it displays. This is important in planning chemical reactions. Generally, “like dissolves like” in terms of intermolecular forces between reactants and solvents. Some important points about solubility and intermolecular forces are below:
    • The dipoles in water molecules are strong enough to dissolve some ionic compounds by interacting with the individual ions. The δ\delta- oxygen atoms can interact with positive ions, and δ\delta+ hydrogen atoms with negative ions. As solvent (water) molecules far outnumber ions, Multiple water molecules can surround ions in a process called hydration. This is why many simple ionic compounds are soluble in water.
    • Alcohols such as ethanol are capable of hydrogen bonding, because of the O-H group present. Ethanol molecules will interact strongly with water molecules through their hydrogen bonding, so small simple alcohols such as ethanol are highly soluble in water through mutual hydrogen bonding.
    • Not all polar molecules are soluble in water. Most haloalkanes for example have permanent dipoles due to the carbon-halogen bond. However, any dipole-dipole interactions they may have with water will not be strong enough to overcome interactions between hydrogen-bonding water molecules interacting among themselves. As such, haloalkanes are only slightly soluble in water because they cannot form the hydrogen bonds necessary to interact with a water solvent.

  • The stronger or more intermolecular forces there are, the more the physical properties are affected – hardness, melting and boiling point and whether a substance conducts electricity are all affected by intermolecular forces. Later, in Polarity, you'll be able to predict these properties of substances based on intermolecular forces!