Introduction to acid-base theory

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  1. What are acids and bases?
  2. Arrhenius theory.
  3. Lewis and Brønsted-Lowry acids and bases.
  4. What happens to H+ ions?
  5. What is a basic solution?
  6. Measuring pH using H3O+ ions.
  1. Recall how pH is measured in solutions.
    1. A solution has an H3O+ concentration of 0.07 mol dm-3. What is its pH?
    2. What is the pH of the solution if the concentration is doubled?
    3. How many times more acidic is a solution at pH 2 than a solution at pH 5?
    1. Understand water's properties to act as an acid and a base.
      Water (H2O) is an amphiprotic molecule.
      1. Explain what this means in terms of acids and bases.
      2. Write two chemical equations to show this property.
      Topic Notes

      In this lesson, we will learn:

      • To understand how acids, bases and salts were originally and currently defined in chemistry.
      • To understand how acid-base theory is related to pH and the amphoteric nature of water.
      • To understand what H+ ions do in solution and how acid-base reactions depend on them.


      • Even though acids, bases and salts are common and well-studied in chemistry, there are different sets of definitions that depend on what you focus on in an acid-base reaction. The chemicals that we call ‘acids’ (e.g. hydrochloric acid) and ‘bases’ (such as sodium hydroxide) all have something in common which leads to the acidic/basic properties that we know.

      • The original definitions of acid, base and salt come from Arrhenius theory (named after Svante Arrhenius), which contains a few basic ideas about how acids and bases are related:
        • An acid is any substance which, in water, produces hydrogen ions (H+).
        • A base is any substance which, in water, produces hydroxide ions (OH-).
        • A salt is any substance which is the product of an acid-base reaction.
          Practically, this means any ionic compound that isn't an acid or base is a salt.

      • These definitions were the first of their kind in describing the properties of acids and bases (Arrhenius did a lot for chemistry!) and are the reason why many chemistry teachers give the following (not perfect) simple rule to 'spot' acids and bases:
        • Any ionic compound beginning with H is an acid.
        • Any ionic compound with an OH group is a base.

      • Today there are two theories or 'points of view’ to acid-base reactions. Brønsted-Lowry theory which is closely related to the original Arrhenius theory is much more common:
        • Brønsted-Lowry acid-base theory is about protons.
          • A Brønsted-Lowry acid is a proton (H+) donor such as HCl.
          • A Brønsted-Lowry base is a proton acceptor.

        • There is also Lewis theory is about electrons.
          • A Lewis acid is an electron pair acceptor such as BH3.
          • A Lewis base is an electron pair donor.

      • Looking at the Arrhenius and Brønsted-Lowry definitions of acids and bases, you will see they are about the hydrogen ion (H+). This is actually not what causes the properties acids are known for. The properties that ‘acids’ have are from the hydronium ion, H3O+. This gets produced rapidly when protons mix with water.
        • Hydrogen atoms have only one proton and one electron. Therefore when 'hydrogen ions (H+) are produced' by a substance (like hydrochloric acid, HCl) dissolving in water, what is actually released is just a single proton without the electron it used to have – quite literally just one tiny proton with nothing surrounding it!
        • The charge density of a lone proton with nothing surrounding it is INCREDIBLY high – an individual proton of +1 charge is extremely small compared to a hydrogen atom with the same positive nucleus with the electron 'cloud' orbiting it. This makes H+ extremely reactive and it will interact with anything remotely negative nearby. Remember, this is all happening dissolved in water.
        • This tiny ultra-concentrated proton immediately interacts with a negative lone pair on the oxygen atom of a nearby water molecule. In doing this, a hydronium ion, H3O+, is formed. This process can be described with the equation:
        • H+(aq) + H2O (l) → H3O+
        • The process can be explained in two ways:
          • Acids release H+ ions in solution which 'protonate' water molecules.
          • Acids release H+ in solution which are ‘hydrated’ (added to by water).

      • The reverse process occurs when bases are added to solution. When bases (that produce OH-) are dissolved, their strong negative charge means they react to neutralize H+ and H3O+ species, which decreases their concentration in the solution. This can also deprotonate neutral water molecules. See the equation below:
        H3O+ (aq) + OH- (aq) → 2H2O (l)
        • This process of decreasing hydronium ion concentration is what makes the pH rise. Eventually a lack of hydronium ions to neutralize the hydroxide ions means there will be free hydroxide (OH-) ions in solution, which causes the basic properties chemists observe.

      • We have seen above that water can act as a base in acidic conditions; it accepts H+ ions to form the hydronium ion. However, water also can act as an acid in basic conditions; it donates H+ to bases in solution, forming a hydroxide ion as a result.
        • Water’s ability to act as both acid and base makes it an amphoteric molecule.

      • Acid-base theory is important for understanding how pH is measured. Remember, the definition of acidic is pH < 7, while basic is pH > 7.
        • The equation to find pH of a solution is:
          pH = -log[H3O+]

        • This is an inverse logarithmic expression which means the following:
          • Inverse: as the concentration of hydronium ions increases, pH decreases.
          • Logarithmic: To change the pH by 1, you have to change [H3O+] by a factor or ten. For example, a solution of pH 4 is ten times more concentrated with H3O+ than a solution of pH 5.
        • This equation explains why acidic solutions have a low pH value, while basic solutions (where [H3O+] is low) have a high pH value.