# Introduction to chemical reactions

### Introduction to chemical reactions

#### Lessons

In this lesson, we will learn:
• The difference between a chemical and physical change
• How to identify evidence for a chemical reaction occurring.
• The methods of writing a chemical reaction.

Notes:
• Chemical reactions (or chemical changes) involve collisions between molecules which rearrange atoms or bonds within molecules, changing the overall composition. They are far too small to see directly with the naked eye. Instead, we make observations which are evidence that a reaction has happened or is happening.
For chemists, an observation is a measurable that can be measured we can tell directly using our senses, for example seeing a color change, feeling a temperature change (like a glass beaker warming up) or seeing gas being released from a test tube.

• Chemical changes are not the same as physical changes. We are only talking about chemical changes in this chapter; know the difference!
• A chemical change changes the composition of a substance(s) by rearranging their atoms or bonds. This creates new substances with new properties.
• Examples of chemical changes are combustion (‘burning’) and acid-base neutralization.
• Evidence of chemical changes are gases being produced, colour changes and forming of an insoluble precipitate.
• A physical change changes the properties of a substance but the substance composition doesn’t fundamentally change (e.g. ice or steam might not be ‘water’ but it is still made of H2O molecules!). The change may be cause by the breaking of intermolecular forces.
• Examples of physical changes are changes of phase/state, like melting, boiling and condensation.

With this said, remember that whatever chemical change occurs, atoms retain their identity in chemical reactions. Bonds between atoms may break/form or otherwise rearrange (this can change molecules and produce new substances), but the individual atoms are not changed, created or destroyed.

• The reason this happens is because different chemical compounds have different properties that we can see when that chemical is produced. For example a color change from brown to colorless might mean a reactant chemical that is brown is being converted into a product that is colorless.

• There are two ways to write that a reaction is taking place:
• a word equation
• a chemical (symbol) equation.

• The $\,$$\,$ arrow found in a chemical equation is very important; it shows that a reaction is taking place. The reactants (what gets put in) of a reaction are always on the left-hand side of it, and the products (the new chemical that gets made) of a reaction are always on the right-hand side.
• Introduction
Introduction to chemical reactions
a)
What is a chemical reaction?

b)
How do we know reactions happen?

c)
How to write a word equation

d)
How to write a chemical equation

• 1.
Write word and chemical equations to describe basic chemical reactions.
Some hydrochloric acid (HCl) was added to a beaker containing sodium hydroxide (NaOH). A reaction occurred and produced sodium chloride (NaCl) and water (H$_2$O).
a)
Write a word equation to show this reaction.

b)
Write a chemical equation to show this reaction happening.

c)
Explain why sodium chloride is written on the right-hand side of the reaction arrow.

• 2.
Recognise evidence of a chemical change or reaction.
Read the notes from each lab experiment below and highlight which part of the experiment notes shows evidence that a chemical reaction is happening:

Experiment 1: Hexene was added to bromine water in a beaker and stirred thoroughly. Afterward, the brown color disappeared and the mixture became colorless.

Experiment 2: Using a delivery tube and gas syringe, carbon dioxide was bubbled through a solution containing lime water. Over a short time, the lime water became cloudy.

Experiment 3: Sodium hydroxide and sulfuric acid were added to a test tube which was shaken thoroughly. The test tube began to feel warm to the touch.

Experiment 4: Bubbling and fizzing occurred when a strip of magnesium metal was dropped in a solution of hydrochloric acid.

Experiment 5: Iron sulfide powder was added to a solution of hydrochloric acid and a bad smell of rotten eggs was noticed.