Chemical Reactions and Equations

C4 Chemical Reactions

This chapter covers:

Chemistry deals with how substances react with each other.
Chemical reactions range from very simple to complex, such as those that keep our bodies alive.

Ice, snow, and water are all made of water molecules (H_2O) but are in different physical forms due to temperature and pressure.
In physical changes, no new chemical substances are formed.
Example: Dissolving sugar in ethanol or water.
Physical changes are often easy to reverse, and mixtures produced are usually easy to separate.

When magnesium burns in oxygen, it produces magnesium oxide (a new substance) and a brilliant white flame. This is an exothermic change, releasing heat and light.
Some chemical reactions, like those in fluorescent glow bracelets, produce chemiluminescence, emitting energy in the form of light.
During the reaction between nitrogen and oxygen to make nitrogen monoxide, heat energy is taken in from the surroundings (endothermic change).
Exothermic reactions are more common than endothermic ones.

C4.01 State whether the following changes are physical or chemical
- a the melting of ice
- b the burning of magnesium
- c the sublimation of solid carbon dioxide
- d the dissolving of sugar in water.
C4.02 State whether the following changes are exothermic or endothermic:
- a the condensation of steam to water
- b the burning of magnesium
- c the addition of concentrated sulfuric acid to water
- d the evaporation of a volatile liquid.
C4.03 What is the most important thing that shows us that a chemical reaction has taken place?

Some chemical reactions are obvious (e.g., explosions), while others are not (e.g., neutralization of an acid with an alkali).
Even if there's no visible change, a reaction has happened if temperature changes and new substances are formed.

Word equations link the names of the reactants with the names of the products.
Example: magnesium + oxygen → magnesium oxide
The reaction between hydrogen and oxygen is another highly exothermic reaction. The reaction has been used to fuel rockets, most notably the now-retired Space Shuttle. The word equation for this reaction is:
hydrogen + oxygen→ water
An equation includes only the chemical substances involved, and energy is not a chemical substance.

Law of conservation of mass: the total mass of all the products of a chemical reaction is equal to the total mass of all the reactants.
During a chemical reaction, atoms of one element are not changed into those of another element, nor do atoms disappear or appear from nowhere.
Reactions involve the breaking and making of bonds between atoms.
Atoms 'change partners' during a chemical reaction.
Each molecule of water (formula H2O) contains only one oxygen atom (O). It follows that one molecule of oxygen (O2) has enough oxygen atoms to produce two molecules of water (H2O). Therefore, two molecules of hydrogen (H2) will be needed to provide enough hydrogen atoms (H) to react with each oxygen molecule. The numbers of hydrogen and oxygen atoms are then the same on both sides of the equation.
The symbol equation for the reaction between hydrogen and oxygen is therefore written: 2H2 + O2 → 2H_2O
A balanced equation has the same number of each type of atom on both sides.

Step-by-step approach to working out the balanced equation for a reaction:
- Step 1: Identify reactants and products.
- Step 2: Write the word equation.
- Step 3: Write the equation using chemical formulas.
- Step 4: Balance the equation by putting multiplying numbers in front of each formula but never alter the formulae of the substances.
Example: Balancing the equation for the reaction between magnesium and oxygen.
- Magnesium burns in air (oxygen) to form magnesium oxide.
- Word equation: magnesium + oxygen → magnesium oxide
- Unbalanced symbol equation: Mg + O_2 → MgO
- Balanced equation: 2Mg + O_2 → 2MgO
Chemical reactions often involve compounds, not just elements.
Potassium metal is highly reactive and produces hydrogen gas when it contacts water.