Introduction: Chemical Equations as Reaction Recipes

Chemistry is fundamentally about change , atoms rearranging to form new substances. To describe and quantify these changes, chemists use chemical equations: symbolic representations that show exactly which substances react and which are produced.

Chemical equation: A chemical equation is a symbolic representation of a chemical reaction, showing reactants (starting substances) on the left, products (substances formed) on the right, and an arrow indicating the direction of the reaction.

Analogy

Think of a chemical equation as a recipe. Just as a cake recipe specifies exact amounts of flour, sugar, and eggs, a chemical equation specifies the exact ratios of reactants needed to produce the desired products. Use too much or too little of an ingredient, and the result won't be what you expected , the same is true in chemistry.

Chemical equations must be balanced to reflect the law of conservation of mass: atoms are rearranged during a reaction, never created or destroyed. This means the number of atoms of each element must be identical on both sides of the equation.

Law of conservation of mass: The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. The total mass of reactants equals the total mass of products.

The study of quantitative relationships in chemical equations is called stoichiometry , and it underpins virtually every calculation in chemistry.

Balancing Chemical Equations

Before you can use a chemical equation to calculate amounts of substances, you must ensure it is correctly balanced. Balancing involves adjusting stoichiometric coefficients (the numbers in front of chemical formulas) until the atom count is equal on both sides.

Steps to balance a chemical equation:

Write the unbalanced equation with correct formulas for all reactants and products.
Count the atoms of each element on both sides.
Add or adjust coefficients to equalise atom counts , start with elements that appear in the fewest compounds, and leave simple diatomic molecules (like $O_{2}$ or $H_{2}$ ) until last.
If fractional coefficients arise, multiply all coefficients by the appropriate integer to obtain whole numbers.
Verify by recounting all atoms on both sides.

Warning

You must only change coefficients , never change the subscripts within a chemical formula. Changing subscripts would alter the identity of the substance entirely.

Example

Balancing the combustion of ethane, $C_{2} H_{6}$

Step 1: Unbalanced equation:
$C_{2} H_{6} + O_{2} \to C O_{2} + H_{2} O$

Step 2: Count atoms:

Reactants: 2 C, 6 H, 2 O
Products: 1 C, 2 H, 3 O

Step 3: Balance carbon first (fewest compounds):
$C_{2} H_{6} + O_{2} \to 2 C O_{2} + H_{2} O$

Step 4: Balance hydrogen:
$C_{2} H_{6} + O_{2} \to 2 C O_{2} + 3 H_{2} O$

Step 5: Balance oxygen (7 O atoms on right, so need 3.5 $O_{2}$ ):
$C_{2} H_{6} + 3.5 O_{2} \to 2 C O_{2} + 3 H_{2} O$

Step 6: Multiply through by 2 to get whole-number coefficients:
$2 C_{2} H_{6} (g) + 7 O_{2} (g) \to 4 C O_{2} (g) + 6 H_{2} O (l)$

Verify: 4 C, 12 H, 14 O on each side. ✓

Exam Tip

When balancing combustion reactions of hydrocarbons, use this order: carbon → hydrogen → oxygen. This approach nearly always leads to a quick balance.

State symbols provide additional information about the physical state of each substance:

Symbol	State
$(s)$	Solid
$(l)$	Liquid
$(g)$	Gas
$(a q)$	Aqueous (dissolved in water)

Always include state symbols in your equations when they are known , they are often required in IB exam answers.

Free preview

9 more sections in this topic

Symbol

State

(s)

Solid

(l)

Liquid

(g)

Gas

(a q)

Aqueous (dissolved in water)

R2.1 How much? The amount of chemical change

Introduction: Chemical Equations as Reaction Recipes

Balancing Chemical Equations

9 more sections in this topic

R2.1 How much? The amount of chemical change

Introduction: Chemical Equations as Reaction Recipes

Balancing Chemical Equations

9 more sections in this topic