Word and Chemical Equations
Why Chemists Write Equations At All
- When a chemical reaction happens, atoms are rearranged into new substances.
- The reaction might look simple in real life (a flame, bubbles, a colour change), but scientists need a way to:
- describe exactly what substances are reacting
- show how many particles react and form (ratios)
- compare reactions fairly between experiments
- communicate the same idea internationally and accurately
- A chemical equation is like a “reaction sentence” that tells the story of what changes, using a standard format that chemists worldwide understand.
Word Equations
What a Word Equation Shows
A word equation describes a reaction using the names of substances.
reactant(s) → product(s)
- Reactants start the reaction (written on the left)
- Products are formed (written on the right)
- The arrow → means “produces” or “forms”
- A plus sign + means “reacts with” or “and”
Example
Hydrogen + oxygen → water
Note
Word equations are especially useful when you are first learning chemistry because they focus on meaning (what substances are involved) without needing formulas yet.
Chemical (Symbol) Equations
What a Chemical Equation Shows
- A chemical (symbol) equation represents the same reaction using chemical formulas.
- It can also include extra information such as state symbols, reaction conditions, and sometimes energy changes.
Example
2H₂(g) + O₂(g) → 2H₂O(l)
This is more precise than a word equation because it shows:
- the formulas (exact identity of substances)
- the ratio of particles reacting (coefficients)
- the physical state of each substance (state symbols)
Key Vocabulary You Must Know
Coefficient vs Subscript
- A subscript is the small number inside a formula (e.g., H₂O has 2 H atoms).
- Subscripts are part of the substance’s identity and must never be changed when balancing.
- A coefficient is the number in front of a formula (e.g., 2H₂O).
- Coefficients tell you how many particles (molecules or formula units) are involved and can be changed to balance.
Example
- 2H₂O means: two molecules of water
- Total atoms: H = 4, O = 2
State Symbols
- State symbols tell the physical state of each substance, which affects:
- how fast the reaction happens
- whether substances can mix/collide
- whether the product is a gas (bubbles), solid (precipitate), etc.
- State symbols:
- (s) solid
- (l) liquid
- (g) gas
- (aq) aqueous (dissolved in water)
Example
- NaCl(s) → Na⁺(aq) + Cl⁻(aq)
- This shows that sodium chloride dissolves in water to form ions (useful later for IB).
Common Mistake
- “(aq)” does not mean “liquid water”.
- It means the substance is dissolved in water.
From a Word Equation to a Balanced Chemical Equation
Atoms Must Balance
- In chemical reactions, atoms are not created or destroyed. They simply rearrange.
- So a correct equation must have the same number of each type of atom on both sides.
- This is the law of conservation of mass.
Example
Step-by-Step Method
Step 1: Write the word equation
Hydrogen + oxygen → water
Step 2: Write the correct formulas (skeleton equation)
Hydrogen is H₂ (diatomic), oxygen is O₂ (diatomic), water is H₂O.
Skeleton equation:
$$H_2 + O_2 \to H_2O$$
- Note that some elements exist naturally as pairs of atoms (diatomic molecules).
- Common ones to remember at this level: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂
Step 3: Count atoms on both sides
A helpful way is a quick table:
| Element | Left side | Right side |
|---|---|---|
| H | 2 | 2 |
| O | 2 | 1 |
Oxygen doesn’t match, so it’s not balanced yet.
Step 4: Balance using coefficients (NOT subscripts)
To increase oxygen on the right, place a coefficient 2 in front of H₂O:
$$H_2 + O_2 \to 2H_2O$$
Now recount:
| Element | Left side | Right side |
|---|---|---|
| H | 2 | 4 |
| O | 2 | 2 |
Hydrogen is now unbalanced, so we fix hydrogen by putting 2 in front of H₂:
$$2H_2 + O_2 \to 2H_2O$$
Recount:
| Element | Left side | Right side |
|---|---|---|
| H | 4 | 4 |
| O | 2 | 2 |
Now it is balanced.
Step 5: Add state symbols (if known)
$$2H_2 (g) + O_2 (g) \to 2H_2O (l)$$
Why Balancing Matters
- It represents what must physically happen
- If atoms are conserved, then the equation must show a rearrangement that is actually possible.
- An unbalanced equation is like a story where characters disappear halfway through.
- It allows fair comparisons
- Balanced equations allow scientists to compare reactions by looking at ratios:
- Which reaction needs more oxygen?
- Which produces more gas?
- Which makes twice as much product?
- Balanced equations allow scientists to compare reactions by looking at ratios:
- It supports quantitative chemistry (bridge to IB)
- Later, you’ll use balanced equations to calculate:
- how many moles react
- limiting reactants
- percentage yield
- Later, you’ll use balanced equations to calculate:
Note
At MYP level, start with this idea:
- Coefficients show ratios.
- For example, in 2H₂ + O₂ → 2H₂O, the ratio H₂ : O₂ : H₂O is 2 : 1 : 2.
What Extra Information Can Be Included in Chemical Equations?
Reaction Conditions (written around the arrow)
Chemists can show special conditions needed for a reaction:
- heat: $\Delta$ (or a temperature)
- light: hν
- catalyst: written above the arrow (e.g., Ni, Pt, MnO₂)
- pressure: especially for gases
Reversible reactions
- Some reactions can go both directions depending on conditions: $$\mathrm{N}_2(\mathrm{g})+3 \mathrm{H}_2(\mathrm{g}) \rightleftharpoons 2 \mathrm{NH}_3(\mathrm{g})$$
- The double arrow ⇌ means the reaction can proceed both ways (equilibrium idea which will be explored later in IB but was also briefly discussed in the article about Reversible Reactions).
Energy changes
- Sometimes equations show whether energy is released or absorbed.
- Exothermic: energy released (often written as heat on the products side)
- Endothermic: energy absorbed (heat on reactants side)
- At this stage, it’s enough to recognise: Energy is part of the reaction story, even if we don’t calculate it yet.
