- Scientists convert everyday biological processes into equations to:
- Simplify complex reactions: Equations reduce lengthy descriptions into concise statements.
- Ensure accuracy: Equations provide a clear and precise way to represent reactions.
- Aid communication: Standardized equations allow scientists worldwide to understand and replicate experiments.
Think of equations as recipes that tell you exactly what ingredients you need and what products you'll get.
A word equation describes the overall chemical change using names only, leaving out enzymes, energy transfers, or intermediate steps:
$$glucose + oxygen → carbon dioxide + water$$
- Word equations are useful when learning the core idea of a reaction before using formulas.
- To read them:
- Reactants appear on the left.
- The arrow means “reacts to form.”
- Products appear on the right.
- Order does not show timing or mechanism, only input and output.
A chemical equation uses symbols and formulas to show how atoms rearrange. It's more precise than a word equation because it shows actual molecular formulas:
$$C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O$$
- Students often swap coefficients and subscripts.
- Coefficients balance the equation.
- Subscripts define the molecule and cannot be changed.
- The number of each type of atom must be the same on both sides.
- This follows the law of conservation of mass.
- You balance equations by adjusting coefficients, not subscripts.
Respiration only becomes correct once oxygen, carbon, and hydrogen all match.
- Never alter subscripts to balance an equation.
- Only adjust the numbers in front of formulas.
- Real biological reactions involve many steps, each controlled by enzymes.
- Writing every step would be too complex to study.
- An equation shows only the net chemical change, which is all that is needed for most analysis.
- Cellular respiration involves many enzymes and dozens of intermediate compounds, but its core chemical change can be represented simply:
$$glucose + oxygen → carbon dioxide + water$$
- A chemical equation provides even more detail:
$$C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O$$
- Respiration: $C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O$
- Photosynthesis: $6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂$
- Lactose digestion: $lactose → glucose + galactose$
- Lactic acid fermentation: $glucose → lactic acid$
- Each highlights the core chemical change, not the enzyme mechanisms beneath it.
- What does a word equation show, and what does it leave out?
- What must be true for a chemical equation to be correctly balanced?
- Why do scientists prefer equations over everyday descriptions of reactions?
- Why is changing a subscript different from changing a coefficient?
- How does the respiration equation summarise a much larger metabolic pathway?
- Explain how equations help biologists compare processes across species.
- 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.
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”
Hydrogen + oxygen → water
Word equations are especially useful when you are first learning chemistry because they focus on meaning (what substances are involved) without needing formulas yet.
- 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.
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)
- 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.
- 2H₂O means: two molecules of water
- Total atoms: H = 4, O = 2
- 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)
- NaCl(s) → Na⁺(aq) + Cl⁻(aq)
- This shows that sodium chloride dissolves in water to form ions (useful later for IB).
- “(aq)” does not mean “liquid water”.
- It means the substance is dissolved in water.
- 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.
Hydrogen + oxygen → water
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
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.
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.
$$2H_2 (g) + O_2 (g) \to 2H_2O (l)$$
- 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?
- It supports quantitative chemistry (bridge to IB)
- Later, you’ll use balanced equations to calculate:
- how many moles react
- limiting reactants
- percentage yield
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.
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
- 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).
- 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.