Year 11 / Chemical reactions — reactants, products and energy change

The coefficients set the recipe.

Balanced equations conserve each element’s atom count. Coefficients describe ratios of particles or moles, not masses. Add state symbols to distinguish solids, liquids, gases and aqueous species. Reaction classes include combination, decomposition, combustion, displacement and acid–base reactions.

To find a limiting reactant, compare available amount divided by its coefficient. The smaller value sets the reaction extent. Calculate product from that limiting amount, then compare an actual collected amount with theoretical yield if needed.

INTERACTIVE MODEL

The reactant that runs out

Available O₂ / mol2

READ THE GRAPH

Explore the relationship

2H₂ + O₂ → 2H₂O. Product is capped when oxygen becomes limiting.

Maximum H₂O / mol

Tap or focus a plotted point to read its values.

View plotted data as a table

Explore the relationship
Series	Available H₂ / mol	Maximum H₂O / mol
Maximum H₂O / mol	0	0
Maximum H₂O / mol	0.166667	0.166667
Maximum H₂O / mol	0.333333	0.333333
Maximum H₂O / mol	0.5	0.5
Maximum H₂O / mol	0.666667	0.666667
Maximum H₂O / mol	0.833333	0.833333
Maximum H₂O / mol	1	1
Maximum H₂O / mol	1.16667	1.16667
Maximum H₂O / mol	1.33333	1.33333
Maximum H₂O / mol	1.5	1.5
Maximum H₂O / mol	1.66667	1.66667
Maximum H₂O / mol	1.83333	1.83333
Maximum H₂O / mol	2	2
Maximum H₂O / mol	2.16667	2.16667
Maximum H₂O / mol	2.33333	2.33333
Maximum H₂O / mol	2.5	2.5
Maximum H₂O / mol	2.66667	2.66667
Maximum H₂O / mol	2.83333	2.83333
Maximum H₂O / mol	3	3
Maximum H₂O / mol	3.16667	3.16667
Maximum H₂O / mol	3.33333	3.33333
Maximum H₂O / mol	3.5	3.5
Maximum H₂O / mol	3.66667	3.66667
Maximum H₂O / mol	3.83333	3.83333
Maximum H₂O / mol	4	4
Maximum H₂O / mol	4.16667	4
Maximum H₂O / mol	4.33333	4
Maximum H₂O / mol	4.5	4
Maximum H₂O / mol	4.66667	4
Maximum H₂O / mol	4.83333	4
Maximum H₂O / mol	5	4
Maximum H₂O / mol	5.16667	4
Maximum H₂O / mol	5.33333	4
Maximum H₂O / mol	5.5	4
Maximum H₂O / mol	5.66667	4
Maximum H₂O / mol	5.83333	4
Maximum H₂O / mol	6	4
Maximum H₂O / mol	6.16667	4
Maximum H₂O / mol	6.33333	4
Maximum H₂O / mol	6.5	4
Maximum H₂O / mol	6.66667	4
Maximum H₂O / mol	6.83333	4
Maximum H₂O / mol	7	4
Maximum H₂O / mol	7.16667	4
Maximum H₂O / mol	7.33333	4
Maximum H₂O / mol	7.5	4
Maximum H₂O / mol	7.66667	4
Maximum H₂O / mol	7.83333	4
Maximum H₂O / mol	8	4
Maximum H₂O / mol	8.16667	4
Maximum H₂O / mol	8.33333	4
Maximum H₂O / mol	8.5	4
Maximum H₂O / mol	8.66667	4
Maximum H₂O / mol	8.83333	4
Maximum H₂O / mol	9	4
Maximum H₂O / mol	9.16667	4
Maximum H₂O / mol	9.33333	4
Maximum H₂O / mol	9.5	4
Maximum H₂O / mol	9.66667	4
Maximum H₂O / mol	9.83333	4
Maximum H₂O / mol	10	4

Probe along the horizontal axis0Maximum H₂O / mol: 0

Explore: Increase hydrogen until oxygen limits the product. Explain the plateau.

2\mathrm H_2(g)+\mathrm O_2(g)\to2\mathrm H_2O(l)\qquad n_{\rm product}=\nu_{\rm product}\min\frac{n_i}{\nu_i}

WORKED EXAMPLE

React 5 mol H₂ with 2 mol O₂.

H₂ allows extent $\frac{5}{2}$ = 2.5; O₂ allows extent $\frac{2}{1}$ = 2.
O₂ limits the reaction, producing 4 mol H₂O and leaving 1 mol H₂.

Assumed knowledge

Balanced equations and the mole concept.

Learning checkpoints & source

YOUR LEARNING CHECKPOINT

Balance equations and use mole ratios.
Find limiting reactants and theoretical yields.

QCAA Chemistry · Unit 1 · Chemical reactions — reactants, products and energy change ↗

READY TO TRY IT?

The coefficients set the recipe.

The reactant that runs out

Explore the relationship

React 5 mol H₂ with 2 mol O₂.

Put the idea to work.