Leftovers

Leftovers: find the limiting reactant by cooking the reaction yourself

This free game is part of our AQA GCSE Chemistry (8462) course for exams in EXAM YEARS 2027-2029.

What is a limiting reactant?

For AQA GCSE Chemistry (spec point 4.3.2.4, Higher tier only), in a chemical reaction involving two reactants it is common to use an excess of one of the reactants to ensure that all of the other reactant is used. The reactant that is completely used up is called the limiting reactant, because it limits the amount of products. Once it runs out, the reaction stops, and some of the reactant in excess is left over. You need to be able to explain the effect of a limiting quantity of a reactant on the amount of products, in terms of amounts in moles or masses in grams.

Leftovers turns this into a game set in the Reaction Kitchen, where every order is a recipe: a balanced equation. The two reactants float in a glass reaction pot as 3D particles, and each particle shown stands for a set amount in moles, for example 0.025 mol. You cook by drawing a loop round one full recipe set, such as 2 Mg + 1 O₂. The atoms join, flash and drop down a chute as product, and you keep cooking until no full set is left. The reactant that has all gone is stamped LIMITING, and the particles still floating glow amber and are swept into the leftovers tub: they are in excess.

Use the ratio, not the bigger number

The big numbers in a balanced equation give the mole ratio. In 2SO₂ + O₂ → 2SO₃, every O₂ reacts with two SO₂. So with 3.0 mol of SO₂ and 2.0 mol of O₂, the sulfur dioxide needs only 1.5 mol of oxygen. Sulfur dioxide runs out first and 0.5 mol of oxygen is left over, even though there was less oxygen to start with. In the game you feel this happen: each oxygen molecule takes two sulfur dioxide molecules, and you watch the oxygen being left behind.

From the second level you predict which reactant runs out first before you cook, then prove it. The orders are chosen to catch the classic mistakes: the reactant with fewer moles is not always the limiting one, and with N₂ + 3H₂ → 2NH₃ the hydrogen can run out first even when there is more of it.

Grams can fool you: change masses to moles first

Real orders come in grams, and you cannot find the limiting reactant by comparing masses. In the Weigh it level the masses sit on digital scales, and you change them to moles with moles = mass ÷ M_r (or A_r). With 6.5 g of zinc and 4.0 g of sulfur, zinc has the bigger mass but only 0.10 mol, while sulfur has 0.125 mol, so zinc is the limiting reactant. The maximum mass of zinc sulfide is 0.10 × 97 = 9.7 g, and 0.80 g of sulfur is left over (9.7 g + 0.80 g = 10.5 g, so mass is conserved). With 9.6 g of magnesium and 3.2 g of oxygen, the oxygen (0.10 mol) is limiting and makes 0.20 mol of magnesium oxide: 8.0 g.

The Kitchen fixes level tests the key idea the spec wants you to explain. Double the limiting reactant and you double the product. Double the reactant that is already in excess and you make no extra product: it is just left over. Using 8.0 g of sulfur instead of 4.0 g still makes 9.7 g of zinc sulfide, and halving the zinc halves the product to 4.85 g. You also build a 3-mark exam answer one mark at a time.

How to play Leftovers

Draw a loop round particles with the mouse or your finger. You can also tap particles one at a time, or press 1 and 2 to add the nearest particle of each reactant. The loop cooks the moment it holds exactly one recipe set; catch too many of one reactant and the extras bounce back into the pot. There are five levels: Prep, Who runs out?, Weigh it, Kitchen fixes and a 70 second Dinner Rush where every right prediction adds 2 seconds. Every wrong prediction or answer opens a short explanation worked from the numbers, with a link back to the lesson. A game takes about 8 minutes, and your best score is saved so you can play again for a higher kitchen rank.

Exam tip: write the mole ratio down before you compare anything. To explain a limiting reactant, use three ideas: name the limiting reactant and the one in excess, use moles and the ratio to show there is not enough of it to react with all of the other one, and link it to the product: once it is used up, the reaction stops and no more product can form.