Rate is how quickly product forms. Yield is how much product forms in total. A graph shows rate in its steepness and yield in its final level, so you can answer each from a different feature.
This lesson is part of energy evidence and industrial chemistry reasoning. It extends comparing rate graphs fairly.
What feature answers which question?
Ask one question at a time and look at one feature.
| Question | Feature to read |
|---|---|
| Which experiment is faster? | Steepness at the start, or time to reach the plateau |
| Which gave more product? | Height of the plateau |
Worked example from supplied data
An original question gives the total gas collected in three experiments with magnesium and acid.
| Experiment | Condition | Time to finish (s) | Final volume (cm³) |
|---|---|---|---|
| A | 0.12 g Mg, excess acid, 30 °C | 100 | 120 |
| B | 0.12 g Mg, excess acid, 50 °C | 40 | 120 |
| C | 0.24 g Mg, excess acid, 30 °C | 100 | 240 |
Speed. B finishes in 40 s and A in 100 s, so B is faster. The rate in B is higher.
Yield. A and B both end at 120 cm³, so they form the same amount. C ends at 240 cm³ because there is twice the magnesium: 0.24 ÷ 24 = 0.010 mol Mg gives 0.010 mol H₂, which is 0.010 × 24 000 = 240 cm³.
The temperature changed the speed in B but not the yield. The mass of magnesium changed the yield in C, because magnesium is the limiting reactant.
The mistake that costs marks
The common slip is to write “B gives more hydrogen because it is faster”. The data show 120 cm³ in both A and B.
| Sentence | Verdict |
|---|---|
| “B reacts faster than A.” | Right: 40 s against 100 s |
| “B gives a higher yield than A.” | Wrong: both give 120 cm³ |
Write the speed sentence and the yield sentence separately, with a number from the data in each.
Check yourself
Experiment D uses 0.12 g Mg with 10 cm³ of 0.50 mol/dm³ hydrochloric acid at 30 °C and finishes in 60 s. Compared with A, say whether D is faster or slower and whether its yield is higher, lower or the same. Mg + 2HCl → MgCl₂ + H₂, and assume 24 000 cm³/mol.
Answer
Moles of HCl = 10 ÷ 1 000 × 0.50 = 0.0050 mol. This needs only 0.0025 mol Mg, so acid is limiting and Mg is in excess.
Hydrogen = 0.0025 mol = 0.0025 × 24 000 = 60 cm³, which is lower than the 120 cm³ in A, so the yield is lower.
Finishing in 60 s compared with 100 s means D is faster. A faster reaction with a lower yield is possible, which is why the two must be judged separately.
What to study next
Go on to explaining a material choice from its supplied structure and properties. You can practise amount calculations with the mole and stoichiometry steps tool.
If you want a teacher to check your separate speed and yield sentences, see online one-to-one Chemistry tuition.