Rates & Equilibrium is a core part of GCSE Chemistry. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
Reaction profileA diagram showing the energy of reactants and products and the activation energy.
Bond breakingEndothermic — energy must be supplied to break chemical bonds.
Bond makingExothermic — energy is released when new bonds form.
Overall energy changeWhether a reaction is exothermic or endothermic overall, found by comparing energy to break and make bonds.
YieldHow much product is formed; reversible reactions reach a balance that limits yield.
Compromise conditionsConditions chosen to balance a good rate, a reasonable yield and cost.
Mean rateTotal quantity ÷ total time taken
GradientSlope of a graph — used to find instantaneous rate
cm³/sUnit for rate of gas produced per second
g/sUnit for rate of mass lost per second
mol/dm³ /sUnit for change in concentration per second
TangentStraight line touching a curve at one point
Initial rateRate at time = 0 — steepest part of the curve
Catalyst mechanismProvides a route with lower activation energy
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
How does a catalyst speed up a reaction?✗ A catalyst lowers the activation energy of the original reaction pathway. ✓ It provides an alternative reaction pathway with a lower activation energy.
In an exothermic reaction, what happens energetically with the bonds?✗ Bond breaking releases energy and bond making absorbs energy. ✓ Energy released forming new bonds exceeds energy absorbed breaking existing bonds.
On a reaction profile, activation energy is shown by:✗ height of products above reactants ✓ height of peak above reactants
At dynamic equilibrium, what is happening to the reactions?✗ Both reactions stop because the system has reached balance. ✓ Forward and reverse reactions continue at equal rates; concentrations remain constant.
Why does increasing temperature increase the rate of reaction significantly?✗ Particles collide much more frequently at higher temperature, which is the main reason for the rate increase. ✓ Mainly because a much larger fraction of collisions have energy at least equal to the activation energy; collision frequency increases only slightly.
Practise Rates & Equilibrium — free games
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