FreeScienceLessons Β· AQA ChemistryRate of Reaction
Cognito Β· AQA ChemistryCollision theory and rate of reaction
Key facts
Collision theory
For a reaction to occur, particles must collide with sufficient energy (β₯ activation energy).
Rate of reaction = how quickly reactants are converted to products.
Measured by: rate of disappearance of reactants OR rate of appearance of products.
Can be calculated from a graph: rate = gradient of concentration-time graph.
Activation energy = minimum energy required for a collision to result in reaction.
Factors affecting rate
Temperature β β particles move faster β more frequent collisions β more collisions with sufficient energy β rate increases significantly.
Concentration β (or pressure for gases) β more particles in same space β more frequent collisions β rate increases.
Surface area β (smaller pieces) β more particles exposed β more frequent collisions β rate increases.
Catalyst β provides an alternative reaction pathway with lower activation energy β more collisions have sufficient energy β rate increases. Not used up.
Reaction: NaβSβOβ + 2HCl β 2NaCl + S + SOβ + HβO. Sulfur precipitate makes the solution cloudy.
Method: mix solutions, time until a cross drawn on paper beneath the flask can no longer be seen.
Variable: concentration of NaβSβOβ (or temperature).
More concentrated / higher temperature β cross disappears faster β faster rate.
Calculate rate: 1 Γ· time (arbitrary units).
Control variables: same volume of solutions, same cross mark, same judge for when cross disappears.
Exam questions β 4 questions Β· 12 marks
β‘ Extended questions included. Read the hint. Show all working. AI marks against the real AQA mark scheme.
1 markTemperature and rateAQA 8462 P2 style
Why does increasing temperature increase the rate of reaction? Select the best answer.
A Higher temperature gives particles more energy so they move faster, collide more frequently and more collisions have energy above the activation energy
B Higher temperature dissolves more particles making the solution more concentrated
C Higher temperature reduces the activation energy needed
D Higher temperature makes particles larger so they collide more easily
4 marksExplain surface area effectAQA 8462 P2 style
A student reacts 5g of marble chips with excess hydrochloric acid. They repeat the experiment with 5g of powdered marble. Explain, using collision theory, why the powder reacts faster. (4 marks)
Hint: Key phrase: greater surface area β more particles exposed β more frequent collisions β more successful collisions per second β faster rate.
+40 XP
4 marksCalculate rate from graphAQA 8462 P2 Jun 2023 style
A graph shows the volume of gas produced (cmΒ³) against time (s) during a reaction. At 30 seconds, draw a tangent to the curve and calculate the rate of reaction at that point. The tangent passes through (10, 0) and (50, 80). Show your working. (3 marks)
Hint: Gradient = Ξy Γ· Ξx using the two points on the tangent. Include units (cmΒ³/s).
+40 XP
1 markCatalyst mechanismAQA 8462 P2 style
How does a catalyst increase the rate of reaction?
A It provides energy to the reactants so they can overcome the activation energy
B It provides an alternative reaction pathway with a lower activation energy, so more collisions result in reaction
C It increases the temperature of the reaction mixture