GCSE Chemistry revision

Catalysts

Rate of reaction

AQA 4.6.1.4 Foundation & Higher

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A catalyst increases the rate of a chemical reaction without being consumed or chemically changed, meaning its total mass and composition remain identical at the end of the reaction.
Example: Adding solid manganese(IV) oxide (MnO₂) to hydrogen peroxide (H₂O₂) rapidly produces oxygen gas, yet the 1.0 g of MnO₂ added can be filtered, dried, and fully recovered at the end.

1. Definition and Chemical Recovery AQA

A catalyst increases the rate of a chemical reaction without being consumed or chemically changed, meaning its total mass and composition remain identical at the end of the reaction.
Example: Adding solid manganese(IV) oxide (MnO₂) to hydrogen peroxide (H₂O₂) rapidly produces oxygen gas, yet the 1.0 g of MnO₂ added can be filtered, dried, and fully recovered at the end.

Catalysts function by offering an alternative reaction pathway with a lower activation energy, allowing a significantly higher fraction of particle collisions to meet or exceed the energy required to react.
Example: In the industrial Contact process, vanadium(V) oxide (V₂O₅) provides an alternative catalytic route that lowers the activation energy required to convert sulfur dioxide into sulfur trioxide.

2. Alternative Pathway and Lower Activation Energy AQA

Catalysts function by offering an alternative reaction pathway with a lower activation energy, allowing a significantly higher fraction of particle collisions to meet or exceed the energy required to react.
Example: In the industrial Contact process, vanadium(V) oxide (V₂O₅) provides an alternative catalytic route that lowers the activation energy required to convert sulfur dioxide into sulfur trioxide.

A reaction profile diagram plots potential energy against reaction progress; activation energy is the vertical height from the reactant line to the top of the curve, whereas enthalpy change (ΔH) is the vertical difference between reactant and product energy levels.
Example: For an exothermic reaction (such as burning methane), the products sit lower on the energy axis than the reactants, giving a negative ΔH value while the curve's peak represents the initial bond-breaking energy barrier.

3. Features of Profile Diagrams AQA

A reaction profile diagram plots potential energy against reaction progress; activation energy is the vertical height from the reactant line to the top of the curve, whereas enthalpy change (ΔH) is the vertical difference between reactant and product energy levels.
Example: For an exothermic reaction (such as burning methane), the products sit lower on the energy axis than the reactants, giving a negative ΔH value while the curve's peak represents the initial bond-breaking energy barrier.

Adding a catalyst lowers the height of the activation energy peak on the diagram without altering the energy levels of the starting reactants, final products, or the overall enthalpy change (ΔH).
Example: A profile diagram comparing the decomposition of hydrogen peroxide displays a distinctly lower peak for the path with MnO₂ compared to the uncatalysed path, while both curves end at the exact same final energy line.

4. Effect of Catalysts on Reaction Profiles AQA

Adding a catalyst lowers the height of the activation energy peak on the diagram without altering the energy levels of the starting reactants, final products, or the overall enthalpy change (ΔH).
Example: A profile diagram comparing the decomposition of hydrogen peroxide displays a distinctly lower peak for the path with MnO₂ compared to the uncatalysed path, while both curves end at the exact same final energy line.

Increasing solid surface area, solution concentration, or gas pressure increases collision frequency per second, while raising temperature increases both collision frequency and the proportion of particles possessing energy equal to or greater than the activation energy.
Example: Reacting powdered calcium carbonate with hydrochloric acid produces carbon dioxide gas much faster than using a single marble chip of equal mass due to the greater exposed surface area.

5. Collision Theory and Factors AQA

Increasing solid surface area, solution concentration, or gas pressure increases collision frequency per second, while raising temperature increases both collision frequency and the proportion of particles possessing energy equal to or greater than the activation energy.
Example: Reacting powdered calcium carbonate with hydrochloric acid produces carbon dioxide gas much faster than using a single marble chip of equal mass due to the greater exposed surface area.

Reaction rates are measured experimentally by tracking product formation or reactant consumption over time, using techniques such as collecting gas volume in a syringe or measuring mass loss on a balance.
Example: Investigating the effect of temperature on the reaction between sodium thiosulfate and hydrochloric acid involves timing how quickly a yellow sulfur precipitate forms to obscure a black cross drawn beneath the flask.

6. Experimental Investigations AQA

Reaction rates are measured experimentally by tracking product formation or reactant consumption over time, using techniques such as collecting gas volume in a syringe or measuring mass loss on a balance.
Example: Investigating the effect of temperature on the reaction between sodium thiosulfate and hydrochloric acid involves timing how quickly a yellow sulfur precipitate forms to obscure a black cross drawn beneath the flask.