1. Model Answers for AS Chemistry

2. Heterogeneous Catalysts Mechanism

3. Four Stages of Heterogeneous Catalysts Reactants adsorb to catalyst surface (1) Bonds in reactants weaken and break (1) New bonds form in products (1) Products diffuse away from catalyst surface (1)

4. Bond Angle / Shape Questions

5. Standard Answer The Bond angle is………. (1) as there are …….. areas of electron density (1) around the central atom (1) electron groups repel to get as far as possible apart to minimize repulsion (1) therefore shape is…… (1)

6. Angles/Shapes 2 areas = 180 = Linear 3 area = 120 = planar triangular 6 areas = 90 = Octahedral 4 areas = 109 Shape : tetrahedral if all bonding (methane) Pyramidal if three bonding and one lone (ammonia) Bent Planar if two bonding, two lone (water) Page 10 revision guide

7. Greenhouse gases Mechanism and effect

8. Key Points – replace CO 2 for any greenhouse gas High Energy UV and visible from the Sun is absorbed by the Earth / heats the Earth (1) Earth radiates/emits lower energy IR (1) CO 2 absorbs IR (1) Which causes bonds to vibrate (more) (1) Energy is transferred to KE that increases atmospheric temperature / molecules radiate IR that warms Earth / atmosphere (1) More CO 2 means more radiation is absorbed and increases warming effect (1)

9. Le Chatelier and Dynamic Equilibrium

10. Dynamic Equilibrium definition When the rate of forward and backward reactions are equal in a closed system (1) And the concentration of products and reactant remain constant