Exam specific answers (rate equation)

Question 1

The rate equation for a reaction is
rate = k[E]
Explain qualitatively why doubling the temperature has a much greater
effect on the rate of the reaction than doubling the concentration of E

Answer 1

• the reaction occurs when molecules have energy greate than or equal to the activation energy
• doubling the temperature by 10 degrees celsius causes many more molecules to have this energy
• doubling [E] only doubles the number with this energy

Question 2

describe the method for continuosly monitoring the concentration of iodine during the iodination of propanone reaction?

Answer 2

1. use a glass pipette to draw a set volume of reaction mixture and place in a conical flask - repeat at regular intervals
2. add excess sodium carbonate to the mixture drawn from the main reaction mixture to neutralise the acid and stop the reaction - keep adding until fizzing stops
3. add starch to the quenched reaction mixture
4. titrate with a known concentration of sodium thiosulfate solution - stop adding when blue black colour disappears
5. use known concentration and volume of sodium thiosulate to calculate moles of sulfate ions, then calculate concentration of iodine using known moles and volume