a) Sulphur pieces do not react with cold concentrated nitric acid but sulphur powder reacts because when surface area increases rate of effective collision increases. So the rate of reaction increases. [1 mark]
b) If you want to increase the rate of reaction, you should heat the mixture.[0.5 mark]
When temperature increases more molecules will get threshold energy. So rate of effective collision and rate of reaction increases. [0.5 mark]