Question

Which change does not increase the speed of reaction between zinc and hydrochloric acid?

Answer 1

When solid zinc is dropped into hydrochloric acid solution, it falls to the bottom due to its higher density. The reaction occurs at the surface of the zinc metal and generates hydrogen gas that bubbles up through the solution. You can tell that the rate increases because the rate of bubble formation increases. This will naturally occur because the reaction is exothermic and will warm the solution around the zinc metal. The higher temperature will increase the reaction rate.

If you want a more quantitative analysis of reaction rate, you can perform the experiment in a eudiometer tube that is inverted and completely filled with liquid. As the hydrogen gas collects in the tube you can measure its accumulated volume with time. That will allow you to determine an approximate kinetic rate law for the process. The rate law determined in this manner will suffer from error due to temperature and surface area changes as well as gas volume error due to water vapor. But, it is an interesting application of chemical kinetics.

Answer 2

Any factor that causes molecules to collide more frequently speeds up the reaction rate. This is achieved by an increase of the reactants concentration, surface area, raised temp. , raised pressure of gaseous reactant, or an addition of catalysts to the reactant