Question

1. what effect does a catalyst have on the reaction rate? explain

2. predict how the reaction rate would change if the concebtration of the sodium meta-bisulfite soltion were changed insted of the potassium iodate concentration.

Answer 1

1. Catalysts increase the rate of reaction by lowering the activation energy required for the reaction to proceed, without being consumed. Activation energy is the minimum energy that reactants must possess to undergo a specific reaction. Catalysts provide an alternative reaction path with a lower activation energy.

In a typical reaction, the majority of particles do not have enough energy to react. The reaction path, with lower activation energy, increases the number of particles that can overcome the energy barrier and react.

Catalysts alter the rate at which an equilibrium is attained by affecting the rates of both the forward and the reverse reaction. But both rates are affected to the same extent, and so catalysts have no effect on the value of an equilibrium constant.

2. Potassium iodate and sodium meta-bisulfite react to form iodine in acidic medium. This reaction proceeds through the following steps:

• Formation of iodate ions and hydrogen sulfite ions .
• Iodate ions react with hydrogen sulfite ions to produce iodide ions.
• In the presence of H+ ions, the iodide ions react with excess iodate ions to produce iodine.
• Before the iodine can react with starch to produce dark blue colored complex, it immediately reacts with any hydrogen sulfite ions still present to form iodide ions.
• Once all the hydrogen sulfite ions have reacted, the iodine is then free to react with the starch to form the colored complex. The color indicates that the sulfite ions have been consumed.

So this reaction can be monitored by adding a known but limited volume of sodium meta-bisulfite solution and starch solution. The rate of reaction is estimated by the time taken for the appearance of blue color. If the bisulfite is in excess, it will prevent the formation of dark blue complex.