[Elmorshedy]

Does heat affect the rate of an exothermic reaction?? In my point of view it shouldn't affect it since exothermic reactions are spontaneous so it already works as fast as it can.

[Rutherford]

Change in temperature affect the rate. Check http://www.chemguide.co.uk/physical/basicrates/temperature.html
Gibbs free energy is the measure of spontaneity of a reaction.

[Elmorshedy]

Change in temperature affect the rate. Check http://www.chemguide.co.uk/physical/basicrates/temperature.html
Gibbs free energy is the measure of spontaneity of a reaction.

In case of exothermic reactions if the temperature is increased the rate constant of the endothermic reactions increase more than the constant of the exothermic reaction leading to a smaller Kc value ??

[Rutherford]

Yes, as the endothermic reaction has a bigger Ea value. Also, see this: http://en.wikipedia.org/wiki/Van_'t_Hoff_equation (pay attention to the plots for endothermic and exothermic reactions). The Van't Hoff equation can be derived from the Arrhenius equation.

[Borek]

exothermic reactions are spontaneous so it already works as fast as it can

Don't mix thermodynamics with kinetics, these are two separate things.

Yes, exothermic reactions often are fast. But not always.