Consider the combustion of methane in oxygen (a reaction we've seen when we've lit soap bubbles on fire), which is a highly exothermic reaction. A successful collision (one that leads to the combustion of methane) will have sufficient energy and any orientation between molecules. Once the activation energy has been reached, increasing the temperature will slow down the rate of the reaction. A mixture of methane and oxygen can be kept in the same container indefinitely at room temperature with no change because the collisions taking place between the molecules do not have enough energy to reach overcome the reaction barrier. Once the activation energy has been reached increasing the concentration of methane and oxygen will increase the rate of the reaction.