The definition of an endothermic reaction is that the products have higher energy than the reactants, resulting in a positive enthalpy of reaction. For the reaction to run, there must be an input of energy. The opposite is true for exothermic reactions: the products have lower energy than the reactants, enthalpy of reaction is negative, and heat is released.

Nothing is can be definitively stated about the rates of either type of reaction without additional information, as that will depend on the specific reactions and their respective activation energies.

In an exothermic reaction, heat has been released to the surroundings from the system. As a result, the molecules are at a lower final energy state after releasing the energy to the surroundings. 

Going from a solid to a gas (as well as liquid in between) is an endothermic reaction. Energy must be absorbed in order to raise the energy of the molecules so that the phase change can take place. Boiling water, melting ice, or sublimating carbon dioxide all require an input of energy.

The opposite is observed when magma cools. The liquid magma releases energy to the surroundings, allowing it to cool and form igneous rock. The magma is essentially "freezing," turning from a liquid to a solid.

When graphically tracking the energy of a reaction, you can see that energy is always needed to start a reaction, regardless of its enthalpy. This necessary energy is called the activation energy. Exothermic reactions, however, have a lower activation energy compared to the reverse endothermic reaction. This is because there is a net energy release from an exothermic reaction because the products have less energy than the reactants. To reverse this reaction would be to go from the low energy products back to the high energy reactants, resulting in a net increase in energy (an endothermic process).

Condensation of steam is an exothermic process. Heat must be released since the high energy steam is becoming lower energy water.

Combustion reactions occur when a compound is oxidized in a highly exothermic reaction. Most commonly, the reactant is a hydrocarbon (such as propane) and the oxidizing agent is oxygen gas. The result is a large release of heat energy, frequently visualized as a flame.

Note that exothermic reactions by definition release heat, while endothermic reactions absorb heat.

When a reaction is exothermic ("exo-" meaning out and "-thermic" having to do with heat), it means that the reaction is giving off heat into the environment. Therefore, the reactants have a net heat loss throughout the process of the reaction.

The change in enthalpy, , is a measure of the change in heat energy during a reaction.  is always negative for an exothermic process because the products always have less heat energy than the reactants.

When a reaction is exothermic ("exo-" meaning out and "-thermic" having to do with heat), it means that the reaction is giving off heat into the environment. Therefore, the reactants have a net heat loss throughout the process of the reaction.

The change in enthalpy, , is a measure of the change in heat energy during a reaction.  is always negative for an exothermic process because the products always have less heat energy than the reactants.

When a reaction is exothermic ("exo-" meaning out and "-thermic" having to do with heat), it means that the reaction is giving off heat into the environment. Therefore, the reactants have a net heat loss throughout the process of the reaction.

The change in enthalpy, , is a measure of the change in heat energy during a reaction.  is always negative for an exothermic process because the products always have less heat energy than the reactants.