Law of Conservation of Energy, Continued

In other words, the energy released to the surroundings during an exothermic reaction comes from the potential energy stored in the bonds of the reacting chemicals. In the case of an exothermic reaction, the reactants possess more potential energy than the products. As the reaction proceeds, that excess energy gets converted to thermal energy and released as heat.

The exact opposite is true for an endothermic reaction. In an endothermic reaction, the reactants possess less potential energy than the products and energy must be absorbed from the surroundings to increase the potential energy of the reactants and get the reaction to proceed. According to the law of conservation of energy, the amount of energy released by the surroundings must be equal to the energy absorbed by the reaction. Some kind of external energy source, such as light or heat, must be applied to the reaction.

Why does a car’s engine get hot when the car is running? Where does the heat come from?

The car’s engine gets hot because the reaction that takes place to power the car is exothermic. During the combustion of gasoline, potential energy stored in the gasoline’s chemical bonds gets released and converted to thermal heat, making the car engine feel hot to the touch.

If an exothermic reaction is expressed as reactants → products + energy, how is an endothermic reaction expressed in terms of reactants, products, and energy?

An endothermic reaction is expressed as reactants + energy → products.