General Chemistry/Energy changes in chemical reactions

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Exothermic and Endothermic Reactions[edit]

The release of energy in chemical reactions occurs when the reactants have higher chemical energy than the products. The chemical energy in a substance is a type of potential energy stored within the substance. This stored chemical potential energy is the heat content or enthalpy of the substance.

The collection of substances that is involved in a chemical reaction is referred to as a system and anything else around it is called the surroundings.

If the enthalpy decreases during a chemical reaction, a corresponding amount of energy must be released to the surroundings. Conversely, if the enthalpy increases during a reaction, a corresponding amount of energy must be absorbed from the surroundings. This is simply the Law of Conservation of Energy.

Endothermic reactions increase their enthalpy by absorbing heat. They feel cold to the touch after they have occurred.

Exothermic reactions decrease their enthalpy by releasing heat. They will get warm, and may even burn or explode if they release enough heat.

You are already familiar with enthalpy: melting ice is endothermic and freezing water is exothermic.

Examples
When methane burns in air the heat given off equals the decrease in enthalpy that occurs as the reactants are converted to products.


\hbox{CH}_{4(g)} + 2\hbox{O}_{2(g)} \to \hbox{CO}_{2(g)} + 2\hbox{H}_2\hbox{O}_{(g)} + energy


The enthalpy difference between the reactants and the products is equal to the amount of energy released to the surroundings. A reaction in which energy is released to the surroundings is called an exothermic reaction. In this type of reaction the enthalpy, or stored chemical energy, is lower for the products than the reactants.

When ammonium nitrate is dissolved in water, energy is absorbed and the water cools. This concept is used in "cold packs".


\hbox{NH}_{4}\hbox{NO}_{3(s)} + water + energy \to \hbox{NH}^+_{4(aq)} + \hbox{NO}^-_{3(aq)}


The enthalpy difference between the reactants and the products is equal to the amount of energy absorbed from the surroundings. A reaction in which energy is absorbed from the surroundings is called an endothermic reaction. In endothermic reactions the enthalpy of the products is greater than the enthalpy of the reactants.

Because reactions release or absorb energy, they affect the temperature of their surroundings. Exothermic reactions heat up their surroundings while endothermic reactions cool them down. The study of enthalpy, along with many other energy-related topics, is covered in the Thermodynamics Unit.

Activation Energy[edit]

Think about the combustion of methane. It releases enough heat energy to cause a fire. However, the reaction does not occur automatically. When methane and oxygen are mixed, an explosion does not instantly occur. First, the methane must be ignited, usually with a lighter or matchstick. This reveals something about reactions: they will not occur unless a certain amount of activation energy is added first. In this sense, all reactions absorb energy before they begin, but the exothermic reactions release even more energy. This can be explained with a graph of potential energy:

Ac com.svg

This graph shows an exothermic reaction because the products are at a lower energy than the reactants (so heat has been released). Before that can happen, the energy must actually increase. The amount of energy added before the reaction can complete is the activation energy, symbolized Ea.