Why are enthalpies important

That last statement is a lot like the description of energetics on the previous page. If a system undergoes a reaction and gives off energy, its own energy content decreases. It has less energy left over if it gave some away. Why does the energy of a set of molecules change when a reaction occurs?

To answer that, we need to think about what happens in a chemical reaction. In a reaction, there is a change in chemical bonding. Some of the bonds in the reactants are broken, and new bonds are made to form the products. It costs energy to break bonds, but energy is released when new bonds are made. Whether a reaction is able to go forward may depend on the balance between these bond-making and bond-breaking steps.

Bond energies the amount of energy that must be added in order to break a bond are an important factor in determining whether a reaction will occur.

Bond strengths are not always easy to predict, because the strength of a bond depends on a number of factors. If so, the reaction is endothermic and the enthalpy change is positive.

If more energy is produced in bond formation than that needed for bond breaking, the reaction is exothermic and the enthalpy is negative. Several factors influence the enthalpy of a system. Enthalpy is an extensive property, determined in part by the amount of material we work with.

The state of reactants and products solid, liquid, or gas influences the enthalpy value for a system. The direction of the reaction affects the enthalpy value. A reaction that takes place in the opposite direction has the same numerical enthalpy value, but the opposite sign. Read the material at the link below and answer the following questions:. Skip to main content. Substituting this in for work in the above equation, we can define the change in internal energy for a chemical system:.

The subscript V is added to Q to indicate that this is the heat transfer associated with a chemical process at constant volume. This internal energy is often very difficult to calculate in real life settings, though, because chemists tend to run their reactions in open flasks and beakers that allow gases to escape to the atmosphere.

To correct for this, we introduce the concept of enthalpy , which is much more commonly used by chemists. The enthalpy of reaction is defined as the internal energy of the reaction system, plus the product of pressure and volume. It is given by:. By adding the PV term, it becomes possible to measure a change in energy within a chemical system, even when that system does work on its surroundings. Most often, we are interested in the change in enthalpy of a given reaction, which can be expressed as follows:.

When you run a chemical reaction in a laboratory, the reaction occurs at constant pressure, because the atmospheric pressure around us is relatively constant.

We will examine the change in enthalpy for a reaction at constant pressure, in order to see why enthalpy is such a useful concept for chemists. It is given as follows:. Endothermic reactions absorb energy from the environment, while exothermic reactions release energy to the environment. All chemical processes are accompanied by energy changes.

When a reaction proceeds, it either releases energy to, or absorbs energy from, its surroundings. In thermodynamics, these two types of reactions are classified as exothermic or endothermic, respectively. An easy way to remember the difference between these two reaction types is by their prefixes: endo- means to draw in, and exo- means to give off. Anne Marie Helmenstine, Ph. Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels.

Facebook Facebook Twitter Twitter. Updated January 04, Featured Video. Cite this Article Format. Helmenstine, Anne Marie, Ph. Enthalpy Definition in Chemistry and Physics. Enthalpy Change of Ice to Water Vapor. Enthalpy Change Definition in Science.