Think and Click: Working With Enthalpy Values

You can find the bond enthalpy values for any type of compound or molecule involved in a chemical reaction using published data tables. You can then determine the amount of energy required to break all the bonds in the reactants and all the energy required to form all the products. Then you can find out how heat energy flows for any given reaction using an equation. Scientists use the symbol "delta" (a triangle) to mean "change in." So to describe the total heat absorbed or released in a reaction, they use the equation:

ΔH = energy required to break bonds in the reactants – energy released by forming the bonds in the products, ΔH is the change in heat energy for the reaction as a whole.

Consider the equation above. ΔH can have either a positive or a negative value. Will it be positive or negative for an exothermic reaction? How about an endothermic reaction?

It will be negative for an exothermic reaction and positive for an endothermic reaction. Work through the problems below to see examples of this.

Suppose the combustion of a fuel requires 2700 kJ of energy to break the bonds of the reactants and releases 3500 kJ of energy to form the bonds of the products.

What is the change in energy for the reaction?

ΔH = 2700 kJ – 3500 kJ = -800 kJ. In this case it took 2700 kJ of energy to break the bonds in the reactants but you get 3500 kJ of energy released by the formation of the products. Therefore there is an excess of 800 kJ of energy. The negative sign tells you that this reaction is exothermic and that heat energy will be given off by the reaction.

Suppose two chemicals are reacted together. It takes 800 kJ of energy to break the bonds of the reactants and 620 kJ of energy are released by the formation of the products.

What is the change in energy for the reaction?
Is it endothermic or exothermic?
Will the container holding the chemicals will become hot or cold to the touch?

ΔH = 800 kJ – 620 kJ = 180 kJ. This reaction is endothermic, as shown by the positive value of ΔH. In this case, you need 800 kJ of energy to break the bonds of the reactants but only get 620 kJ of energy released by formation of the products. The reaction will absorb energy from the surroundings, leaving its container cold to the touch.

Knowing how to calculate the change in energy associated with chemical reactions is an important skill. Scientists and engineers use this skill to predict how much energy can be obtained from or will be required for different chemical reactions. For example, you can compare the amount of heat energy given off by the combustion of different fuels and predict which one will give you the most heat. Propane gas, commonly used in household barbeque grills, gives you less heat when combusted than methane gas, for instance. As a result, you will have to burn a lot more propane than methane to heat your home. Understanding ideas like this demonstrates how the ability to calculate energy changes is useful.

When scientists need to make decisions about products, they can write down the chemical reaction involved, look at the types of bonds that are broken and formed, use bond enthalpy tables to calculate ΔH, and compare the abilities of different reactions to absorb or produce heat.