READ: Heat and Reactions

Energy is Conserved in all Changes

Energy is often divided into two types: kinetic energy and potential energy. Kinetic energy is the energy of motion, while potential energy is the energy of position or stored energy.

Molecules contain potential energy in their physical states and in their chemical bonds. When solid substances are changed into liquid, energy has to be added to provide the heat of melting. That energy was used to pull the molecules further apart, changing solid into liquid. That energy is then stored in the liquid as potential energy due to the greater distances between attracting molecules. For similar reasons, energy also has to be added to convert a liquid into a gas.

Chemical bonds store potential energy in a slightly different way. To understand how chemical bonds store energy, we can view a substance as having maximum potential energy in bonds when all the atoms of the substance are separated from each other and are in atomic form (no bonds). The atoms can then form many different bonds. When bonds form, energy is released and the potential energy of the substance decreases.

The Law of Conservation of Energy (energy cannot be created or destroyed) tells us that whatever energy there is in the beginning, there is an equal amount of energy at the end of any change. Although energy can change form between kinetic energy and potential energy, the amount of energy total does not change. The Law of Conservation of Energy is true in all chemical changes from burning gas in a furnace to metabolizing sugar in your body’s cells.

All Chemical Reactions Involve Energy

Every system or sample of matter has energy stored in it. When chemical reactions occur, the new bonds formed never have exactly the same amount of potential energy as the bonds that were broken. Therefore, all chemical reactions involve energy changes. Energy is either given off to the surroundings or taken into the system by the reaction.

Before any reaction can occur, reactant bonds need to be broken. A minimum amount of energy, that is the activation energy, must be supplied before any reaction can take place. This minimum energy might be in the form of heat or electrical current.

NaCl(aq)+AgNO3(aq)→AgCl(s)+NaNO3(aq)ΔH=−166 kJ

The equation above represents a chemical reaction where energy is produced. This means that there is less energy stored in the bonds of the products than there is in the bonds of the reactants. Therefore, extra energy is left over when the reactants become the products.

What Is an Exothermic Reaction?

All chemical reactions involve energy. Energy is used to break bonds in reactants, and energy is released when new bonds form in products. In some chemical reactions, called endothermic reactions, less energy is released when new bonds form in the products than is needed to break bonds in the reactants. The opposite is true of exothermic reactions. In an exothermic reaction, it takes less energy to break bonds in the reactants than is released when new bonds form in the products.

The word exothermic literally means “turning out heat”. Energy, often in the form of heat, is released as an exothermic reaction proceeds. This is illustrated in the Figure below. The general equation for an exothermic reaction is:

Reactants→Products+Energy

If the energy produced in an exothermic reaction is released as heat, it results in a rise in temperature. As a result, the products are likely to be warmer than the reactants.

Another way which chemists use to indicate chemical change is to use the symbol ΔH. This symbol indicates the change in enthalpy of a reaction. For most purposes, we can think of this as the change in heat for a reaction. If a reaction releases heat (exothermic), ΔH will have a negative value.

All combustion reactions are exothermic reactions. During a combustion reaction, a substance burns as it combines with oxygen. When substances burn, they usually give off energy as heat and light. Look at the big bonfire in the Figure below. The combustion of wood is an exothermic reaction that releases a lot of energy as heat and light. You can see the light energy the fire is giving off. If you were standing near the fire, you would also feel its heat.

Energy Change in Endothermic Reactions

Did you ever use an instant ice pack like? You don’t have to pre-cool it in the freezer. All you need to do is squeeze the pack and it starts to get cold. How does this happen? The answer is an endothermic chemical reaction.

The word endothermic literally means “taking in heat”. A constant input of energy, often in the form of heat, is needed to keep an endothermic reaction going. This is illustrated in the figure below. Energy must be constantly added because not enough energy is released when the products form to break more bonds in the reactants. The general equation for an endothermic reaction is:

Reactants+Energy→Products

In endothermic reactions, the temperature of the products is typically lower than the temperature of the reactants. The drop in temperature may be great enough to cause liquids to freeze.

Q: Now can you guess how an instant cold pack works?

A: Squeezing the cold pack breaks an inner bag of water, and the water mixes with a chemical inside the pack. The chemical and water combine in an endothermic reaction. The energy needed for the reaction to take place comes from the water, which gets colder as the reaction proceeds.

Photosynthesis

One of the most important series of endothermic reactions is photosynthesis. In photosynthesis, plants make the simple sugar glucose (C6H12O6) from carbon dioxide (CO2) and water (H2O). They also release oxygen (O2) in the process. The reactions of photosynthesis are summed up by this chemical equation:

6CO2+6H2O→C6H12O6+6O2

The energy for photosynthesis comes from light. Without light energy, photosynthesis cannot occur. As you can see in the Figure below, plants can get the energy they need for photosynthesis from either sunlight or artificial light.

Example

Which of the following processes are endothermic, and which are exothermic?

  1. water boiling
  2. gasoline burning
  3. water vapor condensing
  4. iodine crystals subliming
  5. ice forming on a pond

Solution:

  1. Endothermic – state change from liquid to a gas absorbs heat from the surroundings.
  2. Exothermic – combustion releases heat to the surroundings.
  3. Exothermic – state change from gas to a liquid releases heat to the surroundings.
  4. Endothermic – state change from solid to a liquid absorbs heat from the surroundings.
  5. Exothermic – state change from liquid to a solid releases heat to the surroundings.
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Last modified: Thursday, 14 July 2016, 11:14 AM