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22.1: Energy Change in Reactions

Created by: CK-12

Lesson Objectives

The student will:

  • define the terms system and surroundings in the context of a chemical reaction.
  • identify the system and surroundings in a chemical reaction.
  • describe how heat is transferred in endothermic and exothermic reactions.

Vocabulary

  • enthalpy
  • surroundings
  • system

Introduction

Recall from the chapter “Matter and Energy” that energy is defined as the ability to do work. 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.

Molecules contain potential energy in their physical states and in their chemical bonds. You will remember that when solid substances were changed into liquid, energy had 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.

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 or taken on 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.

\mathrm{NaCl}_{(aq)} + \mathrm{AgNO}_{3(aq)} \rightarrow \mathrm{AgCl}_{(s)} + \mathrm{NaNO}_{3(aq)} \ \ \ \triangle H = - 166 \ \text{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.

In comparison, the decomposition of mercury(II) oxide requires a net input of energy.

2 \ \mathrm{HgO}_{(s)} \rightarrow 2 \ \mathrm{Hg}_{(l)} + \mathrm{O}_{2(g)} \ \ \ \triangle H = 181.7 \ \text{kJ}

In this reaction, there is less energy stored in the bonds of the mercury(II) oxide than is stored in the bonds of the products. Therefore, extra energy had to be added to the reaction to form the products.

These two equations represent the two types of chemical reactions that involve energy transfer and illustrate that chemical reactions involve energy.

Bond Breaking and Bond Forming

In the previous section, we looked at two different types of reactions that involved energy. \triangle H, or the heat of reaction, measures the change in the internal energy of the reaction. The internal energy is the sum of all the energy of the chemical system, that is, the potential and the kinetic. \triangle H is also known as change in enthalpy. Enthalpy is the amount of energy a system or substance contains and cannot be measured directly. What can be measured is the change in enthalpy (or \triangle H). When the bonds of the reactants contain more energy than the bonds of the products, the reaction is exothermic and \triangle H is negative. Conversely, when the bonds of the products contain more energy than the bonds of the reactants, the reaction is endothermic and \triangle H is positive.

System and Surroundings

Many chemical reactions take place in an open system. Whether the reaction is exothermic or endothermic, there is energy transfer between the system and the surroundings. For example, you take an ice cube out of the refrigerator and place it on a counter. As the ice cube melts, it requires a small amount of heat to be absorbed from the surroundings (the room) in order to produce the liquid.

\mathrm{H}_2\mathrm{O}_{(s)} \rightarrow \mathrm{H}_2\mathrm{O}_{(l)} \ \ \ \ \triangle H = + 6.01\ \text{kJ/mol}

The system in this example is the ice cube melting to form the liquid water. The surroundings are the container, room, and building where the reaction is taking place. In other words, the system involves the reactants and products in the reaction. The surroundings are everything else.

Before moving any further in our discussion, it is important to distinguish between heat and temperature when talking about heat transfer. Heat is the total amount of energy that is transferred between the system and the surroundings, while temperature is the average kinetic energy of a substance. The temperature measures the kinetic energy of the reactant and/or product particles. Consider a cup of water and a bucket of water: both will boil at the same temperature (100^\circ\mathrm{C}), but it will require different amounts of heat to bring these two volumes of water to a boil.

An endothermic reaction system absorbs heat from the surroundings and has a positive \triangle H. Phase changes from a solid to a liquid to a gas are all endothermic. The diagram below illustrates the transfer of heat energy from the surroundings to the system.

An exothermic reaction system releases heat to the surroundings and has a negative value of \triangle H. Just as phase changes from a solid to a liquid to a gas are all endothermic, the reverse of these changes are exothermic. Other reactions that you know as exothermic may be the combustion of fuels. Fuels used to drive your car and heat your home all involve reactions that are exothermic in nature. The figure below illustrates the transfer of heat energy to the surroundings from the system.

The reason that endothermic reactions have positive \triangle Hs and exothermic reactions have negative \triangle Hs is because the definition of \triangle H is:

\triangle H = H_{\text{products}}- H_{\text{reactants}}

Therefore, when the reactants contain more potential energy in their bonds than do the products, energy is given off. Looking at the equation above, since a larger number is subtracted from a smaller one, the answer is negative.

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

Lesson Summary

  • \triangle H, or the heat of reaction, measures the change in the enthalpy of the reaction.
  • The enthalpy is the sum of all the energy in a chemical system.
  • The system involves the reactants and products in the reaction.
  • The surroundings are everything else.
  • An endothermic reaction system absorbs heat from the surroundings. An exothermic reaction system releases heat to the surroundings.

Review Questions

  1. How does a campfire involve energy? Is it endothermic or exothermic? What would be the system and what would be the surroundings?
  2. If a chemical reaction absorbs heat from the surroundings, it is said to be what?
    1. in equilibrium
    2. in a closed system
    3. an exothermic reaction
    4. an endothermic reaction
  3. If a chemical reaction releases heat to the surroundings, it is said to be what?
    1. in equilibrium
    2. in a closed system
    3. an exothermic reaction
    4. an endothermic reaction
  4. Symbolically, change in enthalpy is represented as:
    1. H
    2. \triangle H
    3. E
    4. \triangle E
  5. Which of the following processes would be endothermic?
    1. natural gas burning
    2. melting chocolate
    3. fireworks exploding
    4. Steam condensing
  6. Which of the following processes would be exothermic?
    1. gasoline burning
    2. evaporation of ether
    3. melting butter
    4. boiling water

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