Energy Changes in Chemical Reactions

Energy Relationships in Chemical Reactions

ESSENTIAL CONCEPTS

EnergyThe many different forms of energy are, at least in principle, interconvertible.

First Law of ThermodynamicsThe first law of thermodynamics, which is based on the law of conservation of energy, relates the internal energy change of a system to the heat change and the work done. It can also be expressed to show the relationship between the internal energy change and enthalpy change of a process.

ThermochemistryMost chemical reactions involve the absorption or release of heat. At constant pressure, the heat change is equal to the enthalpy change. The heat change is measured by a calorimeter. Constant-pressure and constant-volume calorimeters are devices for measuring heat changes under the stated conditions.

Standard Enthalpy of ReactionStandard enthalpy of reaction is the enthalpy change when the reaction is carried out at 1 atm pressure. It can be calculated from the standard enthalpies of formation of reactants and products. Hess’s law enables us to measure the standard enthalpy of formation of a compound in an indirect way.

The Nature of Energy and Types of Energy

“Energy” is a much-used term that represents a rather abstract concept. For instance, when we feel tired, we might say we haven’t any energy; and we read about the need to find alternatives to nonrenewable energy sources. Unlike matter, energy is known and recognized by its effects. It cannot be seen, touched, smelled, or weighed.

Chemical energy is stored within the structural units of chemical substances; its quantity is determined by the type and arrangement of constituent atoms. When substances participate in chemical reactions, chemical energy is released, stored, or converted to other forms of energy.

Energy Changes in Chemical Reactions

Often the energy changes that take place during chemical reactions are of as much practical interest as the mass relationships we discussed in Chapter 3. For example, combustion reactions involving fuels such as natural gas and oil are carried out in daily life more for the thermal energy they release than for their products, which are water and carbon dioxide.

Almost all chemical reactions absorb or produce (release) energy, generally in the form of heat. It is important to understand the distinction between thermal energy and heat. Heat is the transfer of thermal energy between two bodies that are at different temperatures. Thus, we often speak of the “heat fl ow” from a hot object to a cold one. Although the term “heat” by itself implies the transfer of energy, we customarily talk of “heat absorbed” or “heat released” when describing the energy changes that occur during a process. Thermochemistry is the study of heat change in chemical reactions.

To analyze energy changes associated with chemical reactions we must first define the system, or the specific part of the universe that is of interest to us. For chemists, systems usually include substances involved in chemical and physical changes. For example, in an acid-base neutralization experiment, the system may be a beaker containing 50 mL of HCl to which 50 mL of NaOH is added. The surroundings are the rest of the universe outside the system.

There are three types of systems. An open system can exchange mass and energy, usually in the form of heat with its surroundings. For example, an open system may consist of a quantity of water in an open container, as shown in Figure 6.1(a). If we close the fl ask, as in Figure 6.1(b), so that no water vapor can escape from or condense into the container, we create a closed system, which allows the transfer of energy (heat) but not mass. By placing the water in a totally insulated container, we can construct an isolated system, which does not allow the transfer of either mass or energy, as shown in Figure 6.1(c).

The combustion of hydrogen gas in oxygen is one of many chemical reactions that release considerable quantities of energy (Figure 6.2):

2H₂(g) + O₂(g) = 2H₂O(l ) + energy

In this case, we label the reacting mixture (hydrogen, oxygen, and water molecules) the system and the rest of the universe the surroundings. Because energy cannot be created or destroyed, any energy lost by the system must be gained by the surroundings.

Thus, the heat generated by the combustion process is transferred from the system to its surroundings. This reaction is an example of an exothermic process, which is any process that gives off heat—that is, transfers thermal energy to the surroundings.

Now consider another reaction, the decomposition of mercury(II) oxide (HgO) at high temperatures:

Energy + 2HgO(s) = 2Hg(l ) + O₂(g)

This reaction is an endothermic process, in which heat has to be supplied to the system (that is, to HgO) by the surroundings.

In exothermic reactions, the total energy of the products is less than the total energy of the reactants. The difference is the heat supplied by the system to the surroundings.

Just the opposite happens in endothermic reactions. Here, the difference between the energy of the products and the energy of the reactants is equal to the heat supplied to the system by the surroundings.

In thermodynamics, we study changes in the state of a system, which is defined by the values of all relevant macroscopic properties, for example, composition, energy,temperature, pressure, and volume. Energy, pressure, volume, and temperature are said to be state functions—properties that are determined by the state of the system,regardless of how that condition was achieved. In other words, when the state of a system changes, the magnitude of change in any state function depends only on the initial and fi nal states of the system and not on how the change is accomplished.

Date: 2015-12-24; view: 1340