We will always be interested in the change in H , rather than the absolute value of H itself. When a chemical reaction occurs, there is a characteristic change in enthalpy.
The enthalpy change for a reaction is typically written after a balanced chemical equation and on the same line. For example, when two moles of hydrogen react with one mole of oxygen to make two moles of water, the characteristic enthalpy change is kJ. We write the equation as:. A chemical equation that includes an enthalpy change is called a thermochemical equation.
A thermochemical equation is assumed to refer to the equation in molar quantities, which means it must be interpreted in terms of moles, not individual molecules. Write the thermochemical equation for the reaction of N 2 g with O 2 g to make 2NO g , which has an enthalpy change of kJ. It means that the system in which the chemical reaction is occurring is gaining energy.
If one considers the energy of a system as being represented as a height on a vertical energy plot, the enthalpy change that accompanies the reaction can be diagrammed as in part a in Figure 7. The products are higher on the vertical scale than the reactants. Endothermic, then, implies that the system gains , or absorbs, energy. An opposite situation exists for an exothermic process, as shown in part b in Figure 7. Exothermic, then, implies that the system loses , or gives off, energy.
We premeasure the mass of the chemicals in a system. If we know the specific heat of the materials in the system typically, we do , we can calculate q. The container in which the system resides is typically insulated, so any energy change goes into changing the temperature of the system, rather than being leaked from the system.
The container is referred to as a calorimeter , and the process of measuring changes in enthalpy is called calorimetry. For example, suppose 4. The two solutions are mixed in an insulated calorimeter, a thermometer is inserted, and the calorimeter is covered see Figure 7. The thermometer measures the temperature change as the following chemical reaction occurs:. For exothermic and endothermic reactions, this added stress is a change in temperature.
The equilibrium constant shows how far the reaction will progress at a specific temperature by determining the ratio of products to reactions using equilibrium concentrations. The equilibrium expression for the following equation. If the products dominate in a reaction, the value for K is greater than 1. The larger the K value, the more the reaction will tend toward the right and thus to completion. Since endothermic reactions draw in heat from their surroundings, they tend to cause their environments to cool down.
They are also generally non-spontaneous, since endothermic reactions yield products that are higher in energy than the reactants. As such, the change in enthalpy for an endothermic reaction is always positive. In order to melt the ice cube, heat is required, so the process is endothermic.
Endothermic reaction : In an endothermic reaction, the products are higher in energy than the reactants. Therefore, the change in enthalpy is positive, and heat is absorbed from the surroundings by the reaction. Whether a reaction is endothermic or exothermic depends on the direction that it is going; some reactions are reversible, and when you revert the products back to reactants, the change in enthalpy is opposite.
Endothermic and exothermic reactions : Paul Andersen explains how heat can be absorbed in endothermic or released in exothermic reactions. An energy diagram can be used to show energy movements in these reactions and temperature can be used to measure them macroscopically. A thermochemical equation is a balanced stoichiometric chemical equation which includes the enthalpy change. The equation takes the form:. Notice that in an endothermic reaction like the one depicted above, we can think of heat as being a reactant , just like A and B.
Therefore, all thermochemical equations must be stoichiometrically balanced. Thermochemical equations : Thermochemical equations can describe endothermic or exothermic reactions. Privacy Policy. Skip to main content. Why is the enthalpy of formation of oxygen zero? Why can enthalpy not be measured directly? How does pressure affect enthalpy? How does enthalpy relate to internal energy?
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