Energy and Chemical Reactions

Thermochemistry (Enthalpy) Remember some reactions are exothermic (produce heat energy) and others are endothermic (absorb heat energy). Chemists determine exactly how much energy is transferred using a energy function called enthalpy, which is designated by H.

When a reaction occurs under conditions of constant pressure, the change in enthalpy (∆H) is equal to the energy that flows as heat. ∆Hp = heat the subscript p indicates the process has occurred under constant pressure ∆ means “change in” The enthalpy change for a reaction under constant pressure is same as the heat (q) for that reaction.

Calorimetry A calorimeter is a device used to determine the heat associated with a chemical reaction. The reaction is run in the calorimeter and the temperature change is observed. Knowing the temperature change in the calorimeter and the heat capacity of the calorimeter the heat energy absorbed by released by the reaction can be calculated. From this information, the ∆H value for the reaction can be calculated.

Hess’s Law Enthalpy is a state function, meaning the change in enthalpy for a given process is independent of the pathway for the process. Hess’s law states that in going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step of a series of steps.

To illustrate Hess’s law the oxidation of nitrogen to produce nitrogen dioxide will be examined. The overall reaction can be written in one step, where the enthalpy change is represented by ∆H1. N2 (g) + 2O2 (g) → 2NO2 (g) ∆H1 = 68 kJ The reaction is carried out in two distinct steps, with the enthalpy changes being designated as ∆H2 and ∆H3. N2 (g) + O2 (g)→ 2NO (g) ∆H2 = 180 kJ 2NO (g) + O2 (g) → 2NO2 (g) ∆H3 = -112 kJ ______________________________________ Net: N2 (g) + 2O2 (g) → 2NO2 (g) ∆H2 + ∆H3 = 68 kJ

Note that the sum of the two steps gives the net, or overall, reaction and that ∆H1 = ∆H2 + ∆H3 = 68 kJ The importance of Hess’s law is that it allows us to calculate heats of reaction that might be difficult to measure directly in a calorimeter.

To use Hess’s law to compute enthalpy changes for reactions, it is important to understand two characteristics of ∆H for a reaction: If the reaction is reversed, the sign of ∆H is also reversed. The sign of ∆H indicates the direction of heat flow: Exothermic reaction = - ∆H value Endothermic reaction = + ∆H value Xe (g) + 2F2 (g) → XeF4 (s) ∆H = -251 kJ The negative sign of ∆H indicates an exothermic reaction. If the reaction is reversed XeF4 (s) → Xe (g) + 2F2 (g) ∆H = +251 kJ the sign of ∆H is reversed because it becomes endothermic.

2nd characteristic: The magnitude of ∆H is directly proportional to the quantities of reactants and products in a reaction. If the coefficients in a balanced reaction are multiplied by an integer, the value of ∆H is multiplied by the same integer. For example, since 251 kJ of energy is evolved the following reaction Xe (g) + 2F2 (g) → XeF4 (s) ∆H = -251 kJ then for a preparation involving twice the quantities of reactants and products, or 2Xe (g) + 4F2 (g) → 2XeF4 (s) ∆H = 2(-251 kJ) = -502 kJ twice as much heat is evolved.

Standard Enthalpies of Formation Since the change in enthalpy for some reactions cannot be determined using a calorimeter another method is to use standard enthalpies of formation. The standard enthalpy of formation (∆Hof) of a compound is defined as the change in enthalpy that accompanies the formation of 1 mole of a compound from its elements with all substances in their standard states. A degree symbol on a thermodynamics function, for example, ∆Ho , means the process has been carried out at standard conditions (certain pressure and temperature conditions).

The enthalpy change for a given reaction can be calculated by subtracting the enthalpies of formation of the reactants from the enthalpies of formation of the products. Remember to multiply the enthalpies of formation by the coefficients in the balanced equation. Elements are not included in the calculation because elements require no change in form.