Hess’s law is one of the most useful relationships in thermochemistry and thermodynamics. Because enthalpy is a state property, any path between a set of reactants and products (under the same initial and final conditions) will produce the same change in enthalpy. If it can be shown that two equations can be combined to give a third (for example, eq 1 + eq. 2 = eq 3), then Hess’s law states that the H’sfor these reactions can be combined similarly to give the H for the combined equation (in this exampleH1 +H2 =H3.) More complicated relationships between equations are handled as follows: if equation x is multiplied by a constant, then Hx is also be multiplied by that constant; if an equation must be reversed, thenHx must be negated.
In this lab you will perform three related reactions using known concentrations of common chemicals. Measuring temperature changes will allow you to determine the heats of reaction. Because the reactions take place at constant pressure, the heat of reaction must equal the enthalpy change, which allows you to write thermochemical equations. By applying Hess’s law to the first two reactions, you will be able to calculate the enthalpy change for the third reaction and compare it to the value you obtained experimentally.
Heat cannot be measured directly (like mass on a balance or volume in a graduated cylinder), but only through indirect methods such as finding the temperature change it creates in a known material. For a pure substance, the temperature change is a function of the specific heat of the material (c), it mass (m) and the amount of heat absorbed or lost (q):
q=m•c•T (for pure substances)
Specific heat is an intensive property and is only defined for a homogeneous material. For heterogeneous mixtures, the more general extensive property of heat capacity (C) is used:
q= C•T (for anything)
A device used to measure heat is called a calorimeter. A calorimeter is designed to...