Energy Relations in Chemistry: Thermochemistry

Enthalpies of Reaction

Heat and Enthalpy Changes

When a chemical reaction occurs in an open container most of the energy gained or lost is in the form of heat. Almost no work is done (i.e. nothing is being moved).

Heat flows between the system and surroundings until the two are at the same temperature.

• When a chemical reaction occurs in which the system absorbs heat, the process is endothermic (it feels cold)
• When a chemical reaction occurs in which the system produces heat it is exothermic (it feels hot)

Enthalpy

Under conditions of constant pressure (e.g. most biological processes under constant atmospheric pressure) the heat absorbed or released is termed enthalpy (or "heat content").

We do not measure enthalpy directly, rather we are concerned about the heat added or lost by the system, which is the change in enthalpy (or DH).

In formal terms: The change in enthalpy, DH, equals the heat, qp, added to or lost by the system when the process occurs under constant pressure:

DH=qp

DH represents the difference between the enthalpy of the system at the beginning of the reaction compared to what it is at the end of the reaction:

DH = Hfinal - Hinitial

We are considering the enthalpic state of the system. Thus:

• if the system has higher enthalpy at the end of the reaction, then it absorbed heat from the surroundings (endothermic reaction)
• if the system has a lower enthalpy at the end of the reaction, then it gave off heat during the reaction (exothermic reaction)

Therefore:

• For endothermic reactions Hfinal > Hinitial and DH is positive (+DH)
• For exothermic reactions Hfinal < Hinitial and DH is negative (-DH)

Enthalpies of Reaction

Because the enthalpy change for a reaction is described by the final and initial enthalpies:

DH = Hfinal - Hinitial

we can also describe DH for a reaction by comparing the enthalpies of the products and the reactants:

DH = H(products) - H(reactants)

The enthalpy change that accompanies a reaction is called the enthalpy of reaction (DHrxn).

It is sometimes convenient to provide the value for DHrxn along with the balanced chemical equation for a reaction (also known as a thermochemical equation):

2H2(g) + O2(g) -> 2H2O(g) DH = -483.6 kJ

Note the following:

• DH is negative, indicating that this reaction results in the release of heat (exothermic)
• The reaction gives of 483.6 kilo Joules of energy when 2 moles of H2 combine with 1 mole of O2 to produce 2 moles of H2O.

The relative enthalpies of the reactants and products can also be shown on an energy diagram:

Properties of enthalpy:

1. Enthalpy is an extensive property. The magnitude of DH is dependent upon the amounts of reactants consumed. Doubling the reactants, doubles the amount of enthalpy.
2. Reversing a chemical reaction results in the same magnitude of enthalpy but of the opposite sign. For example, splitting two moles of water to produce 2 moles of H2 and 1 mole of O2 gas requires the input of +483.6 kJ of energy.
3. The enthalpy change for a reaction depends upon the state of the reactants and products. The states (i.e. g, l, s or aq) must be specified.

CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g) DH = -802 kJ

Given the above thermochemical equation for the combustion of methane, how much heat energy is released when 4.5 grams of methane is burned (in a constant pressure system)?

The negative sign (exothermic) indicates that 225.5 kJ of energy are given off by the system into the surroundings.

1996 Michael Blaber