Enthalpy Changes

Section: Understanding Enthalpy Changes

Enthalpy change (ΔH) is the amount of heat energy taken in or given out during a chemical reaction.
It’s measured in joules (J) or kilojoules (kJ) per mole (mol) and can be either positive or negative.
A positive enthalpy change means the reaction is endothermic (absorbs heat from surrounding), while a negative enthalpy change denotes it as exothermic (releases heat to surrounding).

Section: Types of Enthalpy Change
Enthalpy change of reaction (ΔH°r) is the enthalpy change that accompanies a reaction in the molar quantities shown in the chemical equation.
Enthalpy change of formation (ΔH°f) is the enthalpy change when one mole of a compound is formed from its constituent elements.
Enthalpy change of combustion (ΔH°c) is the enthalpy change when one mole of a substance undergoes complete combustion with oxygen.
Enthalpy change of neutralisation (ΔH°n) is the enthalpy change when one mole of water is formed from the reaction of an acid and an alkali.

Section: Calculating Enthalpy Change
Calorimetry is a common technique used to experimentally determine enthalpy changes.
For instance, to find the enthalpy change of combustion, the fuel is burned under controlled conditions while measuring temperature change.
Calculating enthalpy change involves using the equation: q=mcdT (where q = heat change, m = mass of substance, c = specific heat capacity, and dT = change in temperature).
For the exam remember that the standard conditions for enthalpy change measurements are at a pressure of 1 atmosphere and a temperature of about 25 degrees Celsius.

Section: Hess’ Law and Enthalpy Change
Hess’ law states that the total enthalpy change for a reaction is independent of the route by which the reaction takes place.
It’s often used when the enthalpy change of a reaction cannot be measured directly.
Hess’ Law can be applied in graphical form known as Hess’ Cycles or through algebraic methods using known enthalpies of formation or combustion.
Understanding Hess’ Law will be crucial for working through more complex energetics questions.

A positive enthalpy change means the reaction is endothermic (absorbs heat from surrounding), while a negative enthalpy change denotes it as exothermic (releases heat to surrounding).

Enthalpy change of neutralisation (ΔH°n) is the enthalpy change when one mole of water is formed from the reaction of an acid and an alkali.

For the exam remember that the standard conditions for enthalpy change measurements are at a pressure of 1 atmosphere and a temperature of about 25 degrees Celsius.