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.