1.4 Thermodynamics (AS)
The enthalpy change in a chemical reaction can be measured accurately. It is important to know
this value for chemical reactions that are used as a source of heat energy in applications such as
domestic boilers and internal combustion engines.
- Enthalpy change (∆H) - Reactions can be endothermic or exothermic. Definition Enthalpy change (∆H) is the heat energy change measured under conditions of constant pressure. Standard enthalpy changes refer to standard conditions, ie 100 kPa and a stated temperature (eg ∆H298). Students should be able to: • define standard enthalpy of combustion (∆Hc) • define standard enthalpy of formation (∆Hf).
- Calorimetry - The heat change, q, in a reaction is given by the equation q = mc∆T where m is the mass of the substance that has a temperature change ∆T and a specific heat capacity c. Students should be able to: • use this equation to calculate the molar enthalpy change for a reaction • use this equation in related calculations. [Students will not
- Application of Hess’s law - Hess’s law. Students should be able to:use Hess’s law to perform calculations, including calculation of enthalpy changes for reactions from enthalpies of combustion or from enthalpies of formation. Assessment and practical opportunities Students carry out Hess’s law calculations. Students could be asked to find ∆H for a reaction using Hess’s law and calorimetry, then present data in appropriate ways. Examples of reactions could include: • thermal decomposition of
- Bond enthalpies - Definition Mean bond enthalpy. Students should be able to: • define the term mean bond enthalpy • use mean bond enthalpies to calculate an approximate value of ∆H for reactions in the gaseous phase • explain why values from mean bond enthalpy calculations differ from those determined using Hess’s law and enthalpy cycles. Assessment and practical opportunities Students understand that bond enthalpies are mean