1.0 Introduction| | | | | | | | 1| 2.0 Enthalpy Changes| | | | | | | 2,3| 2.1 The Standard Conditions For Calculating Enthalpy Changes| | | | 3.0 Hess’s Law| | | | | | | | 4,5| 3.1 The Applications of Hess’s Law| | | | | | 4.0 Standard Molar Enthalpy Change of Formation, ΔHof| | | | 6,7| 4.1 The Stability of A Compound| | | | | | | 4.2 Using ΔHof Values To Predict The Relative Stability Of A Compound| | | 5.0 Standard Molar Enthalpy Change of Combustion, ΔHoc| | | 8,9 5.1 Standard Enthalpy Change of Combustion and Molecular Structure| | | 6.0 Standard Molar Enthalpy Change of Neutralisation, ΔHonuet| | | 10,11| 6.1 The Standard Enthalpy Change of Neutralisation For Strong Acids | | | | and Strong Alkalis| | | | | | | |
6.2 The Standard Enthalpy Change of Neutralisation of Weak Acids and | | | | Weak Alkalis| | | | | | | |
7.0 Standard Molar Enthalpy Change of Atomisation, ΔHoat| | | 12| 8.0 Electron Affinity| | | | | | | 13| 9.0 Lattice Energy| | | | | | | 14|
9.1 The Magnitude of Lattice Energy| | | | | | 10.0 The Born-Haber Cycle| | | | | | | 15|
11.0 References| | | | | | | | 16|
Thermochemistry is the study of the energy and heat associated with chemical reactions and/or physical transformations. A reaction may release or absorb energy, and a phase change may do the same, such as in melting and boiling. Thermochemistry focuses on these energy changes, particularly on the system's energy exchange with its surroundings. Thermochemistry is useful in predicting reactant and product quantities throughout the course of a given reaction. It is also used to predict whether a reaction is spontaneous or non-spontaneous, favorable or unfavorable. Endothermic reactions absorb heat. Exothermic reactions release heat. Thermochemistry coelesces the concepts of thermodynamics with the concept of energy in the form of chemical bonds. The subject commonly includes the enthalpy changes of reaction, formation, combustion, solution, neutralisation, atomisation, and lattice energy.
2.0 Enthalpy Changes
Enthalpy change is the name given to the amount of heat evolved or absorbed in a reaction carried out at constant pressure. It is given the symbol ΔH, read as "delta H". Enthalpy (H) is measured in the units of kJ. Hence, for the reaction A + B → C + D,
the enthalpy change is given by ΔH.
ΔH= The total enthalpy of the products – The total enthalpy of the reactants = H2 - H1
where H2 = ( HC + HD ) and H1 = ( HA + HB ).
If the reaction is exothermic, heat is given out to the surroundings, the enthalpy change for the reaction, ΔH has a negative value because the heat content of the products is less than the heat content of the reactants.
If the reaction is endothermic, heat is absorbed from the surroundings, the enthalpy change for the reaction, ΔH has a positive value because the heat content of the products is higher than the heat content of the reactants.
2.1 The Standard Conditions for the Calculating Enthalpy Changes The numerical value of the enthalpy change is influenced by five factors. The first factor is the temperature of the experiment being carried out. The second factor is the physical states ( solid, liquid, gas ) of the reactants. The third factor is the allotropic forms of the reactants. The forth factor is the pressure of gaseous reactants and the last factor is the concentration of the reactants. Under these conditions, the reactants and the products are said to be in their standard states. For example, the standard state of oxygen is a gas whereas the standard state of water is a liquid. Although the...