Students in college always complain about this topic called Chemical Energetics and how difficult the questions are set in their schools.
After teaching batches of students year after year, i realised those students that are weak in this topic tends to be also “lazy”. They are too “lazy” to understand and remember the key definitions of each of the different Standard Enthalpy Changes of Reactions. No wonder they are struggling with this topic.
Students need to have a good grasp of all these fundamentals before they can answer application questions on Hess’ Law, Born Haber Cycle, Energy Level Diagram, etc.
To start, let’s look at Enthalpy Change of a Reaction, ΔH, which is defined as the heat change (heat energy absorbed or evolved) when the reaction takes place between the reagents as indicated by the stoichiometric equation for the reaction.
Let’s now take a look at the definitions of the key Standard Enthalpy Changes of Reactions (with examples):
Standard Enthalpy Change of Formation, ΔHfθ:
Enthalpy change when 1 mole of the compound is formed from its constituent elements under standard conditions
E.g. H2 (g) + 1/2 O2 (g) → H2O (l)
Standard Enthalpy Change of Combustion, ΔHcθ:
Enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions
E.g. C2H4 (g) + 3 O2 (g) → 2 CO2 (g) + 2 H2O (l)
Standard Enthalpy Change of Neutralisation, ΔHneuθ:
Enthalpy change when 1 mole of water is formed in the neutralisation between an acid and an alkali under standard conditions
E.g. HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)
Standard Enthalpy Change of Atomisation, ΔHatomθ:
Enthalpy change when an element or compound is converted into 1 mole of free gaseous atoms under standard conditions
E.g. 1/2 O2 (g) → O (g)
Standard Enthalpy Change of Hydration, ΔHhydθ:
Enthalpy change when 1 mole of the gaseous ion is dissolved in large amount of water under standard conditions
E.g. Na+ (g) + water → Na+ (aq)
Standard Enthalpy Change of Solution, ΔHsolnθ:
Enthalpy change when 1 mole of a substance dissolves in such a large volume of solvent that addition of more solvent produces no further heat change under standard conditions
E.g. NaCl (s) + water → Na+ (aq) + Cl– (aq)
Lattice Energy, ΔHlattθ:
Enthalpy evolved when 1 mole of an ionic crystalline solid is formed from its separate gaseous ions under standard conditions
E.g. Na+ (g) + F– (g) → NaCl (s)
Besides the definitions above, it is important that you know two more basic definitions also in order to solve application type questions:
Electron Affinity, E.A.:
Enthalpy change (measured in kJmol-1) when 1 mole of electrons is added to 1 mole of atoms or ions in the gaseous state
E.g. X (g) + e– → X– (g) => this is known as the 1st E.A.
Bond Energy, B.E.:
Energy required to break 1 mole of a covalent bond between 2 atoms in the gaseous state
E.g. A-A (g) → 2 A (g)
Standard condition (under the new H2 Chemistry syllabus code 9729) refers to:
- Pressure of 1 bar (1 atm for the previous/older H2 Chemistry syllabus 9647)
- Temperature of 298K
- Substance in its most stable physical form e.g. H2 (g), H2O (l), Br2 (l), NaCl (s), etc
Hope you find the above discussion useful. Do share it with your friends / students.