CAIE A-Level Chemistry 5.1.3 Standard Enthalpy Definitions

Practise defining reaction, formation, combustion and neutralisation enthalpies with correct standard conditions and amounts.

Exam points

state the syllabus standard conditions as 298 K and 101 kPa with substances in their standard states
define formation for one mole of compound from constituent elements in their standard states
define combustion and neutralisation using one mole of substance burned or one mole of water formed

[Maximum number: 2]

The Group 17 elements are oxidising agents.

$\mathrm{Cl}_{2}$ and $\mathrm{Br}_{2}$ can each react with $\mathrm{NH}_{3}$ to give $\mathrm{N}_{2}$ and a hydrogen halide, HX .

3 \mathrm{X}_{2}+2 \mathrm{NH}_{3} \rightarrow \mathrm{~N}_{2}+6 \mathrm{HX}

X = Cl or Br

The relative bond strengths of X-X and H-X determine the difference in enthalpy change of the two reactions.

Define enthalpy change of formation, $\Delta H_{\mathrm{f}}$ .

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