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NSWChemistryQuick questions
Module 5: Equilibrium and Acid Reactions
Quick questions on Equilibrium constant Keq explained: HSC Chemistry Module 5
10short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is the equilibrium expression?Show answer
$$aA + bB \rightleftharpoons cC + dD$$
What is interpreting the value?Show answer
The magnitude of Keq tells you where the equilibrium lies.
What is the reaction quotient Q?Show answer
For a system not yet at equilibrium, the same expression is evaluated using the current concentrations and is called the reaction quotient Q.
What is the ICE method?Show answer
Almost every Keq calculation uses an ICE table (Initial, Change, Equilibrium). The change row uses the stoichiometric coefficients with a variable $x$ for the extent of reaction. The same ICE method underpins the Ksp calculations for sparingly soluble salts and the weak-acid pH calculations you meet later in the module.
What is check?Show answer
$\frac{(1.556)^2}{(0.222)(0.222)} = \frac{2.42}{0.0493} \approx 49$. Correct.
What is forgetting the stoichiometric exponent?Show answer
$K_{eq} = \frac{[HI]^2}{[H_2][I_2]}$. The "2" on HI is essential.
What is including pure solids or liquids?Show answer
For $CaCO_{3(s)} \rightleftharpoons CaO_{(s)} + CO_{2(g)}$, $K_{eq} = [CO_2]$ only. Solids are excluded.
What is using moles instead of concentration?Show answer
Always convert moles to mol/L by dividing by volume.
What is wrong direction of Keq when reaction is reversed?Show answer
$K_{reverse} = 1 / K_{forward}$. If a question gives Keq for one direction, you must invert it for the reverse.
What is assuming Keq changes with pressure or concentration?Show answer
It does not. Only temperature changes Keq.