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QLDChemistryQuick questions
Unit 3: Equilibrium, acids and redox reactions
Quick questions on The equilibrium constant Kc and reaction extent (QCE Chemistry Unit 3)
12short 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 general expression?Show answer
For a reaction $aA + bB \rightleftharpoons cC + dD$ at equilibrium:
What is worked Kc set-up?Show answer
For $N_2O_{4(g)} \rightleftharpoons 2NO_{2(g)}$:
What is using an ICE table?Show answer
ICE (Initial, Change, Equilibrium) is the standard tool for IA1 calculation stimulus.
What is interpreting the value of Kc?Show answer
The bigger the Kc, the further the equilibrium lies to the right. Kc does not tell you anything about reaction rate, only about the position once equilibrium is reached.
What is what changes Kc?Show answer
For an exothermic forward reaction, Kc decreases as T increases. For an endothermic forward reaction, Kc increases as T increases. This is the basis of QCAA stimulus questions where you must infer the thermicity of the forward reaction from a Kc vs T table.
What is units of Kc?Show answer
Many QCAA answers omit Kc units and treat the constant as dimensionless. This is correct in strict thermodynamic treatment (concentrations divided by 1 mol/L standard state). QCE Chemistry accepts either approach, but if you assign units they must reflect delta-n = (mol products gas) - (mol reactants gas), with the unit being (mol/L)^delta-n.
What is kc vs Q (the reaction quotient)?Show answer
Q has the same expression as Kc but is calculated for any set of concentrations, not just equilibrium ones. Comparing Q to Kc predicts the direction of the net change:
What is including pure solids or pure liquids in the Kc expression?Show answer
Exclude them. The classic trap is the limestone equilibrium, where the only term in Kc is [CO2].
What is forgetting to use equilibrium concentrations rather than initial ones?Show answer
Substituting initial values gives Q, not Kc.
What is wrong stoichiometric exponents?Show answer
The exponents are the equation coefficients, not the moles consumed. For 2NO2, the exponent is 2.
What is reporting Kc with the wrong unit count?Show answer
Either give the calculated units explicitly or state Kc as dimensionless. Inconsistent unit work loses marks.
What is confusing Kc with rate constant k?Show answer
The rate constant k is a kinetics term linking rate to concentration; Kc is the ratio of equilibrium concentrations. They are unrelated except through specific microscopic relationships beyond Unit 3.