Quick questions on The equilibrium constant Kc and reaction extent (QCE Chemistry Unit 3)

ICE (Initial, Change, Equilibrium) is the standard tool for IA1 calculation stimulus.

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.

The exponents are the equation coefficients, not the moles consumed. For 2NO2, the exponent is 2.

Write the equilibrium expression $K_c$ for $2 \text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2 \text{SO}_3(g)$. State its units. [2 marks]

For $\text{H}_2(g) + \text{I}_2(g) \rightleftharpoons 2 \text{HI}(g)$, $K_c = 54$ at $700 \, \text{K}$. Starting with $1.00 \, \text{mol L}^{-1}$ of each $\text{H}_2$ and $\text{I}_2$, calculate equilibrium $[\text{HI}]$. [4 marks]

$K_c$ for the reaction $\text{N}_2 + 3 \text{H}_2 \rightleftharpoons 2 \text{NH}_3$ is $0.060$ at $500^{\circ}\text{C}$ and $0.50$ at $400^{\circ}\text{C}$. (a) Predict whether forward is exothermic or endothermic. (b) Justify.