Q4. True or false? (a) At equilibrium the forward and reverse reactions stop. (b) At equilibrium the concentrations of reactants and products are constant. (c) At equilibrium the concentrations of reactants and products are equal. (d) Equilibrium can be reached by starting from the products.
(a) False: both continue at equal rates. (b) True. (c) False: they are constant but usually different. (d) True.
Q5. At some moment in a reacting mixture the forward reaction is going twice as fast as the reverse reaction. Is the mixture at equilibrium? What happens next?
No: at equilibrium the two rates are equal. Products are still forming faster than they are being used up, so the concentrations of products rise and those of reactants fall. In the simple fixed-volume mass-action model, that slows the forward reaction and speeds up the reverse one until rates are equal; complicated mechanisms need not approach equilibrium monotonically.
Q6. Flask A starts with 2.00 mol L−1 of HI; flask B starts with 1.00 mol L−1 of H2 and 2.00 mol L−1 of I2. Both are kept at 731 K. Will they reach the same equilibrium mixture?
No. In flask A every H2 and I2 comes from HI, so at equilibrium [H2] = [I2]. In flask B each H2 that reacts uses one I2, so [I2] always stays 1.00 mol L−1 above [H2]. The two mixtures cannot be the same.
Q7. In Haber’s experiments the composition of the N2, H2, NH3 mixture stopped changing while plenty of N2 and H2 were still present. Why did the reaction not use them up?
The reaction had reached equilibrium: ammonia was breaking down into N2 and H2 as fast as it was forming, so there was no net change, even though reactants remained.
Q8. Calcium carbonate, lime and carbon dioxide are at equilibrium in a closed vessel at a high temperature: CaCO3(s) ⇌ CaO(s) + CO2(g). A little CO2 made with radioactive carbon-14 is added, and the system is left to settle again. Where is the carbon-14 found later?
In the CO2 gas and also in the solid CaCO3. The reverse reaction, CaO + CO2 → CaCO3, keeps taking CO2 molecules, including labelled ones, into the solid, while the forward reaction releases others, so the label spreads into the solid even though the amounts stop changing.