(a)(i) Explain what is meant by an effective collision (ii) What does each term represent in the following expression? \(\Delta G\) = \(\Delta H\) — T\(\Del...

(a)(i) An effective collision is a collision between reacting particles in which the particles possess energy equal to or greater than the activation energy and collide with the correct orientation, so that a chemical reaction actually takes place.

(ii) In \(\Delta G = \Delta H - T\Delta S\):

• \(\Delta G\) = the change in Gibbs free energy of the system.
• \(\Delta H\) = the change in enthalpy (heat content) of the system.
• \(T\) = the absolute temperature (in kelvin).
• \(\Delta S\) = the change in entropy (degree of disorder) of the system.

(iii) If \(\Delta G = 0\), the deduction is that the system is at equilibrium (the reaction is neither spontaneous in the forward nor in the reverse direction).

(b)(i) Le Chatelier's principle: when a system in dynamic equilibrium is subjected to a change in temperature, pressure or concentration, the equilibrium shifts in the direction that tends to oppose (reduce) the effect of that change.

(ii) For \(2A_{2(g)} + B_{2(g)} \rightleftharpoons 2A_2B_{(g)}\), \(\Delta H = x\,kJ\) (taking the forward reaction to be exothermic), three conditions for maximum yield of \(A_2B\):

• Low temperature (favours the exothermic forward reaction).
• High pressure (3 moles of gas on the left change to 2 moles on the right, so high pressure shifts the equilibrium to the side of fewer gas molecules).
• Continuous removal of \(A_2B\) as it is formed (and/or use of a catalyst to attain equilibrium faster).

(iii) Equilibrium constant expression:

\[K_c = \frac{[A_2B]^2}{[A_2]^2\,[B_2]}\]

(c)(i) A log of wood burns more slowly than the same mass of sawdust because the sawdust is finely divided and therefore has a much larger surface area exposed to air (oxygen); the rate of reaction increases with surface area, so the sawdust burns out faster.

(iii) Two reactions catalysed by light: the photosynthesis of glucose in green plants, and the reaction of hydrogen with chlorine (or the substitution reaction of methane with chlorine).