Capstone: Rate, yield or both?
The whole of C6 comes down to two separate questions about a reaction: how fast it goes (the rate) and how far it goes (the yield at equilibrium). A change to the conditions can affect one, the other, or both — and they don’t always move together. Take one industrial equilibrium, the reaction that makes ammonia:
N2(g) + 3H2(g) ⇌ 2NH3(g) — forward reaction exothermic, and 4 gas molecules become 2
Sort each change by what it does to this equilibrium. Watch out for temperature — it speeds the reaction up but pushes an exothermic equilibrium the wrong way.
Drag each change into a box — or tap it to step through the boxes. Then press Check.
- Calculating rates: mean rate = quantity of reactant used (or product formed)time taken; units g/s or cm³/s (mol/s H). The graph is steepest at the start; draw a tangent for the rate at an instant, and H calculate its gradient.
- Factors & RP5: rate increases with higher concentration, gas pressure, surface area, temperature and a catalyst. Follow a reaction by gas volume (syringe or over water), loss of mass on a balance, or a colour/turbidity change (the disappearing cross). Required practical 5 links concentration to rate — time isn’t rate, so use 1/time. The graph’s steepness is the rate; its plateau height is set by the amount of the limiting reactant.
- Collision theory: particles react only when they collide with at least the activation energy. Factors raise the frequency of collisions; temperature also makes a bigger fraction successful; smaller pieces react faster because of a larger surface area : volume ratio.
- Catalysts: speed up a reaction by providing a different pathway with a lower activation energy, without being used up — so they are not in the equation and change neither the yield nor the overall energy change. Enzymes are biological catalysts.
- Reversible reactions (⇌): the products can react to re-form the reactants; if the reaction is exothermic one way it is endothermic the other, transferring the same amount of energy each way (e.g. heating ammonium chloride).
- Dynamic equilibrium: reached in a closed system when the forward and reverse reactions occur at the same rate (not zero). Concentrations stay constant — constant does not mean equal.
- Changing conditions H: by Le Chatelier’s principle the position shifts to counteract a change — raise temperature → endothermic direction; raise pressure → side with fewer gas molecules; add a reactant or remove a product → towards products. Real processes balance the rate–yield compromise.
That completes C6. The activation energy and reaction profiles that explain why reactions speed up came from C5 — Energy Changes, and the rate–yield compromise you have just met is put to work on real industrial processes in C10 — Using Resources. Next, C7 — Organic Chemistry turns to crude oil and the family of carbon compounds, where reactions like cracking and combustion bring these rate and energy ideas back again.