Equilibria
Contents: 7 sections
Dynamic equilibrium
A reversible reaction can proceed in both directions. In a closed system it reaches dynamic equilibrium, and both words carry meaning:
- Dynamic: both forward and reverse reactions are still happening, at the same rate.
- Equilibrium: the concentrations of reactants and products stay constant, though not necessarily equal.
Saying the reaction has stopped is wrong, and saying the concentrations are equal is wrong. Nothing appears to change because the two rates match.
Le Chatelier's principle
If a change is made to a system at equilibrium, the position of equilibrium shifts so as to oppose that change.
| Change | Shift |
|---|---|
| Increase concentration of a reactant | Towards the products |
| Increase total pressure | Towards the side with fewer gas molecules |
| Increase temperature | In the endothermic direction |
| Add a catalyst | No shift at all |
Two of those need care.
Pressure only matters if the number of gas molecules differs between the two sides. In the Haber process, N₂ + 3H₂ ⇌ 2NH₃, there are four gas molecules on the left and two on the right, so higher pressure favours ammonia. In H₂ + I₂ ⇌ 2HI there are two on each side, so pressure has no effect on the position at all.
Temperature is the only change that alters the value of the equilibrium constant. Everything else shifts the position while leaving K unchanged.
A catalyst speeds up the forward and reverse reactions equally, so equilibrium is reached sooner but at the same position. This is worth stating precisely, because "a catalyst increases the yield" is a common and costly error.
The industrial compromise
The Haber process is exothermic, so a low temperature would give a higher yield. In practice about 450 °C is used, because at genuinely low temperatures the rate is uneconomically slow. The chosen conditions are a compromise between yield and rate, and that phrase is what the mark scheme wants.
Equilibrium constants
For the general reaction aA + bB ⇌ cC + dD,
Products on top, each concentration raised to its balancing number. Kp is the same expression using partial pressures.
The units are worked out from the expression, and they differ from reaction to reaction. If the powers cancel, Kc has no units.
A large K means the position lies well to the right; a small K means it lies to the left. K changes only with temperature.
Worked example
For H₂ + I₂ ⇌ 2HI at equilibrium, a 1.00 dm³ vessel contains 0.200 mol H₂, 0.200 mol I₂ and 1.60 mol HI.
The powers cancel here, one on top against two below of the same kind, so Kc has no units.
Acids and bases
A Brønsted-Lowry acid is a proton donor; a base is a proton acceptor.
When an acid donates a proton it becomes its conjugate base. The pair differ by exactly one H⁺.
For HCl + H₂O → Cl⁻ + H₃O⁺, the conjugate pairs are HCl and Cl⁻, and H₂O and H₃O⁺. Water accepts a proton here, so it is acting as a base; with ammonia it donates one and acts as an acid.
Strong and concentrated are different words. A strong acid is fully dissociated in water; a weak acid only partially dissociates. Concentration is about how much acid is dissolved. A concentrated weak acid and a dilute strong acid are entirely possible.
- Strong acids: HCl, HNO₃, H₂SO₄
- Weak acids: ethanoic acid, carbonic acid
- Strong bases: NaOH, KOH
- Weak bases: ammonia
The pH scale
The scale is logarithmic, so each whole unit is a factor of ten in hydrogen ion concentration. A solution of pH 2 has one hundred times the hydrogen ion concentration of one at pH 4.
For a strong monobasic acid, [H⁺] equals the concentration of the acid, because dissociation is complete. 0.0100 mol dm⁻³ HCl has [H⁺] = 0.0100, and
pH = -log(0.0100) = 2.00
A weak acid of the same concentration has a higher pH, because only a fraction of its molecules have dissociated.
Reversing the calculation, [H⁺] = 10⁻ᵖᴴ.
Common mistakes
- Saying the reaction stops at equilibrium, or that the concentrations become equal.
- Saying a catalyst increases the yield. It changes the rate only, and reaches the same position sooner.
- Applying the pressure rule when both sides have the same number of gas molecules.
- Confusing strong with concentrated. Strong is about dissociation; concentrated is about amount.
- Saying K changes when concentration or pressure changes. Only temperature changes K.
- Leaving the balancing numbers out as powers in the Kc expression.
Check you have it
Question 1
Sodium is added to water to form solution Y. The pH of solution Y is measured.
When powdered substance X is added to solution Y, the pH falls.
Which two compounds could each be substance X?
Answer: A.
MgCl₂ dissolves to give Mg²⁺, which is small and doubly charged. It polarises the water molecules around it strongly enough to release H⁺, so its solution is slightly acidic. The pH falls.
Al(OH)₃ is amphoteric: it behaves as an acid towards a strong alkali and dissolves in it,
Al(OH)₃ + OH⁻ → [Al(OH)₄]⁻
removing hydroxide from the solution, so again the pH falls.
Both do it, so A.
NaCl is the salt of a strong acid and a strong base, so it is neutral and changes nothing. That rules out C and D at once, and noticing it halves the work.
K₂O is a Group 1 oxide, strongly basic. It reacts with water to give KOH, so it would raise the pH, not lower it.
Al(OH)₃ is the interesting one. On its own in water it is an insoluble, near-neutral hydroxide and does nothing; it only lowers the pH because the solution is already alkaline, which is exactly why the stem sets the scene with sodium and water first.
Question 2
The oxides BaO, CaO, MgO and SrO all produce alkaline solutions when added to water.
Which oxide produces the saturated solution with the highest pH?
Answer: A.
MO + H₂O → M(OH)₂
In a saturated solution the pH is set by how much hydroxide will dissolve, and solubility of the Group 2 hydroxides increases down the group.
The order down the group is Mg, Ca, Sr, Ba, so barium is the lowest of the four offered and Ba(OH)₂ is the most soluble. Most dissolved hydroxide means the highest [OH⁻] and the highest pH. That is A.
C is the opposite end. Mg(OH)₂ is so nearly insoluble that its saturated solution reaches only about pH 10, which is why it is safe to swallow as milk of magnesia while barium hydroxide certainly is not.
The reason for the trend is worth carrying, because Group 2 sulfates run the other way. Going down the group the cation gets bigger, so the lattice energy of the hydroxide falls faster than its hydration enthalpy does, and dissolving becomes more favourable. For the sulfates, with their much larger anion, the balance tips the other way and solubility decreases down the group. Hydroxides up, sulfates down, is the pair to remember.
Question 3
In which equilibrium will an increase in pressure at constant temperature increase the yield of the products on the right-hand side of the equation?
Answer: B.
B. 4HCl(g) + O₂(g) ⇌ 2H₂O(g) + 2Cl₂(g)
5 moles of gas on the left against 4 on the right. Fewer on the right, so the equilibrium moves right and the yield of products increases. That is B.
A. CaCO₃(s) ⇌ CaO(s) + CO₂(g). The calcium compounds are solids, so this is 0 gas moles → 1. Raising the pressure pushes it back to the left.
C. 2HI(g) ⇌ H₂(g) + I₂(g). 2 → 2, so pressure has no effect at all.
D. 3Fe(s) + 4H₂O(g) ⇌ Fe₃O₄(s) + 4H₂(g). Ignoring the two solids, 4 → 4. No effect.
The state symbols are the entire question. In D there are seven species-worth of coefficients, and counting them without checking (s) against (g) gives 7 → 5 and the wrong answer. Only gases can be squeezed, so only gases count.
What the syllabus asks for on this topicSyllabus points
Syllabus points
- Explain what is meant by a reversible reaction and by dynamic equilibrium.
- State and apply Le Chatelier's principle to changes in concentration, pressure and temperature.
- Deduce expressions for Kc and Kp, and carry out calculations using them.
- Explain that a catalyst does not change the position of equilibrium.
- Define acids and bases in Brønsted-Lowry terms, identify conjugate pairs, and use the pH scale.
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