What Is Chemical Equilibrium? The Balance Point
Equilibrium sounds like the reaction has stopped. It hasn't — and that misunderstanding costs more marks in this topic than anything else.
The short answer: chemical equilibrium is the state a reversible reaction reaches in a closed system when the forward and reverse reactions are happening at the same rate, so the concentrations of everything stop changing. It is dynamic: both reactions continue indefinitely, they just cancel out.
What "dynamic" actually means
Picture a sealed flask. At the start there's only reactant, so the forward reaction is fast and the reverse can't happen at all. As product builds up, the forward rate falls (reactant is being used) and the reverse rate rises (there's now product to react back).
Sooner or later the two rates meet. From that moment on, product is being made exactly as fast as it's being destroyed, so the amounts stay constant — even though, at the molecular level, nothing has slowed down at all.
An analogy that survives scrutiny: a down escalator with someone walking up it at exactly the escalator's speed. They stay level with the same advert on the wall. Nothing is still; the two motions are simply matched.
The three conditions for equilibrium:
- The reaction must be reversible.
- The system must be closed — nothing enters or leaves.
- Temperature must be constant.
The two signs of equilibrium
You can tell equilibrium has been reached by either:
- Macroscopic properties are constant. Concentration, colour, pressure and pH stop changing.
- Forward rate = reverse rate. This is the underlying definition.
The observable one is the constant properties. But careful — a reaction that has simply finished also shows constant properties. What distinguishes equilibrium is that both reactants and products are still present, and disturbing the system makes it respond.
The equilibrium constant, Kc
Every equilibrium at a given temperature settles at a characteristic ratio, captured by the equilibrium constant. For a general reaction
aA + bB ⇌ cC + dD
the constant is products over reactants, each raised to its coefficient:
Kc = ([C]ᶜ [D]ᵈ) / ([A]ᵃ [B]ᵇ)
using equilibrium concentrations, not starting ones. What it tells you:
| Kc value | Meaning |
|---|---|
| Kc ≫ 1 (large) | Equilibrium lies to the right — mostly products |
| Kc ≈ 1 | Comparable amounts of both |
| Kc ≪ 1 (small) | Equilibrium lies to the left — mostly reactants |
Two facts to hold onto. Kc depends only on temperature — changing concentrations or pressure moves the position of equilibrium but leaves Kc alone. And a catalyst never changes Kc; it only gets the system to the same equilibrium faster.
A worked calculation
For H₂ + I₂ ⇌ 2HI, a sealed vessel at a fixed temperature is found to contain, at equilibrium:
- [H₂] = 0.20 mol dm⁻³
- [I₂] = 0.20 mol dm⁻³
- [HI] = 1.6 mol dm⁻³
Write the expression, then substitute:
Kc = [HI]² / ([H₂][I₂]) = (1.6)² / (0.20 × 0.20) = 2.56 / 0.040 = 64
Kc = 64, comfortably greater than 1, so the equilibrium favours the product — most of the hydrogen and iodine has ended up as HI. Note that HI is squared because its coefficient is 2, and that Kc here has no units, since the powers on top and bottom happen to cancel.
Worked examples
Predict before you read on.
1. A sealed tube of N₂O₄ ⇌ 2NO₂ turns pale brown and the colour then stays fixed. Has the reaction stopped?
No. It has reached equilibrium. Both molecules are still interconverting; the rates are simply equal.
2. A reaction has Kc = 1 × 10⁻⁵. Where does the equilibrium lie?
Far to the left — almost entirely reactants. Typical of a weak acid.
3. You add a catalyst to a system at equilibrium. What changes?
Nothing about the position or Kc. The system would just have arrived there sooner.
4. At equilibrium, are the concentrations of reactants and products equal?
Almost never. Rates are equal; amounts usually aren't.
Common mistakes to avoid
- Saying the reaction has stopped. Equilibrium is dynamic. Both directions run forever; the answer to look for is "the forward and reverse rates are equal".
- Assuming equal concentrations. Equal rates, not equal amounts. Kc tells you which side is favoured.
- Putting starting concentrations into the Kc expression. Only equilibrium concentrations belong there.
FAQ
What is chemical equilibrium in simple terms?
It is the point in a reversible reaction where the forward and reverse reactions happen at the same rate, so the amounts of reactants and products stop changing.
Why is equilibrium described as dynamic?
Because both reactions continue at the molecular level. Nothing stops — the two opposing changes simply cancel out, so no net change is visible.
Does equilibrium mean equal amounts of reactants and products?
No. It means equal rates. The actual amounts can be heavily weighted to either side, which is what the equilibrium constant tells you.
What does a large Kc mean?
That the equilibrium position lies to the right, so at equilibrium the mixture is mostly products.
The takeaway
Chemical equilibrium is a balance of rates, not a truce. Forward and reverse reactions keep running at identical speeds in a closed system, holding the concentrations steady at a ratio set by Kc and by the temperature. Change the conditions and the balance shifts — which is what the last post of this week is about.
Background → [Reversible vs Irreversible Reactions] (sibling) and What Is a Chemical Equation? Balancing Made Simple. See also → [What Is Reaction Rate?] (sibling) for the rates that end up equal, and [Strong vs Weak Acids] (sibling) for an equilibrium you meet constantly.
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