Kinetics vs Thermodynamics: What's the Difference?
Diamond is not the most stable form of carbon. Graphite is. Every diamond on Earth is slowly, inevitably turning into pencil lead — and will finish somewhere around the heat death of the universe. That gap between should and does is the whole of this post.
The short answer: thermodynamics tells you whether a reaction is energetically favourable and how far it will go; kinetics tells you how fast it gets there. A reaction can be thermodynamically favourable and kinetically impossible at room temperature — which is why diamonds survive.
Quick comparison at a glance
| Feature | Thermodynamics | Kinetics |
|---|---|---|
| Question answered | Will it happen? How far? | How fast will it happen? |
| Key quantities | ΔH, ΔS, ΔG, K | Rate, activation energy (Eₐ) |
| Depends on | Only the start and end states | The pathway between them |
| Shown on an energy profile by | The difference in height between reactants and products | The height of the hump |
| Changed by a catalyst? | No | Yes |
| Changed by temperature? | Yes (ΔG and K both shift) | Yes, usually dramatically |
| Typical exam phrase | "thermodynamically favourable / spontaneous" | "kinetically stable / slow" |
What thermodynamics tells you
Thermodynamics is bookkeeping between two states. It asks whether the products sit at a lower free energy than the reactants, and it doesn't care in the slightest how you get from one to the other.
The deciding quantity is Gibbs free energy:
ΔG = ΔH − TΔS
- ΔH is the enthalpy change — heat released (negative, exothermic) or absorbed (positive, endothermic).
- ΔS is the entropy change — roughly, how much the disorder increases.
- ΔG < 0 means the reaction is spontaneous (thermodynamically favourable) in that direction.
"Spontaneous" is the single most misleading word in the subject. In everyday English it suggests immediately. In thermodynamics it means only energetically downhill — with no promise whatsoever about the timescale.
What kinetics tells you
Kinetics is about the road, not the destination. Between reactants and products sits the activation energy barrier, and its height decides the speed. A tall barrier means very few colliding particles have enough energy to get over, so the reaction crawls no matter how favourable the destination.
Crucially, Eₐ and ΔH are independent. You can have:
- Large negative ΔG, small Eₐ → favourable and fast (sodium in water)
- Large negative ΔG, huge Eₐ → favourable but effectively frozen (a sealed H₂/O₂ mixture; petrol sitting in air)
- Tiny negative ΔG, enormous Eₐ → barely favourable on paper and frozen solid in practice (diamond → graphite)
- Small positive ΔG, small Eₐ → unfavourable but quick to reach its (reactant-heavy) equilibrium
Knowing one tells you nothing about the other. That's the heart of the topic.
The examples that make it click
Diamond → graphite. ΔG° is about −2.9 kJ mol⁻¹ — a small number, but negative, so graphite is genuinely the more stable form under ordinary conditions. But converting means dismantling a rigid three-dimensional covalent lattice, which demands an enormous activation energy. At room temperature the rate is unmeasurably small. Diamonds are kinetically stable, not thermodynamically stable — which is a far more interesting thing to say than "forever".
Petrol and air. A tank of petrol sitting in oxygen is enormously thermodynamically favourable — combustion releases a great deal of energy. It sits there safely for months because nothing supplies the activation energy. One spark does, and then it goes all at once.
Hydrogen and oxygen. A mixed sealed flask of H₂ and O₂ is perfectly stable indefinitely. Introduce a platinum catalyst, which lowers Eₐ, and it reacts explosively. The thermodynamics never changed; only the road did.
The one asymmetry worth memorising
A catalyst changes kinetics only. Never thermodynamics.
It lowers Eₐ, so the reaction is faster. It cannot change ΔH, ΔG, K, or the equilibrium position, because those depend only on where the reactants and products sit — not on the route between them. If an exam answer claims a catalyst improved the yield, it is wrong.
Temperature is the sneaky one: it affects both. It raises the rate (kinetics) and it also changes ΔG and shifts K (thermodynamics), which is why industrial processes so often end up as a compromise between the two.
Kinetic and thermodynamic products
When a reaction can give two different products, the split becomes visible. The kinetic product is the one formed faster — the lower activation energy — and dominates at low temperature and short reaction times. The thermodynamic product is the more stable one, and dominates when the temperature is high enough and there's time for the system to equilibrate.
The textbook case is adding HBr to buta-1,3-diene. The 1,2-product forms faster and dominates at low temperature; the 1,4-product is more stable and takes over when the mixture is warm enough to equilibrate. Same reactants, same flask — the thermometer decides which product you isolate.
Organic chemistry is full of related competitions, which is why reaction conditions get specified so carefully whenever SN1, SN2, E1 and E2 pathways are fighting over the same substrate.
Common mistakes to avoid
- Reading "spontaneous" as "fast". It means ΔG < 0 and nothing more. Rusting is spontaneous and takes years.
- Claiming a catalyst improves yield or makes a reaction possible. It speeds up a reaction that was already thermodynamically allowed. It cannot make an unfavourable one go.
- Assuming exothermic means fast. Enthalpy is the height difference; the activation energy is the hill. A very exothermic reaction with a large Eₐ is slow.
FAQ
What is the difference between kinetics and thermodynamics?
Thermodynamics says whether a reaction is energetically favourable and how far it goes; kinetics says how fast it happens. They are independent.
Can a reaction be thermodynamically favourable but not happen?
Yes — that's kinetic stability. Diamond converting to graphite is favourable but has such a high activation energy that the rate is negligible.
Does a catalyst affect thermodynamics?
No. A catalyst lowers the activation energy and speeds the reaction up, but leaves ΔH, ΔG, the equilibrium constant and the yield unchanged.
What does "spontaneous" mean in chemistry?
That ΔG is negative, so the reaction is energetically downhill. It says nothing about speed.
The takeaway
Thermodynamics decides the destination; kinetics decides the journey time. Ask both questions of every reaction — favourable and fast is a different answer from favourable but stuck — and remember that a catalyst can only ever change the second one.
Background → What Is Activation Energy? The Barrier Every Reaction Faces and Endothermic vs Exothermic Reactions Explained. See also → [What Is Reaction Rate?] (sibling), and SN1 SN2 Comparison for competing pathways in practice.
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