Temperature
Simple Explanation
Raising the temperature increases reaction rate for two reasons: particles move faster (so they collide more often), and — more importantly — a much larger fraction of collisions now have enough energy to overcome the activation energy.
Why Do We Need It?
Temperature is often the single most powerful lever for controlling reaction rate — a modest temperature increase can double or triple a reaction's rate, far more dramatically than a similar percentage increase in concentration.
Formula
The Arrhenius Equation
k = A·e^(−Ea/RT)
How the rate constant of a reaction depends on temperature and activation energy — the mathematical basis for why raising temperature speeds up reactions so dramatically.
- k
- — the rate constant of the reaction
- A
- — the pre-exponential (frequency) factor — related to how often molecules collide with the correct orientation
- Ea
- — activation energy, in J/mol
- R
- — the gas constant, 8.314 J/(mol·K)
- T
- — absolute temperature, in kelvin (K)
When to use it: Whenever you need to explain or calculate how much a reaction rate changes with temperature, or compare the activation energies of different reactions.
Worked Example
Estimate the effect of temperature using the 'rule of thumb'
A reaction at 20°C has a rate of 0.010 mol/(L·s). Using the common approximation that reaction rate roughly doubles for every 10°C rise, estimate the rate at 40°C.
Why Does This Work?
This is a direct consequence of the Maxwell–Boltzmann distribution: raising the temperature shifts the whole energy distribution toward higher energies, and because the activation energy cutoff is out in the tail of the distribution, even a modest temperature rise disproportionately increases the fraction of particles with enough energy to react.
Real-Life Example
Refrigeration slows food spoilage
Food spoils far more slowly in a refrigerator than left out at room temperature.
The chemical and biological reactions that cause spoilage have a real activation energy — cooling the food significantly reduces the fraction of molecules with enough energy to react, slowing spoilage dramatically even though the temperature drop is modest.
Practice
A reaction at 10°C has a rate of 0.005 mol/(L·s). Using the "rate roughly doubles every 10°C" rule, estimate the rate at 30°C.
MediumCommon mistake
Treating the "doubles every 10°C" rule as an exact law — it is a rough approximation that works reasonably well for many reactions near room temperature, but the real relationship (given by the Arrhenius equation) depends on the specific activation energy of each reaction.
Quick Review
- Higher temperature increases both collision frequency AND the fraction of successful collisions.
- The second effect dominates — this is why rate is so sensitive to temperature.
- A common rough rule: rate roughly doubles for every 10°C rise.