Start with the recap, study the fully worked examples, then use the practice problems to
check your understanding of Collision Theory.
This page combines explanation, solved examples, and follow-up practice so you can move
from recognition to confident problem-solving in Chemistry.
Concept Recap
A model explaining that chemical reactions occur only when reactant particles collide with sufficient kinetic energy (at least equal to the activation energy) and in.
Molecules must hit each other the right way and hard enough for bonds to break.
Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.
What to Focus On
Core idea:Collision Theory starts by naming reactants and products, then checks conservation with a balanced equation.
Common stuck point:Students often know a formula related to collision theory but skip the recognition step: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation? That leads to a correct-looking substitution attached to the wrong chemical model.
Sense of Study hint:Ask: Am I tracking reactants, products, atom conservation, evidence of new substances, and the balanced equation?
Worked Examples
Example 1
easy
State the three conditions required for a successful chemical reaction according to collision theory.A collision must supply at least Eₐ to cross the energy barrier and form products.
Answer
Collision + sufficient energy + correct orientation = reaction
First step
1
Condition 1: Reactant particles must collide with each other — no collision means no reaction.
Full solution
2
Condition 2: The collision must have sufficient energy (at least equal to the activation energy Ea) to break existing bonds.
3
Condition 3: The particles must collide with the correct orientation so that bonds can form between the right atoms.
Collision theory explains reaction rates at the molecular level. Not all collisions lead to reactions — only those meeting all three criteria are 'effective collisions' that produce products.
Example 2
medium
Use collision theory to explain why increasing the concentration of reactants increases the reaction rate.
Example 3
medium
Why does grinding zinc into a powder make it react faster with HCl than a single piece?
Example 4
hard
Two reactions A and B have the same Ea. Reaction A is run at 300 K and B at 310 K. Predict and justify which is faster.
Practice Problems
Try these problems on your own first, then open the solution to compare your method.
Example 1
medium
Using collision theory, explain why a finely ground powder reacts faster than a large chunk of the same solid.
Example 2
hard
Two reactions have the same activation energy, but Reaction A involves small, linear molecules while Reaction B involves large, complex molecules. Using collision theory, predict which reaction has a higher proportion of effective collisions and explain why.
Example 3
easy
According to collision theory, what two conditions must a collision meet to cause a reaction?
Example 4
easy
Most collisions between reactant molecules do not produce a reaction. True or false?
Example 5
easy
What is activation energy?The bracket marks the minimum energy a collision must supply — identify this quantity.
Example 6
easy
Does raising temperature increase or decrease the fraction of collisions that have enough energy to react?
Example 7
easy
Does a catalyst increase reaction rate by raising or lowering the activation energy?Does the catalyst raise or lower the activation energy barrier?
Example 8
easy
Increasing concentration of a reactant increases reaction rate. Which collision-theory factor explains this?
Example 9
easy
Why do powdered solids react faster than a single lump of the same mass?
Example 10
easy
If a high-energy collision happens but the molecules are poorly aligned, will it react?
Example 11
medium
A reaction speeds up when both temperature and concentration are raised. Separate which factor changes collision energy and which changes collision frequency.
Example 12
medium
Reaction P has Ea=80kJ/mol; reaction Q has Ea=40kJ/mol. At the same temperature, which is faster and why?At the same temperature, which reaction — P (taller barrier) or Q (shorter barrier) — proceeds faster?
Example 13
medium
Explain, using collision theory, why food spoils slower in a refrigerator.
Example 14
medium
Why does a catalyst speed up a reaction even though the reactant concentrations are unchanged?The catalyst lowers Eₐ so more collisions clear the barrier — no extra particles needed.
Example 15
medium
Two gases react. At fixed temperature, the pressure is doubled by compression. Use collision theory to predict the effect on rate.
Example 16
medium
Why does increasing temperature speed up a reaction far more than the small rise in collision frequency alone would explain?
Example 17
medium
A reaction needs molecules to collide with the −OH end facing a specific carbon. Which collision-theory requirement is this, and how does it limit rate?
Example 18
medium
Sketch in words why a reaction's rate is highest at the start: relate concentration to collision frequency.
Example 19
medium
Why does stirring or shaking a reaction mixture of two liquids speed up the reaction?
Example 20
challenge
Reaction R has Ea=50kJ/mol. A catalyst lowers it to 30kJ/mol. Qualitatively, why does this raise the rate, and does it change how much product forms at completion?The catalyst lowers Eₐ (faster rate) but leaves ΔH = 0 unchanged — the same final yield.
Example 21
challenge
At room temperature very few molecules exceed Ea, yet heating 20°C can double the rate. Explain using the energy distribution.
Example 22
challenge
Two reactants are very dilute and at low temperature, yet the reaction is fast. Which single change to collision conditions could explain this, and why?
Example 23
easy
True or false: increasing surface area changes the energy of collisions.
Example 24
easy
Which factor does NOT increase reaction rate: (a) higher temperature, (b) more concentrated reactants, (c) cooler temperature, (d) a catalyst?
Example 25
easy
Does a catalyst get used up during a reaction?
Example 26
easy
Two gas molecules approach each other at high speed. What other condition must be true for a reaction to occur?
Example 27
medium
A reaction has 1 in 1,000,000 collisions effective at 25°C. Raising the temperature 20°C makes 2 in 1,000,000 effective. Approximately how much faster is the reaction (if collision frequency is unchanged)?
Example 28
medium
A gas reaction's volume is halved at constant temperature. By collision theory, how should the rate change?
Example 29
medium
Catalyst C lowers Ea from 75kJ/mol to 50kJ/mol. Does the equilibrium constant (or final yield) change?
Example 30
medium
Why is the rate of a reaction usually highest at the start and slows as time passes?
Example 31
medium
A reaction is heated from 300K to 310K. Empirically, the rate roughly doubles. Identify two collision-theory reasons.
Example 32
medium
Two reactant molecules collide with energy above Ea but the rate stays low. What variable might explain this?
Example 33
medium
Compare reactions of H2+Cl2 (small linear molecules) vs C20H42+Cl2 (long chain). Which is expected to have a higher steric (orientation) factor and why?
Example 34
medium
Why are gas-phase reactions usually faster than the same reaction in solid form at the same temperature?
Example 35
hard
A heterogeneous catalyst works by providing a surface where reactants adsorb. Which two collision-theory factors does this improve?
Example 36
hard
Use the Arrhenius idea: at T1 the rate constant is k1, at T2>T1 it is k2. Why is k2>k1 when Ea>0?
Example 37
hard
If concentration is doubled and the orientation factor is halved (e.g., by switching to a bulkier reactant), what is the approximate net effect on rate (other factors equal)?
Example 38
hard
A reaction in solution is slowed by adding inert solvent. Use collision theory to explain.
Example 39
hard
Why does adding a catalyst speed up a reaction even at very low temperature when very few molecules have Ea energy?
Example 40
hard
An enzyme accelerates a biological reaction 106 times. Using collision theory, what two effects is it likely producing?
Example 41
hard
If a gas is heated in a sealed (rigid) container, both pressure and temperature rise. Which factor (frequency or energy) is most responsible for the rate increase?
Example 42
challenge
Justify why doubling [A] in a reaction A+B→ products doubles the rate, but doubling T (in K) often more than doubles the rate.