What can a chemist change to make a reaction go faster or slower?
Factors affecting reaction rate: describe how concentration, temperature, surface area, a catalyst and the nature of the reactants change the rate of a reaction.
A focused Virginia SOL Chemistry answer on rate factors under CH.6: how concentration, temperature, surface area, catalysts and the nature of the reactants change reaction rate, each explained with collision theory.
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What this topic is asking
Standard CH.6 asks you to describe the factors that change a reaction's rate and to explain each with collision theory. Virginia expects you to handle concentration, temperature, surface area, a catalyst, and the nature of the reactants, and to say why each speeds up or slows down a reaction in terms of effective collisions.
Concentration
More crowded particles collide more often, so more effective collisions occur each second. Diluting a solution or lowering a gas pressure does the opposite, slowing the reaction.
Temperature
Temperature is especially powerful because it affects both the frequency and the energy of collisions. This is why food spoils faster when warm (reactions speed up) and is preserved by refrigeration (reactions slow down).
Surface area
Only the particles at the surface of a solid can collide with a surrounding liquid or gas. Breaking the solid into smaller pieces exposes more particles at once, so more collisions occur. This is why powdered solids react faster than a single block of the same mass.
Catalysts and the nature of the reactants
A catalyst speeds up a reaction by providing an alternative pathway with a lower activation energy, so a greater fraction of collisions are effective. A catalyst is not consumed and can be recovered at the end. The nature of the reactants also matters: reactions between ions in solution are usually very fast (the particles are already separate and need only meet), while reactions that require breaking strong covalent bonds tend to be slower. The state of the reactants and how they are mixed contribute as well.
Try this
Q1. State the effect on rate of increasing the concentration of a reactant, and why. [1 point]
- Cue. The rate increases, because more particles per volume collide more frequently.
Q2. How does a catalyst increase the rate of a reaction? [1 point]
- Cue. It lowers the activation energy, so a greater fraction of collisions have enough energy to be effective.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SOL (multiple choice)1 marksWhich change will increase the rate of a reaction between a solid and a solution? (A) using a single large lump of solid (B) grinding the solid into a powder (C) lowering the temperature (D) diluting the solutionShow worked answer →
The answer is (B) grinding the solid into a powder.
Grinding a solid into a powder increases its surface area, exposing more particles to collide with the solution, so the rate increases. A single large lump (A) has less surface area; lowering the temperature (C) slows the reaction; diluting the solution (D) lowers the concentration and slows it.
The trap is choosing a change that reduces collisions; only increasing surface area, concentration or temperature (or adding a catalyst) speeds the reaction.
SOL (tech-enhanced, drag and drop)3 marksState the effect on reaction rate of each change, using collision theory: (a) increasing the concentration of a reactant; (b) adding a catalyst; (c) lowering the temperature.Show worked answer →
A 3-point item linking factors to collision theory.
(a) Increasing concentration (1 point): more particles per volume means more frequent collisions, so the rate increases.
(b) Adding a catalyst (1 point): a catalyst lowers the activation energy, so a greater fraction of collisions are effective and the rate increases.
(c) Lowering temperature (1 point): particles move slower, colliding less often and with less energy, so fewer effective collisions occur and the rate decreases.
Markers reward both the direction of the effect and a collision-theory reason for each.
Related dot points
- Reaction rates and collision theory: explain reaction rate using collision theory, including effective collisions, orientation and the activation energy.
A focused Virginia SOL Chemistry answer on collision theory under CH.6: what reaction rate measures, why particles must collide with enough energy and the correct orientation, the role of activation energy, and the meaning of an effective collision.
- Potential energy diagrams and activation energy: interpret a potential energy diagram, identify the activation energy and the energy change, and explain the effect of a catalyst.
A focused Virginia SOL Chemistry answer on energy diagrams under CH.6: reading a potential energy diagram, identifying the activation energy, the energy of the products versus reactants, and how a catalyst lowers the activation energy.
- Endothermic and exothermic reactions: distinguish endothermic and exothermic processes by the direction of energy flow and the sign of the enthalpy change.
A focused Virginia SOL Chemistry answer on reaction energy under CH.6: the difference between endothermic and exothermic reactions, the direction of energy flow, the sign of the enthalpy change, and how temperature change signals each type.
- Chemical equilibrium and Le Chatelier's principle: describe dynamic equilibrium in a reversible reaction and predict the shift when concentration, temperature or pressure changes.
A focused Virginia SOL Chemistry answer on equilibrium under CH.6: reversible reactions and dynamic equilibrium, and using Le Chatelier's principle to predict how an equilibrium shifts when concentration, temperature or pressure changes.
- Solutions, solubility and concentration: describe solutes, solvents and the dissolving process, the factors that affect rate of dissolving and solubility, and how to read a solubility curve.
A focused Virginia SOL Chemistry answer on solutions under CH.5: solute and solvent, the dissolving process and like dissolves like, the factors that change the rate of dissolving and solubility, saturated and unsaturated solutions, and reading a solubility curve.
Sources & how we know this
- 2018 Science Standards of Learning - Chemistry — Virginia Department of Education (2018)
- Chemistry Curriculum Framework — Virginia Department of Education (2018)