Why do reactions happen at all, and what does collision theory require?
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.
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What this topic is asking
Standard CH.6 asks you to explain reaction rate using collision theory. Virginia expects you to know that particles must collide with enough energy and the correct orientation for a reaction to happen, to connect this to the activation energy, and to understand the idea of an effective collision. Collision theory is the model that explains why the factors in the next dot point change the rate.
What reaction rate means
Rate can be followed by watching a measurable change: gas produced over time, a color forming or fading, a mass decreasing, or a precipitate appearing. The rate usually decreases as a reaction proceeds, because the reactants get used up and collisions become less frequent.
Collision theory
Most collisions are not effective. Many are too gentle (below the activation energy) and the particles simply rebound; others have enough energy but the wrong geometry, so no bonds rearrange. Only the fraction of collisions that are both energetic enough and correctly oriented actually produce a reaction.
Energy, orientation and the activation energy
The activation energy is the minimum energy a collision must supply to break the existing bonds and form the activated complex. If colliding particles carry less than this, the collision fails no matter how well aligned. The orientation requirement means the particles must approach so that the reacting parts make contact; a molecule struck on the wrong side will not react even with ample energy.
Because temperature is a measure of average kinetic energy, raising the temperature increases both how often particles collide and the fraction of collisions that exceed the activation energy, which is why heating speeds reactions. This link between collision theory and the rate-changing factors is the basis of the next dot point.
Try this
Q1. Name the two conditions a collision must meet to cause a reaction. [1 point]
- Cue. Enough energy (at least the activation energy) and the correct orientation.
Q2. What name is given to a collision that successfully leads to a reaction? [1 point]
- Cue. An effective collision.
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 marksAccording to collision theory, for a reaction to occur, particles must collide (A) at any speed (B) with enough energy and the correct orientation (C) only head-on at low energy (D) without touchingShow worked answer →
The answer is (B) with enough energy and the correct orientation.
Collision theory says particles must collide with at least the activation energy and in a suitable orientation for the collision to lead to a reaction. A collision that meets both conditions is called an effective collision. Collisions that are too gentle or poorly oriented simply bounce apart without reacting.
The trap is thinking any collision causes a reaction; only collisions with sufficient energy and the right orientation are effective.
SOL (tech-enhanced, fill in the blank)2 marks(a) Define the rate of a chemical reaction. (b) State the two requirements for an effective collision.Show worked answer →
A 2-point definition item.
(a) Rate (1 point): the rate of a reaction is how fast reactants are used up or products are formed (the change in amount per unit time).
(b) Requirements (1 point): the colliding particles must have enough energy (at least the activation energy) and collide with the correct orientation.
Markers reward defining rate as a change per unit time and naming both the energy and orientation conditions for an effective collision.
Related dot points
- 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.
- 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.
- States of matter and kinetic molecular theory: describe solids, liquids and gases in terms of particle arrangement and motion, and state the assumptions of kinetic molecular theory.
A focused Virginia SOL Chemistry answer on the states of matter under CH.4: how particles are arranged and move in solids, liquids and gases, the link between temperature and average kinetic energy, and the assumptions of kinetic molecular theory.
Sources & how we know this
- 2018 Science Standards of Learning - Chemistry — Virginia Department of Education (2018)
- Chemistry Curriculum Framework — Virginia Department of Education (2018)