In this explainer, we will learn how to predict and explain the effects of changing concentration, pressure, and surface area on the rate of reaction using the collision theory.
The reaction rate describes the speed at which reactants turn into products during a chemical reaction. Chemical reactions can happen very slowly and they can happen very rapidly. Metals can slowly oxidize if they are left outside, and metals can almost instantly produce gas products if they are dipped in acid. The reaction rate is usually quite difficult to predict because it depends on lots of different factors such as temperature and pressure.
Collision theory states that a chemical reaction can only happen if two reactant particles collide with enough energy to overcome the activation energy barrier. The reaction rate will be quite low if collisions are not happening particularly often or if the energy of the collisions is lower than the activation energy barrier. The figure below shows how chemical reactions are only induced during effective particle collision processes.
Definition: Activation Energy
Activation energy is the minimum amount of energy required for a reaction to occur.
Collision theory can be used to understand how temperature affects the reaction rate. Higher temperatures make reactant particles move around quickly and with a lot of energy. Particles that move faster and with more energy have more frequent and more energetic collisions. The reaction rate will increase if the temperature is increased because the reactant particles collide more frequently and with more energy; the reactant particles overcome the activation energy barrier more often and the reactant particles transform into product particles at a faster rate.
We can extend the application of collision theory to explore the relationship between the reaction rate and other physical properties such as pressure, reactant concentrations, and reactant surface areas.
It is important to state here that the rate of reaction is also affected by catalysts. Molecules tend to react more rapidly in the presence of a positive catalyst because the catalyst has the ability to lower activation energy barriers. Reactant molecules transform into products more frequently when they are interacting with a positive catalyst. The rate of reaction is also affected by the bonding properties of reactant molecules. Ionic compounds tend to react faster than covalently bonded compounds. The rate of reaction is even affected by light and intense ionizing energy. Electromagnetic energy can affect chemical bonds, and it can make particles transform into products at a faster rate. There are quite a few factors that can affect the rate of a chemical reaction and not all of them will be covered in this explainer.
Pressure and reaction rate are directly related to each other because pressure determines the space between reactant particles. The particles in a high-pressure reaction vessel are very close together and they tend to collide very often. The particles in a low-pressure reaction vessel are very far apart and they do not tend to collide very often. The particles in the high-pressure reaction vessel overcome the activation energy barrier more frequently.
The following image shows how there is less space between reactant gas particles in containers that have higher pressure values. One of the reactant gas particle types is blue and the other reactant particle type is red. The walls of the container are shown as a thin black line.
Example 1: Determining Which Combination of Temperature and Pressure Values Will Make Ethene Gas Turn into Polyethene Most Rapidly
Polyethene is formed from ethene gas. Which of the following combinations of conditions would lead to the fastest rate of reaction?
- Low temperature and low pressure
- High temperature and high pressure
- High temperature and low pressure
- Low temperature and high pressure
This question is asking us to select the temperature and pressure states that will increase the reaction rate. To answer this question, we need to know how temperature and pressure values affect the reaction rate.
The temperature and reaction rate are directly related to each other. Particles move more rapidly and collide more frequently when the temperature is high. Particles move less rapidly and collide less frequently when the temperature is lower. We can use these statements to determine that the ethene gas molecules will react more frequently if the temperature is high.
The pressure and reaction rate are also directly related to each other. Particles are squashed into a smaller volume when the pressure is high. The particles are able to collide more frequently and react more often when the pressure is high and the particles are close together. We can use these statements to determine that the ethene gas molecules will react more frequently if the pressure is high.
We can use our two logical deductions to determine that option B is the correct answer for this question.
Collision theory can also be used to understand how surface area values affect reaction rates. Particles can only react and form product molecules if they collide with each other. Chemical reactions can happen more frequently if a large amount of one reactant surface is exposed to other reactant particles. Reactant particles overcome the activation energy barrier more often when more of the reactant surfaces are exposed to each other.
The following image shows how the reaction rate can be increased by increasing the surface area of one reactant molecule type. The image shows how there can be more collisions between the two different particle types when the blue particle block is broken up into smaller pieces.
Definition: Surface Area
The surface area measures the total area that is occupied by the surface of an object.
Example 2: Identifying the System with the Highest Total Surface Area Value
The following combinations of shapes all have the same total volume. Which has the greatest total surface area?
This question is asking us to compare different combinations of shapes and find the combination with the highest total surface area.
One way to solve this problem would be to add the surface areas of each separate shape, but it is simpler to solve this problem intuitively. An explanation of the direct calculation method can be found at the bottom of this example.
The intuitive method involves comparing the figures to see which has a higher or lower total surface area. For example, if we compare choice B (the cube) with choice A (the two rectangular prisms), we can observe that choice A has all of the same exterior faces as choice B, but with two added faces in the center. Choice A must have a higher surface area than choice B.
We can use a similar logic to determine that choice D (the four rectangular prisms) has a higher surface area than choice A.
Lastly, choice C (the eight cubes) is the same as choice D but with additional vertical cuts that expose even more surface area. Choice C has the highest surface area of the listed options.
Calculating the total surface area of each figure is also a correct, but very time-consuming, method. We first have to determine the surface area of each exposed square or rectangular face type. We have to calculate the surface area of each different type of exposed square or rectangular face, and we also have to multiply and add many of these values together. For option A, we have to determine the surface area of the two different types of square and rectangular face types. We then have to multiply these numbers by four or eight and add the two product terms together. The calculations are shown below:
The intuitive and direct computation approaches both indicate that option C is the correct answer for this question.
Collision theory can also be used to understand how concentration values affect reaction rates. There are more collisions between reactant particles when the concentration of one or both of the reactant particles is high. There are fewer collisions between reactant particles when the concentration of one or both of the reactant particles is low. The reactant particles will overcome the activation energy barrier more often if the concentration of at least one reactant particle is high because effective collisions happen more frequently.
This line of reasoning can be used to understand why zinc dissolves slowly in one solution of hydrochloric acid and rapidly in another solution of hydrochloric acid. The chemical reaction happens rapidly in the solution that has a high concentration of hydrogen ions because there are more hydrogen ion–zinc collisions in any given time frame. The zinc and hydrogen ion reactant particles overcome the activation energy barrier more frequently.
Example 3: Determining the Temperature, Concentration, and Surface Area Combination That Minimizes the Reaction Rate
In which of the following diagrams will the rate of reaction be slowest?
This question is asking us to determine which reaction will occur the least quick. The question provides information about the concentration, temperature, and surface area of the reactant particles. We need to pick the setup that is associated with the lowest reaction rate. It is easier to consider properties like concentration and temperature one after another rather than all at the same time.
Collision theory states that reaction rates are low when the concentration of the reactant molecules is low. We use this statement to eliminate the choices that have 2.0 M hydrochloric acid () solutions and concentrate on the choices that have the hydrochloric acid concentration as being 0.5 M.
Collision theory states that reaction rates are low when the temperature is low. We can use this statement to eliminate the choices that have the temperature of the hydrochloric acid solution as being and concentrate on the choices that have the temperature of the hydrochloric acid solution as being .
Collision theory states that reaction rates are low when there is less surface area for collisions between reactant particles. Lumps have less surface area than ground-up powder and this suggests that we should concentrate on the options that have lumps of magnesium carbonate.
Choice B must have the lowest reaction rate because it shows a solution with a 0.5 M concentration of hydrochloric acid that has a temperature of and it is reacted with lumps of magnesium carbonate.
Many physical properties are proportional, or directly related, to the rate of reaction. The reaction rate will be halved when the pressure is halved, and the reaction rate will be doubled when the surface area of contact is doubled. The reaction rate will be reduced by a factor of ten when the concentration of the reactant particles is reduced by ten.
Collision theory can be applied to make chemical reaction processes safer and to avoid industrial hazards. Dust explosions can occur in factories that handle flammable materials such as coal and flour. The combustible materials can react explosively if they are turned into airborne dust because the flammable materials have a high total surface area and they can react at a very fast rate.
Collision theory states that the risk of a dust explosion can be reduced if there are fewer and less energetic collisions between the combustible particles. The number of explosive reactions can be reduced by ensuring that the air is properly ventilated because wind tends to blow combustible particles apart. The number of explosive reactions can also be reduced by ensuring that the air contains lots of water vapor. The combustible particles cannot react with each other if they are colliding with intervening water molecules.
Example 4: Determining the Pressure and Temperature Combination That Maximizes the Reaction Rate
Dust explosions in flour mills are a serious safety concern. Why is the reaction between the flour particles and the oxygen in the air so fast?
- The flour dust acts as a catalyst.
- The flour dust has a large surface area resulting in a high collision rate.
- Dust explosions create pressure pockets in the air, increasing the rate of reaction.
- A dust explosion is exothermic.
- The flour dust is highly flammable and concentrated in the air.
This question describes an explosive reaction between flour particles and oxygen and asks us to identify why the reaction rate is so fast.
Choice A is incorrect. The dust takes part in the reaction, so it cannot be a catalyst. We would not expect dust to remain after an explosion.
Choice D is incorrect. Exothermic and endothermic reactions can each happen quickly or slowly.
Choice C is incorrect. Dust explosions can create pressure pockets, but only after the explosion has occurred. This answer does not explain why the initial reaction is so fast.
Choice E is incorrect. Piles of dust would be more concentrated than airborne dust, but they do not explode as vigorously.
The correct answer is choice B. The flour dust has a high surface area resulting in a high collision rate. The dust particles are exposed to oxygen molecules from all directions. Before flour is ground into a powder, its particles are much larger and more packed together, exposing only an outer layer to collide with oxygen. The high surface area of the airborne dust form results in a high reaction rate and a potentially explosive reaction.
Collision theory can be used to understand and reduce the risk of dangerous explosions in underground mines. Methane is produced through natural processes over the course of millions of years, and it can sometimes become concentrated in small pockets somewhere deep underground. Miners have to be wary of these concentrated pockets of methane because they will react explosively if they happen to be ignited with a spark or combusted with an exposed flame. Collision theory states that the risk of an explosive reaction is directly related to the concentration of the explosive methane molecules in gas pockets underground. The risk of an explosive reaction can be significantly reduced if miners take some precaution and find suitable ways to lower the concentration of the gaseous methane molecules in the air.
Let us summarize what has been learned in this explainer.
- The collision theory model states that chemical reactions can only happen if reactant molecules collide with enough energy to overcome the activation energy barrier.
- Particles collide and react more often when the temperature is high.
- Particles collide and react more often when the pressure is high.
- Particles collide and react more often when a large amount of one reactant surface is exposed to other reactant particles.
- Particles collide and react more often when the concentration of the reactant particles is high.
- The rate of reaction is usually proportional to the ambient temperature and pressure.
- The rate of reaction is usually proportional to the concentration of reactant particles in a liquid solution.
- The rate of reaction is usually proportional to the surface area of the reactants.
- Dust explosions and mine explosions are examples of dangerous reactions that have very high reaction rates.