Reaction Rates and Collision Theory
HSC Chemistry Syllabus
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Conduct a practical investigation, using appropriate tools (including digital technologies), to collect data, analyse and report on how the rate of a chemical reaction can be affected by a range of factors, including but not limited to:
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Investigate the role of activation energy, collisions and molecular orientation in collision theory
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Explain a change in reaction rate using collision theory (ACSCH003, ACSCH046)
Reaction Rates and Collision Theory
This video discusses how reaction rates can be affected by a variety of different factors such as surface area, concentration, and temperature. It also explores how reaction rates can be explained in terms of collision theory,
What is Reaction Rate?
The reaction rate is a measure of how fast a chemical reaction occurs. It can be understood in two ways:
- The rate at which products are formed
- The rate at which reactants are depleted
Consider two reactions taking place simultaneously: one where the product appears rapidly (a quick reaction) like combustion, and one where the product forms slowly (a slow reaction) like the oxidation (rusting) of iron.
By graphing these reactions, with the product amount on the y-axis and time on the x-axis, we get a clear visual of their speeds with a steeper gradient indicating a quicker reaction.
Measuring Reaction Rates
There are several methods which can be used to measure the reaction rate:
- Volume of gas produced: A syringe setup can measure the gas volume which is produced from combustion and acid-metal carbonate reactions. The graph above shows the production of gas from a chemical reaction of two different rates.
- Color changes: Spectrophotometry, or colourimetry, measures how much light a solution is able to absorb over time to indicate concentration changes.
- Temperature changes: As reactions release or absorb energy, the surrounding temperature changes. The rate of temperature change can indicate the progress of a reaction.
Factors Affecting Reaction Rate
Several variables play a crucial role in determining how quickly reactions proceed:
- Concentration of reactants
- Pressure and volume
- Temperature
- Surface area of reactants
- Presence of catalysts
What is Collision Theory?
Collision theory is used to predict and explain the rate of chemical reactions. It is based on the kinetic theory of gases and asserts that for a reaction to occur, reactant particles (molecules, atoms, ions) must collide. However, not all collisions lead to a chemical reaction; certain criteria must be met for the reactants to transform into products effectively. The theory states that the rate of reaction is dependent on three factors:
- Rate of collision: How often molecules collide
- Activation Energy: The energy threshold that must be reached for a reaction to occur.
- Molecular Orientation: The specific alignment needed for molecules to react upon collision.
Factors Affecting Collision Rate
An increased rate of molecular collisions typically leads to a higher reaction rate. Factors influencing this include:
Concentration
Higher concentration means there are more particles in a volume which in turn results in more frequent collisions or more collisions per second. This generally applies to aqueous and gaseous substances.
The effect of concentration on reaction rate is commonly observed in reactions involving solutions. As a reaction proceeds, the concentration of reactant decreases, resulting in fewer collisions per second and hence the reaction rate decreases with time. The reaction rate will reach zero when the limiting reagent is depleted at which point collisions no longer occur. This is shown in the graph above as the rate of gas production decreases with time, representing a decrease in reaction rate.
Pressure/Volume
For gases, increasing pressure (or decreasing volume) raises collision frequency because the same number of gaseous particles would be moving about in a smaller space.
Temperature
Temperature is a measurement of the average kinetic energy of particles in a system. Kinetic energy is given by `K = 1/2 mv^2` where m is the particle's mass and v is its speed.
A higher temperatures means the average kinetic energy is greater and particles on average are moving faster. Consequently, the collision rate increases. This is why heating a chemical reaction generally will increase its rate.
Surface Area of Solids
Increasing the surface area of particles will increase the likelihood of collision with other reactants. This can be achieved by grinding solid state metals or ionic compounds. For example, a chemical reaction using iron metal powder would have a greater reaction rate compared to one using an iron block.
Activation Energy and Reaction Rate
Activation energy is the energy that is needed to initiate a reaction. This is represented by the 'bump' in energy between reactants and products on the energy profile diagram.
Left: activation energy of an exothermic reaction. Right: activation energy of an endothermic reaction.
Regardless of the enthalpy change of the reaction (whether it's endothermic or exothermic), activation energy is always present. In collision theory, molecules or particles must collide with sufficient energy, that is equal or greater than the activation energy of the reaction, to result in a chemical reaction. Conversely, collisions do not lead to chemical reactions if molecules have insufficient energy.
What is Catalyst?
A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. Catalysts work by providing an alternative reaction pathway with a lower activation energy, making it easier for reactants to transform into products. This is depicted on the energy profile diagram as follows:
By lowering the activation energy, a catalyst increases the proportion of reactant particles that have enough energy to react when they collide. This means that more collisions will lead to a successful reaction, which accelerates the reaction rate.
The Maxwell-Boltzmann curve depicts how energy is distributed across all the reactant particles. It helps visualise the proportion of particles that is available to react upon collision. This is the region to the right of the activation energy line. The addition of a catalyst decreases the activation energy and shifts the dashed line leftwards, increasing the area on the right.
Reaction rate can similarly be increased by increasing the temperature of a system. Increasing the temperature raises the proportion of molecules with enough energy to overcome the activation barrier, thus speeding up the reaction. Conversely, lowering temperature has the opposite effect.
Therefore, temperature influences reaction rate via two ways. Firstly, it affects the average speed of reactant particles and molecules, which in turn affects the collision rate between them. Secondly, it affects the average energy possessed by particles upon collision, and the proportion of collisions that will result in a chemical reaction.
Molecular Orientation
The orientation of molecules during a collision is a factor that remains unaffected by the other variables like concentration or temperature. In collision theory molecules must align in a correct orientation in order for particular reactions to occur. This ‘correct’ orientation is one that is favourable, or conducive, to forming the product. The example below demonstrates how carbon dioxide cannot from a collision between carbon monoxide `CO` and oxygen gas `O_2` if the oxygen gas does not contact the carbon monoxide particle on the carbon face. This is because the `C=O` double bond needs to form on the carbon face.
While molecular orientation can be manipulated using some catalysts like platinum in catalytic converters, this aspect is not be explored in the NESA NSW HSC Chemistry Syllabus.