AS Chemistry 3.1.5 Kinetics PDF

AS CHEMISTRY 3.1.5 KINETICS Reaction kinetics is all about how changes in reaction conditions can affect the speed at which a chemical reaction takes place. In order to understand how these changes in conditions affect the rate of reaction, we must...

AS CHEMISTRY 3.1.5 KINETICS Reaction kinetics is all about how changes in reaction conditions can affect the speed at which a chemical reaction takes place. In order to understand how these changes in conditions affect the rate of reaction, we must first understand what must happen in order for a reaction to take place. COLLISION THEORY In order for a chemical reaction to take place, two things must happen: 1. The particles (atoms / ions / molecules) must physically collide 2. The particles must have sufficient energy to react. There is a minimum amount of energy that particles must have to do so. This is known as the Activation Energy (Eact). The Eact for every reaction is different. Reactant particles collide Products are formed with sufficient energy So, in terms of collision theory, there 3 changes that will affect the rate of chemical reaction… a) Increase the Frequency of collisions between the particles - The greater the frequency of collisions, the faster the products will be formed b) Increase the amount of Energy that the particles have - The greater the amount of energy the particles have, the greater the proportion of the particles that have energy greater than the Eact. c) Lower the Activation Energy (Eact) - The lower the Eact, a greater proportion of the particles have energy greater than the Eact. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.5 KINETICS DISTRIBUTION OF ENERGY In any chemical system, there is a distribution in energy that the particles have. In We can use a Maxwell-Boltzmann Distribution Curve to show this. B E C Number of Particles Energy A D A: Some particles may have very little energy. The only way any particles could have zero energy, is if the temperature was at 0K (absolute zero) B: This is the modal amount of energy that the particles have. In other words, it is the most common amount of energy. C: This is the average amount of energy that the particles have. It is always found just to the right of the mode (peak). D: The curve never actually touches the X axis here. It is left open. The reason is that we cannot say with any certainty that some particles do not have an infinite amount of energy. E: The activation energy for the reaction Only particles that have more energy than this (the area to the right of the line) will react when they collide. Area under the curve = the total number of particles AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.5 KINETICS Practically, there are five different condition that can be changed that will affect the rate of a chemical reaction. The Concentration of Reactants (in solution) The Pressure of Reactants (if gases) The Surface Area of Reactants (if solids) Temperature Adding a Catalyst You must be able to describe the effect that each has and explain why using particle theory. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.5 KINETICS CHANGING CONCENTRATION Higher Concentration Eact Number of Particles Energy The higher the concentration, the higher the rate of reaction Explanation: A greater concentration means that there are more reactant particles per unit volume. Therefore particles are closer together and the frequency of collisions increases. A greater number of particles have sufficient energy to react (the shaded area to the right of the line). Notice how the general shape of the distribution curve does not change here. This is because the particle haven’t been given any more energy here. There’s just more of them! Remember! This only applies to reactants that are solutions! AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.5 KINETICS CHANGING PRESSURE The higher the pressure, the higher the rate of reaction Explanation: A greater pressure means that there are more reactant particles per unit volume. Therefore particles are closer together and the frequency of collisions increases. There is no distribution curve to show this. Remember! This only applies to reactants that are gases! CHANGING SURFACE AREA The greater the surface area, the higher the rate of reaction Explanation: A greater surface area means that more solid reactant particles are available to collide with the other reactant (usually a solution). Therefore frequency of collisions increases. There is no distribution curve to show this. Remember! This only applies to reactants that are solids! AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.5 KINETICS CHANGING TEMPERATURE Higher Temperature Eact Number of Particles Energy The higher the temperature, the higher the rate of reaction Explanation: When temperature increases, particles gain kinetic energy. This means frequency of successful collisions increases. In other words, the particles collide more frequently and a greater proportion of those collisions result in a reaction. This is why increasing the temperature has such a large impact on the rate of a chemical reaction. Notice how the shape of the distribution curve has changed with increased temperature. 1. The peak (mode) moves down and to the right from where it was. So the modal energy is higher, but the number of particles that have that energy is less. 2. The average energy that the particles have has increased. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.5 KINETICS ADDING A CATALYST Catalyst Eact Eact Number of Particles Energy Adding a catalyst increases the rate of a reaction Explanation: When a catalyst is added, is lowers the activation energy for the reaction. This means a greater proportion of the particles now have sufficient energy to react (to the right of the Eact line) How To Understand Maxwell-Boltzmann Curves AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.5 KINETICS HOW CATALYST WORK Here’s some important facts about catalysts that you need to remember: Catalysts are not included in the equation for the chemical reaction Catalysts are not used up or chemically changed during the reaction, so they can be re-used Each chemical reaction has its own specific catalyst e.g. Fe is the catalyst for making ammonia (NH3) N2 + H2 → 2 NH3 Catalysts increase the rate of reaction by providing a different pathway for the reaction that has a lower activation energy. Remember, the activation energy is the minimum amount of energy required for a reaction to occur. So, by lowering that amount of energy, more particles are able to react. (Think of it like lowering the pass grade of an exam!) As well as a Maxwell-Boltzmann distribution curve, we can show this using a Reaction Profile Diagram: AQA www.chemistrycoach.co.uk © scidekick ltd 2024