Introduction to kinetics

In this lesson, we will learn:

• To define activation energy and its importance in kinetics.
• To define reaction rate and understand the idea of a rate of a process.
• To use equations to find the rate of reaction.
• To identify ways to measure rate of reaction.

• Kinetics as an idea is about rate – it asks the question "how fast is that happening?" In chemistry, it is chemical processes; how fast does any chemical reaction happen?
• Remember that whenever chemical reactions happen, nearly every process can either be described as exothermic or endothermic:
• Exothermic reactions are reactions which have the overall effect of releasing heat energy to the environment. In other words, the energy put in that was needed to break up the reactant bonds was less than the energy given out when the new bonds in the products formed. This is shown by the energy diagram below:
  
• Endothermic reactions are chemical reactions which have the overall effect of absorbing heat energy from the environment. In other words, the energy given out when the products formed was less than what was needed to break up the bonds in the reactant substances. This is shown by the energy level diagram below:
  
• Look at the energy diagram of an exothermic reaction. If the change in energy is literally exactly like that, scientists would expect that any reaction that is exothermic happens spontaneously – without any outside influence at all. This is not observed in the world though; burning fuels is generally very exothermic but it needs igniting, which involves a spark of initial energy to start the reaction.
• Therefore, there must be some form of barrier or wall that stops exothermic (and endothermic) reactions from happening spontaneously. Given this, energy diagrams are more accurately drawn like this:
  
  
• This wall or 'hill' of energy that must be overcome for ANY reaction to overcome is called the activation energy of a reaction.
• The activation energy is the minimum amount of energy required for reactants to successfully collide and convert to products in a chemical reaction.
• Think of it like the process of a ball going over a hill. A ball won't roll over a hill by itself; you need to put in enough energy to get it to the top. Once it's at the top and reaches a decline, it starts to roll down – the rest is done by itself! The energy you put in to get it to the top is the activation energy.
• One way to learn more about this activation energy 'wall' is by observing either side of it – the reactants and products in the reaction! The reaction rate is the rate at which a chemical process converts reactants to products, and can be measured in different ways:
• $Rate\;of\;reaction=\frac{amount\;of\;product\;formed}{time}$
• $Rate\;of\;reaction=\frac{amount\;of\;reactant\;used}{time}$
• Both of these are specific versions of the general:
  $Rate = \frac{change\;in\;amount}{change\;in\;time}$
• Do you notice no units in these equations? Rate is just a general idea! As long as you are measuring a change in quantity over a period of time, you are measuring rate!
• A chemical reaction might measure change in mass of reactants used per minute, which is rate of reaction.
• An athlete might measure their change in distance they run per second – you know this as a rate called speed!
• A business person might measure the change in growth in a business per year. This is a rate called annual growth (normally later turned into a percentage by comparing it to previous years).
• Depending on what chemical reaction you are doing, there are different methods for measuring rate of reaction which include:
• Using a gas syringe to measure the volume of gas being created (as one of your products is a gas produced from a solution).
• Using a colorimeter to measure the change in colour (as colored reactants produce colorless products or vice versa).
• Using a weighing balance to measure the change in mass of your reaction mixture (if a solid reactant is being converted to a gas product(s), a drop in mass will be observed).
• When finding a rate of reaction, make sure you are clear what units you are giving it in – rate can be measured in a lot of different ways and you may need to convert from one unit to another. If you need to do this, use the unit conversion method (See lesson C1.3).