Rates of Reaction
Introduction to Rates of Reaction
Definition: "Change in concentration of a reactant or product per unit time"
The study of rate of reaction is known as chemical kinetics.
Different chemical reactions occur at different speeds; some can react very quickly, others can react very slowly. The rate that a chemical reaction occurs can be affected by changing the conditions.
E.g Glucose will burn rapidly in air, but when in the body, to be used as an energy source, is oxidised very slowly. However in both reactions the products are carbon dioxide and water.
For chemical reactions the term rate is used instead of speed to describe how quickly a reaction occurs. The rate of reaction is found by measuring the amount, in moles, of a reactant is used up or the amount of product produced per unit time.
What is the point in studying rates of reaction?
- To improve the rate of production of a chemical
- To help understand the processes going on in out bodies or in the environment
- To gain an insight into the mechanism of a reaction
During the manufacture of a chemical the reaction rate is one of the contributing factors which determine the overall rate of reaction. An understanding of how fast a reaction occurs helps chemists and chemical engineers to choose the optimum conditions to manufacture a chemical in. It also helps to manage environmental issues.
E.g. The rate of formation of the ozone in the stratosphere is dependent on the intensity of the UV radiation travelling from the Sun to Earth. The ozone layer normally helps to filter out this UV radiation from sunlight, but the destruction of the ozone layer leads to high levels of UV radiation reaching the Earth and causing issues such as skin cancer. A knowledge of the rates of various reactions happening in the ozone layer (chlorine free radicals being formed by UV radiation on chloroflurocarbons - refer to Chapter 14) has enabled chemists to contribute greatly to an understanding of this environmental issue, highlighting the urgent need to control the use of chloroflurocarbons in industry.
There are many organic reaction mechanisms that have been discovered by a study of reaction rates. It is the slowest step in the mechanism which determines the overall rate of reaction - this step is called the rate-determining step.
E.g. In the formation of ozone in the stratosphere, the slowest step involves photodissociation of oxygen molecules by high energy UV radiation into oxygen atoms.
Environmental chemists have studied the rates of many of the reactions which occur in the atmosphere; such research has contributed to our understanding of these reactions and the effects of pollutant gases. This work has both demonstrated the need to control of artificial pollutants and led to the development of more environmentally friendly products.
Factors that affect the rate of a reaction
The four main factors considered to affect the rate of a reaction are: the concentration of reactants, the temperature of the experiment, whether or not a catalyst has been used, and the surface area of the reactants. By altering different factors in the below animation, you can see how they affect the rate of reaction - whether they increase or decrease it.
Concentration of reactants: Increasing the concentration of one of the reactants will increase the rate of reaction. The same is true for an increase in pressure of any reactants in the gaseous state.
Temperature: In most reactions, an increase in temperature will increase the rate of reaction. In many experiments, an increase of just 10C is likely to double the rate of reaction.
Catalysts: A catalyst speeds up a chemical reaction while remaining chemically unchanged, meaning that it is not used up or reacted in the reaction.
Surface area: An increase in surface area will cause an increase in the rate of reaction. For example, if a solid reactant is changed from one lump into a powder, the surface area will have been increased and, as a result, the rate of reaction will also increase. Catalysts usually work more effectively if they have a large surface area.
To understand why the rate of reaction increases due to these factors, see Liam and Jake's tackk on The Collision Theory.