Understand Reaction Rate

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Basic Descriptive Topic

The Collision Theory

Discusses the collision theory of reaction rates, including the importance of activation energy and the Maxwell-Boltzmann distribution.

The effect of surface area on rate of reaction

Describes and explains the effect of surface area on the rate of a reaction between a solid and a liquid or a gas.

The effect of concentration on rate of reaction

Describes and explains the effect of changing the concentration on the rate of a reaction involving liquids or gases. If you are interested in rate equations, orders of reaction and rate constants, these are explained separately - see below.

The effect of pressure on rate of reaction

Describes and explains the effect of changing the pressure on the rate of a reaction involving gases.

The effect of temperature on rate of reaction

Describes and explains the effect of changing the temperature on the rate of a reaction, including the importance of activation energy and the Maxwell-Boltzmann distribution. The Arrhenius equation is dealt with separately - see below.

The effect of a catalyst on rate of reaction

Describes and explains the effect of adding a catalyst to a reaction. This is an introductory look only. You will find a separate section dealing with catalysts in detail - link via the physical chemistry menu (see below).

Rate equations and Orders of reaction

Orders of reaction and rate equations 

An introductory look at orders of reaction, rate equations and the rate constant.

The relationship between order and mechanism

Looks at some simple cases to show how orders of reaction can sometimes give useful information about the mechanism of a reaction. Also discusses the difference between the sometimes confusing terms "order " and "molecularity " of a reaction.

A look at energy profile for reactions

Takes a closer look at simple energy profiles for reactions, and shows how they are slightly different for reactions involving an intermediate or just a transition state.

Order of reaction experiments

A survey of some of the methods of finding orders of reaction experimentally.

The Arrhenius Equation

Discusses the mathematical relationship between the rate constant and temperature and activation energy.

SPM Past Years' Exam Papers

Kimia SPM Chemistry

2014_Chemistry_P1
2014_Chemistry_P2,3
2013_Chemistry_P1
2013_Chemistry_P2,3
2012_Chemistry_P1
2012_Chemistry_P2,3
2011_Chemistry_P1,2,3
2010_Chemistry_P1
2010_Chemistry_P2
2010_Chemistry_P3
2008_Chemistry_P2
2006_Chemistry_P1
2006_Chemistry_P2
2006_Chemistry_P3
2005_Chemistry_P1
2005_Chemistry_P2
2005_Chemistry_P3

Rate of Reaction

UNDERSTANDING RATE OF REACTION

Scientific concept

The rate of a reaction is the speed at which a chemical reaction happens. If a reaction has a low rate, that means the molecules combine at a slower speed than a reaction with a high rate. Some reactions take hundreds, maybe even thousands, of years while others can happen in less than one second. Examples of very slow reaction that takes place in plants and ancient fish to become fossils (carbonization). The rate of reaction also depends on the type of molecules that are combining. If there are low concentrations of an essential element or compound, the reaction will be slower. 

Rates of reaction involves collision theory. The collision theory says that as more collisions in a system occur, there will be more combinations of molecules bouncing into each other. If you have more possible combinations there is a higher chance that the molecules will complete the reaction. The reaction will happen faster which means the rate of that reaction will increase.

Reactions take place as a result of particles (atoms or molecules) colliding and then undergoing a reaction. Not all collisions can cause reaction, even in a system where the reaction is spontaneous. When temperature of a system is high, the molecules bounce around a lot more. They have more energy. When they bounce around more, they are more likely to collide. That fact means they are also more likely to combine. When lowering the temperature, the molecules are slower and collide less.  

If there is more of a substance in a system, there is a greater chance that molecules will collide and speed up the rate of the reaction. If there is less of substance, there will be fewer collisions and the reaction will probably happen at a slower speed.

Pressure affects the rate of reaction, especially in the gases. When increasing the pressure, the molecules have less space in which they can move. That greater density of molecules increases the number of collisions. When decreasing the pressure, molecules don't hit each other as often and the rate of reaction decreases.