Rate of reaction INTRODUCTION
Some chemical reactions take place extremely quickly. For example, when petrol is ignited it combines with oxygen almost instantaneously. Reactions like these have a high rate. Other reactions are much slower, for example when an iron bar rusts in the air; reactions like these have a low rate. Chemical reactions can be controlled and made to be quicker or slower. This can be very important in situations like food production, either by slowing down or increasing the rate at which food ripens, or in the chemical industry where the rate of a reaction can be adjusted to an optimum level.
Δ Fig. 2.34 Petrol igniting.
KNOWLEDGE CHECK ✓ Know the arrangement, movement and energy of the particles in the three states of matter: solid, liquid and gas. ✓ Understand how the course of a reaction can be shown in an energy level diagram. ✓ Be able to write and interpret balanced chemical equations.
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RATE OF REACTION
LEARNING OBJECTIVES ✓ Be able to describe a practical method for investigating the rate of a reaction involving the evolution of a gas. ✓ Be able to interpret data obtained from experiments concerned with rate of reaction. ✓ Be able to describe the effects of concentration, particle size, catalysts and temperature on the rates of reactions. ✓ EXTENDED Be able to suggest apparatus, given information, for experiments, including collection of gases and measurement of rates of reaction. ✓ EXTENDED Be able to describe and explain the effect of changing concentration in terms of collisions between reacting particles. ✓ EXTENDED Be able to explain that an increase in temperature causes an increase in collision rate and more of the colliding particles have sufficient energy (activation energy) to react, whereas an increase in concentration only causes an increase in collision rate.
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SCIENCE LINK
BIOLOGY ENZYMES
• the factors that affect how quickly a chemical reaction happens link directly to the role of enzymes in the maintenance of body processes • describing how the energy of the particles changes at higher temperatures also allows us to explain why enzymes will not work above a certain temperature
PHYSICS SIMPLE KINETIC MODEL • explaining why the different factors affect the rate of a chemical reaction uses the same particle model that gives us the simple structure of solids, liquids and gases • thinking about the forces between the particles and the energy involved in the interactions between particles leads to a common explanation in terms of particle speed and kinetic energy EXTENDED
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Δ Fig. 2.35 Particles must collide with sufficient energy to make an effective collision.
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PHYSICAL CHEMISTRY
COLLISION THEORY For a chemical reaction to occur, the reacting particles (atoms, molecules or ions) must collide. The energy involved in the collision must be enough to break the chemical bonds in the reacting particles – or the particles will just bounce off one another. A collision that has enough energy to result in a chemical reaction is an effective collision.
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