Grade 12 science summary presentation for chapter 7: rate and extent of reaction

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7. Rate and extent of reaction
Chemistry
Grade 12

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Reaction rate
The average rate of a reaction describes how quickly reactants are used, or how quickly
products form. The units used are usually moles per second.
We can use one of the following two equations to calculate reaction rate (depending on if we
have information about the products or about the reactants).
Collision theory
Reactant particles must collide with the correct energy and orientation for the reactants to
change into products.
Successful collisions are necessary to break the existing bonds (in the reactants) and form
new bonds (in the products).
reaction rate=
moles product formed
reaction time (s)
reaction rate=
moles reactant used
reaction time (s)

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Factors that affect the average rate of reaction
A number of factors can affect the average rate of a reaction. These include:
● Nature of the reactants: Substances have different chemical properties and therefore
react differently, and at different rates (e.g. the rusting of iron vs. the tarnishing of silver).
● Concentration of solutions: As the concentration of the reactants increases, so does the
reaction rate.
● Surface area of solids: Large lumps of solid reactants have smaller surface areas than
powdered solids.
● Pressure of gases: As the pressure of the reactants increase, so does the reaction rate.
● Temperature of the reaction: If the temperature of the reaction increases, so does the
average rate of the reaction
● Presence or absence of a catalyst: Adding a catalyst increases the reaction rate by
lowering the energy required for a successful reaction to take place. A catalyst speeds up a
reaction and is released at the end of the reaction, completely unchanged.

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Measuring rates of reactions
Different methods can be used to measure the average rate of a reaction. The method
used will depend on the nature of the product or products. Reactions that produce gases
can be measured by collecting the gas in a syringe or by measuring a change in mass of
the reaction vessel. In reactions that produce a precipitate the mass of the dried precipitate
can be measured.
A change in colour can show that the product has formed or that the reaction has reached
completion. (This is known as colorimetry).

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Mechanism of reaction and catalysis
For any reaction to occur, a minimum amount of energy is needed so that bonds in the
reactants can break, and new bonds can form in the products. The minimum energy that is
required is called the activation energy of a reaction.
In reactions where the particles do not have enough energy to overcome this activation
energy, one of two methods can be used to facilitate a reaction to take place: increase the
temperature of the reaction or add a catalyst.

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Mechanism of reaction contd.
Increasing the temperature of a reaction means that the average energy of the reactant
particles increases and they are more likely to have enough energy to overcome the activation
energy and the chances of two molecules having the correct orientation are increased.
A catalyst is used to lower the activation energy so that the reaction is more likely to take
place. A catalyst does this by providing an alternative, lower energy pathway for the reaction.
A catalyst therefore speeds up a reaction but remains unchanged after the reaction is
complete.

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