CHAPTER 3. REACTION RATES 3.9
Chapter 3 — Summary
See the summary presentation ( Presentation: VPmwm at http://www.everythingscience.co.za)
- The rate of a reaction describes how quicklyreactants are used up,or how
quickly products form. The units used are molesper second.
- A number of factors can affect the rate of a reaction. These include the nature
of the reactants, the concentration of reactants, temperature of the reaction, the
presence or absence ofa catalyst and the surface area of the reactants.
- Collision theory provides one way of explaining why each of these factors can
affect the rate of a reaction. For example, higher temperatures mean increased
reaction rates because the reactant particles havemore energy and are more likely
to collide successfully with each other.
- Different methods canbe used to measure the rate of areaction. The method
used will depend on the nature of the product. Reactions that produce gases
can be measured by collecting the gas in a syringe. Reactions that produce a
precipitate are also easyto measure because theprecipitate is easily visible.
- For any reaction to occur, a minimum amountof energy is needed so that bonds
in the reactants can break, and new bonds canform in the products. The mini-
mum energy that is required is called the activation energy of a reaction.
- In reactions where theparticles do not have enough energy to overcome this
activation energy, one of two methods can be used to facilitate a reactionto take
place: increase the temperature of the reaction or add a catalyst.
- Increasing the temperature of a reaction means that the averageenergy of the
reactant particles increases and they are morelikely to have enough energy to
overcome the activationenergy.
- 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, lowerenergy
pathway, for the reaction.
- A catalyst therefore speeds up a reaction but does not become part of the reaction
in any way.
- Chemical equilibrium is the state of a reaction, where the concentrations of the
reactants and the products have no net changeover time. Usually thisoccurs
when the rate of the forward reaction is the same as the rate of the reverse reac-
tion.
- The equilibrium constant relates to reactions at equilibrium, and can becalcu-
lated using the followingequation:
Kc=
[C]c[D]d
[A]a[B]b
where A and B are reactants, C and D are products and a, b, c, and dare the
coefficients of the respective reactants and products.
- A high Kcvalue means that the concentration of products at equilibrium is high
and the reaction has a high yield. A low Kcvalue means that the concentration
of products at equilibrium is low and the reaction has a low yield.
