Reaction Rates

REACTION RATES The reaction rate or rate of reaction for a reactant or product in a particular reaction is intuitively defined as how fast a reaction takes place. For example, the oxidation of iron (iron rusting) under the atmosphere is a slow reaction which can take many years, but the combustion of butane in a fire is a reaction that takes place in fractions of a second that is a fast reaction rate.

Iron rusting – slow reaction rate

Wood burning – fast reaction rate Consider a typical chemical reaction:

aA + bB → pP + qQ The lowercase letters (a, b, p, and q) represent stoichiometric coefficients, while the capital letters represent the reactants (A and B) and the products (P and Q).

Rate of a reaction is always positive. '-' sign is present in the reactant involving terms because the reactant concentration is decreasing. Factors that affect the rate of reaction: Concentration: Reaction rate increases with concentration, as described by the rate law and explained by collision theory. As reactant concentration increases, the frequency of collision increases. Temperature: temperature increases the rate of reaction is that more of the colliding particles will have the necessary activation energy resulting in more successful collisions. The influence of temperature is described by the Arrhenius equation. Pressure: The rate of gaseous reactions increases with pressure. For condensed-phase reactions, the pressure dependence is weak. A catalyst: The presence of a catalyst increases the reaction rate by providing an alternative pathway with lower activation energy.

EMPOWER There’s a factor that influence the rate of reaction such as temperature, concentration, catalysts, surface area and pressure.

1) Temperature : By heating the mixture, the energy levels of the molecules involved

in the reaction will be raised. Increasing temperature means the molecules move faster. 2) Concentration : Increasing the concentration of the reactants will increase the

frequency of collisions between the two reactants. 3) Catalysts

: Catalysts speed up chemical reactions.

4) Surface area

: Smaller particles have a bigger surface area than larger particle for

the same mass of solid. The larger the surface area of solid, the faster the reaction. 5) Pressure

: Increasing the pressure will squeeze the molecule together and

increase the frequency of collisions between the molecules.

1) Surf the internet. 2) Search for phet edu.com

3) Open the web

4) Search for the chemistry simulation

5) Look for the topic Reaction and Rates

6) Download and the simulation can be used

Questions 1. What is meant by rate of reaction? 2. Explain what happen when to the reaction rate when temperature is increases? 3. How does catalyst do in the reaction and its definition?

Answers: 1. Rate of a reaction is the decrease in reactant concentration and increase in a

product reaction per unit time.

2. When temperature increases, overall kinetic energy of all particles of reactants increases, so more molecules will have energy greater than activation energy. So, the overall frequency of collisions increases, therefore frequency of effective collisions increases thus the reaction rate increases. 3. Catalyst is used in the reaction by decreasing the value of activation energy and provide a pathway to increase the rate of reaction by increasing the quantity of effective collisions between reactants without itself being consumed.