2012 topic 6
TRANSCRIPT
Lesson
Rates of Reaction
Collision Theory
IB Chemistry Power Points
Topic 06
Kineticswww.pedagogics.ca
Technically, the rate of a chemical reaction is a measure of the change in [concentration] per unit time:
a decrease in [reactant] per unit time
OR
an increase in [product] per unit time
Rate of Reaction – Data analysis exercise
rate = -D[reactant]
Dtrate =
D[product]Dt
Rates of reaction can be determined in different ways. For example:
1. a decrease in mass can be measured if the reaction has a gas product
2.The volume of gas produced can be measured if the reaction has a gas product
3.absorption of light can be measured if there is a color change in the reaction mixture
4.can also measure changes in conductivity or pH
Measuring Rate of Reaction
Graphs of concentration vs time are plotted to illustrate how reactions proceed.
Rate of Reaction
For example: consider the following reaction A B
rate = -D[A]Dt
rate = D[B]Dt
time
Rate of Reaction – Instantaneous vs Average Rates
Rate of Reaction – Instantaneous vs Average Rates
An average rate would be determined by the change in concentration over a longer time interval. This is represented by the slope of the dashed line
The rate of a reaction changed over time!
Kinetic Molecular Theory – a review
Basic KMT- particles are in constant random motion (have
KE)
- absolute temperature is proportional to kinetic energy
Solid Liquid Gas
Enthalpy Review
The activation energy is the energy difference between the reactants and the minimum energy required for the reaction. In an endothermic reaction
Enthalpy ReviewAnd in an exothermic reaction
Label (a), (b) and (c)
Collision Theory
Collision Theory
If a reaction between the particles of two reactants occurs:- particles must collide with correct orientation
(geometry)
- there must be sufficient energy to initiate the reaction E>Ea
NOT all collisions result in reactions!
Collision Theory
According to collision theory, increasing a reaction rate would result by increasing the number of SUCCESSFUL collisions per unit of time.
To increase a reaction rate- increase the frequency of collisions
- increase the energy of collision
Factors affecting the rate of reactionThere are four factors that affect the rate
of a chemical reaction. Each effect can be explained using collision theory. The 4 factors are: Concentration
Surface Area
Temperature
Catalysts
Concentration
The more concentrated the reactants are, the more particles there are per unit volume of the reaction medium (the mixture of the reactants).
More particles means a greater collision frequency (more collisions per unit time) and therefore a greater reaction rate.
When one reactant is in solid pieces, the other reaction can only occur at the surface.
No reaction occurs
If the solid pieces are subdivided, there is more surface area and more collisions per unit time. Therefore the reaction occurs faster.
Surface Area
Temperature
Increasing temperature increases the rate of reaction for two reasons.
1. frequency of collision increases because particles are moving faster.
2. increase in energy which increases the proportion of successful collisions – a larger % of particles have E > Ea
Temperature - Maxwell Boltzmann curves
Recall, a temperature measurement represents the average kinetic energy of a group of molecules.
As temperature increases, the proportion of
molecules with the required velocity
(minimum energy) also increases
Catalysts
Catalysts increase reaction rates by lowering activation energy. When a catalyst is present, the proportion of successful collisions increases because more molecules have sufficient energy to react.
Temperature, Concentration and Surface Area effects on reaction rates.