IB CHEMISTRY:
TOPIC 6 KINETICS NOTES
6.1 Collision theory and rates of reactions
Collision Theory
• For a reaction between two particles to occur three conditions must be met
o The particles must collide
o They must collide with the appropriate geometry or orientation
o They must collide with sufficient energy to bring about the reaction (bigger than
or equal to Ea)
• The minimum amount of energy required is known as the activation energy (does not
change, not dependent on temp)
Factor Effect of increasing Explanation
factor
Surface Area
Decreases rate The greater the particle size, the smaller the exposed surface area.
Particle Size
Reactions require collisions for reactions to occur, so if the surface
area is smaller, fewer collisions will occur per unit time
Temperature Increases rate As the temperature increases, the particles will move faster so
(increase Ke) there will be more collisions per second. However, the main reason
why an increase in temperature increases the rate is that more of the
colliding particles will possess the necessary activation energy
resulting in more successful collisions. Generally, an increase of
10°C doubles the rate of a chemical reaction
Concentration Increases rate Increasing the concentration increases the frequency of collisions
which increases the frequency of successful collisions too, thus
increasing the rate.
Pressure Increases rate Higher pressure compresses the gas, effectively increasing the
concentration, so increasing the frequency of collisions and therefore
the rate too.
, Rate of reaction
• Chemical kinetics is the study of the factors affecting the rate of a chemical reaction
• The rate of a chemical reaction can be defined as the change in concentration of the
products/reactants per unit time or Increase/decrease in concentration of
product/reacts per unit time
• Concentration changes in a reaction can be indirectly monitored by monitoring :
o Change in volume of a gas, mass, concentration through titration, gas pressure
or electrical conductivity
o Calorimetry and light absorbance (spectrometer)
o pH change (e.g. with data loggers or litmus indicator), colour change
• The results obtained is usually then plotted in a graph of concentration against time
• The rate at any point in time is the gradient of the graph at that time
∆𝒄𝒐𝒏𝒄𝒆𝒏𝒕𝒓𝒂𝒕𝒊𝒐𝒏
• Thus, find the gradient at 𝑡 by using 𝑹𝒂𝒕𝒆 𝒐𝒇 𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 =
∆𝒕𝒊𝒎𝒆
• The rate of reaction (gradient) will generally be the steepest at the start of the
curve
o This is because a high concentration of reactant particles, therefore a high
frequency of successful collisions
• The rate of reaction (gradient) decreases over time as time as the concentration of
reactant particles decreases, thus the frequency of successful collisions also
decreases
TOPIC 6 KINETICS NOTES
6.1 Collision theory and rates of reactions
Collision Theory
• For a reaction between two particles to occur three conditions must be met
o The particles must collide
o They must collide with the appropriate geometry or orientation
o They must collide with sufficient energy to bring about the reaction (bigger than
or equal to Ea)
• The minimum amount of energy required is known as the activation energy (does not
change, not dependent on temp)
Factor Effect of increasing Explanation
factor
Surface Area
Decreases rate The greater the particle size, the smaller the exposed surface area.
Particle Size
Reactions require collisions for reactions to occur, so if the surface
area is smaller, fewer collisions will occur per unit time
Temperature Increases rate As the temperature increases, the particles will move faster so
(increase Ke) there will be more collisions per second. However, the main reason
why an increase in temperature increases the rate is that more of the
colliding particles will possess the necessary activation energy
resulting in more successful collisions. Generally, an increase of
10°C doubles the rate of a chemical reaction
Concentration Increases rate Increasing the concentration increases the frequency of collisions
which increases the frequency of successful collisions too, thus
increasing the rate.
Pressure Increases rate Higher pressure compresses the gas, effectively increasing the
concentration, so increasing the frequency of collisions and therefore
the rate too.
, Rate of reaction
• Chemical kinetics is the study of the factors affecting the rate of a chemical reaction
• The rate of a chemical reaction can be defined as the change in concentration of the
products/reactants per unit time or Increase/decrease in concentration of
product/reacts per unit time
• Concentration changes in a reaction can be indirectly monitored by monitoring :
o Change in volume of a gas, mass, concentration through titration, gas pressure
or electrical conductivity
o Calorimetry and light absorbance (spectrometer)
o pH change (e.g. with data loggers or litmus indicator), colour change
• The results obtained is usually then plotted in a graph of concentration against time
• The rate at any point in time is the gradient of the graph at that time
∆𝒄𝒐𝒏𝒄𝒆𝒏𝒕𝒓𝒂𝒕𝒊𝒐𝒏
• Thus, find the gradient at 𝑡 by using 𝑹𝒂𝒕𝒆 𝒐𝒇 𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 =
∆𝒕𝒊𝒎𝒆
• The rate of reaction (gradient) will generally be the steepest at the start of the
curve
o This is because a high concentration of reactant particles, therefore a high
frequency of successful collisions
• The rate of reaction (gradient) decreases over time as time as the concentration of
reactant particles decreases, thus the frequency of successful collisions also
decreases
