Topic 6: Chemical Kinetics Flashcards
Define the rate of reaction.
It is the change in concentration of reactants or products per unit time.
Units:
mol dm^(-3) s^(-1)
mol dm^(-3) min^(-1)
etc
What are the experimental measurements of reaction rates?
1) change in pH (for acid-base reactions)
2) change in conductivity (for reactions involving solids or gases)
3) change in mass or volume (for reactions involving solids or gases)
4) change in colour (for reactions involving transition metals or other coloured compounds)
List three ways in which the rate of reaction can be expressed.
- average rate
- instantaneous rate
- initial rate
What is a rate equation?
Equation showing rate expressed in terms of concentration.
e.g. in the reaction
A + B → C + D
a rate equation can be written as:
rate = -(d[A])/dt = -(d[B])/dt = +(d[C])/dt = +(d[D])/dt
xA + yB → qC + pD
rate = -(1/x)(d[A])/dt = -(1/y)(d[B])/dt = +(1/q)(d[C])/dt = +(1/p)(d[D])/dt
What is the kinetic-molecular theory of gases?
- gases move at high velocities in random directions
- the size of gaseous particle is very small.
- collisions between gaseous particles are completely elastic, ie. energy is transferred not lost.
- average kinetic energy of the particles is proportional to the absolute temperature in kelvin, ie. the movement of atoms or molecules is due to their thermal energy.
What is the collision theory?
Theory that explains why rates of chemical reactions depend on temperature and is based on the kinetic-molecular theory.
For chemical reaction to occur between two reacting particles, these conditions must be fulfilled:
- two particles must collide with each other
- the colliding particles must have the correct mutual orientation
- the reacting particles must have sufficient kinetic energy to initiate the reaction.
Define activation energy, Ea.
The activation energy is the minimum energy that colliding particles need for a reaction to occur.
Define a catalyst.
A catalyst is a substance that increases the rate of a chemical reaction, but is not consumed in the reaction itself.
A catalyst provides an alternative pathway for the reaction and lowers the activation energy.
May be either homogeneous or heterogeneous.
What is a homogeneous catalyst?
Catalyst in the same physical phase or state as the reactants.
e.g. chlorine atoms are the catalysts in the ozone destruction.
What is a heterogeneous catalyst?
Catalyst in a different physical phase or state from the reactants.
e.g. a catalytic converter used in exhaust system of a car.
What is a Maxwell-Boltzmann energy distribution curve?
The curve shows the distribution of velocities of particles. Particles constantly collide and thus the velocities change.
Kinetic energy (x-axis) vs fraction of particles with kinetic energy (y-axis).
The area under the curve represents the total number of gaseous particles in a sample. At a certain temperature, the majority of particles will have a kinetic energy near the mean value.
Describe the temperature effects on kinetic energies.
With an increasing temperature, the proportion of particles that have sufficient kinetic energy to overcome the Ea barrier will increase.
As temperature increases, the mean velocity increases.
As temperature increases, the frequency of collisions increases.
Typically with an increase of 10 °C, the reaction rate will double.
List 4 factors that affect the rate of a chemical reaction.
- increasing the temperature at which the reaction is conducted.
- addition of a catalyst
- increasing the concentration of the reactants.
- decreasing the particle size of reactants in the solid phase.
List 4 ways to measure the rate of a chemical reaction.
- change in pH
- change in conductivity
- change in mass or volume
- change in colour
