List the main factors that affect the rate of a chemical reaction
- Temperature
- Concentration
- Pressure
- Surface area
- Catalysts
Define rate of reaction
The change in concentration of any of the reactants or products per unit time.
What does the rate expression tell us
How each of the species involved in a chemical reaction affects the reaction rate
What is the rate equation for a reaction
- rate = k multiplied by the concentrations of each reaction raised to the power of the order of that reactant.
What is k in the rate equation and how does it vary
- K is the rate constant for the reaction.
- it varies with temperature- so the temperature at which it is measured needs to be stated.
How does the rate constant K vary with temperature
As temperature increases, the value of K increases
What is the order of a reaction with respect to one of the species
The order of reaction with respect to one of the species is the power to which the concentration of that species is raised in the rate expression. It tells us how the rate depends on the concentration of that species.
What is the overall order of a reaction
The overall order of the reaction is the sum of the orders of all of the species that appear in the rate expression.
What does the rate expression tell us and what does it include
The rate expression tell you how the rate of a reaction depends on the concentration of the species involved. It only includes the species that affect the rate of the reaction.
If the concentration of a species does not affect the rate of the reaction what is its order
Zero order
If the concentration of a species is directly proportional to the rate of the reaction what is the order of that species
1st order
If the rate of the reaction is directly proportional to the square of the concentration of a species, what is the order of that species
2nd order
What are the two experimental methods by which you can determine the order of a reaction with respect to a particular species
1) Finding the order by using rate- concentration graphs
2) Using an initial rate method
Describe how the order of reaction of a species can be determined by using rate-concentration graphs
- Plot a graph of rate against concentration
- If the graph is a horizontal straight line, the order of the species is 0
- If the graph is a straight line through the origin, then the order of the species is 1
- If the graph is a parabola/not a straight line then the order cannot be found directly- but is most likely two (at A-level). If the concentration of the species squared is plotted against rate, it should produce a straight line graph if the order is two.
Describe how the order of reactants are determined by using the initial rate method
- A series of reactions is carried out at a constant temperature.
- Each experiment starts with a different combination of concentrations of reactants and catalysts and they are designed so that between any pair of experiments, the concentration of only one species varies.
- The initial rate of each of the experiments is determined and comparing the concentrations and initial rates allows us to work out the order of each reactant.
What are two ways of measuring the initial rate of a reaction
1) Plot a concentration-time graph for the reactant and draw a tangent to the graph at time=0. The gradient of this tangent is the initial rate
2) Use 1/T. This works because the rate constant remains the same as long as temperature is kept constant.
Why does the rate constant depend on temperature
- Temperature is a measure of the average kinetic energy of the particles
- As temperature increases, a higher proportion of the particles have the activation energy to react.
- So reaction rate increases with temperature which is what the rate constant represents.
How is activation energy linked to the rate constant
By the Arrhenius equation
What is the Arrhenius equation
k= Ae ^ -Ea/RT
Where:
- k= rate constant
- A= the pre-exponential factor which is related to the number of collision between reactant molecules.
- e ^ -Ea/RT = The fraction of collisions with enough energy to react.
What is the logarithmic form of the Arrhenius equation
lnk= - Ea/RT + lnA
Why is the logarithmic form of the Arrhenius equation useful- what does it help us to do
- The logarithmic form of the Arrhenius equation allows us to see that a graph of lnk against 1/T will be a straight line of the gradient -Ea/R
- This can be used to find a value for Ea experimentally by measuring the rate of reactions at different temperatures.
Explain the meaning of reaction mechanism
- Most reactions take place in more than one step- these separate steps that lead to the formation of reactants and products are together called the reaction mechanism.
- The steps in between invoke very short-lived intermediates.
- These intermediate species, which give information about the mechanism of the reaction are usually difficult to isolate and identify which means other methods need to be used.
- This is the utility of the rate determining step
What is the rate determining step
- The rate determining step is the slowest step in a reaction.
- It may form a ‘bottleneck’ and governs the rate of the whole process.
Which stages of a reaction appear in the rate equation
- Only the rate determining step appears in the rate equation
- No stages after the rate determining step appear in the rate equation.
- Because the reactants for the rate determining step May depends on the stages before it, previous stages do occur in the rate equation.
