Chemical Kinetics Flashcards
What is meant by the rate of reaction?
The change in concentration of reactants or products over time
What is the rate equation?
Rate = k[A]m[B]n
M and n represent the orders with respect to each reactant
K is the rate constant
What is a rate constant?
The constant of proportionality linking the rate of reaction and the concentration of the reactants raised to the power of their order in the rate equation
How do you calculate the units of a rate constant ?
- Rearrange the rate equation to make k the subject
- Substitute units into the equation
- Cancel the common units to find the units for k
What is meant by the ‘order of reaction’?
- The order with respect to a reactant is the power to which the concentration of the reactants raised is raised in the rate equation
- Overall order of a reaction = the sum of all the individual orders of the reactants
What do the different orders of reaction mean ?
Zero Order = If the rate [A]0 then the rate is unaffected by changing [A]
First Order = If the rate [A]1 then the rate of reaction increases at the same rate as the [A] increases
Second Order = If the rate [A]2 then the rate will increase by the square of the factor [A] increases by
What is activation energy ?
The minimum energy required for a particular reaction to occur
What experimental techniques allow you to obtain rate data?
- Titration
- Measuring the volume of gas released from a reaction over time
- Measuring the change in mass of reactant over time
- Colorimetry
- Measuring the time taken for a colour change
What are 2 methods that can be used to investigate reaction rates?
Initial-rate method = Experiments are carried out where different initial concentrations of one reagent are used
Continuous monitoring = The conc/volume is continuously monitored so that a concentration or volume time graph can be produced from the data
What does the rate-concentration graph look like for a zero order reactant?
Straight line at the top of the graph
What does a rate-concentration graph look like for a first order reactant?
Line goes straight through the origin and concentration is proportional to the rate of reaction increases
How do you calculate the gradient of a line/tangent ?
Gradient = change in y / change in x
What is the rate determining step?
- The slowest step in the reaction
- Only the species that take part in the rate determining step affect the rate
What is the relationship between the rate-determining step and the orders with respect to the reactants?
- The species present in the rate equation are those that part take in the rate determining step
- For any reactant in the rate equation, the order related to it tells you how many molecules of it are involved in the rate determining step
What equation is used to calculate activation energy?
The Arrhenius equation
What is sampling and quenching?
A small sample of the reacting solution is taken at regular intervals and then the reaction is stopped or quenched as quickly as possible, then the sample is analysed
What is the Arrhenius equation?
lnk = lnA - Ea / RT
What values make up the Arrhenius equation?
K = the rate constant
A = Arrhenius constant
Ea = activation energy in J mol-1
R = gas constant = 8.314 J K mol-1
T = Temperature in K
E = 2.718
How do you find the activation energy from a graph?
Ea = Gradient x R
