Reaction Kinetics A2 Flashcards
Define rate of reaction of reaction
We can calculate rate of reaction by measuring a decrease in concentration of particular reactant or an increase in the concentration of a particular product over a period of time.
Rate of reaction= change in concentration/time taken for change
Unit: mol dm-3 s-1
Usually measured in seconds
What is the rate constant?
The proportionality constant, k, in a rate equation
E.g. K[NO2]^2
What is a rate equation?
An equation showing the relationship between the rate constant and the concentrations of the species that affect the rate of the reaction.
Rate equations can only be determined from experimental data and not stoichiometric equation.
The rate equations for some reactions may include compounds that are not present in the chemical equation.
What is the order of reaction?
The power to which the concentration of reactant is raised in the rate equation. If a rate is directly proportional to concentration it is first order. If the rate is directly proportional to the square of the concentration it is second order. The overall order of reaction is the sum of these powers.
Many reactions involving free radicals have fractional order of reaction.
What is the unit of the rate constant
The units of the rate constant must be worked out from the rate equation in the form:
K=mol dm-3/units for the product of the concentration terms
How can we identify the order of reaction?
We can identify the order of a reaction in three ways:
*Plot a graph of reaction rate against concentration of reactant
*Plot a graph of concentration of reactant against time
*Deduce successive half-lives from graphs of concentration against time
How can we determine the order of reaction from a graph of reaction rate against concentration
Zero order reaction:
The plot of reaction rate against concentration is a horizontal straight line. The reaction rate does not change with concentration. For a zero order reaction, k is numerically equal to the reaction rate.
Rate=K
This is because any number to the power of zero=1
First order reaction
The plot of reaction rate against concentration is an inclined straight line going through the origin. The rate is directly proportional to the concentration.
Second order reaction
The plot of reaction rate against concentration is an upwardly curved line
Reaction rate is directly proportional to the square of the concentration.
How can we use graph of concentration of reactant against time to determine order of reaction.
For a zero order reaction, the graph is descending straight line. The rate of reaction is the slope (gradient) of the graph. The reaction proceeds at the same rate whatever the concentration of the reactant.
For first and second order the graph is a curve. The curve for second order is much deeper than for a first order reaction. It also appears to have a longer tail as it levels off.
How can we use half life and reaction rates to calculate order of reaction
Half-life, t 1/2 is the time taken for the amount (or concentration) of the limiting reactant in in a reaction in a reaction to decrease to half its initial value.
A zero order reaction has successive half lives which decrease with time.
A first order reaction has a half-life which is constant
Second-order reactions have successive half-lives which increase with time.
What is a limiting reactant?
The reactant which is not in excess. The reaction will stop when the limiting reactant is all used up.
How can we calculate K from initial concentrations and initial rate
Step 1. Write out the rate equation
Step 2. Rearrange in terms of K
Step 3. Substitute the values
How can we calculate K from half life
For a first order reaction, half life is related to the rate constant by the expression
t1/2=0.693/K
Where t1/2 is the half life measured in s.
How can we deduce order of reaction from course of a reaction
Steps in analysing the date:
Step 1; plot a graph to show the concentration of a particular reactant or product changes with time
Step 2; take tangents at various points along the curve which correspond to particular concentrations of the reactant
Step 3; calculate the gradient at each concentration selected. The rate of reaction is calculated from the gradient
Step 4: Plot a graph of rate of reaction against concentration
How can we deduce the order of reaction using initial rates
This method is often used when the rate of reaction is slow
*Carry out several experiments with different known initial concentrations of each reactant
*Measure the initial rate of reaction by either:
*Taking the tangent of the curve at the start of each experiment or
*Measuring the concentration of a reactant after the experiment has started
*For each reactant plot a graph of initial rate against concentration of that particular reactant.
What is the rate-determining step?
A reaction occurs in a number of steps. We call this reaction mechanism. These steps do not take place at the same rate. The overall rate of reaction depends on the slowest step. We call this the rate determining step.
If there is only a single species in the rate determining step, we called the reaction unimolecular. If two species are involved in the rate determining step, we call the reaction bimolecular.
What is a catalyst?
Catalyst increase the rate of a chemical reaction by providing an alternative pathway for the reaction with low activation energy. Many transition elements act as catalysts. We can divide catalyst into two main classes homogeneous and heterogeneous catalysts.
What is a homogeneous catalyst?
The type of catalysis in which the catalyst and reactants are in the same phase. For example, examples of uric acid catalyse the formation of an ester from an alcohol and carboxylic acid.
Most examples of homogeneous catalysis involve redox reactions.
In order for this catalysis to work the standard electron potential for the reactions involving the catalyst must lie between the electrode potential involving the reactants.
What is a heterogeneous catalyst?
The type of catalysis in which the catalyst is in a different phase from the reactants for example iron in the Haber process.
The mechanism of heterogeneous catalyst can be explained using the theory of adsorption.
Adsorb means to bond to the surface of a substance. Absorb means to move right into the substance.
Heterogeneous catalysis in iron in the haber process.
- Diffusion: nitrogen gas and hydrogen gas diffuse to the surface of iron.
- Adsorption: the reactant molecules are chemically adsorbed onto the surface of the iron. The bonds form between the reactant molecules and the iron are:
* strong enough to weaken the covalent bonds within nitrogen and hydrogen molecule so the atoms can react with each other
* weak enough to break and allow the products to leave the surface - Reaction: the adsorbed nitrogen and hydrogen atoms react on the surface of the iron to form ammonia.
- Desorption: the bonds between the ammonia and the surface of the iron weak and are eventually broken.
- diffusion: ammonia diffuses away from the surface of the iron.
Describe the possible steps of transitional elements in catalytic converters?
catalytic converters are fitted to car exhaust to speed up the conversion of nitrogen oxide to harmless nitrogen gas and conversion of poisonous carbon monoxide to carbon dioxide.
The honeycomb structure inside the catalytic converter contain small beats coated with platinum palladium or rhodium. These act as heterogeneous catalyst. The steps and catalytic process include:
1. Adsorption of nitrogen oxide and carbon monoxide to the catalyst surface.
2. Weakening of the covalent bonds within nitrogen oxide and carbon monoxide.
3. Formation of new bonds between
* adjacent nitrogen atoms (to form nitrogen molecules)
*Carbon monoxide and oxygen atoms to form carbon dioxide
4. Desorption of nitrogen molecules and carbon dioxide molecules from the surface of the catalyst.
