Topic 16- Further kinetics Flashcards
Define the term rate of reaction
change in concentration (of any reactant or product) per unit of time. state what is being monitored, usually production of a product
How could you calculate the rate of a reaction at a given instant?
rate of reaction = change in concentration/ change in time
What are the methods of determining the rate of reaction experimentally?
- measuring the volume of gas evolved
- measuring the change in mass of a reaction mixture
- monitoring the change in intensity of colour of a reaction mixture (colorimetry)
- measuring the change in concentration of a reactant or product using titration
- measuring the change in pH of a solution
- measuring the change in electrical conductivity of a reaction mixture
How can you determine the rate constant and expression for a reaction?
only experimentally
What affects the value of a rate constant for a specific reaction?
temperature
Do species need to be in the chemical equation to be in the rate equation?
no- species in the chemical equation may be excluded and species not in the chemical equation e.g. catalysts,
Define the term order of reaction (with respect to a given product)
the power to which a species’ concentration is raised in the equation
Define the term overall order of reaction
the sum of the orders of reaction of all species in the rate expression e.g. total order= x + y (x and y are the indices)
How would you calculate the units of the rate
constant?
units of rate= moldm-3s-1
units of concentration= moldm-3
rearrange equation to get k= and sub in the units and cancel out
What would a rate concentration graph look like for a zero order of reaction?
parallel straight line across- no curve, all the points plotted in a line
What would a rate concentration graph look like for a first order of reaction?
straight line going from the origin diagonal (equal between the y and x axis)
What would a rate concentration graph look like for a second order of reaction?
curved line upwards, not equal
What would a concentration time graph look like for a zero order of reaction?
negative correlation, straight line going down
What would a concentration time graph look like for a first order of reaction?
curved line with negative correlation (steeper than zero order)
What would a concentration time graph look like for a second order of reaction?
curved line with negative correlation (steeper than first order)
How can you tell the difference from first order and second order concentration time graphs?
- the concentration time graph for a first order reaction has a constant half life
- the concentration time graph for a second order reaction has increasing half lives with a decreasing concentration
How could you find the rate expression using the initial rate method?
- do a series of experiments whilst varying the concentration, so the concentration of just one reactant changes each time
- plot a graph of concentration against time for each reactant and use a tangent at t=0 to find the initial rate of the reaction
- compare rates and concentrations between each experiment to find order of reactions and overall rate equation
What must you add to react with iodine as it is produced for an iodine clock reaction?
known moles of sodium thiosulfate and a little starch (reacts with a 1:2 ratio)
When does the starch turn a blue-black colour in an iodine clock reaction and why?
When all the sodium thiosulfate (Na2S2O3) has been used up and so iodine is produced- which reacts with starch leading to a blue-black colour
How can you calculate the rate of reaction from the data from an iodine clock reaction?
record the time taken for the colour change to occur and use rate=1/t (used as the initial rate)
What is roughly the effect of a 10K temperature change on the rate of reaction?
doubles the rate
What is true of the half life of a first order reactant (concentration against time graph)?
half life is constant
What is the rate-determining step?
the slowest step in a reaction mechanism, which determines the overall rate of the equation
How does the rate determining step link to the species involved in the rate equation?
any species involved in the rate determining step appear in the rate expression, species only involved after the rate determining step do not appear in the rate expression
Define a homogeneous catalyst
In the same phase as the reactants
Define a heterogenous catalyst
In a different phase to the reactants
What is activation energy, Ea
The minimum energy that colliding particles must possess for a reaction to occur
What does activation energy represent?
The amount of energy that the colliding particles must obtain to reach the energy level of the transition state
What is a catalyst?
A substance that increases the rate of a chemical reaction but is chemically unchanged at the end of the reaction
How does a catalyst work?
Provides an alternative route for the reaction that has a lower activation energy than the original route
What is the half-life of a reaction?
The time taken for the concentration of the reactant to fall to one-half of its initial value
Define the instantaneous reaction rate
The gradient of a tangent drawn to the line of the graph of concentration against time. The instantaneous rate varies as the reaction proceeds, except for zero order
What is an elementary reaction?
When a reaction takes place by a single collision between the two reactant particles to form the products
What can you determine from the equation of an elementary reaction?
The rate equation
Can you determine the rate equation from any stoichiometric equation for a reaction?
No, only if it’s elementary
What is the rate determining step?
The slowest step in a reaction
What is the Arrhenius equation?
k=Ae(-Ea/RT)
State the meaning of each component in the Arrhenius equation
A=pre-exponential factor, a measure of the rate at which collisions occur
Ea=activation energy
R=gas constant
T=absolute temperature, in kelvin
What is the natural log of the Arrhenius equation?
lnk=-Ea/RT 1/T + lnA
What would the graph look of lnk against 1/T?
a straight line with a gradient of -Ea/R
What are the different methods for measuring the rate of reaction experimentally?
- colorimeter at suitable intervals to measure colour change
- gas syringe to measure gas evolved
- change in mass of the reaction mixture
- titration to measure the time it takes fir end-point to occur
How would you draw a rate concentration graph?
plot [A] against time, draw tangents at different values —> draw a secondary graph of rate against [A]
