Rate Equations (Kinetics A Level) Flashcards
(19 cards)
What are rate equations? And reaction orders?
The rate equation relates mathematically
the rate of reaction to the conc of the reactants.
- For the following reaction,
- aA + bB → products, the generalised rate equation is:
-> r = k[A]^m[B]^n
- r is rate; units (mol dm-3 s-1)
- [square brackets] is conc (mol dm-3)
- k is rate constant
..
m, n are called reaction orders
>Orders are usually integers 0,1,2
> O means reaction is zero order with respect to that
> reactant 1 means first order
> 2 means second order
>
The total order for a reaction is worked by adding all the individual order together (m+n)
How do reaction orders show how conc has an effect on rate of reaction
For zero order: the concentration of A has no effect on rate of reaction
r=k[A]º=k
For first order: rate of reaction is directly proportional to conc of A
r = k[A]^1
For second order: rate of reaction is proportional to conc of A squared
r = k[A]^2
diagram1
What are rate constants ? (k)
- The units of k depend on the overall order of reaction.
>must be worked out from rate equation - The value of k is independent of concentration and time.
It is constant at a fixed temperature - The value of k refers to a specific temperature and increases if we increase temperature
.
for a 1st order overall reaction the unit of k is s-1
For 2nd order overall reaction the unit of k is mol-1 dm3 s-1
For 3rd order overall reacion for the unit of k is mol-2 dm6 s-1
Examples: finding units of k
Example 1st order:
Rate = k[A][B]0
- overall order m=1 and n=0
- 0 + 1 =1 so overall is first order
- first orders written as rate=k[A]
To find units
- rearrange to make k subject
K = rate/[A]
- insert units to cancel
- mol dm-3 s-1 / mol dm-3
- = s-1
Whats comtinuous monitoring
Continuous Monitoring
- When we follow one experiment over time recording change in conc
- we call it a continuous rate method.
- The gradient represents rate of reaction.
- The reaction is fastest at start where gradient is steepest.
- The rate drops as the reactants start to get used up; their conc drops.
- The graph will eventually become horizontal and the gradient
- becomes zero which represents the reaction having stopped.
How do we Measure the change in volume of a gas
This works if theres a change in number of moles of gas in reaction.
- Using a gas syringe is a common way of following this.
- works well for measuring continuous rate but a typical gas syringe only measures 100ml of gas
- so you don’t want a reaction to produce more than this volume.
-
- Quantities of reactants need to be calculated carefully.
Mg + 2HCl → MgCl +H2
Eg of Typical Method
• Measure 50 cm’ of the hydrochloric acid;add to conical flask.
• Set up gas syringe in stand : Weigh 0.20 g of magnesium,
• Add magnesium ribbon to the conical flask, place bung firmly into top of the flask and start the timer.
•
• Record volume of hydrogen gas collected every 15 seconds for 3 minutes.
..
• Large excess of reactants
- In reactions with several reactants, if the conc of one of the reactant is kept in a large excess
- then reactant wont affect rate and will be pseudo-zero order.
- its conc stays virtually constant and does not affect rate.
• The initial rate
- rate at the start of the reaction, where it is fastest.
- calculated from the gradient of a continuous monitoring conc vs time graph at time = zero.
- A measure of initial rate is preferable as we know the concs at the start of the reaction.
diagram 2
how to Compare continuous rate curves
- The higher the conc/ temp/ SA, the faster the rate (steeper the gradient)
- If magnesium or marble chips is in excess of acid, then
- final vol of gas produced will be proportional to amt of moles of acid.
..
analysing diagram 3
Different vols of same initial concs will have same initial rate (if other conditions are same)
>but will end at different amounts
Need to calculate and compare initial moles of reactants
> to distinguish between diff finishing volumes.
Whats the initial rate method? And what are clocks reactions
- The initial rate can be calculated from taking gradient
- of a continuous monitoring conc vs time graph at time = zero
- Initial rate can also be calculated from clock reactions where time taken to reach a fixed conc is measured.
Initial rate represented as 1/t
..
- In clock reactions there are often two successive reactions.
- end points achieved when one limited reactant runs out, resulting in sudden colour change.
- By repeating experiment several times, varying conc of a reactant e.g. I- (keeping other reactants constant conc)
- you can determine order of reaction with respect to that reactant
Clock reaction example
- Hydrogen peroxide reacts w iodide ions to form iodine.
- thiosulfate ion then immediately reacts iodine formed, in second reaction as shown below.
H202(aq) + 2H+(aq) + 2l-(aq) → I2(aq) + 2H20(l)
2S2032-(aq) + I2(aq) → 2I-(aq) + S4062-(aq)
- When the I2 produced has reacted with all of limited amount of thiosulfate ions present,
- ## excess I2 remains in solution.
- Reaction with starch then suddenly forms a dark blue-black colour.
- A series of experiments is carried out, in which the conc of iodide ions is varied,
- while keeping concs of all of the other reagents the same.
- In each experiment, time taken (t) for reaction mixture to blue is measured.
Working out orders from experimental initial rate data
Normally to work out rate equation we do a series of experiments
- where initial concs of reactants are changed (one at a time) and
- measure initial rate each time
Working order out graphically
- in an experiment where conc of one reagent is changed and reaction rate measured,
- its possible to find order graphically
-
diagram 4 TRY BEST TO UNDERSTAND 🚨
How to deduce rate equation for a reaction, using an initial rate data in a table
To calculate order for a particular reactant, its easiest to
- compare two experiments where only that reactant is being changed.
- If conc is doubled and rate stays same: order= 0
- If conc is doubled and rate doubles: order= 1
- If conc is doubled and rate quadruples : order= 2
Add orders of all reactants in equation to find equation’s order
Working out orders when two reactant concs change at the same time
- in most questions its possible to compare 2 experiments where
- ## only one reactant has its initial conc changed
- if two reactants are changed then the effect of both individual changes
- of conc are multiplied to give overall change in rate
Calculating a value for k using initial rate data? And when does k change?
Using the inital rate data tables, choose any one of the experiments
- and substitute values into rate equation and rearrange to give k
Remember k is same for all experiments done at same temp.
> Increasing the temperature increases the value of the rate constant k
Calculating k from conc time graphs with zero order
For zero order reactants, rate stays constant as the reactant is used up.
> means the conc of that reactant has no effect on rate.
> Rate = k [A]º so rate = k
If the graph is directly proportional, gradient = value of k constant
Effect of temperature on rate constant: the Arrhenius equation
Increasing temp increases value of the rate constant k
- the relationship is given by Arrhenius equation (k = Ae^-Ea/RT
- where A is Arrhenius constant,
- R is the gas constant,
- and Ea is activation energy
diagram 5
Using the Arrhenius equation (equations will be given in the exam )
k = Ae-Ea/RT
The Arrhenius equation is usually rearranged to
Ln k= Ln A -Ea/(RT)
> shd be able to rearrange and substitute values into both these equations.
Units:
-Temp uses the unit K
-R = 8.31 J mol-1K-1
-Activation energy will need to be in J mol-1 to match the units of R
-The unit of Arrhenius constant A will be same as the unit of rate constant
diagram 6 for examples
Calculating the activation energy graphically from experimental data
Using the rearranged version
Ink=In A - Ea/(RT)
k is proportional to rate of reaction so ln k can be replaced by ln(rate)
From plotting a graph of ln(rate) or ln k against 1/T the activation energy
- can be calculated from measuring
- gradient of the line
..
Eg.
- in a graph: y is ln(rate) and x is 1/T
- gradient = Ea / R
- Ea = gradient x R (rearranged)
» gradient is always negative
diagram 7 for example
Rate equations and mechanisms
- A mechanism is a series of steps through which reaction progresses,
- often forming intermediate compounds.
-> If all steps added together theyll add up to overall equation for reaction - Each step can have a diff rate of reaction.
- slowest step will control overall rate of reaction.
- The slowest step is rate-determining step.
- The molecularity (number of moles of each substance) of molecules
- in the slowest step will be same as order of reaction for each substance. -> e.g. 0 moles of A in slow step wd mean A is zero order.
1 mole of A in the slow step would mean A is first order
diagram 8 lots of examples
