Kinetics II - topic 16 Flashcards
What are the techniques used to investigate rates of reactions
- Measurements of the change in volume of a gas
- Measurement of change of mass
- Titrating samples of reaction mixture with acid, alkali, sodium thiosulfate
- Colorimetry
- Measuring change in electrical conductivity
- Measurement of optical activity.
when do we use the Measurement of the change in volume of a gas to investigate rate of reaction
When there is a change in the number of moles of gas in the reaction.
draw the apparatus used in the Measurement of the change in volume of a gas
pg 1 chemrevise
when do we use the Measurement of change of mass to investigate rate of reaction
when there is a gas produced which is allowed to escape
when do we use Colorimetry to investigate rate of reaction
If one of the reactants or products is coloured then a colourimeter can be used to measure the change in colour of the reacting mixtures and then plot a calibration curve e.g iodine clock reaction
how do we use titrations to investigate rate of reaction
Small samples are removed from the reaction mixture, quenched (which stops the reaction) at regular intervals and the titrated with a suitable reagent.
What do we titrate the mixtures of sodium hydroxide, acids and iodine with
The NaOH could be titrated with an acid
The H+ (acid) could be titrated with an alkali
The I2 could be titrated with sodium thiosulfate
when do we measure the change in electrical conductivity to investigate the rate of reaction
if there is a change in the number of ions in the reaction mixture as this will change the level of electrical conductivity
when do we measure the optical activity to investigate the rate of reaction
If there is a change in the optical activity through the reaction this could be followed in a polarimeter
what is the rate equation to work out order
r = k[A]^m[B]^n
m and n = reaction orders
[A] and [B] = reactants
how many different orders are there
0 order
1st order
2nd order
how to work out the total order
m+n
what do each order mean
- For zero order: the concentration of A has no effect on the rate of reaction r = k[A]0 = k
- For first order: the rate of reaction is directly proportional to the concentration of A r = k[A]1
- For second order: the rate of reaction is proportional to the concentration of A squared r = k[A]2
how can the half life from continuous data determine the order
- If half-lives are constant then the order is 1st order
- If half-lives rapidly increase then the order is 2nd order
- If the half life decreases then it it’s 0 order
what does k represent in the rate equation
The value of k refers to a specific temperature and
it increases if we increase temperature
units of k depending on the order of the reaction
- For a 1st order overall reaction the unit of k is s-1
- For a 2nd order overall reaction the unit of k is
mol-1dm3s-1 - For a 3rd order overall reaction the unit of k is mol-2dm6s-
rules for determining order of a reactants
If conc is doubled and rate stays the same: order= 0
If conc is doubled and rate doubles: order= 1
If conc is doubled and rate quadruples : order= 2
learn how to found out different order of reactants
pg 4 chemrevise
https://www.youtube.com/watch?v=BN1A9s0E0FI
what is rate constant k changed by
temperature
increasing temp = increase in rate constant k
Arrhenius equation
k = Ae^-EA/RT
or
ln k = ln A – EA/(RT)
units of Arrhenius equation
- rate constant is 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 the Arrhenius constant A will be the same as the unit of the rate constant k
- e is found on the calc (shift ln)
- Temp is in kelvin
- A is Arrhenius rate
what is the rate determining step
the slowest step that controls the overall rate of reaction
how to determine the order of reactants using the mechanism for a reaction
The number of moles of each substance in the slowest step will be the same as the order of reaction for each substance.
write the order of reaction for this mechanism
overall reaction
A + 2B + C —-> D + E
Mechanism Step1 Step 2 Step 3
A+B —> X+D fast
X + C —> Y slow
Y + B —> E fast
r = k [x]1 [c]1
The intermediate X is not one of the reactants so must be replaced with the substances that make up the intermediate in a previous step
A + B —-> X + D
r = k[A]1[B]1[C]1 == ANSWER
write the order of reaction for this mechanism
overall reaction
A + 2B + C—–>D + E
Mechanism
Step1 A+B —-> X+D slow
Step 2 X + C—->Y fast
Step 3 Y + B—->E fast
r = k [A]1[B]1 = ANSWER
C is zero order as it appears in the mechanism in a fast step after the slow step
how to determine the rate determining step in sn1 and sn2 reactions
sn1 = 1 molecule in rate determining step
sn2 = 2 molecules in rate determining step
when plotting the Arrhenius graph what represents the gradient
- -Ea/R
how to work out the activation energy using the Arrhenius graph
find the gradient to work out -Ea/R
then find Ea by: -(gradient x R (8.31))
how to work out what A is using the Arrhenius graph
1) find the gradient to work out -Ea/R
2) Then select a point on the graph and use the coordinates to find out what lnk (y-axis) and 1/T (x-axis) is
3) Use Arrhenius equation: lnk = lnA - Ea/RT to find A by doing lnk (coordinate from the graph) = lnA - Ea/R (gradient) x 1/T
4) then rearrange to find A and make sure to use e button on calc to do the inverse of ln
-Ea/R x 1/T = -Ea/RT
how does gradient equation y = mx + c link to Arrhenius graph
y = lnk
mx = -Ea/R
c = InA
What does rate of reaction mean
The change of concentration/amount of a reactant or product per unit time
How to calculate rate
Amount of product used or made / time
How to quench reactions
- Diluting with deionized water
- cool it down
- add a chemical (quenching agent) to stop reaction
What is the order
The power to which the concentration is raised in the rate equation, it tells us how the concentration of the substrate affects the rate
How to conduct an experiment to work out initial rate and order
1) Repeat the experiment but changing the concentrations of each reactant one at a time one each experiment to find out its effect on the rate of
2) plot a graph with time on the x axis and concentration on the y and work out the the initial rate by finding the gradient at time = 0
3) record the concentrations of reactants for each experiment and their initial rates in a table, from this we can work out the order with respect to each reactant and write a rate equation
What is a clock reaction and what is its assumptions
Clock reaction = where you time how long it takes for a reaction to occur
Assumptions:
- the temperature of the reaction remains constant
- the concentration of reactants doesn’t change significantly during the time period of the reaction
- reaction has not proceeded too far where the end point is seen
Based on these assumptions we can say that the rate has remained constant during the time of measuring
How does the Iodine clock reaction (Harcourt-Esson) work
1) add sodium thiosulfate and starch (which acts as an indicator iodine (blue black when positive)) to excess hydrogen peroxide — H2O2(aq) + 2H+(aq) + 2I-(aq) —-> 2H2O (l) + I2 (aq)
2) The sodium thiosulfate reacts immediately with the I2 that is produced in the reaction — 2S2O3(aq) + I2(aq) —> 2I-(aq) + S4O62- (aq)
3) When there is no more sodium thiosulfate left the I2 reacts with starch to give a deep blue black colour
4) the rate of reaction is monitored by conducting the experiment in a conical flask on a white tile with a cross on it and seeing how long it takes for the cross to disappear through the flask
How does rate graphs/time conc graphs help us find the order of the reaction
-if the line is straight (flat/horizontal) ON RATE GRAPH = 0 order
- if line is inversely proportional ON TIME CONC GRAPH = 0 order
- if the line is directly proportional ON RATE GRAPH = 1 order
- if the line is inversely exponential ON TIME CONC GRAPH = 1 order OR 2nd order
- if the line is directly exponential ON RATE GRAPH = 2nd order
look on https://cognitoedu.org/coursesubtopic/c3-alevel-ocr_wonPntOe
What is the half life
The time it takes for half of the reactants to be used up
Why do halogens form polar bonds with carbon
- halogens are more electronegative than carbon so they pull electrons towards themselves in a covalent bond leading to the formation of a polar bonds
What reactions do primary tertiary and secondary halogenoalkanes undergo (SN1 or SN2)
Primary = SN2
Secondary = SN1 + SN2
Tertiary = SN1
Why does the rate of reaction increase when activation decreases
As many more particles will have enough energy to react when they collide
how to work out -Ea/R without using a graph or the equation, just using data
difference in InK / difference in 1/T == -Ea/R
