Topic 16: Further Kinetics COPY Flashcards
How do you find reaction rate from a graph?
RR is measured by recording how quickly chemical curves change. As this varies throughout the reaction we always consider the initial rate, which is the gradient at 0.
What is the rate eqn?
The rate eqn tells us the relationship between the concs of the reagents and the reaction rate. eg
Rate= k[A]^x[B]^y
k= rate eqn.
x and y= order of the reaction with respect to A and B
What does the rate eqn tell us?
If you increase the concentrations of A or B, the rate would increase by some factor. X and Y tell us by how much changing the concentration will affect the rate.
We cannot tell X and Y from the overall equation, only from experimental data.
What are the types of orders?
First order: if you double A the rate is doubled. The value of X=1. Rate is directly proportional to [A]^1
Second order: if you double A the rate is squared, increasing by a factor of 4. X=2. Rate is directly proportional to [A]^2
Zero order= if I change the concentration of a it has no effect on the overall rate. Rate is directly proportional to [A]^0
State the definition for the order of a reaction
Is the power to which the concentration of a given reagent is raised in the rate equation.
What is the overall order of a reaction?
The overall order of reaction equals the sum of the orders, x+y. eg overall order for [A][B]^2 = 3
Describe k, the rate constant.
k is large for fast reactions and small for slow reactions. Increase in temperature is accompanied by an increase in the value of k.
k is a temperature dependent constant (like Kc!)
what are the units for k?
The unit of k will change depending on the expression for the rate eqn.
Deduce the units for k in the following rate eqn:
rate= k[B][C]
Units for rate= moldm^3s-1 units for [B]= moldm^-3 units for [C]= moldm^-3 Substitute units into equation and solve: moldm^3s-1= k x moldm^-3 x moldm^-3 s-1 = k x moldm^-3
therefore k = mol^-1dm^3s-1
Do catalysts appear in the rate equation?
Don’t appear in chemical equations but may appear in the rate of expression or rate eqn.
Describe reagent CONCENTRATION time graphs for 0, 1st and 2nd orders
see sheet- 0 order is a straight line gradient downwards
1st order is a more curved downwards line
2nd order is most curved
Describe RATE-CONCENTRATION graphs for 0, 1st and 2nd orders
On the x axis 1/time is plotted. on y axis, plot the conc of a REAGENT.
0 order= a flat line
1st order= a straight, positive gradient
order>1= a curved, positive gradient
2nd order= a straight, positive gradient
To find an order>1, plot 1/time against the concentration SQUARED. If the line is straight, then it is second order
How can we find the orders from the INITIAL rate method?
A series of experiments are carried out at constant temp, changing only the concs of the reagents. By comparing initial concs and rates of pairs of experiments the order can be found
What is the half life of a reaction?
The time taken for the concentration of the reagent to become half of its initial value.
How can we find the orders using half lives in RATE CONC graphs?
To find the half life, find half of the highest concentration. Dot along the graph until you hit the curve. Mark down the time taken, eg 10mins. Find half of the concentration again, and do the same again. Is the half lives are constantly the same (always 10mins), the order of the reagent is one
What is the rate determining step?
Many reactions consist of separate steps.
The overall rate of a multi step process is governed by the slowest step. This is the rate determining step.
The rate eqn shows ALL involved species up to and including the RDS. Therefore the RDS is the step with ALL the species that are in the rate eqn!!
The RDS for a 3 step reaction is this:
H2O2 + I- ——> IO- +H20
What is the rate eqn for the reaction?
Every reactant in the RDS is in the rate eqn. So the answer is:
Rate = k[H202][I-]
The RDS for a 3 step reaction is this:
AB + AB —-> A2B2
What is the rate eqn for the reaction?
Every reactant in the RDS is in the rate eqn. So the answer is:
Rate = k[A]^2[B]^2
The reason it is squared is bc there are 2 A’s and 2 B’s in the RDS.
X + Y +2Z ——> XYZ2
The rate eqn for the following reaction: Rate= k[X][Y]
Suggest the mechanisms for the reaction. Give the fast steps and the Rate determining step.
X + Y —–> XY SLOW (RDS)
XY + Z —–> XYZ FAST
XYZ + Z —–> XYZ2 FAST
Describe the reaction between iodine and propanone
The reaction between iodine and propanone:
CH2COCH3(aq) + I2(aq) —> CH3COCH2I(aq) + H+ +I-
The reaction investigates the change in iodine conc as the reaction takes place. By varying the starting conc of iodine, it is possible to find the order with respect to iodine.
The reaction is acid catalysed. A colorimeter measures the rate at which the iodine is being decolourised.
Describe the method for the reaction between iodine and propanone
In 100 cm3 conical flasks, make four mixtures of HCl, propanone solution and water. Label the flasks A, B, C, and D.
From a burette, measure out the volumes of iodine solution required for each run into four test tubes. Label the tubes a, b, c, d.
Start the first run by adding a to flask A. Al mismo tiempo, start the timer and measure the time taken in seconds for the colour of iodine to disappear.
Repeat step 3 for the other three runs.
Water is added to some of the mixtures to ensure the total volume for all mixtures is the same. Therefore the conc of each reactant is proportional to the volume.
Describe the method to find the rate of the iodine-propanone reaction using a TITRIMETRIC method
Mix 25cm3 of 1 mol dm−3 (aq) propanone with 25 cm3 of 1 mol dm−3 sulfuric acid in a beaker.
Start stop clock as soon as you add 50 cm3 of 0.02 mol dm−3 iodine solution. Shake the beaker to mix.
Pipette 10cm3 the mixture and transfer it to a conical flask.
Stop the reaction by adding a spatula of sodium hydrogencarbonate, noting the exact time you add it.
Titrate the remaining iodine present in the sample with 0.01 mol dm−3 sodium thiosulfate(VI) solution, using starch indicator. Record results.
6. Continue withdrawing 10 cm3 samples every 5 mins and treat them similarly. Always note the exact time at which the Na hydrogencarbonate is added.
How can you use a clock reaction to find the order of reaction with respect to iodide ions?
- Measure 10.0 cm3 ( 0.8M) of potassium iodide solution into a small beaker standing on a white tile.
- Add 5.0 cm3 of sodium thiosulfate solution to the potassium iodide solution.
- Add 10 drops of starch solution to the mixture in the small beaker. Starch acts as the indicator and must be used in each experiment.
- Measure out 10.0 cm3 (0.05M)of the sodium peroxodisulfate solution. Pour this into the mixture prepared in steps 1 and 2. Start the stop clock.
- Stop the clock when a blue colour appears in the beaker and note the time taken.
- Make a copy of Table 2 from the ‘Analysis of results’ section. Use this to record your results.
- Repeat steps 1–5 using the volumes of sodium peroxodisulfate and potassium iodide solutions shown in Table 1. The total volume, including the sodium thiosulfate solution, must add up to 25.0 cm3 , which can be achieved by adding the correct volume of distilled/deionised water.
How can you use a clock reaction to find the order of reaction with respect to iodide ions?
Measure 10.0 cm3 of KI solution into a small beaker on a white tile.
Add 5.0 cm3 of Na thiosulfate solution to the KI solution.
Add 10 drops of starch solution to act as the indicator.
Measure out 10.0 cm3 of sodium peroxodisulfate solution. Pour this into the mixture. Start the stop clock.
Stop the clock when a blue colour appears in the beaker and note the time taken.
record your results.
Repeat using diff volumes of sodium peroxodisulfate and potassium iodide solutions shown in Table 1. The total volume, including the sodium thiosulfate solution, must add up to 25.0 cm3 , which can be achieved by adding the correct volume of distilled/deionised water.
