Topic 16 kinetics 2 Flashcards
What is the rate formula?
Rate = Amount of reactant used or product made / time
How can rate be measured in experiments?
Electrical conductivity:
During a reaction, there may be a change in the number of ions. This change will affect the level of electrical conductivity
Amount of mass lost:
For reactions that produce a gas you should place reaction on a balance and measure the mass loss as gas is lost. Fairly accurate method however use a fume cupboard if gas is harmful or toxic.
Volume of gas produced:
Another way of measuring the rate of gas is produced. Measure the amount of gas produced using a gas syringe. Measure this over a specified time.
The change in PH of a reaction:
The PH of a reaction may change over time if H+ ions are used up or produced. A PH meter can be used to measure the PH of a reaction at regular intervals. You can then calculate the H+ ion concentration
Titration:
We can monitor the change in concentration of a reactant or product by taking small samples at regular time intervals and titrating them. When we take the sample, we must slow down the reaction immediately. If we didn’t, the reaction would continue as normal and the conc would change as we try to conduct the titration
To slow down this reaction we can :
1. Dilute with deionized water
2. Cool it down
3. Add a chemical to stop the reaction (quenching agent)
Colour change:
A colorimeter measures the absorbance of light by a colored sample. The more concentrated a sample is, the darker its color and hence the more light absorbed. A common reaction is between propane and iodine
I2 + CH3COCH3 —-> CH3COCH2I + I- + H+
First we have to plot a calibration curve. This is created by making up a range of known different concentrations of iodine. The absorption is measured for each one and the results are plotted.
An experiment like the one above is set away and samples are taken regularly. The samples are tested for absorbance. We then use the calibration graph and absorbance to find out the conc of I2
How can you calculate rate from a graph?
Find gradient
Find gradient of tangent to a point on a curved line of best fit
The rate equation orders
Rate = k[A]^a[B]^b
small letters are orders of the reaction
Big letters are conc of substances
k = rate constant
What are the orders of reaction?
Orders of reaction:
An order is the power to which a concentration is raised to in the rate equation. It tells us how the concentration of the substance affects the rate.
Order 0:
Changes in conc have no effect on rate. If [A] doubles then the rate doesn’t change
1st order:
Changes in conc has a proportional change on rate. If [A] doubles then rate doubles
2nd order:
Changes in conc has a squared proportional change on rate. If [A] doubles then rate quadruples.
Orders can only be determined by experiment, not by just looking at an equation
The rate constant is a number that allows us to equate rate and concentration. It is only fixed at a particular temperature. If the temp changes then so does the rate constant (k increases as temp increases)
The larger the value of k the faster the rate of reaction. This is as we increase temp the particles have more KE and they collide more often. This increases the rate. To make this equation balance, then the value of K must increase. But the conc of substances remains constant
How do you calculate initial rate from a graph?
Initial rate is rate right at the start. we take the gradient of the tangent at 0 mins
The initial rates method
Let’s say we want to work out the rate equation for A+B+C —-> D+E
Repeat the experiment several times but changing the conc of ABC one at a time in each experiment
We need to work out the initial rate for each experiment. We calculate this using the graph.
We record the concentrations of reactants used for each experiment and their initial rates in a table. From this we can work out the orders with respect to each reactant and write a rate equation
Look at paper for examples
Clock experiments
A clock reaction is where you can rime how long it takes for a reaction to occur.
Most reactions like iodine clock can be monitored by sitting the reaction vessel on som epaper with a cross on. We time how long it takes until we can’t see the cross through the beaker. There is a colour change. This is known as the end point.
Essentially, the quicker the clock reaction, the faster the initial rate of reaction is
What 3 assumptions d we make about clock reactions?
1) The temp of the reaction remains constant
2) The conc of reactants doesn’t change significantly during the time period of the reaction
3) Reaction hasn’t proceeded too far when the end point is seen
On the basis of the assumptions we can say the rate of reaction remains constant during the time period you are measuring
As a result, the rate of the clock reaction is a good estimate of the initial rate of reaction
What is the iodine clock reaction (Harcourt - esson)?
H2O2 + 2H+ + 2I- —-> 2H2O + I2
All we do is add sodium thiosulphate and starch (which acts as an indicator) to excess hydrogen peroxide
The sodium thiosulphate reacts immediately with the I2 that is produced in this reaction
2S2O32- + I2 —–> 2I- + S4O62-
When there is no more sodium thiosulphate left, then they I2 reacts with the starch ehich gives a deep blue / black colour.
Varying the conc of I2 and or H2O2 and keeping everything else constant will result in the time taken for the blue/black colour to appear, to change. We can then work out the order of the reaction
Rate concentration graphs
Rate - concentration graphs can help us identify the order
A rate concentration graph is created by knowing the rate. The rate is found by taking the gradient at various points on a conc - time graph.
The rate on a straight line graph is constant. The rate - rate-concentration graph shows a horizontal line. Changing concentration doesn’t change the rate. This is 0 order
The rate on a shallow curve graph changes in equal amounts. The rate-concentration graphs show a straight diagonal line. Changing conc changes the rate equally. This is 1st order
The rate on a steep curve changes in unequal amounts. The rate concentration graph shows a curved line. Changing conc changes the rate squared. This is 2nd order
Look at the paper for graphs
Half life
It is the time it takes for half the reactant to be used up. It can be calculated by using a conc time graph.
0 order means half life decreases
1st order means half life is constant
2nd order means half life increases
Rate determining step
It is the slowest step in a multistep reaction.
Most reactions occur via a few different steps. Out of these steps, on eof them is the rate determining step. The whole reaction depends on how quick this step happens.
In chemical reactions, we have a catalyst or change temp etc to speed up a step
The rate equation
Reactants that appear in the rate equation affect the rate of the reaction
These reactants must appear in the rate-determining step
It’s not just reactants that can appear in the rate equation - catalysts can too.
Substances not in the rate equation won’t be in the rate determination step
The rate equation can be found from a multistep reaction
1) A+B —->2C (fast
2) 2C —–> D (slow)
3) D+ E —–> F+G (fast)
We know the rate-determining step is number 2 as it is the slowest
One of the reactions is C so this must appear in the rate equation. Notice the 2 next to it tells us the power is 2. 2nd order with respect to C
C is an intermediate which is formed from A + B. C is a catalyst as it doesn’t appear in the overall reaction. This reaction is an example of autocatalysis
Rate equation is rate = k[A][B][C]^2
Mechanisms of halogenoalkanes
They can be hydrolysed (split using water) by hydroxide ions. They follow different mechanisms depending if they are primary, secondary or tertiary
