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THE EFFECT OF TEMPERATURE ON k

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Presentation on theme: "THE EFFECT OF TEMPERATURE ON k"— Presentation transcript:

1 THE EFFECT OF TEMPERATURE ON k
L.O.: Be able to explain the effect of changes in temperature on the rate constant k.

2 2H2 (g) + 2NO(g) 2H2O + N2(g) This reaction is second order with respect to NO and first order with respect to H2. Write the rate equation. rate = k[H2][NO]2 What is k? Is it always constant? k is the rate constant for a given reaction at a particular temperature.

3 Remember the particle theory. How do reactions happen?
What would you expect to see if the temperature increases? Why? Insert animation from wiki. Explain Ea

4 Find animation (boardworks?)
Figure 4, page 13. distribution of molecular energies.

5 Figure 4, page 13. distribution of molecular energies.

6 THE EFFECT OF TEMPERATURE ON k
L.O.: Be able to explain the effect of changes in temperature on the rate constant k.

7 Finding the order of a reaction by using rate-concentration graphs.
L.O.: Interpret rate-concentration graphs

8 GRAPHICAL DETERMINATION OF RATE
The variation in rate can be investigated by measuring the change in concentration of one of the reactants or products, plotting a graph and then finding the gradients of the curve at different concentrations. RATE CALCULATION The rate of reaction at any moment can be found from the gradient of the tangent at that point. The steeper the gradient, the faster the rate of reaction Place a rule on the outside of the curve and draw a line as shown on the graph. y x gradient = y / x In the reaction… A(aq) + B(aq) ——> C(aq) + D(aq) the concentration of B was measured every 200 minutes. The reaction is obviously very slow!

9 GRAPHICAL DETERMINATION OF RATE
The variation in rate can be investigated by measuring the change in concentration of one reactants or product, plotting a graph and then finding the gradients of tangents to the curve at different concentrations. RATE CALCULATION The rate of reaction at any moment can be found from the gradient of the tangent at that point. The steeper the gradient, the faster the rate of reaction Place a rule on the outside of the curve and draw a line as shown on the graph. y x gradient = y / x concentration = mol dm-3 gradient = mol dm-3 1520 min rate = x 10-3 mol dm-3 The rate is negative because the reaction is slowing down

10 GRAPHICAL DETERMINATION OF RATE
The variation in rate can be investigated by measuring the change in concentration of one of the reactants or products, plotting a graph and then finding the gradients of the curve at different concentrations. The gradients of tangents at several other concentrations are calculated. Notice how the gradient gets less as the reaction proceeds, showing that the reaction is slowing down. The tangent at the start of the reaction is used to calculate the initial rate of the reaction. RATE CALCULATION The rate of reaction at any moment can be found from the gradient of the tangent at that point. The steeper the gradient, the faster the rate of reaction Place a rule on the outside of the curve and draw a line as shown on the graph. y x gradient = y / x

11 FIRST ORDER REACTIONS AND HALF LIFE
One characteristic of a FIRST ORDER REACTION is that it is similar to radioactive decay. It has a half-life that is independent of the concentration. It should take the same time to drop to one half of the original concentration as it does to drop from one half to one quarter of the original. The concentration of a reactant falls as the reaction proceeds

12 FIRST ORDER REACTIONS AND HALF LIFE
The concentration of reactant A falls as the reaction proceeds The concentration drops from 4 to 2 in 17 minutes

13 FIRST ORDER REACTIONS AND HALF LIFE
The concentration of reactant A falls as the reaction proceeds The concentration drops from 4 to 2 in 17 minutes 2 to 1 in a further 17 minutes

14 FIRST ORDER REACTIONS AND HALF LIFE
The concentration of reactant A falls as the reaction proceeds The concentration drops from 4 to 2 in 17 minutes 2 to 1 in a further 17 minutes 1 to 0.5 in a further 17 minutes

15 FIRST ORDER REACTIONS AND HALF LIFE
The concentration of reactant A falls as the reaction proceeds The concentration drops from 4 to 2 in 17 minutes 2 to 1 in a further 17 minutes 1 to 0.5 in a further 17 minutes

16 FIRST ORDER REACTIONS AND HALF LIFE
A useful relationship k t½ = loge 2 = where t½ = the half life Half life = 17 minutes k t½ = k = k = = min-1 17

17 ORDER OF REACTION – GRAPHICAL DETERMINATION
The order of reaction can be found by measuring the rate at different times during the reaction and plotting the rate against either concentration or time. The shape of the curve provides an indication of the order.

18 ORDER OF REACTION – GRAPHICAL DETERMINATION
The order of reaction can be found by measuring the rate at different times during the reaction and plotting the rate against either concentration or time. The shape of the curve provides an indication of the order. PLOTTING RATE AGAINST CONCENTRATION RATE OF REACTION / mol dm-3 s-1 CONCENTRATION / mol dm-3

19 ORDER OF REACTION – GRAPHICAL DETERMINATION
The order of reaction can be found by measuring the rate at different times during the reaction and plotting the rate against either concentration or time. The shape of the curve provides an indication of the order. PLOTTING RATE AGAINST CONCENTRATION ZERO ORDER – the rate does not depend on the concentration. The line is parallel to the x axis. RATE OF REACTION / mol dm-3 s-1 CONCENTRATION / mol dm-3

20 ORDER OF REACTION – GRAPHICAL DETERMINATION
The order of reaction can be found by measuring the rate at different times during the reaction and plotting the rate against either concentration or time. The shape of the curve provides an indication of the order. PLOTTING RATE AGAINST CONCENTRATION ZERO ORDER – the rate does not depend on the concentration. The line is parallel to the x axis. RATE OF REACTION / mol dm-3 s-1 FIRST ORDER – the rate is proportional to the concentration so you get a straight line of fixed gradient. The gradient of the line equals the rate constant for the reaction. CONCENTRATION / mol dm-3

21 ORDER OF REACTION – GRAPHICAL DETERMINATION
The order of reaction can be found by measuring the rate at different times during the reaction and plotting the rate against either concentration or time. The shape of the curve provides an indication of the order. PLOTTING RATE AGAINST CONCENTRATION SECOND ORDER – the rate is proportional to the square of the concentration. You get an upwardly sloping curve. ZERO ORDER – the rate does not depend on the concentration. The line is parallel to the x axis. RATE OF REACTION / mol dm-3 s-1 FIRST ORDER – the rate is proportional to the concentration so you get a straight line of fixed gradient. The gradient of the line equals the rate constant for the reaction. CONCENTRATION / mol dm-3

22 ORDER OF REACTION – GRAPHICAL DETERMINATION
The order of reaction can be found by measuring the rate at different times during the reaction and plotting the rate against either concentration or time. The shape of the curve provides an indication of the order. PLOTTING RATE AGAINST CONCENTRATION SECOND ORDER – the rate is proportional to the square of the concentration. You get an upwardly sloping curve. ZERO ORDER – the rate does not depend on the concentration. The line is parallel to the x axis. RATE OF REACTION / mol dm-3 s-1 FIRST ORDER – the rate is proportional to the concentration so you get a straight line of fixed gradient. The gradient of the line equals the rate constant for the reaction. CONCENTRATION / mol dm-3

23 ORDER OF REACTION – GRAPHICAL DETERMINATION
The order of reaction can be found by measuring the rate at different times during the reaction and plotting the rate against either concentration or time. The shape of the curve provides an indication of the order. PLOTTING RATE AGAINST TIME ZERO ORDER A straight line showing a constant decline in concentration. FIRST ORDER A slightly sloping curve which drops with a constant half-life. RATE OF REACTION / mol dm-3 s-1 SECOND ORDER The curve declines steeply at first then levels out. Leave out? TIME / s

24 GRAPHICAL DETERMINATION
ORDER OF REACTION GRAPHICAL DETERMINATION Calculate the rate of reaction at 1.0, 0.75, 0.5 and 0.25 mol dm-3 Plot a graph of rate v [A] Calculate the time it takes for [A] to go from... 1.00 to 0.50 mol dm-3 0.50 to 0.25 mol dm-3 Deduce from the graph that the order wrt A is 1 Calculate the value and units of the rate constant, k

25 Rate graph sorting game
sort out graphs (look at laura’s folder)

26

27 The rate-determining step
L.O. Explain what the rate-determining step of a reaction. Explain the connection between the rate equation for a reaction and the reaction mechanism.

28

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30 Reaction of 1-bromobutane and 2-bromo-2-mentylpropane with alkali.
In pairs write both reactions Explain why the first is a first a second order reaction while the second is a first order reaction.

31 Many reactions consist of a series of separate stages.
RATE DETERMINING STEP Many reactions consist of a series of separate stages. Each step has its own rate and rate constant. The overall rate of a multi-step process is governed by the slowest step (like a production line where overall output can be held up by a slow worker). This step is known as the RATE DETERMINING STEP. If there is more than one step, the rate equation may not contain all the reactants in its format.

32 THE REACTION BETWEEN PROPANONE AND IODINE
RATE DETERMINING STEP THE REACTION BETWEEN PROPANONE AND IODINE Iodine and propanone CH3COCH I CH3COCH2I HI react in the presence of acid The rate equation is... r = k [CH3COCH3] [H+] Why do H+ ions appear in the rate equation? Why does I2 not appear in the rate equation?

33 THE REACTION BETWEEN PROPANONE AND IODINE
RATE DETERMINING STEP THE REACTION BETWEEN PROPANONE AND IODINE Iodine and propanone CH3COCH I CH3COCH2I HI react in the presence of acid The rate equation is... r = k [CH3COCH3] [H+] Why do H+ ions appear in The reaction is catalysed by acid the rate equation? [H+] affects the rate but is unchanged overall Why does I2 not appear The rate determining step doesn’t involve I2 in the rate equation?

34 THE REACTION BETWEEN PROPANONE AND IODINE
RATE DETERMINING STEP THE REACTION BETWEEN PROPANONE AND IODINE Iodine and propanone CH3COCH I CH3COCH2I HI react in the presence of acid The rate equation is... r = k [CH3COCH3] [H+] Why do H+ ions appear in The reaction is catalysed by acid the rate equation? [H+] affects the rate but is unchanged overall Why does I2 not appear The rate determining step doesn’t involve I2 in the rate equation? The slowest step of any multi-step reaction is known as the rate determining step and it is the species involved in this step that are found in the overall rate equation. Catalysts appear in the rate equation because they affect the rate but they do not appear in the stoichiometric equation because they remain chemically unchanged

35 RATE DETERMINING STEP HYDROLYSIS OF HALOALKANES
Haloalkanes (general formula RX) are RX OH ROH X- hydrolysed by hydroxide ion to give alcohols. With many haloalkanes the rate equation is... r = k [RX][OH-] SECOND ORDER This is because both the RX and OH- must collide for a reaction to take place in ONE STEP

36 RATE DETERMINING STEP HYDROLYSIS OF HALOALKANES
Haloalkanes (general formula RX) are RX OH ROH X- hydrolysed by hydroxide ion to give alcohols. With many haloalkanes the rate equation is... r = k [RX][OH-] SECOND ORDER This is because both the RX and OH- must collide for a reaction to take place in ONE STEP but with others it only depends on [RX] r = k [RX] FIRST ORDER The reaction has taken place in TWO STEPS... - the first involves breaking an R-X bond i) RX R X- Slow - the second step involves the two ions joining ii) R OH ROH Fast The first step is slower as it involves bond breaking and energy has to be put in. The first order mechanism is favoured by tertiary haloalkanes because the hydroxide ion is hindered in its approach by alkyl groups if the mechanism involves the hydroxide ion and haloalkane colliding.

37 RATE DETERMINING STEP The reaction H2O H3O I¯ I H2O takes place in 3 steps Step 1 H2O I¯ IO¯ H2O SLOW Step 2 IO¯ H3O HIO H2O FAST Step 3 HIO H3O I¯ I H2O FAST The rate determining step is STEP 1 as it is the slowest

38 RATE DETERMINING STEP The reaction H2O H3O I¯ I H2O takes place in 3 steps Step 1 H2O I¯ IO¯ H2O SLOW Step 2 IO¯ H3O HIO H2O FAST Step 3 HIO H3O I¯ I H2O FAST The rate determining step is STEP 1 as it is the slowest The reaction N2O NO O2 takes place in 3 steps Step 1 N2O NO NO3 SLOW Step 2 NO NO NO + NO O2 FAST Step 3 NO NO NO2 from another Step 1 FAST The rate determining step is STEP rate = k [N2O5]


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