THE EFFECT OF TEMPERATURE ON k

L.O.:

Be able to explain the effect of changes in temperature on the rate constant k.

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.

2H2 (g) + 2NO(g) 2H2O + N2(g)

• Remember the particle theory.

• How do reactions happen?

What would you expect to see if the temperature increases? Why?

• Animation

Find animation (boardworks?)

THE EFFECT OF TEMPERATURE ON k

L.O.:

Be able to explain the effect of changes in temperature on the rate constant k.

Finding the order of a reaction by using rate-concentration graphs.

L.O.:

Interpret rate-concentration graphs

GRAPHICAL DETERMINATION OF RATE GRAPHICAL DETERMINATION OF RATE

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!

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.

GRAPHICAL DETERMINATION OF RATE GRAPHICAL DETERMINATION OF RATE

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 = 1.20 mol dm-3

gradient = - 1.60 mol dm-3

1520 min

rate = - 1.05 x 10-3 mol dm-3

The rate is negative becausethe reaction is slowing down

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.

GRAPHICAL DETERMINATION OF RATE GRAPHICAL DETERMINATION OF RATE

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

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.

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.

FIRST ORDER REACTIONS AND HALF LIFEFIRST 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

The concentration of reactant A falls as the reaction proceeds

The concentration drops from

4 to 2 in 17 minutes

FIRST ORDER REACTIONS AND HALF LIFEFIRST ORDER REACTIONS AND HALF LIFE

The concentration drops from

4 to 2 in 17 minutes2 to 1 in a further 17 minutes

FIRST ORDER REACTIONS AND HALF LIFEFIRST ORDER REACTIONS AND HALF LIFE

The concentration of reactant A falls as the reaction proceeds

The concentration drops from

4 to 2 in 17 minutes2 to 1 in a further 17 minutes1 to 0.5 in a further 17 minutes

FIRST ORDER REACTIONS AND HALF LIFEFIRST ORDER REACTIONS AND HALF LIFE

The concentration of reactant A falls as the reaction proceeds

The concentration drops from

4 to 2 in 17 minutes2 to 1 in a further 17 minutes1 to 0.5 in a further 17 minutes

FIRST ORDER REACTIONS AND HALF LIFEFIRST ORDER REACTIONS AND HALF LIFE

The concentration of reactant A falls as the reaction proceeds

A useful relationship

k t½ = loge 2= 0.693

where t½ = the half life

FIRST ORDER REACTIONS AND HALF LIFEFIRST ORDER REACTIONS AND HALF LIFE

Half life = 17 minutes

k t½ = 0.693

k = 0.693 t½

k = 0.693 = 0.041 min-1 17

ORDER OF REACTION – GRAPHICAL DETERMINATION 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.

ORDER OF REACTION – GRAPHICAL DETERMINATION 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 CONCENTRATIONPLOTTING RATE AGAINST CONCENTRATION

RA

TE

OF

RE

AC

TIO

N /

mo

l dm

-3 s

-1

CONCENTRATION / mol dm-3

ORDER OF REACTION – GRAPHICAL DETERMINATION 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 CONCENTRATIONPLOTTING RATE AGAINST CONCENTRATION

RA

TE

OF

RE

AC

TIO

N /

mo

l dm

-3 s

-1

CONCENTRATION / mol dm-3

ZERO ORDER – the rate does not depend on the concentration. The line is parallel to the x axis.

ORDER OF REACTION – GRAPHICAL DETERMINATION 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 CONCENTRATIONPLOTTING RATE AGAINST CONCENTRATION

RA

TE

OF

RE

AC

TIO

N /

mo

l dm

-3 s

-1

CONCENTRATION / mol dm-3

ZERO ORDER – the rate does not depend on the concentration. The line is parallel to the x axis.

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.

ORDER OF REACTION – GRAPHICAL DETERMINATION 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 CONCENTRATIONPLOTTING RATE AGAINST CONCENTRATION

RA

TE

OF

RE

AC

TIO

N /

mo

l dm

-3 s

-1

CONCENTRATION / mol dm-3

ZERO ORDER – the rate does not depend on the concentration. The line is parallel to the x axis.

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.

SECOND ORDER – the rate is proportional to the square of the concentration. You get an upwardly sloping curve.

ORDER OF REACTION – GRAPHICAL DETERMINATION 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 CONCENTRATIONPLOTTING RATE AGAINST CONCENTRATION

RA

TE

OF

RE

AC

TIO

N /

mo

l dm

-3 s

-1

CONCENTRATION / mol dm-3

ZERO ORDER – the rate does not depend on the concentration. The line is parallel to the x axis.

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.

SECOND ORDER – the rate is proportional to the square of the concentration. You get an upwardly sloping curve.

ORDER OF REACTION – GRAPHICAL DETERMINATION 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 TIMEPLOTTING RATE AGAINST TIME

RA

TE

OF

RE

AC

TIO

N /

mo

l dm

-3 s

-1

TIME / s

ZERO ORDERA straight line showing a constant decline in concentration.

FIRST ORDERA slightly sloping curve which drops with a constant half-life.

SECOND ORDER The curve declines steeply at first then levels out.

ORDER OF REACTIONORDER OF REACTION

GRAPHICAL GRAPHICAL DETERMINATION DETERMINATION

Calculate the rate of reaction at1.0, 0.75, 0.5 and 0.25 mol dm-3

Plot a graph of rate v [A]

Calculate the time it takesfor [A] to go from...1.00 to 0.50 mol dm-3 0.50 to 0.25 mol dm-3

Deduce from the graphthat the order wrt A is 1

Calculate the value andunits of the rate constant, k

Rate graph sorting game

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.

http://blogs.theage.com.au/lifestyle/fashion/Shot5.jpg

Reaction of 1-bromobutane and 2-bromo-2-mentylpropane with alkali.

1)In pairs write both reactions

2)Explain why the first is a first a second order reaction while the second is a first order reaction.

RATE DETERMINING STEPRATE 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.

RATE DETERMINING STEPRATE DETERMINING STEP

THE REACTION BETWEEN PROPANONE AND IODINE

Iodine and propanone CH3COCH3 + I2 CH3COCH2I + HIreact in the presence of acid

The rate equation is... r = k [CH3COCH3] [H+]

Why do H+ ions appear inthe rate equation?

Why does I2 not appear in the rate equation?

RATE DETERMINING STEPRATE DETERMINING STEP

THE REACTION BETWEEN PROPANONE AND IODINE

Iodine and propanone CH3COCH3 + I2 CH3COCH2I + HIreact in the presence of acid

The rate equation is... r = k [CH3COCH3] [H+]

Why do H+ ions appear in The reaction is catalysed by acidthe 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?

RATE DETERMINING STEPRATE DETERMINING STEP

THE REACTION BETWEEN PROPANONE AND IODINE

Iodine and propanone CH3COCH3 + I2 CH3COCH2I + HIreact in the presence of acid

The rate equation is... r = k [CH3COCH3] [H+]

Why do H+ ions appear in The reaction is catalysed by acidthe 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

RATE DETERMINING STEPRATE 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 ORDERThis is because both the RX and OH- mustcollide for a reaction to take place in ONE STEP

RATE DETERMINING STEPRATE 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 ORDERThis is because both the RX and OH- mustcollide 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.

RATE DETERMINING STEPRATE DETERMINING STEP

The reaction H2O2 + 2H3O+ + 2I¯ I2 + 4H2O takes place in 3 steps

Step 1 H2O2 + I¯ IO¯ + H2O SLOW

Step 2 IO¯ + H3O+ HIO + H2O FAST

Step 3 HIO + H3O+ + I¯ I2 + 2H2O FAST

The rate determining step is STEP 1 as it is the slowest

RATE DETERMINING STEPRATE DETERMINING STEP

The reaction H2O2 + 2H3O+ + 2I¯ I2 + 4H2O takes place in 3 steps

Step 1 H2O2 + I¯ IO¯ + H2O SLOW

Step 2 IO¯ + H3O+ HIO + H2O FAST

Step 3 HIO + H3O+ + I¯ I2 + 2H2O FAST

The rate determining step is STEP 1 as it is the slowest

The reaction 2N2O5 4NO2 + O2 takes place in 3 steps

Step 1 N2O5 NO2 + NO3 SLOW

Step 2 NO2 + NO3 NO + NO2 + O2 FAST

Step 3 NO + NO3 2NO2 from another Step 1 FAST

The rate determining step is STEP 1 rate = k [N2O5]