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Chemistry > Topic 16: Kinetics II (chemical kinetics) > Flashcards

Topic 16: Kinetics II (chemical kinetics) Flashcards

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1
Q

Why does the rate of reaction decrease over time?

A

The concentration of reactants decreases

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2
Q

Units for rate of reaction

A

moldm^-3s^-1

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3
Q

How to find the rate of reaction at a particular time

A

Draw a tangent - t=0 for initial rate

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4
Q

Experimental methods to measure reaction rates

A

Colorimetry - change in colour or precipitate formed
Titration - if acids/alkalis are used which can be neutralised or if iodine is used/produced
Loss in mass with balance - gas formed
Volume of gas produced with syringe - gas formed
pH measurements with pH probe - change in pH if acid/alkali is produced/consumed

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5
Q

Issues with measuring gas produced

A

There is gas in syringe - higher value than expected
Bung is not placed quickly enough after adding reactant - lower value than expected
Uncertainty in syringe

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6
Q

Issues with measuring loss of mass

A

Place on open vessel on mass balance
Delay on adding cotton wool - acid escapes - lower value than expected
Loss of mass may be small if mass eg gas is small eg hydrogen
Uncertainty on balance

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7
Q

When does the rate constant (k) not remain constant

A

If temperature is changed or a catalyst is used

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8
Q

What do the orders determine

A

How the concentrations of reactants affects the rate of reactions

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9
Q

What is the effect of zero order on the rate of reactions

A

It has no effect
If A doubles, the rate of reaction doesn’t change
Doesnt appear in rate equations as (A)^0=1

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10
Q

What is the effect of first order on rate of reaction

A

The rate of reaction is proportional to conc. A^1
If A doubles, rate doubles

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11
Q

What is the effect of second order on rate of reaction

A

The rate of reaction is proportional to conc. A^2
If A doubles, rate quadruples
If A were multiplies by 4, reactions would multiply by 4^2 (16)

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12
Q

How can orders of reactions be found

A

By experiment

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13
Q

Units of k

A

Depend on rate equation - have to cancel out

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14
Q

Effect on raising temperature on value of k

A

Increases

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15
Q

Effect of adding a catalyst on value of k

A

Increases

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16
Q

Effect of increasing concentration of value of k

A

No change

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17
Q

Rate equation includes reactants/products?

A

reactants only

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18
Q

Half life

A

The time taken for the concentration of a reactant to half

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19
Q

How to find reaction orders from experiments

A
  1. Continuous monitoring - measure the quantity of a reactant at interval (every 30 secs) during the course of a reaction
  2. Using initial rates - change conc of one reactant at a time: find from graph or clock reaction (how long it takes for a certain amount of product to be made)
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20
Q

Concentration-time graph for first order

A

Straight line of best fit

21
Q

Concentration-time graph for first order

A

Curve - constant half life

22
Q

Concentration-time graph for second order

A

Curve - half life is not constant (increases significantly)

23
Q

Rate-concentration graph for zero order

A

Straight line across - no effect

24
Q

Rate-concentration graph for first order

A

Directly proportional

25
Q

Rate-concentration graph for second order

A

Curve - exponential

26
Q

How to obtain a rate-concentration graph

A

Make tangents of the time-concentration graph

27
Q

How to plot a log graph

A

log(rate) on y axis
log(conc) on x axis
gradient is order
log(rate)= logk + nlog[A]

28
Q

How to write answers to deducing order from experiments

A

Order wrt [A] from experiments 1 and 2
[A] 0.5 –> 1.0 doubles
[B] is unchanged
Rate 0.2 –> 0.4 doubles
Rate (initial) is proportional to [A]
Therefore order wrt [A] is first

29
Q

Initial rate of reaction =

A

1/time when a fixed concentration (C1) is used (same time)

30
Q

Why is initial rate=1/t an approximation

A

The rate could be changing as we can’t calculate rate at t=0 exactly

31
Q

What does the Arhenius equation show

A

The effect of changing temperature/catalyst on the rate constant
Can work out the activation energy from it

32
Q

Arrhenius equation

A

ln(k) = ln(A) - Ea/RT
can use ln(rate)

33
Q

Graph to find activation energy

A

ln(rate) on y axis
1/T in Kelvin on x axis - (often 10^-3)

34
Q

What is the reaction mechanism

A

The sequence of elementary steps that leads to the formation of a product

35
Q

What types of elementary steps are there

A

Unimolecular
Bimolecular
Termolecular

36
Q

What order is unimolecular elementary step

A

A –> product
rate = k[A] first

37
Q

What order is bimolecular elementary step

A

A+A or A+B –> product
rate = k[A]^2 or k[A][B]

38
Q

What order is termolecular elementary step

A

A+A+A or A+A+B –> product
rate = k[A]^3 or k[A]^2[B]

39
Q

The slowest step

A

The Rate Determining Step - determines the overall rate of reaction

40
Q

What species appear in the rate equation

A

The ones in the RDS

41
Q

What is the overall order of an SN1 reaction

A

First as there is only one species in the RDS

42
Q

What is the overall order of an SN2 reaction

A

Second as there are two species in the RDS

43
Q

What are four examples of clock reactions

A

Iodine clock reaction
Bromine clock reaction
Sulphur clock reaction
Reaction of iodide ions with peroxodisulphate (VI) ions

44
Q

Describe the sulphur clock reaction

A

Add a dilute solution of sodium thiosulphate to a beaker with water, making it 25cm in total
Add 25cm of HCl and stir and record the time it takes for enough sulphur to be produced to conceal the X below the beaker
Repeat with different amount of sodium thiosulphate
S2O3^2- + 2H+ –> S(s) + H2O + SO2

45
Q

Describe the iodine clock reaction

A

Oxidation of I- by H2O2 in acid solution
50cm of H2O2 and water (50:50)
Mix with 25cm of KI (aq) and 5cm of sodium thiosulphate
H2O2 + 2I- + 2H+ –> 2H20 + I2
I2 reacts instantaneously with sodium thiosulphate until it us used up
I2 + 2S2O3^2- –> 2I- + S4O6^2-
I- is colourless
When all sodium thiosulphate is used up, I2 reacts with starch indicator to produce a blue/black colour
Repeat with different relative amounts of H2O2 (still 50cm total)

46
Q

Describe the bromine clock reaction

A

Beaker with bromate (V) ions and water and methyl red indicator
Another beaker with Br- ions and a little bit of phenol
Mix beakers and record time for colour change to occur
5Br- + BrO3- + 6H+ –> 3H2O + 3Br2
Br2 reacts instantaneously with phenol
3Br2 + C6H5OH –> C6H2OHBr3 + 3H+ + Br- (colourless)
When phenol is used up, Br2 reacts with methyl red indicator (pink in acidic solution) and bleaches it

47
Q

Describe the reaction of iodide ions with peroxodisulphate (VI) ions

A

S2O8^2- + I- –> 2SO4^2- + I2
I2 + S2O3^2- –> 2I- + S4O6^2-
I2 + starch –> blue/black complex

48
Q

Why does H+ appear in the rate equation for the reaction between iodine and propanone

A

It is a catalyst for the reaction, known as autocatalysis

Chemistry (8 decks)

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