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Chemistry Module 5 Physical Chemistry And Transition Elements > Chapter 18 Rates Of Reaction > Flashcards

Chapter 18 Rates Of Reaction Flashcards

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1
Q

What is the equation for the rate of reaction?

A

Rate= change in concentration / time

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2
Q

What is the shorthand representation of concentration?

A

Square brackets enclosing the reagent
Eg, [A] means the concentration of chemical A

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3
Q

How does the order of reaction affect rate?

A

The rate of reaction is proportional to the concentration of a particular reactant raised to a power.
Rate ∝[A]^n

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4
Q

What is zero order?

A

When changing the concentration of a reactant has no affect on the rate of reaction
Rate ∝ [A]^0

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5
Q

What is first order?

A

When concentration of a reactant doubles, rate increases by a factor of x 2
Rate ∝ [A]^1

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6
Q

What is second order?

A

The rate is proportional to the concentration of a reactant to the power of 2
Rate ∝[A]^2

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7
Q

What is the rate equation?

A

Rate= k[A]^m [B]^n
K= rate constant
[A]= concentration of A
[B]= concentration of B
M= order of reaction with respect to A
N= order of reaction with respect to B

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8
Q

What is the equation for rate constant?

A

K= rate / [A] [B]

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9
Q

What is overall order?

A

The overall effect of the concentrations of all reactants have on the rate of reaction

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10
Q

How is the overall order calculated?

A

Sum of all order with respect to each reactant

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11
Q

What is continuous monitoring?

A

Taking continuous measurement of data during the course of the reaction

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12
Q

What type of graph can be formed by using continuous monitoring?

A

Concentration- time graphs

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13
Q

Describe the different method of continuous monitoring to create a concentration time graph

A

-monitoring gas volume
-monitoring mass loss
-monitoring colour change using a colorimeter which measures the amount of light absorbed by a solution

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14
Q

What is the shape of a zero order concentration- time graph?

A

Straight line with negative gradient
(Rate of reaction unchanged)

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15
Q

How is k calculated from a zero order concentration time graph?

A

K= gradient of straight line

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16
Q

Describe the shape of a first order concentration time graph

A

Downward curve with decreasing gradient over time
(Rate of reaction gradually slows down)

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17
Q

How can k be calculated from a first order concentration time graph?

A

K calculated from half life
K= ln2 / t1/2

K calculated from rate and substituted into equation
-Tangent drawn at particular concentration
-gradient calculated = rate of reaction
-rate equation rearranged and concentration at position of the tangent and rate is substituted into equation

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18
Q

Describe the shape of a second order concentration time graph

A

Downward curve which is steeper at the start and then tails off more slowly

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19
Q

What is half life?

A

The time taken for the concentration of a reactant to halve its original value

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20
Q

Describe how a first order relationship can be determined from a concentration time graph

A

If successive half lives are constant then the reaction is first order with respect to the reactant.

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21
Q

What are the units for rate?

A

Moldm^-3s-1

22
Q

How does the rate of reaction change when the concentration is multiplied by 1/2 for zero order?

A

1/2^0 = x 1

23
Q

How does the rate of reaction change when the concentration is multiplied by 1/2 for first order?

A

1/2^1 = x 1/2

24
Q

How does the rate of reaction change when the concentration is multiplied by 1/2 for second order?

A

1/2^2 = x 1/4

25
Q

Describe the shape of a zero order rate concentration graph

A

Horizontal straight line with zero gradient
(Rate unchanged)

26
Q

How is k calculated from a zero order rate-concentration graph?

A

K= the intercept on y axis

27
Q

Describe the shape of a first order rate concentration graph

A

Straight line through the origin

28
Q

How is k calculated from a first order rate concentration graph?

A

K= gradient of straight line

29
Q

Describe the shape of a second order rate concentration graph

A

Upward curve with increasing gradient

30
Q

How is k calculated from a second order rate concentration graph?

A

The graph is a curve so k cannot be directly obtained.
A second graph needs plotting with rate against concentration squared resulting in a straight line through origin.
K= gradient of new line

31
Q

What is initial rate?

A

The instantaneous rate at the beginning of an experiment

32
Q

How can the initial rate of a reaction be measured?

A

Drawing a tangent when t=0 and measuring gradient

Clock reaction- changing the concentration of a reactant while keeping the concentration of other reactants constant.
At each concentration the time taken from the beginning of an experiment is measured until a visual change is observed.
Initial rate is proportional to 1/t
Values of 1/t are calculated for each time at different concentrations.

33
Q

What is the Rate determining step?

A

The slowest step in a multi step mechanism reaction

34
Q

What equation is used to identify the rate determining step?

A

Rate equation

35
Q

What does the rate equation help to identify in the rate determining step?

A

The reactants involved in the RDS
The number of each reactant involved from the order

36
Q

When predicting the two step mechanisms what conditions needs to be satisfied?

A

The RDS and second step add up to overall equation
The RDS is consistent with the rate equation
Any charged must balance out on reactants and products

37
Q

Why does rate constant increase when temperature increases?

A

Increasing temperature shifts the Boltzmann distribution curve to the right, increasing the proportion of particles that exceed activation energy
Particles have more kinetic energy so more effective collisions occur

38
Q

Determine the components of the Arrhenius equation
K=Ae^-Ea/RT

A

K= rate constant (same units as A)
A= pre exponential factor (same units as k)
-Ea= activation energy in Jmol^-1
May need to change into KJmol^-1
R= gas constant 8.31
T= temperature in Kelvin

39
Q

What does the Arrhenius equation predict?

A

How rate constant changes with activation energy or temperature

40
Q

Using the Arrhenius equation, how does k change when activation energy increases?

A

Increased activation energy= lower k

41
Q

Using the Arrhenius equation, how does k change when activation energy decreases?

A

Decreasing activation energy= higher k

42
Q

Using the Arrhenius equation, how does k change when temperature increases?

A

Increasing temperature = higher k

43
Q

Using the Arrhenius equation, how does k change when temperature decreases?

A

Decreasing temperature = lower k

44
Q

Using the Arrhenius equation, how does k change when the pre exponential factor increases?

A

Increasing A= higher k

45
Q

Using the Arrhenius equation, how does k change when the pre exponential factor decreases?

A

Decreasing A= lower k

46
Q

What is the intermediate of a two step mechanism?

A

A product formed in one of the steps but is used up again later

47
Q

Explain why it is unlikely for a reaction to proceed by one step only

A

An effective collision is unlikely to occur between more than two species

48
Q

What is the logarithmic form of the Arrhenius equation used to predict?
ln k = -Ea/ RT +lnA

A

The activation energy and pre exponential factor can be determined graphically
ln k = -Ea/ R x 1/ T +lnA
Y = M X + C
Gradient = -Ea/ R
-Ea= gradient x m

lnA- pick a particular concentration point and isolate Ln A by substituting in the value of ln k, -Ea/ R and 1/ T

49
Q

Describe how the activation energy and frequency factor can be found graphically

A

A graph is first plotted of ln k against 1/t
Activation energy can then be calculated by measuring the gradient of the line
Ln A is isolated by substituting in the value of ln k, -Ea/ R and 1/ T
A is measured independently by doing the inverse of ln= e on calculator

50
Q

Describe the shape of the logarithmic Arrhenius equation graph?

A

LnA against 1/T
Straight line with negative gradient

51
Q

What is the equation for half life?

A

T 1/2 = ln2 / k

Chemistry Module 5 Physical Chemistry And Transition Elements (3 decks)

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