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Chemistry module 5 > rates of reaction > Flashcards

rates of reaction Flashcards

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1
Q

How are reaction rates measured

A
  • by observing the changes in the quantaties of reactants or products over time
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2
Q

What are the equations for rate of reaction

A
  • quantity reacted or produced/time
  • change in concentration/time
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3
Q

what is the unit for rates

A

moldm-3s-1

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4
Q

what does the power represent for each reactant

A

order of reaction

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5
Q

what are the common orders

A
  • zero
  • first
  • second
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6
Q

What is zero order

A

when the concentration of a reactant has no effect on the rate

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7
Q

what happens in a zero order reaction

A
  • any number raised to the power zero is 1
  • concentration does not influence the rate
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8
Q

what is the first order

A

a reaction is first order respect to a reactant when the rate depends o its concentration when raised to the power of 1

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9
Q

What is a first order reaction

A
  • if concentration is doubled the reaction rate increases by a factor of 2
  • if concentration is triples the reaction rate increases by a factor of 3
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10
Q

What is second order

A

a reaction is second order with respect to a reactant when the rate depends on its concentration raised to the power of 2

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11
Q

What happends in a second order reaction

A
  • if concentration is doubles the reaction rate increases by a factor of 4
  • if concentration is triples the reaction rate increases by a factor of 9
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12
Q

What is the rate equation

A

the rate equation fives the mathematical relationship between the concentrations of the reactants and the reaction rate

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13
Q

What is the rate constant k

A
  • is the proportionality constant it is the umner that mathematically converts the rate of reaction and concentration and orders
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14
Q

What is the overall order

A
  • the overall order of a reaction gives the overall effect of the concentrations of all reactats on the rate of reaction
  • sum of orders with respect to each reactant
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15
Q

How do you solve the units of the rate constant k

A
  • rearranging the equation to make k the subject
  • substitute units into the expression for k
  • cancel common unit and show the final units on a single line
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16
Q

What is the initial rate

A

the initial rate is the instantaneous rate at the beginning of an experiment t=0

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17
Q

How can concentration time graph be plotted

A
  • from continuous measurement taken during the ccourse of a reaction - continuous monitoring
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18
Q

How can you monitor reactions that produce a gas

A
  • monitoring by gas collection
  • monitoring by mass loss
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19
Q

How can you monitor a reaction that does not produce gas and how

A
  • colour change
  • by eye or monitored with a colorimeter
20
Q

How is a colorimeter used to detect rate of reaction

A
  • measures the wavelengthh of the light passing through a coloured solution is using a filter. the amount of light is absorbed by a solution is measured
21
Q

How can you tell the rate of reaction from a concentration time graph

A

measure the gradient

22
Q

Describe the concetration time graph for zero order

A
  • produces a straight line with a negative gradient
  • reaction rate does not change at all during the course of the reaction
  • value of the gradient is equal to the rate constant k
23
Q

Describe the concentration time graph for first order

A
  • a first order reaction produces a downward curve with a decreasing gradient over time
  • as gradient decreases with time the reactopm gradually slows down
  • the time or the concentration of the reactant to halve is constant
  • this is called half life
  • rate constant can be determined using it value
24
Q

Describe the concentration time graph for second order

A
  • downward curve
  • steeper at the start
  • tailing off more slowly
25
Q

What is half life

A

time taken for half of a reactant to be used up

26
Q

What is the first order concentration time graph pattern referred to

A

exponential decay

27
Q

how are rate concentration graphs plotted

A
  • measurements of the rate of reaction at different concentrations
28
Q

Why are rate concentration graphs important

A
  • offer a route into direct link between rate and concentration in the rate reaction
29
Q

describe the rate concentration graph for zero order

A
  • produces a horizontal straight line with zero gradient
  • intercept on y axis gives the rate constant k
  • reaction rate does not change with increasing concentration
30
Q

describe the rate concentration graphs for first order

A
  • reaction produces a straight line graph through the origin
  • rate is directly proportional to concentration for first order relationship the rate constant can be determined by measuring the gradient of the straight line of this graph
31
Q

describe the rate concentration graphs of second order

A
  • second order reactant produce an upward curve with increasing gradient
  • as this rate concentration graph is a curve the rate constant cannot be obtained directly from this graph
  • by plotting second graph of rate against concentration squared result is a straight line through the origin
  • gradient of this straight line graph is equal to the rate constant k
32
Q

what is a clock reaction

A
  • a convenient way of obtaining the intial rate of reaction by taking a single measurement
  • time t from the start of an experiment is measured for a visual change to be observed often a colour or precipitate
33
Q

What is the initial rate proportional to

A

1/T

34
Q

What are iodine clocks

A
  • relies on formation of iodine
  • as aqueous iodine is coloured orange brown the time from the start of the reaction and appearance of iodine coloured can be measured
  • starch is usually added since it forms a complex with iodine which is an intense blue black colour
35
Q

How accurate are clock reactions

A
  • in clock reactions you are measuring the average rate during the first past of the reaction
  • over this time you can assume that the average rate of reaction is constant and is the same a the initial rate
  • you are measuring an average rate of a change in reactant over time
  • the shorter the period time over which an average rate is measured the less the rate changes over that period of time
  • initial rate measured during clock reaction is approximation but it is sill reasonably accurate
36
Q

Describe multi step reactions give an example

A
  • a reaction can only take place when particles collide
  • for example one molecule of H2O2 and two I- ions and two H+ ions would have to collide togetherr simultaneously which is an unlikely event
  • so it is more likely to take place in a series of steps and it is unlikely more than two particles will collide together at the same time
37
Q

What is a reaction mechanism

A

the series of steps tthat make up an overall reaction

38
Q

What is the rate determining step

A
  • slowest step in the sequence
39
Q

How do we know wether a reaction mecahnism is likely to be correct?

A
  • the rate equation only includes reacting species involved in the rate determining step
  • the orrders in the rate equation match the number of species involved in the rate determining step
40
Q

Describe the hydrolysis of haloalkane and what it is used to show

A
  • they are hydrolysed by hot aqueous alkali
  • RBr+Oh–>ROH+Br-
  • hydrolysis reactions of haloalkanes can be investigated experimentally to determine the overall order of reaction adn the rate equation as well as a possible mechanism for the reaction
41
Q

Describe what happens to the rate constant when temperature increases

A
  • as temperature increases, the rate increases and the value of the rate constant k will also increase
  • as 10 degrees celcius rise the rate constant will double as well as the rate of reaction
42
Q

What two factors affect the rate constant when temperature is increases

A
  • increasing temperature shifts the boltzmann distribution to the right increasing amount of particles that exceed activation energy
  • temperature increases particles move faster and collide more frequently
43
Q

What is the exponential form of the arrhenious equation

A

k=Ae^Ea/RT

44
Q

What is the logarithmic form of the arrhenious equation

A

lnk=-Ea/Rx1/T+ln A
y=mx+c

45
Q

How to calculate the activation energy from a graph

A
  • calculate the gradient
  • Ea=- Rx gradient

Chemistry module 5 (2 decks)

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