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A2 Chemistry > Chapter 18 > Flashcards

Chapter 18 Flashcards

1
Q

How do you calculate rate of reaction?

A

quantity reacted/produced
rate= —————————————
time

                        or 

       change in concentration  rate= --------------------------------------
                     Time
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2
Q

What is [A] short for?

A

Concentration of A

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3
Q

Order of reaction

A

For each reactant, the power is the order of reaction for that reaction
In a reaction, different reactants can have different orders and each may affect the rate in different ways
Common orders are zero order (0), first order(1) and second order (2)

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4
Q

What is a zero order reactant?

A

When the concentration of a reactant has no effect on the rate, the reaction is zero order with respect to the reactant
rate ∝ [A]^0
- Any number raised to the power of 0 is 1
- Concentration does not influence rate

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5
Q

What is a first order reactant?

A

A reaction is first order with respect to a reactant when the rate depends on its concentration raised to the power of one:
rate ∝ [A^]1
- If the concentration of A is doubled (x2), the reaction rate increases by a factor of 2^1=2
If the r=concentration of A is tripled (x3), the reactant rate increases by a factor of 3^1=3

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6
Q

What is a second order reaction?

A

A reaction is second order with respect to a reactant when the rate depends on its concentration raised to the power of two:
second order: rate ∝ [A]^2
- If the concentration of A is doubled (x2), the reaction rate increases by a factor of 2^2=4
- If the concentration rate is tripled (x3), the reaction rate increases by a factor of 3^2=9

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7
Q

What is a rate equation?

A

Gives the mathematical relationship between the concentrations of the reactant and the reaction rate

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8
Q

What is the rate constant (k)?

A

The proportionality constant. It is the number that mathematically converts between the rate of reaction and concentration and orders

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9
Q

define overall order

How is it calculated?

A

Gives the overall effect of concentrations of all reactants on the rate of reaction
Overall order= sum of orders with respect to each reactant

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10
Q

Orders from experimental results

Can orders be found directly from the chemical reaction?

A

Orders of reaction must be determined experimentally by monitoring how a physical quantity changes over time. Orders cannot be found directly from the chemical equation

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11
Q

Why is the initial rate mostly used?

A

The initial rate is the instantaneous rate at the beginning of an experiment when t=0
This is useful as all the reactions are of known concentrations at this point

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12
Q

Continuous monitoring of rate

A
  • Concentration -time graphs can be plotted from continuous measurements taken during the course of a reaction. This is called continuous monitoring and it can be done by:
  • monitoring be gas collection
  • monitoring be mass loss
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13
Q

How can rate be measure~with a calorimeter?

A
  • In a calorimeter, the wavelength of the light passing through a coloured solution is controlled using a filter.
  • A filter is chosen so that it is complementary in colour to the colour being absorbed in the reaction.
  • Absorbance is recorded with is proportional to the concentration of the solution
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14
Q

Concentration-time graph~ orders from shapes:

Zero order

A
  • A zero order reaction produces a straight line line with a negative gradient
  • The reaction rate does not change at all during the course of the reaction
  • The value of the gradient is equal to the rate constant k
  • The straight-line graph makes a zero order easy to identify
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15
Q

Concentration-time graph~ orders from shapes:

First order

A
  • A first order reaction produces a downward curve with a decreasing gradient over time
  • As the gradient decreases with time, the reaction gradually slows down
  • In a first order concentration-time graph, the time take for the reactant to halve is constant
  • This is the half life and the rate constant of a first order reaction can be determined using its value
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16
Q

Concentration-time graph~ orders from shapes:

Second order

A
  • The graph for a second order is also downward curve, steeper at the start but tailing off more slowly
17
Q

What are ‘half lives’?

How can a first order relationship be confirmed using it?

A
  • Half life is the time taken for half of a reactant to be used up
  • First order relationships have a constant half life with the concentration halving every half life
  • This pattern is called exponential decay
  • A first order relationship can be confirmed from a concentration-time graph by measuring successive half lives
  • If they are the same, the reaction is first order with respect to the reactant
18
Q

What are the two ways that k can be determined for a first order reaction ?

A
  1. Draw a tangent to the curve on the concentration-time graph at a particular concentration. The gradient of the tangent is calculated giving the rate of the reaction. The rate constant is calculated by rearranging the rate equation
  2. Use the exponential relationship for a constant half life:
    ln2
    k =——-
    t1/2
19
Q

Rate-concentration graphs~ orders from shapes:

Zero order

A

A zero order reactant produces a horizontal straight line with zero gradient

  • The intercept on the y-axis gives the rate constant k
  • The reaction rate does not change with increasing concentration
20
Q

Rate-concentration graphs~ orders from shapes:

First order

A

A first order reactant produces a straight-line graph with a positive gradient that goes through the origin

  • Rate is directly proportional to concentration for a fist order relationship
  • The rate constant= gradient of line
21
Q

Rate-concentration graphs~ orders from shapes:

Second order

A

A second order reactant produces an upward curve with increasing gradient
- Rate cannot be obtained directly from graph
- Plot a second graph of rate against concentration squared, the result is a straight line through the origin
gradient= rate constant

22
Q

briefly describe the initial rates method

A

The initial rate is the instantaneous rate at the start of the reaction t=0. The initial rate can be found by measuring the gradient of a tangent drawn t=0 on a concentration-time graph

23
Q

briefly describe a clock reaction

A

A clock reaction is a more convenient way of obtaining the initial rate of reaction by taking a single measurement

  • The time t from the start of an experiment is measured for a visual change to be observed, often a colour change or precipitate
  • Provided that there is no significant change in rate during this time, it can be assumed that the average rate of reaction over this time will be the same as the initial rate
  • The initial rate is then proportional to 1/t
  • The clock reaction is then repeated several times with different concentrations and values of 1/t are calculated for each experimental run
24
Q

Explain iodine clocks

A
  • A common type of clock reaction relies on the formation of iodine
  • As aqueous iodine is coloured orange-brown, the time from the start of the reaction and the appearance of the iodine colour can be measured
  • Starch is usually added since it forms a complex with iodine which is an intense blue-black colour
25
Q

How accurate are clock reactions?

A
  • In a clock reaction you are measuring the average rate during the first part of the reaction.
  • Over this time,you can assume that the average rate of reaction is constant and is the same as the initial rate
  • In a clock reaction, you are measuring an average rate of change in a reactant over time.
  • The shorter the period of time over which an average rate is measured, the less the rate changes over that time period
  • The initial rate measured during a clock reaction is an approximation but it is still reasonably accurate provided that less than 15% of the reaction has taken place
26
Q

briefly describe multi-step reactions

What is a reaction mechanism?

A
  • An overall chemical equation compares the reactants and products. The balancing numbers give the stoichiometry, the relative amounts of the species in the reaction
  • Many reactions take place in a series of steps and it is unlikely that more than two particles will collide together at the same time
  • The series of steps that make up an overall reaction is called the reaction mechanism
27
Q

what is the ‘rate determining step’?

A

The steps in a multi-step reaction will take place at different rates. The slowest step is the rate demining step

28
Q

How do you know whether a reaction is likely to be correct?

A
  • The rate equation only includes reacting species involved in the rate-determining step
  • The orders in the rate equation match the number of species involved In the rate determining step
29
Q

What is the effect of temperature on the rate constants?

A

As temperature increases, factors contribute to the increased rate and rate constant

  • Increasing the temperature shifts the Boltzmann distribution to the right, increasing the proportion of particles that exceed the activation energy, Ea
  • As the temperature increases, particles move faster and collide more frequently
30
Q

What is the Arrhenius equation?

A

It is an exponential relationship between the rate constant and the temperature T

k=Ae-^(-Ea/RT)
Where
k is the rate constant
T is the absolute temperature (in kelvins)
A is the pre-exponential factor, a constant for each chemical reaction that defines the rate due to frequency of collisions in the correct orientation
Ea is the activation energy for the reaction (in Joules)
kB is the Boltzmann constant

31
Q

What is the exponential factor (-Ea/R*T)?

What is the pre-exponential factor? (A)

A

It represents the proportion of molecules that exceed Ea and have sufficient energy for a reaction to take place

  • A takes into account the frequency of collisions with the correct orientation
  • The frequency factor essentially gives the rate if there was no activation energy
32
Q

What is the logarithmic form of the Arrhenius equation?

Why Is it useful?

A

Ea
ln k= ——– + ln A
RT

It is very useful as it enables Ea and A to be determined graphically. A plot of ln k against 1/T gives a straight line graph of the type y=mx + c

  • gradient m of -Ea/R
  • intercept c of ln A

A2 Chemistry (1 decks)

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