Chapter 18 Flashcards
How do you calculate rate of reaction?
quantity reacted/produced
rate= —————————————
time
or
change in concentration rate= --------------------------------------
Time
What is [A] short for?
Concentration of A
Order of reaction
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)
What is a zero order reactant?
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
What is a first order reactant?
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
What is a second order reaction?
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
What is a rate equation?
Gives the mathematical relationship between the concentrations of the reactant and the reaction rate
What is the rate constant (k)?
The proportionality constant. It is the number that mathematically converts between the rate of reaction and concentration and orders
define overall order
How is it calculated?
Gives the overall effect of concentrations of all reactants on the rate of reaction
Overall order= sum of orders with respect to each reactant
Orders from experimental results
Can orders be found directly from the chemical reaction?
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
Why is the initial rate mostly used?
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
Continuous monitoring of rate
- 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
How can rate be measure~with a calorimeter?
- 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
Concentration-time graph~ orders from shapes:
Zero order
- 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
Concentration-time graph~ orders from shapes:
First order
- 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
Concentration-time graph~ orders from shapes:
Second order
- The graph for a second order is also downward curve, steeper at the start but tailing off more slowly
What are ‘half lives’?
How can a first order relationship be confirmed using it?
- 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
What are the two ways that k can be determined for a first order reaction ?
- 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
- Use the exponential relationship for a constant half life:
ln2
k =——-
t1/2
Rate-concentration graphs~ orders from shapes:
Zero order
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
Rate-concentration graphs~ orders from shapes:
First order
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
Rate-concentration graphs~ orders from shapes:
Second order
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
briefly describe the initial rates method
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
briefly describe a clock reaction
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
Explain iodine clocks
- 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
