Module 5: Chapter 18 (Reaction Rates) Flashcards
What affects the rate of a chemical reaction?
concentration / pressure
temperature
surface area of a solid
use of a catalyst
Explain collision theory
only effective collisions lead to chemical reactions:
- particles must collide with the correct orientation
- the particles have sufficient energy to overcome the activation energy Ea barrier of the reaction (E > Ea)
Define activation energy
Activation energy Ea is the minimum energy required to start a reaction by the breaking of bonds.
the rate of a chemical reaction = …?
how fast a reactant is used up
OR
how fast a product is formed
(conc/time)
What is the order of a reaction?
Changing the concentration often changes the rate of reaction
the rate of reaction is proportional to the concentration of a particular reactant raised to a power
rate α [A]n
Different reactants in a chemical reaction can have different orders and hence affect the rate in different ways Common orders: Zero order (0) First order (1) Second order (2)
What is zero order?
rate α [A]0
The concentration of the reactant has no effect on the rate
any number raised to the power zero is 1: [A]0 = 1
What is first order?
rate α [A]1
The concentration of the reactant affects the rate of reaction:
doubling [A] (x2) increases reaction rate by a factor of 2 (21 = 2)
tripling [A] (x3) increases reaction rate by a factor of 3 (31 = 3)
What is second order?
rate α [A]2
The concentration of the reactant affects the rate of reaction:
doubling [A] (x2) increases reaction rate by a factor of 4 (22 = 4)
tripling [A] (x3) increases reaction rate by a factor of 9 (32 = 9)
What is the rate equation?
rate = k x [A]m x [B]n (k = rate constant (is the proportionality constant)
What is the overall order?
overall order = sum of orders with respect to each reactant
- gives the overall effect of the concentrations on the rate of reaction
What methods must be taken to produce a concentration-time graph?
- Concentration-time graphs can be taken from continuous measurements of a suitable property during the course of a reaction
- This property must be proportional to the concentration of the reactant or product
Continuous measuring of rate: gas
- Monitoring by gas collection (volume of gas evolved)
- using a gas syringe to collect gas produced
- The gradient of a curve produced tells us the rate of the reaction
Continuous measuring of rate: mass
- Monitoring of mass loss
- Using a balance and time intervals
- graph of mass loss against time
Continuous measuring of rate: colour
- Monitoring by colour change
- If one of the reacting substances or products has a colour, the intensity of this colour will change during the reaction.
- The absorbance reading of a colorimeter provides a measure of the concentration.
- The amount of light absorbed by a solution is measured
What is the method for colorimeters?
- Choose a suitable filter (complimentary colour)
- Zero the colorimeter with water/solvent
- Make up a range of samples with known concentrations
- Take absorbance readings and plot a calibration curve
- Set up your reaction, take a small sample, put in a cuvette and place in colorimeter
- Take absorbance readings at set time intervals
- Read concentrations of reaction mixture off curve or just use absorbance values
(-The order of reaction with respect to a coloured chemical can then be determined from the concentration-time graph)
Describe the concentration-time graphs for orders
- rate of reaction is the gradient of a concentration- time graph
- order with respect to a reactant can only be determined if all other reactant concentrations remain constant
Zero order = straight line with negative gradient, rate of reaction does not change, value of gradient is equal to rate constant k
First order = downward curve with decreasing gradient and the time for the concentration of the reactant to half (half-life) is constant
Second order = downward curve, steeper at start but tailing off more slowly
Half life and first order
- First order reactions have a constant half-life with the concentration halving every half-life (exponential decay)
- t1/2 is the time taken for the concentration of a reactant to decrease to half its original value
Determination of k for a first order reaction from a concentration-time graph for a first order reaction
rate = k x [A]
k = ln 2 / t1/2
Rate-concentration graphs: Zero order
- produces a horizontal straight line with no gradient
Rate = k[A]^0
rate = k - the interpet on the y-axis give the rate constant
- reaction rate doesn’t change with increasing conc
Rate-concentration graphs: First Order
- produces a straight-line graph through the origin
rate = k[A]^1
rate = k[A] - rate is directly proportional to the conc for a first order relationship
- rate constant can be determined by measuring the gradient of the straight line of this graph
Rate-concentration graphs: Second Order
- produces an upwards curve with increasing gradient
rate = k[A]^2 - rate constant cannot be obtained directly from the graph
- by plotting a second graph of the rate against conc squared, the result is a straight line (gradient of this line is equal to rate constant k)
What is initial rate?
The instantaneous rate at the start of a reaction when t=0
- can be found using a tangent on t=0 on a conc-time graph
What is the clock reaction?
Convenient way of obtaining the initial rate of a reaction by taking a single measurement
- the time t from the start of a reaction to when a visual hange is observed is measured
- provided that there is no signif change in rate during this time, assumed that average rate over time (1/t) is the same as initial rate (initial rate is proportional to 1/t)
What is half-life?
The time taken for the concentration of a reactant to decrease to half of its original value
What is exponential decay?
First order reactions having a constant half-life with the concentration halving every half life
How can a first order relationship be confirmed on a concentration-time graph?
By measuring successive half-lives, if they are the same the reaction is first order with respect to the reactant
How can the rate constant be calculated from the half-life of a first order reaction (k)?
k (rate constant) = ln2 / half-life
What is an iodine clock?
- type of clock reaction
- relies on formation of iodine in the presence of starch (colourless to blue-black colour)
- aqueous sodium thiosulfate is added to delay the colour change (remove iodine as it forms)
- initial rate proportional to 1/t
- plot results on a rate-concentration graph
Why do thiosulfate ions delay the colour change?
Thiosulfate ions react with iodine forming I- ions which are not coloured
As soon as all the thiosulfate has reacted, iodine will start to build up in solution producing colour
Why is it important to use the same volume of thiosulfate ions each time?
The amount of iodine is removed each time by thiosulfate should be constant
This means that the rate is proportional to 1/t
What is the reaction mechanism?
The series of steps that make up an overall reaction
What is the rate-determining step?
The slowest step in the sequence
Why is the rate-determining step important?
- rate equation only includes reacting species involved in the RDS
- order in the rate equation match the number of species involved in the RDS
- therefore provides important evidence in supporting or rejecting a proposed reaction mechanism
N2 + 3H2 -> 2NH3 why would this reaction likely be a multi-step reaction?
A one step reaction would require 4 molecules colliding at the same time, which is very unlikely
slow H2O2(aq) + Br-(aq) -> H2O(l) + BrO-(aq) fast H2O2(aq) + BrO-(aq) -> H2O(l) + Br-(aq) + O2(g)
Determine the rate equation for this reaction?
rate = k[H2O2(aq)][Br-(aq)]
- the rate equation only includes the reacting species in the RDS
slow H2O2(aq) + Br-(aq) -> H2O(l) + BrO-(aq) fast H2O2(aq) + BrO-(aq) -> H2O(l) + Br-(aq) + O2(g)
Determine the overall reaction?
2H2O2(aq) -> 2H2O(l) + O2(g)
slow H2O2(aq) + Br-(aq) -> H2O(l) + BrO-(aq) fast H2O2(aq) + BrO-(aq) -> H2O(l) + Br-(aq) + O2(g)
Identify the intermediate?
BrO-
slow H2O2(aq) + Br-(aq) -> H2O(l) + BrO-(aq) fast H2O2(aq) + BrO-(aq) -> H2O(l) + Br-(aq) + O2(g)
What is the role of Br-?
A catalyst as it is used in the first step but regenerated in the second step, not used up overall
2N2O(g) -> 2N2(g) + O2(g)
rate = k[N2O(g)]
Suggest a two-step mechanism
slow step: N2O(g) -> N2(g) + O(g)
fast step: N2O(g) + O(g) -> N2(g) + O2(g)
What increases when temperature increases?
The rate constant, k
What factors affect the rate constant k?
When temperature increases 2 factors contribute to increased rate and k…
- increasing energy shifts Boltzmann diagram distribution to the right, increasing proportion of particles exceeding Ea (MAIN DETERMINANT)
- particles move faster and collide more frequently
What is the Arrhenius equation?
k = A e^-Ea/RT (e^ button on the calculator)
A is the pre-exponential factor (frequency factor)
e^etc is the exponential factor (linked to activation energy and temp)
R is the gas constant (8.314 Jmol^-1K^-1)
T is for temperature in Kelvin
What is the gas constant?
8.314 J mol^-1 K^-1
What is the exponential factor?
e^-Ea/RT
- represents the proportion of molecules that exceed Ea and sufficient energy for a reaction to take place
What is the pre-exponential factor?
A
- takes into account the frequency of collisions with the correct orientation
- essentially constant over small temperature ranges
- gives the rate if there were no activation energy
How can the Arrhenius equation be expressed in a logarithmic form?
ln k = -Ea/RT + ln A
Why is the logarithmic form of the Arrhenius equation useful?
- enables Ea and A to be determined graphically
- A plot of ln k on y-axis and 1/T on the x-axis gives a straight line graph, y=mx+c
ln k = y
-Ea/R = m
1/T = x
ln A = c
What does a plot of ln K and 1/T give us?
A straight downward line
- gradient m of -Ea/R
- intercept c of ln A on the y-axis
Describe how the activation energy and frequency factor can be found graphically?
A graph of ln k against 1/T is plotted
The gradient of the straight line graph is equal to -Ea/R
The intercept of the y-axis is ln A
In the Arrhenius equation, state the effect on k of an increase in A?
Increases k
In the Arrhenius equation, state the effect on k of an increase in Ea?
Descreases k
In the Arrhenius equation, state the effect on k of an increase in T?
Increases k
How do you convert between k and ln k?
ln(value of k given)
How do you convert between T and 1/T
1 divided by the value of T
How do you calculate activation energy from the gradient of the straight line?
Work out gradient (y-axis divided by x-axis)
Since you now know the gas constant and gradient then:
Ea = R x m = 8.314 x gradient
(units are Jmol^-1, divide by 1000 for kJmol^-1)
