Flashcards in Module 5: Chapter 18 (Reaction Rates) Deck (55)
What affects the rate of a chemical reaction?
concentration / pressure
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
how fast a product is formed
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
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