R.P 7 (Measuring rate of reaction) Flashcards
What are the 2 ways of measuring the rate of a reaction
An initial rate method
A continuous monitoring method
What are the 2 ways of measuring the rate of reaction: by an initial rate method
By taking the gradient of a continuous monitoring conc vs time graph at time = zero
From clock reactions where the time taken to reach a fixed concentration is measured
What is a clock reaction
A reaction where a sudden visible change (e.g. color) occurs after a fixed time, used to measure reaction rates
How can we determine the order I- using the iodine clock reaction
By repeating the experiment several times,
varying the concentration of a reactant e.g.
I– , ( keeping the other reactants at constant
concentration ) you can determine the
order of reaction with respect to that reactant
Outline the 5 stages in the iodine clock reaction
(1) Put each of the chemicals in the table in separate burettes.
(2) In each experiment, measure out required volumes of the potassium iodide, sodium thiosulphate, starch and water into a small conical flask from the burettes
(3) Measure the hydrogen peroxide into a test tube
(4) Pour the hydrogen peroxide from the test tube into the conical flaks and immediately start the timer. Stir the mixture.
(5) Time until the first hint of blue/ black colour appears
How do we find initial rate after we find the time taken for blue or black colour of iodine to appear
Measure the time until the colour change happens.
Initial rate = 1 ÷ time taken (assuming iodine amount stays constant).
Repeat with different concentrations to see how rate changes
Then can deduce orders and work out rate equation
When can we work out rate graphically using the iodine clock reaction
In an experiment where the concentration of one of
the reagents is changed and the reaction rate measured it is possible to calculate the order graphically
How do we calculate the order of I- graphically in the iodine clock reaction
Taking rate equation
Rate = k [I- ]^n
Log both sides of equation
Log rate = log k + n log [Y]
Y = c + m x
A graph of log rate vs log [I- ] will yield a straight line where the gradient is equal to the order n
During the iodine clock reaction, what scenario will produce the highest percentage error
In this experiment high concentrations with quick
times will have the biggest percentage errors
How do we calculate initial rate in iodine clock reaction
Initial rate = 1 ÷ time taken for blue back colour to appear
How do we find k in iodine clock reaction
Find rate from experiment (e.g.
1÷time 1÷time in a clock reaction).
Determine order of I- (assuming conc of all other reactants is constant) m by changing its concentration and seeing how rate changes.
Choose one experiment and plug in values for rate, concentrations, and orders.
Rearrange to solve for 𝑘
What is the rate equation for the iodine clock reaction assuming concentration of all other reactants is constant
rate=k[I −]^n
Where:
rate = 1/time until colour change
k = rate constant (depends on temperature),
[I⁻] = concentration of iodide ions,
n = order of reaction with respect to I⁻ (found from experiment)
When do we start and stop the timer in the iodine clock reaction
As soon as the two solutions are mixed
When the colour suddenly changes (usually blue-black)
What is the difference between initial rate and continuous monitoring method of finding rate
I.R
Measure time for a visible change (e.g. colour), repeat with different concentrations, use 1 ÷ time to estimate initial rate
C.M
Measure change in concentration or volume throughout the reaction , collect data over time, plot a concentration time graph to find rate at any point
What does the gradient at any given point in a concentration time graph represent
Rate of reaction
Why does the rate of reaction eventually go to 0 in a concentration time graph
Reactants are all used up
Outline the method of continuously monitoring by measurement of the change in volume of a gas produced in this reaction
Mg + HCl —> MgCl2 +H2
Measure 50 cm³ of 1.0 mol dm⁻³ hydrochloric acid and add to a conical flask.
Set up the gas syringe in stand
Weigh 0.20 g of magnesium.
Add magnesium to the flask, quickly insert the bung, and start the timer
Record the volume of hydrogen gas collected every 15 seconds for 3 minutes
Plot a volume time graph to find rate at any given point
Why do we need to carefully measure quantities of reactants when continuously monitoring by measurement of the change in volume of a gas
A typical gas syringe only measures 100ml of gas so
you don’t want at reaction to produce more than this volume
Why should we use a large excess of reactants apart from 1 when continuously monitoring by measurement of the change in volume of a gas
In reactions with several reactants, if one is in large excess, its concentration stays nearly constant and does not affect the rate — it behaves as zero order.
Apart from initial rates and continuous monitoring, outline 3 other methods of measuring the rate of a reaction and state when they are most applicable
Colorimetry
If one of the reactants or products is coloured
then colorimetry can be used to measure the
change in colour of the reacting mixtures
Measurement of change of mass
This works if there is a gas produced which is allowed to escape. Works better with heavy gases such as CO2
Measuring change in electrical conductivity
Can be used if there is a change in the number
of ions in the reaction mixture
Measurement of optical activity.
If there is a change in the optical activity through the reaction this could be followed in a polarimeter
Give 2 ways of quenching/stopping a reaction
By dilution
Cooling
