Topic 16: Kinetics II (chemical kinetics) Flashcards
Why does the rate of reaction decrease over time?
The concentration of reactants decreases
Units for rate of reaction
moldm^-3s^-1
How to find the rate of reaction at a particular time
Draw a tangent - t=0 for initial rate
Experimental methods to measure reaction rates
Colorimetry - change in colour or precipitate formed
Titration - if acids/alkalis are used which can be neutralised or if iodine is used/produced
Loss in mass with balance - gas formed
Volume of gas produced with syringe - gas formed
pH measurements with pH probe - change in pH if acid/alkali is produced/consumed
Issues with measuring gas produced
There is gas in syringe - higher value than expected
Bung is not placed quickly enough after adding reactant - lower value than expected
Uncertainty in syringe
Issues with measuring loss of mass
Place on open vessel on mass balance
Delay on adding cotton wool - acid escapes - lower value than expected
Loss of mass may be small if mass eg gas is small eg hydrogen
Uncertainty on balance
When does the rate constant (k) not remain constant
If temperature is changed or a catalyst is used
What do the orders determine
How the concentrations of reactants affects the rate of reactions
What is the effect of zero order on the rate of reactions
It has no effect
If A doubles, the rate of reaction doesn’t change
Doesnt appear in rate equations as (A)^0=1
What is the effect of first order on rate of reaction
The rate of reaction is proportional to conc. A^1
If A doubles, rate doubles
What is the effect of second order on rate of reaction
The rate of reaction is proportional to conc. A^2
If A doubles, rate quadruples
If A were multiplies by 4, reactions would multiply by 4^2 (16)
How can orders of reactions be found
By experiment
Units of k
Depend on rate equation - have to cancel out
Effect on raising temperature on value of k
Increases
Effect of adding a catalyst on value of k
Increases
Effect of increasing concentration of value of k
No change
Rate equation includes reactants/products?
reactants only
Half life
The time taken for the concentration of a reactant to half
How to find reaction orders from experiments
- Continuous monitoring - measure the quantity of a reactant at interval (every 30 secs) during the course of a reaction
- Using initial rates - change conc of one reactant at a time: find from graph or clock reaction (how long it takes for a certain amount of product to be made)
Concentration-time graph for first order
Straight line of best fit
Concentration-time graph for first order
Curve - constant half life
Concentration-time graph for second order
Curve - half life is not constant (increases significantly)
Rate-concentration graph for zero order
Straight line across - no effect
Rate-concentration graph for first order
Directly proportional
Rate-concentration graph for second order
Curve - exponential
How to obtain a rate-concentration graph
Make tangents of the time-concentration graph
How to plot a log graph
log(rate) on y axis
log(conc) on x axis
gradient is order
log(rate)= logk + nlog[A]
How to write answers to deducing order from experiments
Order wrt [A] from experiments 1 and 2
[A] 0.5 –> 1.0 doubles
[B] is unchanged
Rate 0.2 –> 0.4 doubles
Rate (initial) is proportional to [A]
Therefore order wrt [A] is first
Initial rate of reaction =
1/time when a fixed concentration (C1) is used (same time)
Why is initial rate=1/t an approximation
The rate could be changing as we can’t calculate rate at t=0 exactly
What does the Arhenius equation show
The effect of changing temperature/catalyst on the rate constant
Can work out the activation energy from it
Arrhenius equation
ln(k) = ln(A) - Ea/RT
can use ln(rate)
Graph to find activation energy
ln(rate) on y axis
1/T in Kelvin on x axis - (often 10^-3)
What is the reaction mechanism
The sequence of elementary steps that leads to the formation of a product
What types of elementary steps are there
Unimolecular
Bimolecular
Termolecular
What order is unimolecular elementary step
A –> product
rate = k[A] first
What order is bimolecular elementary step
A+A or A+B –> product
rate = k[A]^2 or k[A][B]
What order is termolecular elementary step
A+A+A or A+A+B –> product
rate = k[A]^3 or k[A]^2[B]
The slowest step
The Rate Determining Step - determines the overall rate of reaction
What species appear in the rate equation
The ones in the RDS
What is the overall order of an SN1 reaction
First as there is only one species in the RDS
What is the overall order of an SN2 reaction
Second as there are two species in the RDS
What are four examples of clock reactions
Iodine clock reaction
Bromine clock reaction
Sulphur clock reaction
Reaction of iodide ions with peroxodisulphate (VI) ions
Describe the sulphur clock reaction
Add a dilute solution of sodium thiosulphate to a beaker with water, making it 25cm in total
Add 25cm of HCl and stir and record the time it takes for enough sulphur to be produced to conceal the X below the beaker
Repeat with different amount of sodium thiosulphate
S2O3^2- + 2H+ –> S(s) + H2O + SO2
Describe the iodine clock reaction
Oxidation of I- by H2O2 in acid solution
50cm of H2O2 and water (50:50)
Mix with 25cm of KI (aq) and 5cm of sodium thiosulphate
H2O2 + 2I- + 2H+ –> 2H20 + I2
I2 reacts instantaneously with sodium thiosulphate until it us used up
I2 + 2S2O3^2- –> 2I- + S4O6^2-
I- is colourless
When all sodium thiosulphate is used up, I2 reacts with starch indicator to produce a blue/black colour
Repeat with different relative amounts of H2O2 (still 50cm total)
Describe the bromine clock reaction
Beaker with bromate (V) ions and water and methyl red indicator
Another beaker with Br- ions and a little bit of phenol
Mix beakers and record time for colour change to occur
5Br- + BrO3- + 6H+ –> 3H2O + 3Br2
Br2 reacts instantaneously with phenol
3Br2 + C6H5OH –> C6H2OHBr3 + 3H+ + Br- (colourless)
When phenol is used up, Br2 reacts with methyl red indicator (pink in acidic solution) and bleaches it
Describe the reaction of iodide ions with peroxodisulphate (VI) ions
S2O8^2- + I- –> 2SO4^2- + I2
I2 + S2O3^2- –> 2I- + S4O6^2-
I2 + starch –> blue/black complex
Why does H+ appear in the rate equation for the reaction between iodine and propanone
It is a catalyst for the reaction, known as autocatalysis
