Topic 16 Kinetics 2 Flashcards
How can you use titrations to measure the rate of reaction
Take small samples (aliquot) from an ongoing reaction and immediately quench the reaction. Then titrate the aliquot to see how much has reacted already
What do you use to measure colour change
Colorimiter
What does the rate equation link
Rate of reaction and concentrations of substances
Identify each letter in this rate equation
Rate = k[A]^a[B]^b
k is the rate constant
A and B are reactants
a and b are the order of their respective reactants
Units of rate in the rate equation
moldm^-3s^-1
What does an order of reaction tell us
The power to which a reactant affects the rate of reaction
In terms What happens if a reactant is 0 order
Change in conc has no effect on rate
What is 1st order
Change in conc is proportional to the change in rate
What is 2nd order
Change in concentration has a squared proportional change on rate
Can you work out an order from the molar value in an equation
No you can only find it experimentally
When does the rate constant change
When temperature changes
What happens when k is larger in a rate equation
The rate of reaction is higher
When using a clock reaction (disappearing cross) to estimate initial rate what three assumptions are made
- Temp is constant
- Concentrations of reactants doesn’t change significantly during reaction period
- Reaction has not proceeded too far by the time the end point is reached
What do the three approximations of a clock reaction allow us the estimate
That rate is constant from the start to the endpoint so rate across this period roughly equals the initial rate
What is the iodine clock reaction
And what indicator mixture is used for this reaction. How does this provide a result
H2O2 + 2H+ + 2I- —> 2H2O + I2
Starch and sodium thiosulfate.
Thiosulfate reacts with iodine immediately to reform I- ions. When all the thiosulfate is used up the I2 formed can react with the starch giving a blue/ black colour
What do rate concentration graphs help us identify
The order of the reaction
How do you find a rate concentration graph
From a concentration time graph
What is the order when a horizontal line is on a rate concentration graph
0
The rate concentration graph is a straight diagonal line - what is the order
First order - rate changes proportionally to concentration
The rate concentration graph is an upwards curve - what is the order
2nd - rate squares the increase in concentration
What is half life
Time it takes for half of the reactant to be used up
How can you find the order from concentration time graphs
0 order - half life decreases
1st order - half life is constant
2nd order - half life increases
What is the rate determining step
The slowest step in a multi step reaction
What does the whole reaction rate depend on in a multi step reaction
Rate determining step
What can use use to work out the rate determining step and why
The rate equation - reactants in the rate equation impact the rate of reaction and the only step affecting the rate of reaction is the rate determine step
Can catalysts appear in the rate equation and why
Yes they affect the rate of reaction
When does a reactant that is not in the rds appear in the rate equation
They react to form an intermediate in the rds
What is the Arrhenius equation and what are all the letters
k = Ae^(-Ea/RT)
k- rate constant
A - Arrhenius constant
Ea - activation energy
R - gas constant
T - temp
What does the Arrhenius equation tell us about the relationship between activation energy and rate constant
As activation energy gets smaller rate constant gets bigger
How does the Arrhenius equation mostly appear in exam questions.
The
ln k = ln A - Ea/RT which is used to model straight lines
When ln k = ln A - Ea/RT is used to model straight lines what does each thing represent
ln k - y axis
1/T - x axis
-Ea/R - gradient
lnA - y intercept
