Rates of reaction Flashcards
Rates of reaction
Change in conc/time
Units of rate
mol dm-3 s-1
Zero order
When the change in conc has no effect on rate
Rate c [A]0
First order
When the conc is doubled, rate is doubled
Rate c [A]1
Second order
When the conc is doubled, rate is squared
Rate c [A]2
The relationship between the rate equation and rate constant
Rate=k[A]m[B]n
m and n=order with respect to A, B
k=rate constant
m+n=overall order
Units of rate equation
mol dm-3 s-1
Cancel out over mol dm-3
Positive indices before negatives
Working out orders from experimental results
Determine orders, overall order and rate equation
Write rate equation and calculate k, including units
How to determine the rate of reaction
Continuous monitoring
Gas collection, mass lost, color changes with a colorimeter
Concentration time graph and line shapes
0=straight desc line
1=desc curve decreases over time
2=desc curve, steeper at start, falls slowly
Half life and first order reactions
Exponential decay
If curse is 1st order, will be a half life
If not, not 1st order
Calculating rate constant form rate
Rate=gradient, use tangent
Use rate constant equation
Calculating rate constant from half life
More accurate than tangent
k=Ln2/t 1/2
Rate concentration graphs
0=no change in grad
1=steady grad, directly proportional
2=upwards curve, if graph of conc2, line will be straight
Log log graphs to find order and rate constant
Log(rate) = n log[A] + log k
y=mx+c
Initial rates
Instantaneous rate at start of reaction when t=0
Find gradient with tangent at t=0
How do iodine clocks work
Hydrogen peroxide reacts with iodine in acid to form iodine.
Starch used to show presence
Can obtain initial rate with single measurement, 1/T against conc plotted
Repeat with different concs and plot graph
Accuracy of an iodine clock reaction
Average rate=constant, same as initial as long as less than 15% of reactant consumed
After 15%, rate changes
Rate determining step
If there are multiple reactants, more likely to take place in more steps
Series of steps that make up an overall reaction is the reaction mechanism
The slowest step is the rate determining step
Hydrolysis of haloalkanes and the mechanisms
If given the rate equation and the balanced equation, you can work out the reaction mechanism
The rate equation shows what is involved in the slow step, anything else is in the fast step
Compare the overall equation with the information from the rate equation
Steps must add up to the overall equation
Identify the intermediate and work out the fast step
