Initial Rates Method Flashcards
Initial rate of reaction
The rate of reaction (the change in concentration or amount of product/reactant per unit time) at the very start of a reaction
When time = 0s
What is the initial rates method?
A way to find the order of reaction with respect to certain reactants using multiple experiments of the same reaction but the initial starting concentration of the reactant of interest is varied
finding the initial rate (the rate at start of reaction) and comparing how the rate changes when the conc of one reactant initially changes
How can the initial rates method be carried out?
Carry out an experiment to produce a concentration time graph of a reactant, find the initial rate
Repeat the experiment but change the concentration of a different reactant and find the initial rate
See how changing the concentration of this reactant affects initial rate measured = gives us order of reaction
Problem with using initial rates method
Involves a lot of experiments, continuous monitoring methods and graph drawing
And the tangent drawn at time =0s is slightly off
Method to find order of reaction once we have found the initial rates of reaction?
Use a table of initial conc of both reactants and its initial rate
How can we use the initial rate table?
Find order with respect to one reactant: compare to 2 reactions where it changes but other reactants stay constant
When one reactant changes, see how this affects rate. Does the rate not change, does it increase proportionally or exponentially? = Order
What if the table doesn’t have a concentration of a reactant that remains constant in any experiments, so for the other reactants the order isn’t easily worked out?
Work out order of 1 reactant using prev method
Draw an additional column which will show, according to orders, what the rate will be if the change is due to that reactant only given the concentration
Compare this value to the actual rate
Finding a missing value of initial rate?
Use rate equation we formed from previous steps
Calculate k with complete data set
Then use k and the rate equation with incomplete data set to find missing value
K = rate / concentrations x concentrations
(To power of 2 if 2nd order)
If the initial rate of reaction stays the same when we change a concentration, what does this tell us about this reactant?
It is 0 order: changing this reactant has no effect on rate of reaction
If the initial rate of reaction changes by the same amount as the changed concentration, what does this tell us about this reactant?
It is 1 order because the rate of reaction will change proportionally to the concentration of this reactant
If the initial rate of reaction increases by the squared amount that we increased the concentration by, what does this tell us about this reactant?
It is 2 order with respect to the changed reactant
Because changing the concentration has a squared proportional change
Difficult initial rate tables: the hypothetical rate is equal to the real rate
Then the reactant we are trying to find order of has no effect on the rate thus is 0 order
Difficult initial rates tables: when the conc of a reactant changes, the hypothetical rate to the real rate changes by same amount
1 order to show the effect this reactant has on rate of reaction
After carrying out multiple experiments that varied the initial concentration of a reactant, what do we do?
Find the initial rate for each concentration
Plot a rate- concentration graph and see which order with respect to this reactant it is based on this graph
Using initial rates method for gas production
Using different starting concentrations of reactant:
Measure mass/volume of gas produced immediately and compare for different concentrations
Or time how long it takes for a certain volume to be produced in a given time (1/time=rate) at different starting concentrations
Using colour change reactions with the initial rate method
Time how long it takes for a specific colour change for each different starting concentration of reactant
Rate = 1/time
How can we ensure we have same ending point colour change?
Use a colorimeter
Using cloudiness to find order of a specific reactant
Time how long it takes until it is no longer possible to see through a reaction mixture eg see through to the cross
Divide by 1 = rate
Repeat for multiple concentrations
Plot rate concentration graph
Using quenching of a reaction to measure initial rate
Add a lot of water to dilute it and stop the reaction then use titration to determine the concentration
