Rates of Reaction Flashcards
How do you calculate the initial rate of reaction? Why is this possible?
Calculate the rate (concentration/time) at t=0, where the graph is roughly linear, and where the other reactants are in excess.
Define first order reaction w.r.t [A]
Give a more general definition for orders of reaction
A reaction where doubling [A] means the rate of reaction is doubled
The power to which the concentration of a reactant is raised in the rate equation
Define second order reaction w.r.t [A]
doubling the concentration of [A] means the rate is increased by 2^
2=4 times
Define zero order reaction w.r.t [A]. Give an example
The concentration of [A] does not affect the reaction, for example decomposition of H2O2
What is the rate equation?
An equation that links to the rate of reaction to the concentration of each reactant, raised to their order, linked by the rate constant.
How can we determine the order based on C-T graph?
Innitial rates method - Assuming the concentration of the reactant is roughly linear and has a small amount of change, the rate is proportional to 1/time.
gradient of C-T graph is rate of reaction
repeat for several different concentrations,
plotting rate / [A] graph to show order of reactant A
How do you measure the half life of reactions with a first order conc-time graph?
Measure the time for the concentration to halve, and repeat this process three times. By comparing the half lives of three separate divisions, we can see if it is constant. If so, it is first order.
What is characteristic of half life of zero order reaction?
It decreases with time
What is characteristic of the half life of second order reactions?
It increases with time.
What is the effect of termperature on the rate of reaction? Does anything else affect the rate of reaction?
For every 10oC increase, the rate doubles.
No.
Define molecularity
The number of molecules in the rate determining step.
How do you determine order of reaction
by inspection?
Fine two experiments where there is a change in only one reactant. Find the effect this has on the initial rate, and hence the proportionality: 0,1,2 order. Then state that between exp.1 and 2, the concentration of A doubled, and the rate doubled also, so rate is proportional to [A], hence first order wrt A.
Define the half life of a reaction
The time taken for the concentration of a reactant to halve.
What is characteristic of the half life of first order graphs?
The half life is constant. This is a defining feature as it is an exponential decrease in terms of e.
When working out a half life of a first order graph, what must you do at the end to prove the half life is constant?
Find the average of at least 3 half lives. If the average is close to the three values, the half life can be said to be constant, so the graph is first order.
If paracetamol has a biological half life of 380 seconds, how long does it take for the concentration to fall to 1/16 its original concentration?
1/1 –> 1/2 –> 1/4 –> 1/8 –> 1/16
4 half lives = 4*380 = 1520
What are we comparing when we plot a clock reaction for various concentrations of a reactant?
the proportionality of 1/T vs concentration. Since 1/T is proportional to the rate, we can determine the order of the graph, if not the actual rate at these points.
What measures can we use for clock reactions to determine the time taken?
Time taken for: Precipitate to form Solid to disappear sudden colour change change in pH
What are the species in the rate equation all involved in>
The rate determining step
What is the rate determining step?
The slow step of a reaction which determines the rate, in a multi step reaction.
In this example, is the rate determining step mono or bimolecular?
r=k[NO2]^2
Bimolecular, because there are 2 molecules of NO2 reacting.
Suggest an overall reaction mechanism for NO2 + CO = NO + CO2 using the rate equation:
r=k[NO2]^2
Identify any intermediates in the reaction
Slow step:
2NO2 –> 2NO + O2
Fast step
2CO + O2 –> 2CO2
O2 is the intermediate
What does k express in terms of the overall reaction?
How quickly the reaction occurs. A small value of k means a slow reaction, whilst a large value of k means a fast reaction.
Can the speed of a reaction be determined mathematically?
No, because the rate equation only concerns the slow step. This means k only refers to the slow step, and not the overall reaction.
What equation could you use to calculate the overall speed of the reaction?
Arrhenious equation.
How can you tell if the reaction is 1st order given a rate concentration graph? What two things must you say?
Straight line AND goes through origin.
If the rate equation is r=k[H2][NO]^2, what happens if both H2/NO both increase in concentration by 4x?
increase by 64x
What is the rate of disappearance of I- if the rate of formation of I2 is 1E-5 moldm-3s-1
Since two moles of I- formed from 1 mole of I2, the rate is 2E-5
What is the half life in a first order reaction if the pressure is halved?
The same, as it is independent of initial concentration. Only dependent on temperature.
What does molecularity refer to?
The number of molecules in an ELEMENTARY REACTION.
For a multi step reaction, this is the rate determining step
