Chapter 18 - Rates of Reaction Flashcards
What are the equations for rate?
Rate = quantity reacted or produced/time
Rate = change in concentration/change in time
How can you rearrange the rate equation to work out k?
k = rate/[A]m[B]n
What is concentration measured in?
mol dm^-3
What is zero order?
When the concentration of a reactant has no effect on the rate, the reaction is zero order with respect to the reactant
Concentration does not influence rate
What is first order?
When the rate depends on a reactant’s concentration raised to the power of 1
If concentration doubles, rate also doubles
What is second order?
When rate depends on a reactant’s concentration raised to the power of 2
If concentration doubles, reaction rate increases by 2 to the power of 2 = 4
What is the rate equation?
Gives the mathematical relationship between the concentrations of the reactants and the reaction rate
rate = k [A]m [B]n
Where k = rate constant, m= order of reaction for A and n= order of reaction for B
What is the overall order?
The overall effect of all reactants on the rate of reaction.
Overall order = sum of orders with respect to each reactant
How does the rate constant change with concentration and pressure?
It does not change
What is the initial rate?
Th instantaneous rate at the beginning of an experiment when t =0
What are the methods for continuous monitoring of reactions to determine rate?
Monitoring by gas collection
Monitoring by mass loss
Monitoring rate with a colorimeter
What shape concentration time graph does zero order reaction produce?
A straight line with a negative gradient
What shape concentration time graph does a first order reaction produce?
A downward curve with a decreasing gradient over time
What shape concentration time graph does a second order reaction produce?
A downward curve, steeper at start but tailing off more slowly
What is half life?
The time taken for half of a reactant to be used up.
How can a first order relationship be determined from a concentration time graph?
By measuring successive half lives. If they are the same, then the reaction is first order with respect to the reactant. HALF LIFE IS CONSTANT FOR FIRST ORDER
How can you determine k from a first order reaction?
- calculating rate constant from a curve
Find gradient, then use that in k = rate (gradient)/reactant - calculating rate constant from half life
k = ln2/ half life
What shape rate concentration graph do zero order reactions produce?
A horizontal straight line with zero gradient
What shape rate concentration graph does a first order reaction produce?
A straight line graph through the origin
What shape rate concentration graph does a second order reaction produce?
An upward curve with increasing gradient
How can the initial rate be found?
By measuring the gradient of a tangent drawn at t = 0 on a concentration-time graph
What is a clock reaction, and how can this be used to determine initial rate?
The time from the start of an experiment is measured for a visual change to be observed (usually a colour or precipitate). Initial rate is then proportional to 1/time.
What is an iodine clock?
Aqueous iodine is coloured orange brown, so the time from the start of the reaction and the appearance of the iodine colour can be measured. Starch is usually added as it forms a complex with iodine which is an intense dark blue black colour
How accurate are clock reactions?
You are measuring the average rate during the first part of reaction (assumption that average rate is constant and same as initial rate). Initial rate is therefore an approximation, but still reasonably accurate provided that less than 15% of reaction has taken place
What is the rate determining step?
When there is a multi-step reaction, they take place at different rates. The slowest step is called the rate determining step
How can you predict reaction mechanisms?
The rate equation only includes reacting species involved in the rate determining step.
The orders in the rate equation match the numbers of species involved in the rate determining step.
Why is it unlikely that some reaction mechanisms only take place in one step?
Stochiometry in rate equation does not always match stochiometry in the overall equation
Collisions are unlikely with more than 2 ions/species
What 2 factors affect the rate constant when temperature increases?
- an increase in temp shifts Boltzmann distribution to right to increase proportion of particles exceeding Ea
- as temp increases, particles move faster and collide more frequently
What is the Arrhenius equation?
An exponential relationship between the rate constant k and temperature T
k = A e^ -Ea/RT
GIVEN ON DATA SHEET
What are the different parts of the Arrhenius equation, and what units are used?
k = rate constant
A = pre-exponential factor, always same units as k
Ea = activation energy, J mol-1
R = gas constant, J K-1 mol -1
T = temperature, Kelvin
e^ -Ea/RT = exponential factor
What is the logarithmic form of the Arrhenius equation?
lnk = -Ea/RT + lnA
How can Ea and A be determined graphically?
Via a plot of lnk againsts 1/T which gives a straight line graph with
- gradient -Ea/R
- y intercept of lnA
How does the logarithmic form of the Arrhenius equation match up to y = mx+c?
lnk = -Ea/R x 1/T + lnA
y = m x + c
How do you go from lnA to A?
If lnA = x, A = e^x
