C18 - Rates of Reaction Flashcards
How’s rate of reaction calculated?
Quantity reacted or produced / time
Change in concentration / time
What is the rate of reaction proportional to?
Rate of reaction is proportional to the concentration of a particular reactant raised to a power.
The power is the order of the reaction.
What occurs in a zero order reaction?
Any number to the power zero is 1 (e.g. 2^0 = 1)
Concentration does not influence the rate
What’s a first order reaction?
One where the rate of reaction of a reactant depends on the concentration raised to the power one (e.g. 2^1 = 2)
If concentration is doubled, rate increases by a factor of two.
If concentration is tripled, rate increases by a factor of three.
What occurs in a first order reaction?
If concentration is doubled, rate increases by a factor of two.
If concentration is tripled, rate increases by a factor of three.
What’s a second order reaction?
One where the rate of reaction of a reactant depends on it’s concentration raised to the power two.
What occurs in a second order reaction?
Reaction rate increases by a factor of its concentration squared.
If concentration is doubled, reaction rate increases by a factor of 4.
If concentration is tripled, reaction rate increases by a factor of 9.
What’s the rate equation?
A mathematical relationship between concentration of reactants and reaction rate.
rate = k [A]^m [B]^n
What’s the overall order of reaction?
It gives the overall effect of the concentrations of reactants on the reaction rate.
Overall order = sum of orders with respect to each reactant
What are two methods of continuous monitoring reactions producing gas?
Monitoring gas collection
Monitoring mass lost
How can the orders of reactants be identified?
By experimental results or the shape of the line of concentration-time graphs.
What does a zero order concentration-time graph look like?
A straight line with a negative gradient.
The gradient is equal to the rate constant, k.
What does a first order concentration-time graph look like?
A downward curve with a decreasing gradient over time.
Time taken for the concentration to halve is constant (known as the half life - showing exponential decay) which can be used to determine rate constant, k.
What does a second order concentration-time graph look like?
A downward curve steep at the start but tailing off slowly.
What’s a zero order?
When the concentration of a reactant has no effect on the rate of reaction.
Any number to the power zero is 1
Concentration does not influence the rate
What are the (2) methods for determining the rate constant from a concentration-time graph for first order reactions?
1) Calculate the gradient of the tangent to the curve, giving the rate of reaction.
The rate constant is then calculated by rearranging the rate equation and substituting the value of the rate and concentration.
2) Use the exponential relationship for a constant half life where k = ln 2 / t (1/2)
What does the rate-concentration graph of a zero order reaction look like?
A horizontal straight-line with zero gradient.
The intercept on the y-axis gives the rate constant k.
Reaction rate does not change with increasing concentration.
Rate = k[A] 0 so rate = k
What does the rate-concentration graph of a first order reaction look like?
A positive straight line graph through the origin.
Rate is directly proportional to concentration.
The rate constant can be determined by measuring the gradient of the straight line.
Rate = k[A] 1 So rate = k[A]
What does the rate-concentration graph of a second order reaction look like?
An upward curve with an increasing gradient.
Being a curve, rate constant cannot be obtained directly from the the graph.
By plotting a second graph of rate against concentration square, a straight line through the origin is produced. The gradient of the straight line is equal to the rate constant.
Rate = k[A] 2
How can a clock reaction be used to find initial rate of reaction?
The time is recorded from the start of an experiment until a visual change occurs.
Initial rate is then proportional to 1/t.
What’s an iodine clock?
A clock reaction relying on the formation of iodine.
Since aqueous iodine is coloured orange-brown, the time from the start of the reaction and appearance of the iodine colour can be measured.
Starch can be added as it forms a complex with iodine which is dark blue-black.
What’s the rate-determining step of a multi step reaction?
The slowest step in the sequence.
How can the correct mechanism for reaction be identified?
The rate equation only includes reacting species involved in the rate-determining step.
The orders in the rate equation match the number of species involved in the rate-determining step.
How are haloalkanes hydrolysed?
By a hot aqueous alkali.
RBr + OH- -> ROH + Br-
When temperature is increased, what (2) factor contribute to the increased rate and rate constant?
Increasing temp shifts the Boltzmann distribution to the right, increasing the proportion of particles that exceed the activation energy.
As temperature increases, particles move faster and collide more frequently.
Increasing frequency of collisions is comparatively small compared with the increase in proportion of molecules above Ea from the shift in Boltzmann distribution. (So rate mainly determined by Ea).
What is the Arrhenius equation?
It is an exponential relationship between rate constant k and temperature T.
k = Ae ^ -(Ea/RT)
Rate constant = pre-exponential frequency factor * exponential factor
What does the pre-exponential and exponential factor [e^-Ea/RT] of the Arrhenius equation represent?
Pre-exponential: takes into account the freq of collisions with correct orientation.
Exponential: The proportion of molecules that exceed the Ea with sufficient energy for a reaction to take place.
What is the logarithmic form of the Arrhenius equation and how does it allow Ea and A to be determined graphically?
ln k = -Ea/RT + ln A
ln k against 1/T is plotted to give a straight line graph like y = mx + c where:
m is -Ea/R
x is 1/T
c is ln A
How is half life, t (1/2), calculated?
t (1/2) = ln 2 / k
What is t (1/2)?
Half life
t (1/2) = ln 2 / k
What is half life?
Time taken for the concentration of a substance to halve.
How can k (rate constant) be calculated using half life?
k = ln 2 / t(1/2)
How can the rate constant, k, of a first order reaction be calculated?
The gradient of its concentration/time graph shows half life.
Half life can be used to find k.
k = ln 2 / t(1/2)
How can the rate constant, k, of a zero order reaction be calculated?
The appearance of its concentration/time graph appears a straight line.
It’s gradient shows rate constant, k.
For a reaction, rate constant, k, is determined at different temperatures, T.
How would a graph of ln k against 1/T be plotted and what shape would it be?
1/T along the x axis and ln k along the y axis.
It would be a straight line with a negative gradient.
The y intercept would be ln A (A being the frequency factor)
The gradient is equal to -(Ea/R)
What are the features of an arrenhius plotted graph
1/T along the x axis and ln k along the y axis.
It would be a straight line with a negative gradient.
The y intercept would be ln A (A being the frequency factor)
The gradient is equal to -(Ea/R)
How can an iodine clock reaction be used to identify the orders of both I- and S2O8- in:
2I- + S2O8 2- —> I2 + 2SO4 2-
using the initial rates method?
(4)
Time how long it takes for the solution to turn from colourless to blue/black.
Vary [S2O8 2-] whilst keeping [I-] constant
Vary [I-] whilst keeping [S2O8 2-] constant
Rate of reaction is proportional to 1/t OR rate = concentration / time
Rate-concentration graph gives a straight line through the origin (demonstrating that they’re both first order).
