5.1.1 Orders, Rate Equations And Rate Constants Flashcards
Key notes
Rate = quantity reacted or produced/time
Measured in moldm-3s-1
Changing conc often changed rate of reaction.
Zero order
When conc of reactant has no effect on the rate, reaction is zero order either respect to the reactant.
[A]^0
Any number raised to the power of 0 is 1.
Conc does not influence the rate.
First order
If conc of A is doubled, the reaction rate increases by factor of 2^1= 2
If conc A is tripled, reaction rate increases by factor 3^1= 3
Second order
If conc of A doubled, reaction rate increases by factor 2^2 = 4
If conc of A tripled, RR increases by factor 3^3 = 9
Rate equation
Rate = k[A]^m[B]^n
K= rate constant
M= order of R respect to A
N= order of R respect to B
Overall order
Sum of orders with respect to each reactant
Rate = k[A]^m[B]^n, overall order= m+n
Continuous monitoring
Concentration-time graphs plotted from continuous measurements during the course of a reaction.
3 types of monitoring
By gas collection
By mass loss
With a colorimeter
Determining order from shapes of graphs
Zero order: straight line with a negative gradient, value of gradient equal to rate constant k.
First order: produces a downward curve with a decreasing gradient over time - time for conc of the reactant to halve is constant - half-life.
Second order: also downward curve, steeper at the start but taking off more slowly.
Half-life t1/2
Time taken for half of a reactant to be used up - exponential decay.
If successive half-lives are the same, the reaction is first order in respect to the reactant.
Determination of k for a first order reaction
- Tangent to the curve drawn at a particular conc.
Gradient of the tangent calculated giving the ror.
Rate constant calculated by rearranging the rate equation + substituting the value of rate + conc at the position where the tangent lines been drawn. - Make use of the exponential relationship for a constant half life. K = ln^2/t1/2 s^-1
Rate-conc graphs 0
Zero order - y intercept
Zero order reactant produces a horizontal straight line with zero gradient
Rate = k
The intercept on the y axis gives the rate constant k.
The reaction rate does not change with increasing conc.
Rate-conc graphs 1
First order - y/x
First order reactant produces a straight line graph through the origin
Rate = k[A]
Rate is directly proportional to conc for a first order relationship.
The rate constant can be determined by measuring the gradient of the straight line of this graph.
Rate-conc graphs 2
Second order - y/x^2 (gradient)
Second order reactant produces an upward curve with increasing gradient.
Rate = k[A]^2
Must plot another graph, rate against conc SQUARED
Result is a straight line through the origin.
Initial rates method
Initial rate can be found by measuring the gradient if a tangent drawn at t=0 on a conc-time graph.
A clock reaction is a convenient way of obtaining the initial rate of. a reaction by taking a single measurement. The time from the start of an experiment is measured for a visual change to be observed, a colour or precipitate.
Can be assumed that the average rate over this time will be the same as the initial rate.
Initial rate is proportional to 1/t
Multi-step reactions
Reaction mechanism: series of steps that make up an overall reaction.
The slowest step is called the rate-determining step
Rate equation only includes reacting species involved in the rate-determining step
Effect of temp on rate constant
As temp increases, the value of the rate constant k will also increase.
Increasing the temp shifts the Boltzmann distribution to the right, increasing the proportion of particles that exceed the activation energy.
As temp increases, particles move faster and collide more frequently.
Arrhenius equation
K= A e^-Ea/RT
A = pre-exponential factor (frequency factor)
e^-Ea/RT = exponential factor (linked to activation energy + temp)
e^x on calculator
Represents the proportion of molecules that exceed Ea and have sufficient energy for a reaction to occur.
A takes into account the frequency of collisions with the correct orientation.
Logarithmic form of Arrhenius equation
lnk = -Ea/RT + lnA
lnk = -Ea/R 1/t + lnA
y = m x + c
