Chapter 18 - Rate of reactions Flashcards
How are reaction rates measured ?
By observing the changes in quantities of reactants or products over time
What is the equation for the rate of reaction ?
Change in concentration / change in time
Why is change in concentration and time used ?
For consistency throughout the reaction
What is concentration measured in ?
Mol/dm^3/s
What is the shorthand format for concentration ?
- [A] = concentration of reactant A
- It has the usual units of concentration
How are rate of reaction and concentration of a particular reactant raised to a power related ?
They are proportional
What is the equation for rate of reaction and concentration ?
rate is proportional to [A]^n
What is the power ?
The order of the reaction
What are the common orders of reaction ?
- Zero order
- First order
- Second order
When is the order zero ?
When the concentration of a reactant has no effect on the rate
What is the equation for zero order reactions ?
rate is proportional to [A]^0
What happens in a zero order reaction ?
- Any number raised to the power of zero is 1
- Concentration does not influence the rate
When is the order one ?
When the rate depends on its concentration raised to the power of one
What is the equation for first order reactions ?
rate is proportional to [A]^1
What happens in a first order reaction ?
If the concentration of A is doubled, the reaction rate increases by a factor of 2^1 = 2
When is the order two ?
When the rate depends on its concentration raised to the power of two
What is the equation for second order reactions ?
rate is proportional to [A]^2
What happens in a second order reaction ?
If the concentration of A is doubled, the reaction rate increases by a factor of 2^2 = 4
What does the rate equation give ?
It gives the mathematical relationship between the concentrations of the reactants and the reaction rate
What is the rate equation ?
rate = k [A]^m [B]^n
What is the rate constant also known as ?
- The proportionality constant
- It is the number that mathematically converts between the rate of reaction and the concentration of orders
What does the overall order of a reaction give ?
The overall effect of the concentrations of all reactants on the rate of reaction
How is the overall order of the reaction calculated ?
Overall order = sum of orders with respect to each reactant
What do the units of the rate constant depend upon ?
The number of concentration terms in the rate equation
How can you determine the units of k ?
- Rearranging the equation to make k the subject
- Substitute units into the expression for k
- Cancel common units and show the final units
What are the units for overall order 0 ?
mol/dm^3/s
What are the units for overall order 1 ?
/s
What are the units for overall order 2 ?
dm^3/mol^1/s
What is initial rate?
The instantaneous rate at the beginning of an experiment when t=0.
In a concentration time graph, and a rate time graph, what would zero order look like?
- Concentration time - linear to 0 concentration
- Rate time - Horizontal line
In a concentration time graph, and a rate time graph, what would first order look like?
- Concentration time - curve getting closer to zero
- Rate time - Linear proportional
In a concentration time graph, and a rate time graph, what would second order look like?
- Concentration time - steeper curve
- Rate time - proportional to x^2
What are the two ways of continuous monitoring when a gas is produced?
- Gas collection
- Mass loss
What is a way of utilising light to monitor rate?
- Using a colorimeter.
- Measuring the amount of light absorbed by a solution.
What is half life?
The time taken for half of a reactant to be used up.
In a concentration time graph, how can you deduce whether the reactant is first or second order?
- Measure multiple half lives.
- If the half life is constant, it is first order.
- If the half life is increasing, it is second order.
How do you calculate the rate constant from the rate of reaction ?
k = rate/concentration
How do you calculate the rate constant from the half life ?
k = ln|2| / t(1/2)
How can rate concentration graphs be plotted ?
From measurements of the rate of reaction at different concentrations
Why are rate concentration graphs important ?
They offer a route into the direct link between the rate and concentration in the rate equation
What graph does a zero order reactant produce ?
Horizontal straight line with zero gradient
What does the rate equal in a zero order reaction ?
rate = k
How do you find the rate constant from a zero order graph ?
The intercept on the y axis gives the rate constant k
What is the relationship between the rate and concentration in a zero order reaction ?
The reaction rate does not change with increasing concentration
What graph does a first order reactant produce ?
Straight line graph through the origin
What does the rate equal in a first order reaction ?
rate = k[A]
How do you find the rate constant from a first order graph ?
The rate constant can be determined by measuring the gradient of the straight line of this graph
What is the relationship between the rate and concentration in a first order reaction ?
Rate is directly proportional to the concentration
What graph does a second order reactant produce ?
Upward curve with increasing gradient
What does the rate equal in a second order reaction ?
rate =k[A]^2
How do you find the rate constant from a second order graph ?
- Plot a second graph of rate against concentration squared
- This will give u a straight line through the origin
- The gradient of this straight line graph is equal to the rate constant
What is the relationship between the rate and concentration in a second order reaction ?
Rate increases as concentration increases
What is a clock reaction ?
A more convenient way of obtaining the initial rate of a reaction by taking a single measurement
What do you look for/measure in a clock reaction ?
The time t from the start of an experiment is measured for a visual change to be observed
What else are you measuring in a clock reaction ?
- An average rate of a change in reactant over time
- The shorter the period of time over which an average rate is measured, the less the rate changes over that time period
What is the initial rate proportional to ?
1/t
As temperature increases, how does the rate of the reaction change ?
The rate increases
As temperature increases, how does the rate constant increase ?
The rate constant increases
For many reactions, what does a 10 degree rise in temperature result in ?
- Doubles the rate constant
- Doubles the rate of the reaction
When temp increases, what two factors contribute to the increase in rate ?
- Boltzmann distribution shifts right, increasing the proportion of particles that exceed the activation energy
- As the temp increases, particles move faster and collide more frequently
What is the change in rate mainly determined by ?
Activation energy
Why is the change in rate mainly determined by the activation energy ?
The increased frequency of collisions is comparatively small compared with the increase in the proportion of molecules that exceed Ea from the shift in the boltzmann distribution
What is the Arrenhius equation ?
An exponential relationship between the rate constant and temperature
What is the formula for the Arrenhius equation ?
k = A x e^(-E(a)/RT)
What is A known as in the Arrenhius equation ?
Pre exponential factor
What is E(a)/RT known as in the Arrenhius equation ?
Exponential factor
What does the exponential factor represent ?
The proportion of molecules that exceed the activation energy and have sufficient energy for a reaction to take place
What is the pre exponential term also called ?
The frequency factor
What does the frequency factor take into account ?
It takes into account the frequency of collisions with the correct orientation
When does the frequency factor increase ?
Increases slightly with temperature but it is essentially constant over a small temperature range
What does the frequency factor essentially give us ?
The rate if there were no activation energy
What is the logarithmic form of the Arrhenius equation?
ln|k| = -E(a)/RT + ln|A|
What can the log form of the Arrhenius equation be modelled as?
- A straight line graph in y=mx + c
- ln|k| is y
- 1/T is x
- E(a)/R is gradient
- ln|A| is y intercept
Define stoichiometry
- The balancing numbers
- The relative amounts of the species in the reaction
Is it likely for more than two particles to collide at the same time ?
No it is unlikely that more than two particles will collide at the same time
Why is a single step reaction unlikely ?
Multiple ions colliding simultaneously is an extremely unlikely event
How are reactions likely to take place ?
In a series of steps
Define reaction mechanism
The series of steps that make up an overall reaction
Define rate determining step
The slowest step in a multi step reaction pathway
How can you predict reaction mechanisms (RDS) ?
- The rate equation includes reacting species involved in the RDS
- The orders in the rate equation match the number of species involved in the RDS
Why is the RDS important ?
It provides important evidence in supporting or rejecting a proposed reaction mechanism
What does investigating the hydrolysis of haloalkanes determine ?
- The overall order of reaction
- The rate equation
- Possible mechanism for the reaction
