Chapter 18 - Rates of reaction Flashcards
What is the ‘rate equation’?
Rate = Quantity reacted or produced / time
or
Rate = change in concentration / time
What are the units for rate?
mol dm^-3 s^-1
What are the 3 ‘orders’ of a reaction?
Zero order, first order and second order
What is ‘zero order’?
When [A] is risen to the power of 0 and the concentration doesn’t effect the rate
What is ‘first order’?
When [A] is risen to the power of 1 and any change to the concentration gives the same factor effect to the rate
What is ‘second order’?
When [A] is risen to the power of 2 and any change to the concentration gives a squared factor of the value on the effect on the rate
How do you work out the overall order?
Add all the order values together
(eg. rate=k[A]+[B]^2 , the over rate order = 2+1 = 3)
How do you work out the units for the rate constant k?
Make k the subject of the rate equation then divide the rate (mol dm^-3 s^-1) by how the orders amount of concentration (eg second order = mol dm^-3 mol dm^-3)
What are the rate constant k units for a first order equation?
s^-1
What are the rate constant k units for a second order equation?
mol^-1 dm^3 s^-1
What are the rate constant k units for a zero order equation?
mol dm^-3 s^-1
What is ‘continuous monitoring’?
When you monitor the measurements throughout a reaction (usually to create a concentration-time graph)
This can be done by monitoring the mass lost, gas collection or by colour change
What apparatus is used to measure colour change?
Colorimeter (measures the amount of light absorbed by a solution)
What would the orders look like on a concentration-time graph?
Zero order - straight line with a negative gradient
First order - downward curved line with a decreasing gradient (have a constant concentration half life)
Second order - similar to first order except the line starts off steeper and levels off faster
What is a ‘half-life’?
The time taken for half of the reactants to be used up (pattern is called exponential decay)
How can you work out the rate from a first order graph?
By working out the gradient of a tangent at a given point (gradient = change in y/ change in x)
or by working out k from the natural log2 (ln2)/ half-life value
What would the orders look like on a rate-concentration graph?
Zero order - horizontal line (concentration of zero order have no effect on the rate, rate = k)
First order - straight line graph (y=x, as rate is directly proportional to the concentration)
Second order - upwards curve with an increasing gradient
How do you work out the rate constant k of a second order rate-concentration graph?
Create another graph of the values of rate against the concentration squared, then the gradient of the new line will be the rate constant k
What is the ‘initial rate of reaction’?
The instantaneous rate of reaction at the start of a reaction (when t=0)
How can you find the initial rate?
By finding the gradient of a tangent at t=0 on a concentration-time graph
What is another way of finding the initial rate of reaction with one measurement?
A clock reaction (recording the time it takes for a colour change or precipitate to be formed in a reaction), therefore initial rate is 1/t
What is the ‘reaction mechanism’?
The series of steps that make up an overall reaction.
What is the ‘rate determining step’?
The slowest step in a multi-step reaction mechanism
How can you predict reaction mechanisms?
- Rate equations only include 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 can you determine the overall reaction from the multi-step mechanism?
Cross cancel any compounds that appear on the left and right hand side of the reaction until you are left with your overall reactants and products
Describe what effect temperature has on the rate constant, making references to the Boltzmann distribution curve
Increasing the temperature causes the curve to shift to the right, increasing the proportional number of particles that are able to overcome the Ea (activation energy) as particles move faster under higher temperatures which leads to a higher frequency of collisions (at the right energy and orientation)
What does the Arrhenius equation suggest?
The exponential relationship between the rate constant k and temperature T
What is the Arrhenius equation and how can it be written as a logarithmic equation?
k = Ae ^(Ea/RT)
or ln k = ln A - (Ea/RT)
What value does R, T, A and Ea represent?
R - 8.314 J mol-1 K-1
T - temperature (K)
A - frequency factor / pre-exponential factor
Ea - activation energy (exponential-factor)
What is y-intercept in the logarithmic equation for Arrhenius?
ln A
What is the gradient value in the logarithmic equation for Arrhenius?
-Ea/RT
What is the x value in the logarithmic equation for Arrhenius?
1/T
What is the label that goes on the y-axis of a graph with the logarithmic Arrhenius equation?
ln k
