Chapter 18 - Rates of Reactions Flashcards
(57 cards)
What is the shorthand for the concentration of A
[A]
What is the order of reaction?
The power of the concentration. It can change with different reactants and affects the rate of the reaction in different ways.
What is the equation of rate to concentration
Rate ∝ [A] ^n
What does zero order mean
This means that the concentration of a reactant has no effect on the rate.
The reaction is zero order with respect to the reactant.
What does first order mean
A reaction is first order with respect to a reactant when the rate depends on its concentration raised to the power of 1.
If the concentration is multiplied by 2, then the rate if also multiplied by 2.
What does second order mean
If the concentration is multiplied by 2, the rate of the reaction is multiplied by 4.
The rate equation
Rate = k [A] ^m [B] ^n
It gives a mathematical relationship between the concentration of the reactants and the reaction rate. K is the reaction rate.
Overall order
M+N
How to find out the units of k
- Rearrange to make k the subject
- Substitute units into that expression
- Cancel the common units.
How can you calculate the overall order from experimental orders
You should get the tangent and the rate of the reaction. Then compare each repeat of the experiment. How does changing the concentration of a reactant change the initial rate? If you *2 the conc, and the rate is *2, then that is the first order of that reactant.
What is continuous monitoring
They can be plotted from continuous measurements taken during the course of a reaction.
Examples of continuous monitoring
Monitoring by gas collection
Monitoring by mass loss
If no gases are produced, then a colour change?
Explain a colorimeter
The wavelength of the light passing through a coloured solution is controlled using a filter. The amount of light absorbed by a solution is measured.
Zero order in a conc-time graph
A straight line with a negative gradient.
The gradient = k.
First order in a conc-time graph
Downward curve with a decreasing gradient over time.
Half life: the time for the concentration to halve is constant and called the half life.
The rate constant can be figured out using this value.
Second order in a conc-time graph
Also a downward curve, steeper at the start and tailing off more slowly.
Half life.
T1/2.
Time taken for half the reactant to be used up.
1st order relationship can be confirmed from a conc-time graph by measuring the number of successive half-lives. If they are the same, then the reaction is 1st order with respect to that reactant.
Determining k for a 1st order reaction by calculating rate constant from the rate.
Draw tangent and calculate rate. Then put this in the rate equation to calculate k.
Determining k for a 1st order reaction by using half-life
K= ln2/ t1/2.
PAG for using a colorimeter
- Prepare solution of known conc.
- Select filter
- Zero the colorimeter with water
- Measure the absorbance
- Plot a calibration curve
- Carry out reaction
- Measure absorbance of unknown curve
- Using calibration curve, get the concentration.
Why are rate- concentration graphs good?
They offer a direction route in the link between rate and the concentration in the rate equation.
On a rate- concentration graph, what does zero order look like?
A horizontal straight line with 0 gradient.
This means that the y-intercept is equal to k.
On a rate- concentration graph, what does first order look like?
A straight line through origin.
Where the rate is proportional to the concentration.
The gradient is equal to the gradient.
On a rate- concentration graph, what does second order look like?
An upward curve with an increasing gradient.
Because the rate is equal to the K * concentration squared.
So plot another graph, rate against the concentration^2.
This gives you a straight line graph, going through the origin, where the gradient = k.
