Chapter 18 - Rates of Reaction Flashcards
What does changing the concentration of a reactant change in terms of reactions?
The rate of reaction
But not always
What is the rate of reaction proportional to?
The concentration of a particular reactant raised to a power, ‘the order of reaction’
What is zero order and what do it look like?
Any number raised to the power zero is one
So changing the concentration of ‘A’ in this case does not affect the rate of reaction
What is first order and what does it look like?
If a reactant is order 1, whatever factor is changed to the concentration, the rate will change by same factor
e.g. if the conc. of ‘A’ is trebled the rate will treble
What is 2nd order and what does it look like?
If a reactant is order 2 then the rate will change by the square of whatever factor you change the concentration by
e.g. if the conc. of ‘A’ is double the rate will go up by a factor of 2²
What is meant by the rate equation?
Gives the mathematical relationship between the rate and the conc. of each reactant
What is the rate constant?
k, is temperature dependant
What does the rate constant’s units depend on?
The actual orders for each reactant
How do we find the units for the rate constant, k?
Rearrange the equation in terms k
Substitute in all other units, cancelling also
What is meant by the overall order of reactio?
Gives the overall effect of the concentrations of all reactants on the rate of reaction
e.g.:
Worked example:
What is meant by colorimetry?
Quantitative technique
Used to measure the intensity of a colour
How is the wavelength of the light controlled when passing through the solution?
Using a filter
How do we convert absorbance unit into concentration values?
A calibration curve is needs to be constructed
What is the practical for working out a calibration curve?
Prepare standard solutions of known concentration of the coloured chemical
Select a suitable filter with the complementary colour to that of the coloured chemical
Zero the colorimeter with distilled water
Measure the absorbance readings of the standard solutions
Plot a calibration curve of absorbance against concentration
Carry out reaction taking a.u readings at time intervals
What does a zero order concentration-graph look like?
The line is straight with a -ve gradient
The gradient is constant
The value of the gradient is equal to the rate constant, k
What does a first order concentration-time graph look like?
The gradient/rate decreases as the reaction progresses. It is proportional to the decreasing concentration of A
How do we prove the order is one in a concentration-time graph?
Test to see if the graph has a constant half life
What is meant by the half-life?
Time taken for the concentration of a reactant to decrease to half its original value
How do we calculate the rate constant from half-life?
k=ln2/t1/2
How do we find the order of a reactant from experimental data?
We plot rate vs concentration
Observe at general shape of graph
e.g.
Describe the rate-concentration graph for first order
Rate is directly proportional to the concentration of reactant
Describe the rate-concentration graph for rate vs concentration²
