C18 - Rates of reactions Flashcards
Equation of rate of reaction in terms of reactants / products
Rate = quantity reacted or produced / time
Equation of rate of reaction in terms of concentration
Rate = change in concentration / change in time
Units for concentration
Mol/dm^3/s
What does [A] mean?
Concentration of A
What is zero order?
Concentration of a reactant has no effect on the rate
What is 1st order?
Rate is proportional to the concentration raised to the power of 1
What is 2nd order
Rate depends on its concentration raised to the power of 2
What is the rate equation
Rate = k [A]^m [B]^n
What is ‘k’?
Rate constant
What does the ‘m’ and ‘n’ mean?
The order of the reaction with respect to that reactant
How do you calculate the units of ‘k’?
Rearrange formula
Substitute units into expression or k
Cancel common units
How do you calculate the overall order of a reaction?
Add all orders
How can you determine orders experimentally?
Measuring concentrations at equal time intervals from a titration or colorimeter practical
Compare concentrations
Determine relationship between concentration and rate
When should rate be measured during experiment?
As close to the start of experiment
(Initial rate)
What is the initial rate
Instantaneous rate at the beginning of an experiment when t=0
What is continuous monitoring?
Continuous measurements taken over the course of a reaction
What are the two methods of continuous monitoring
Monitory gas collection
Monitoring mass loss
Monitoring colour change
How are concentration-time graphs plotted
Continuous monitoring of reaction
How do you monitory colour change?
Using a colorimeter
What does a colorimeter do?
Wavelength of light passing through coloured solution is controlled using a filter
Amount of light absorbed by solution is measured
How do you use colorimetry
1) Prepare standard solutions
2) Select filter complimentary to colour of chemical
3) Zero the colorimeter with water
4) Measure the absorbance readings of the solutions
5) Plot a calibration curve
6) Carry out reaction to produce product that needs to be tested
7)Take absorbance readings of reacting mixture at measured time intervals
8) Use calibration curve to identify concentration at each absorbance reading
9) Plot concentration time graph to identify order
Concentration-time graph shapes
Zero order - Straight line with negative gradient
First order - Downwards curve with decreasing gradient over time (constant half life)
Second order - Downward curve that’s steeper at start but tails off more slowly (non-constant half life)
What is a half life?
Time taken for the concentration of a reactant to decrease by half
T(1/2)
What is exponential decay
Concentration halves every half life (constant half life)
How do you determine k of a first order reaction?
K = (ln2) / t(1/2)
Rate constant = (ln2) / half life
OR
Tangent to curve drawn at particular concentration
Gradient calculated giving rate of reaction
Rate constant determined is calculated by rearranging rate equation ( k = rate / (conc of reactants) )
What are the shapes of rate-concentration graphs?
Zero order - Horizontal straight line, rate = k
First order - Straight line graph, rate = k[A]
Second order - Upward curve, rate = k[A]^2
What reaction can you use to obtain the initial rate?
Clock reaction
Measure time to see visible change
1/t is proportional to initial rate
Repeated with different concentrations
Describe the iodine clock reaction
Colour change delayed by aqueous sodium thiosulfate (Na2S2O3) which removes iodine as it forms, when it’s used up the colour change occurs
Blue black colour occurs when aqueous iodine forms
Concentration of one of the reactants changed
Time measured for colour change to occur
1/t proportional to initial rate
Graph of 1/t against conc drawn
Graph’s shape will tell you order of reaction
How accurate are clock reactions
Approximate, but reasonably accurate given that less than 15% of the reaction has taken place
What’s a reaction mechanism
Series of steps that make up an overall reaction
What is the rate determining step
The slowest step in a reaction mechanism
How can you predict reaction mechanism
Order and species involved in rate equation only involve those that are true for the rate determining step
Hydrolysis if haloalkanes half equation
RBr + OH- —> ROH +Br-
Conditions for hydrolysis of haloalkanes
Hot aqueous alkali
How do you determine a rate mechanism for a reaction
What happens to rate constant when temp increases
Shifts Boltzmann distribution to right, Inc proportion of particles exceed Ea
(Particles move faster and collide more frequently)
What is the Arrhenius equation
What does the exponential factor represent
Proportion of molecules that exceed Ea, have sufficient energy for reaction to take place
What does the pre exponential factor represent
Frequency of collisions with correct orientation
Logarithmic form of the Arrhenius equation
How can the logarithmic form of the Arrhenius equation tell us the equation of a straight line
