18 - Rates Flashcards
Two ways of determining rate
• Measuring the decrease in the concentration of a reactant over time
• Measuring the increase in the concentration of a product over time
How can rate equations be determines
can only be determined experimentally not by mole ratios
Rate-concentration = 0 order
o Changing the concentration of the chemical has no effect on the rate of the reaction
Is the 0 order included in rate equation
No
Rate from 0 order rate-concentration graph
o Rate = K // y-intercept
Rate - concentration 1st order
o The concentration of the chemical is directly proportional to the rate of reaction, e.g. doubling the concentration of the chemical doubles the rate of reaction
Rate from a first order rate-concentration graph
o Rate = gradient
Rate - concentration 2nd order
o The rate is directly proportional to the square of the concentration of that chemical, e.g. doubling the concentration of the chemical increases the rate of reaction by a factor of four
Rate from 2nd order rate-concentration graph
o Upward curve = plot rate against conc squared = straight line = k = gradient
- gradient of straight line
Overall order of a rate equation
sum of the powers of the reactants in a rate equation
Order with respect to [(CH3)3CBr]
• From the above table, that is experiments 1 and 2
o The [(CH3)3CBr] has doubled, but the [OH-] has remained the same
o The rate of the reaction has also doubled
o Therefore, the order with respect to [(CH3)3CBr] is 1 (first order)
Order with respect to [OH-]
• From the above table, that is experiments 1 and 3
o The [OH-] has doubled, but the [(CH3)3CBr] has remained the same
o The rate of reaction has increased by a factor of 4 (i.e. increased by 22)
o Therefore, the order with respect to [OH-] is 2 (second order)
Rate equation of this
Rate = k [(CH3)3CBr] [OH-]2
Draw 0 order rate concentration graph
Draw second order rate-concentration graph
Draw third order rate-concentration graph
How to find k
Use one row from table
Units of k
Monitoring rate with a colorimeter
• In colorimeter – wavelength of light passing through a coloured solution is controlled using a filter
• Amount of light absorbed is measures
How to use a colorimeter - 6 marks
Concentration - time graph 0 order
• In a zero-order reaction, the concentration of the reactant is inversely proportional to time
• When the order with respect to a reactant is 0, a change in the concentration of the reactant has no effect on the rate of the reaction
Rate constant of concentration time graph
gradient= rate constant k
Draw 0 order concentration time graph
1st order concentration-time graph
• In a first-order reaction, the concentration of the reactant decreases with time
o The graph is a curve going downwards and eventually plateaus:
Draw a first order concentration time graph
How to deduce order of reaction from graph
Half life
Half life is only for…
Concentration - time graph
Zero order half life
successive half-lives decrease with time
First order half life
half-life of a first-order reaction remains constant throughout the reaction
Second order half life
half-life increases with time
Define half life
the amount of time taken for the amount (or concentration) of the limiting reactant in a reaction to decrease to half its initial value
Determination of K for a first order reaction - two ways - concentration-time
- rate
-half life
1st order reaction has… (concentration-time)
Constant half life
Calculating the rate constant from the rate - first order concentration - time
Calculating the rate constant from half-life - first order concentration - time
Clock reaction
Idk
What is the rate determining step
Reactant in rate-determining step is in the rate equation
Feature of rate determining step
Slowest step of
Example of rate determining step
Increasing temp on rate
• Increasing temp increases rate of reaction (may not impact yield due to la chateliers principle)
• Therefore increasing temp increases the value of the rate constant, k, assuming that the concentration of the reactants remains unchanged
relationship between the rate of reaction and temperature
exponential
When temp of a reaction mixture increases,
particles to move around faster resulting in more frequent collisions
o proportion of successful collisions increases, meaning a higher proportion of the particles possess the activation energy to cause a chemical reaction
Boltzmann distribution
Arrhenius equation
Calculate the activation energy of a reaction which takes place at 400 K, where the rate constant of the reaction is 6.25 x 10-4 s-1.
A = 4.6 x 1013 and R = 8.31 J mol-1 K-1.
Take natural logs of both sides
Arrhenius plot:
• A graph of ln k against 1/T can be plotted, and then used to calculate Ea
How to draw Arrhenius plot - remember equation (look at it in form y=mx+c)
How to find rate constant from concentration time graph - method 2
what is the rate constant
a constant of proportionality in the rate equation
how would you prove that the gradient of the graph gives you the rate constant
For a first-order reaction of a reactant A:
rate = k[A]
Therefore, the gradient of a rate–concentration graph gives the value of the rate constant, k.
if you equate the rate to be 1/t what assumption is made + how valid is this
The assumption is that the reaction proceeds at constant rate to the measured
point.
The assumption is reasonable so long as the point is soon after the start of the reaction.
how could you follow the progress of a reaction between gaseous hydrogen and gaseous bromine to make gaseous hydrogen bromide
A colorimeter
could be used to detect the loss of colour of the bromine as it formed hydrogen bromide
when predicting equations what should you focus on
the number of moles in the rate equation ( the power) and the number of moles in the overall equation
does the order of the reactant in the rate equation match up to the number of moles in the rate determining step
?
what is the pre-exponential factor
A
units of the parts of the Arrhenius equation ???
R - Jmol−1K−1
A -
K -
Ea - Jmol-1
T - K
