Rates Flashcards
Rate of reaction
The change in concentration per unit of time
Activation energy
Minimum energy required for a reaction to occur
Why does a higher temperature increase the rate of reaction
Increase kinetic energy, more successful collisions
Greater frequency of successful collisions
Why does higher conc/pressure increase the rate of reaction
More particles within the space
More collisions
Frequency of successful collisions
Why does breaking a solid into smaller particles increase the rate?
Increase surface area
More collisions
Greater frequency of successful collisions
What is a catalyst
Substance with increases the rate of reaction without being used up
How does a catalyst work
Provides an alternative route with a lower activation energy
Rate equation
Rate = change in concentration/time
What is zero order
Changing the concentration of a reactant has no effect on the rate of reaction
What is first order
Concentration of reactant is directly proportional to the rate
[A] x2 = rate x 2
What is second order
Rate proportional to [reactant]^2
[A] x2 = rate x 4
Describe conc vs time graphs
Initially there is a high conc of reactants so a large frequency of collisions
As reactants start to get used up successful collisions become less frequent
Once one or more reactants are used up there can be zero successful collisions
What are the two types of experiments that can be done to determine the order with respect to different reactants
Continuous monitoring method (following course of a single reaction)
Initial rates method (doing multiple experiments)
What are the two ways to measure the progress of a reaction in continuous monitoring method
1 - by taking samples at regular intervals
2 - by using a visible indicator/physical property such as gas volume
Continuous monitoring method enables you to plot a…
…concentration time and tangent graph CTT
From the concentration time tangent graph how do you work out initial rate
Draw a tangent at t=0
Work out the gradient using change in y over change in x
How do you measure the rate after a certain time
Draw tangent at t = 600s (example)
Work out gradient using change in y over change in x
How do you work out orders from CTT graph
Take 2 tangents at different concentrations
Compare factor of change in concentration then factor of change in rates (gradients that you calculated)
How to carry out the iodine clock initial rates practical
1) measure known volumes of reactants. Keep them separate so reactant doesn’t start
2) measure known volume of sodium thiosulphate
3) start the stop clock when the last of reactants is added to conical flask. Do not add sodium thiosulphate last
4) stop timer when mixture turns blue/black and record
5) repeat the experiment using same volume and altering volume of KI
6) Rate = 1/time
7) plot a graph of 1/t against volume of potassium iodide
How to work out orders using initial rates experiment data
Compare the difference in concentrations to the difference in the rates
What is the Arrhenius equation
K = Ae ^(-Ea/RT)
K ~ rate constant
A ~ Arrhenius constant
e ~ e button on calculator
Ea ~ activation energy
R ~ molar gas constant
T ~ temperature
What can you rearrange the Arrhenius equation to
lnk = lnA - Ea / RT
How can you use Arrhenius equation and compare it to equation for straight line graph
When written as lnk = -Ea/RT + lnA
It has same form as Y = mx + c
Y = lnk
M = -Ea/R
X= 1/t
C= lnA
Ink = -(ᵉᵃ⁄ᵣ) ¹⁄ₜ + lnA
How do you work out the activation energy from the gradient
Calculate the gradient using ∆y/ ∆x
then x R and divide by 1000
What is meant by rate determining step
When a chemical reaction takes place there is always one step which is slower than the others
The species that take part in the rate determining step must…
….match the species in the rate equation
Example for rate determining:
(CH3)3CBr + OH- ——> (CH3)3COH + Br-
step 1: (CH3)3CBr ——> (CH3)3C+ + Br-
step 2: (CH3)3C+ + OH- ———> (CH3)3COH
what is the rate determining step
step 1 depends on (CH3)3CBr whereas step 2 depends on (CH3)3C+ and OH-.
As step 1 is dependent on the same species as the rate equation step 1 must be the rate determining species
