(2) Rate Equations Flashcards
Define rate of reaction.
the change in concentration of a reactant or product per unit time
Define Activation energy.
the minimum amount of energy needed to start a chemical reaction.
Why does high temperature increase rate of reaction?
- More particles have energy greater of equal than activation energy
- There is higher frequency of successful collisions
Why does a higher conc/pressure increase the rate of reaction?
- There are more particles within the same amount of space
- so more frequent and successful collisions
Why does breaking a solid into smaller particles increase rate?
Increased surface area so more frequent, successful collisions
- more surface exposed for collisions
What is a catalyst?
- A substance which increases the rate of reaction without being used up
How does a catalyst work?
- Provides an alternate reaction route with a lower activation energy
Maxwell-Boltzmann distribution key points?
- Peak is the most probable energy
- total amount of particles = area under graph
- area to the right of Ea represents the particles which have enough energy to react
Majority of reactions start off with a fast rate of reaction and then slow down before the reaction stops. Why is this?
- Initially there’s a high concentration of reactants so large frequency of successful collisions
- as reactants start to get used up, successful collisions become less frequent
- once one or more reactants are used the can be zero successful collisions
Define order.
sum of the order with respect to each reactant in the rate equation
(how changing the concentration affects the rate).
Define 0 order.
rate is independent of concentration.
Define 1st order.
rate is directly proportional to concentration.
Define 2nd order.
rate is proportional to the square of the concentration.
How do you show the three orders in an equation? Use [X] for reference.
Zero order = Not written in equation
First order= [X]
Second order = [X]2
What’s the overall order of:
R=k[X][Y]2
3rd
What’s the overall order of:
R=k[X][Y]2
3rd order
What is the rate constant?
‘k’
- Specific for different reactions
- Only affected by temperature
- k=rate / reactants
How do you determine order of reactions?
- Experimentally
How can you determine the units for k?
Powers of rate minus Powers of reactants in rate equation
so for : Rate= k[X]2[Y]
1-3 = -2
mol dm-3 x -2 = mol-2 dm6 s-1
What are the 2 types of experiment which can be done to determine the order with respect to different reactants?
- Continuous monitoring
- comparing initial rates
How does the continuous monitoring method work?
There are two ways to measure the progress of a reaction:
- Take samples at regular intervals
- Using a visible indicator
How would you solve the problem of the reaction mixture still reacting?
- use COLD DISTILLED water and COOL and DILUTE the reaction to stop it reacting further.
(QUENCHING the reaction)
What is a problem when finding order by monitoring a physical property, such as volume of gas produced at regular intervals?
- Orders of reaction are for reactants
- We measure production of a product
- Volume would need to be converted to moles to calculate rate
Where is concentration and time plotted on a CTT graph?
Concentration always on Y axis
Time on X axis
Where do you draw tangent when asked to measure the initial rate and rate at a specific time?
Initial rate = point where the time is at 0 but and conc is at highest
Specific time= tangent just touches the specific time
Rate = gradient of tangent
What is the order when conc change from 0.1 to 0.03 and rate changed from 2x10^-5 to 6x10^-4?
0.1 –> 0.03 was x3.3 (0.1/0.03)
2x10^-5 –> 6x10^-4 was x3.3 (2x10^-5/ 6x10^-4)
change in rate is directly proportional to change in conc so its FIRST ORDER
What is the order when conc change from 0.1 to 0.03 and rate changed from 2x10^-5 to 2x10^-5?
0.1 –> 0.03 was x3.3 (0.1/0.03)
2x10^-5 –> 2x10^-5 had no change
- change in conc has no change in rate so its ZERO ORDER
What is the order when conc change from 0.1 to 0.2 and rate changed from 2x10^-5 to 8x10^-5?
0.1 –> 0.2 was x2
2x10^-5 –> 8x10^-5 was x4
(4 is 2^2)
rate is proportional to the square of the concentration so its SECOND ORDER
What do you have to make sure of when working out order for a specific reactant on a table with multiple reactants?
- Working out order from experiments where other reactants have CONSTANT CONCENTRATION
What’s the scale factor when concentration of a reactant is halved?
x0.5
Outline the iodine clock reaction.
- Measure known volumes of reactants. Keep them separate so the reaction doesn’t start.
- Measure a known volume of sodium thiosulphate.
- Start the stop clock when the last of the reactants is added to a conical flask.
- Do not add sodium thiosulphate last.
- Stop the timer when the mixture turns blue/black and record.
- Repeat the experiment using the same total volume altering the volume of KI
Rate = 1/time
Plot a graph of 1/t against vol of KI
What is the equation for Arrhenius and what does each symbol stand for?
K= Ae ^-Ea/RT
K= rate constant
A= Arrhenius constant
e= exponential
Ea= activation energy
R= gas constant
T= Temp (K)
What is the opposite of exponential?
natural log
What is the simplified equation of Arrhenius?
lnK = lnA - Ea/RT
If Ea increases what happens to K?
K will decrease
- less particles with sufficient energy for a successful collision to occur
What happens to K when Temperature increases?
K will increase
- particles have more KE so higher chance of successful collisions
What is the rearranged formula to find Ea from Arrhenius?
Ea= (lnA - lnK) x RT
always work them out separately then multiply for more accurate answer
Compare Arrhenius equation to the straight line equation..
y= mx + c AND lnk= lnA- Ea/RT
y= lnK
m= -Ea/R
X= 1/T
c= LnA
What is the equation to calculate Ea using gradient?
> work out gradient using diif y/diff x
> gradient = -Ea/R
> Ea = -(gradient x R)
What is the equation to calculate A (from lnA) on a graph?
Pick two point from graph eg (-1.1, 3.2x10^-2)
lnk= - (Gradient x X axis value) + lnA
> add gradient on lnk to get lnA
> use e button and input lnA to get A on its own
Suggest why initial rates of reaction are used to determine these orders rather than rates of reaction at other times during the experiments.
The concentrations are known at the start.
Key practical notes?
1) The concentration does not itself need to be known; It is assumed that the volume is proportional to the concentration
2) In initial rates experiments the rate of reaction is proportional to 1/time
Rate = 1/t
Define ‘Rate determining step’.
THE SPECIES IN THE RATE DETERMINING STEP MUST MATCH THE SPECIES IN THE RATE EQUATION.
