Rate Equations Flashcards
Define the rate of reaction
Change in concentration per unit of time
Define activation energy
The minimum energy required for a reaction to occur
Why does a higher temperature increase the rate of reaction
More particles have more energy than activation energy, there are more frequent successful collisions so particles react
Why does a higher concentration/pressure increase the rate of reaction
More particles in the same volume, more frequent successful collisions
Why does breaking a solid into smaller powder particles increase the rate?
More surface area, more frequent successful conditions
What is a catalyst
A substance which increases the rate of a reaction
Without being used up
How does a catalyst work
Provides an alternative reaction route, with a lower activation energy
Key points for the maxwell-Boltzmann distributions
Must start at the origin-> no particles have 0 energy
The highest peak is the most probable energy
Never touches the X axis at the end
Area under the curve tells you the number of particles
What is the calculation of rate
Rate= change in concentration/ time
Why do chemical reactions start off with a fast rate and then slow down
high conc of reactants means a high rate of collisions. As reactants get used up successful collisions become less frequent, once one or more reactants are used up there are 0 successful collisions
Zero order
If changing the concentration of a reactant has no effect on the rate of reaction it is said to be zero order with respect to the rate of reaction.
For example if the concentration of A doubles, the rate remains the same
As a graph this is a horizontal line
1st order
Changing concentration is directly proportional to rate of reaction
For example if the concentration of A is doubled, the rate is also doubled
As a graph this is a vertical line
Second order
Changing the concentration increases the rate of the reaction by the square of the concentration
For example if concentration A is doubled, the rate is squared.
As a graph this starts of steep and curves
How could you express rate for X+Y—> Z
Rate= K[X][Y]2
How do you calculate the units for an order reaction
1-the overall order
Moldm-3s-1
Why would you use a large excess of reactant
Means the order of reaction is zero with respect to that reactant as the reactants concentration is effectively constant
What are the two types of experiment done to determine the order of a reaction
Continious monitoring- following the course of a single reaction
Initial rates method- doing multiple experiments
Continious monitoring method
Take samples at regular intervals by using a visible indicator such as gas volumes
What is the problem with the continious monitoring method
The reaction is still reacting- quench the reaction, add a large volume of cold distilled water, it will cool and dilute the reaction, decreasing the rate.
Monitoring using a physical property and its problem
The volume of gas produced could be measured at regular intervals
Problem- it records the product produced and the order of reaction is about reactants. Volume recorded would need converting to reactant concentration
How to measure a reaction that changes colour
Colourimetry
Draw graphs for zero, first and second order for continious monitoring method
Reference to picture on phone
How to measure the initial rate using a graph produced from the continuous monitoring method
Draw a tangent, find the gradient- this will give the rate for that specific time.
How to work out an order from a CTT graph- use the example:
Concentration went from 0.1 to 0.03. The rate changed from 2x10-4 to 6x10-5
Take two tangents at different concentrations and compare the factor of change in concentration to rates.
Conc and rate have decided by 3
Rate is proportional to change so 1st order
How does an initial rate reaction occur
The time it takes to get to a specific point in the reaction is recorded for an experiment. E.g- time is recorded when 20cm3 gas is produced or when mixture changes or a ppt is formed
Assumptions made in initial rates
When changing volume of A, concentration is proportional to volume of sample because total volume is kept the same using water
Rate is 1/ts-1
The thiosulfate- acid reaction
Na2S2O3 + 2HCl —> 2NaCl + SO2 + H2O + S(makes it cloudy)
Calculate the time it takes for the solution to go cloudy, the volume of S2O32- is changed every time to measure order
Why does the concentration not need to be known in this experiment
Total volume must be constant. [reagent] is proportional to volume
What is the rate of reaction in initial rates
1/time. Mass of sulfur formed is not known. However, it is the same each time you repeat it.
iodine clock initial rates reaction
Measure known volumes of some reagent
Measure known amounts of X or use a calorimeter
In separate containers add A B and C
Start timer at the point of mixing
Time recorded for appearance of blue colour
Use the same conc of B and C and same volume of X
Keep temperature same and constant
Repeat with different concentrations of A
1/Time taken to measure the rate
Plot graph of 1/t against volume of A
Straight line of best fit through origin and find order
Zero order from initial rates
Change in concentration X has no effect on the rate so x is zero order
Horizontal line
First order for initial rates
Rate is proportional to change in [H2O2] so H2O2 is zero order
Second order for initial rates
Rate is proportional to [NH3]2- so is second order
Rate and temperatures
The higher the temp the faster the rate of reaction, particles have more kinetic energy so a greater proportion of particles have activation energy so greater frequency of successful collisions
Arrhenius equation for rate
k= Ae^-Ea/RT
Equation to find activation energy
Find the gradient and multiply by 8.31. Then divide by 1000
rearrangement to form A
A= K/e^Ea/Rt
Rearrangement for Ea
Ea= RT(LnA-lnK)
Rearrangement for T
T= Ea/R x (LnA-lnK)
How do you determine which step is the rate determining step
Must match the species in the rate equation.
Example of the rate determining step
(CH3)3CBr + OH- —> (CH3)3COH + Br-
Step 1- (CH3)3CBr —> (CH3)3C+ + Br-
Step 2- (CH3)3C+ + OH- —> (CH3)3COH
Step one only depends on [(CH3)3CBr] whereas step two depends on [(CH3)3C+] + [OH-]
As step one is dependant on the same species at the rate equation, step 1 must be the rate determining step
By considering both rate and yield, suggest why the reaction is carried out at 300 and not any higher
Would increase rate and decrease yield, chosen temperature compromises both factors
The effect of less molecules in a reaction
Particles spread apart so less frequent successful collisions between molecules
Explain why doubling the temperature has a greater effect on the rate of reaction than doubling the concentration of E
Reaction occurs when molecules have energy greater than activation energy- doubling T causes many more molecules to have this whereas doubling E only causes a change in the conc of E
State one method that you would use to distinguish these solutions other than smell
Measure pH with a meter, methylamine has a higher pH
Suggest the role of the cyanide ion and explain your answer
Catalyst- appears in the rate equation but not the written
State and explain why different volumes of water are added to mixtures in the experiment
To make volume constant for all mixtures so that volume of propanone is proportional to concentration.
Suggest the purpose of adding sodium Hydrogencarbonate to the reaction
To quench the reaction by neutralisation- reaction is ongoing and adding excess will decrease the rate of reaction
Suggest why the order with respect to iodine is 0
iodine is not involved in the rate determining step
Explain why using excess of a concentration is useful
Large excess means that concentration of reactant is effectively constant
Suggest why initial rates of reaction are used to determine these orders rather than rates of reaction
Concentrations are known at 0
State the significance of this zero order for the mechanism of the reaction
No significance- doesn’t occur in a rate determining step, only A does.
State one way this method could be improved over than repeating
Establish a constant temp using a water bath, increases accuracy of time for colour change
