Module 5 Section 1: Rates, Equilibrium and pH Flashcards
Expression for equilibrium constant
Remember [ ]
Calculate Kc (with units) for the reaction:
Kc =
What is a homogeneous mixture
All the reactants and products are put into the expression for the equilibrium constant
All reactants and products are in the same state
What is a heterogeneous mixture
Only gases and aqueous substances go into the expression for the equilibrium constant (solids and liquids are left out)
However, if the solvent isn’t aqueous you must include any water in the Kc expression
Reactants and products are different states
Expression for the equilibrium constant for:
Reactants and products are a mixture of aqueous and solid (heterogeneous) so only aqueous substances are included in Kc
Find the units when (mol dm-3)^2 / (mol dm-3)^4
(mol dm-3)^2 / (mol dm-3)^4 = 1 / (mol dm-3)^2 = 1 / mol^2 dm^-6 = mol^-2 dm^6
Equation: A(g) ⇌ 2B(g)
4.00 moles of A was placed in a 20.0dm3 container and heated to 320K until equilibrium had been established
The equilibrium mixture was found to contain 1.50 moles of A
Calculate Kc at this temperature
A(g) ⇌ 2B(g) 1 : 2
What does the order of reaction tell you
With respect to a particular reactant, the order of reaction says how the reactants concentration affects the rate
What does the zero order mean
If you double the reactant’s concentration, the rate stays the same
Concentration does not influence rate
Increases by 2^0 = 1 (stays same)
What does the first order mean
If you double the reactant’s concentration, the rate also doubles
Increases by 2^1 = 2 (doubles)
Tripling the concentration triples the rate
What does the second order mean
If you double the reactant’s concentration, the rate multiplies by 4
Increases by 2^2 (multiplies by 4)
Tripling the rate increases the rate by a factor of 3^2
What is the overall order
The sum of the orders of all the different reactants
What is the only way to find out the orders of reaction
Can only be found from experiments
Not from chemical equations
Methods to work out the order of reaction for a specific reactant in a reaction
Continuously monitor the change in concentration of the chosen reactant against time to construct a rate-concentration graph
Or
Use an initial rates method to find out how the initial rate changes as you vary the concentration of the chosen reactant
For each method, the concentrations of other reactants must be in excess so the change in rate is only due to changing the conc of the chosen reactant
How to construct a rate-concentration graph from a concentration-time graph
Find the gradient (represents the rate) at various points along the conc-time graph
This gives a set of points for the rate-conc graph
Plot points and join them up with line or smooth curve
Shape of the graph shows the order
What the different orders look like on a concentration-time graph
Zero order: horizontal line to show that changing the concentration doesn’t change the rate
First order: straight line through origin, shows the rate is proportional to [X]
Second order: curve meaning that the rate will be proportional to [X]^2
How to work out the order of reactions for reactants using experimental data
Look at what happens when the conc doubles for a specific reactant and see how this affects the rate
If rate stays the same, order is zero
If rate doubles, order is first
If rate quadruples, order is second
What do rate equations mean
They tell us how the rate is affected by the concentrations of reactants
Rate equation with units
k: rate constant, bigger it is the faster the reaction
m, n: the orders of the reaction with respect to reactants A and B
m says how the conc of A affects the rate (same for n and B)
Units of rate are mol dm-3 s-1
Find rate equation for reaction:
Rate equation: k[CH3COCH3]^1 [H+]^1 [I2]^0
[X]^1 can be written as [X] and [X]^0 = 1 so can be left out
Simplified: k[CH3COCH3] [H+]
Steps to work out units for a rate equation
1.Rearrange the equation to make k the subject
2.Substitute units into the expression for k
3.Cancel common units and show the final units on a single line
How to find rate constant from rate-concentration graph of first order reactions
If the overall reaction is first order the rate constant is equal to the gradient of the rate-concentration graph of that reactant
What does [A] represent
Concentration of A
What do colorimeters do
Measure the absorbance of a particular wavelength of light by a solution
How to measure the rate of reaction using a colorimeter
Can be set to measure the absorbance of a wavelength that is absorbed by one of the reactants but not by the products
The change in absorbance over the course of the reaction can be used to measure the rate
How to use a colorimeter to produce concentration time graph
- Produce standard solutions of known concentrations
- Select correct filter
- Zero colorimeter with water
- Plot calibration curve (from absorbance readings of standard solutions)
- Carry out reaction of unkown solution and take absorbance readings at regular intervals
- Use calibration curve to measure the concentration of iodine at each absorbance reading
- Plot concentration time graph
How to plot and work out rate of reaction from a concentration time graph
You can plot a concentration-time graph by repeatedly taking measurements during a reaction
The rate at any point in the reaction is given by the gradient at that point on the graph
If the graph is a curve, the draw a tangent to the curve and find the gradient of that
Sign in front of gradient (+or-) doesn’t matter, take positive value
How to find rate from the gradient
Gradient = change in y/change in x
Shape of order of reaction for concentration-time graphs
Zero order: negative straight line with unchanging gradient
First order: downward curve with a decreasing gradient over time
Second order: downwards curve which is steeper at the start, but falling off more slowly
What is the half-life for a reactant
This is the time taken for half of the reactant to be used up
Half life for first order of reaction
First order: this is independent of the concentration
So each half life will be the same length every time the reactant concentration halves
Equation to find the rate constant for a first order reaction using the half life
k = ln2/t 1/2
Find concs
Answers
Find Kc
0.5 mol B and 0.3 mol C mixed in 10dm3
At equilibrium there was found to be 0.1 mol of A present
Calculate Kc
What is total pressure
The total pressure of a gas mixture is the sum of all the partial pressures of individual gases
Equation for mole fraction of a gas in a mixture
Number of moles of gas / total number of moles of gas in the mixture
Equation for partial pressure
Mole fraction of gas x total pressure of the mixture
Find partial pressure
Answer
Find partial pressure
Use mole fraction equation
Ans
How to find Kp
Calculate Kp for decomposition of PCl5
Calculate partial pressure of monomer in the equilibrium and find total pressure exerted by equilibrium mixture
What to do when calculating kP expressions for heterogeneous equilibrium
You do not include solids or liquids, only gases
With: NH4HS (s) ⇌ NH3 (g) + H2S (g)
Expression for kP is kP = p(NH3) p(H2S)
(No bottom line as reactant is solid)
Draw each concentration-time graph for different orders
Draw each rate-concentration graph for different order
Work out rate constant:
What is the initial rate of a reaction and how do you find it on a graph
This is the rate right at the start of the reaction
Calculate the gradient of the tangent at time = 0
How to show that the value of initial rates depend on the reactant concentration
Carry out reaction, continuously monitoring one reactant
Use this to draw concentration-time graph
Repeat experiment using different initial concentration of the reaction
Keep the concentrations of other reactants the same, draw another concentration time graph
Use the graphs to calculate the initial rate for each experiment
Repeat process for each reactant (different reactants may affect the rate differently)
How to use clock reactions to measure initial rates
In a clock reaction, you measure how the time taken for a set amount of product to form changes as you vary the concentration of one of the reactants
There is usually an easily observable endpoint, such as colour change, to tell you when the desired amount of product has formed
The quicker the clock reaction finishes, the faster the initial rate of the reaction
(Known conc of reactant, rate stays the same, so time taken for end point to finish can be used in change in conc/time to find initial rate (rate stays same)) ask Pritchard?
Assumptions for clock reactions
The concentration of each reactant doesn’t change significantly over the time period of your clock reaction
The temperature stays constant
When the endpoint is seen, the reaction has not proceeded too far
If these assumptions are reasonable for your experiment, you can assume that the rate of reaction stay constant during the time period of your measurement, so the rate of your clock reaction will be a good estimate for the initial rate of you reaction
How does the iodine clock reaction work
Small amount of sodium thiosulfate solution and starch are added to an excess of hydrogen peroxide and iodide ions in acid solution
Starch is used as an indicator as it turns blue black in the presence of iodine
The sodium thiosulfate that is added to the added to the reaction mixture reacts instantaneously with any iodine that forms
To begin, all the iodine that forms in the first reaction is used up straight away in the second reaction
But once all the sodium thiosulfate is used up, any more iodine that forms will stay in solution, so the starch indicator will suddenly turn the solution blue-black, this is the end of the clock reaction
Varying iodide or hydrogen peroxide concentration while keeping the other constant will give different times for the colour change
Two reactions of iodine clock reaction
What is a rate determining step
Reaction mechanisms can have one step or a series of steps
In series of steps, each step can have a different rate
The overall is decided by the step with the slowest rate
This is the rate determining step
How to pick the reactants from the chemical equation are involved in the rate determining step
If a reactant appears in the rate equation, it must affect the rate
So this reactant, or something derived from, must be in the rate determining step
If a reactant doesn’t appear in the rate equation, then it isn’t involved in the rate determining step (and neither is anything derived from it)
Important points about rate determining steps and mechanisms
The RDS doesn’t have to be the first step in a mechanism
The reaction mechanism can’t usually be predicted from just the chemical equation
How to tell how many molecules are involved in the rate determining step from the order
The order of a reaction with respect to a reactant shows the number of molecules of that reactant which are involved in the rate determining step
So, if a reaction’s second order with respect to X, there’ll be two molecules of X in the rate determining step
Predict rate equation from this mechanism:
Predict the mechanism from the rate equation for the reaction:
(CH3)3CBr + OH- -> (CH3)3COH + Br-
Rate equation: k[(CH3)3CBr]
Possible exceptions to suggesting a mechanism for this reaction
- From the chemical equation, it looks like 2 N2O5 molecules react with each other so the reaction may be predicted as second order with respect to N2O5
- However the rate equation shows that the reaction is first order
- There’s only 1 molecule of N2O5 in the RDS
So there’s only one mechanism
What needs to happen for a reaction to occur
Particles need to:
Collide with eachother
Have enough energy to react (exceed activation energy)
Correct orientation
How does increasing the temperature increase the rate in terms of particles
Gives particles more kinetic energy so they speed up and collide more often
More reactant particles have the required activation energy so there are more successful collisions
How does increasing the temperature increase the rate
According to the rate equation, reaction rate depends only on the rate constant and reactant concentrations
So changing the temperature must change the rate constant
How does temperature increasing the rate constant result in a higher rate of reaction
The rate constant applies to a particular reaction at a certain temperature
At a higher temperature, the reaction will have a higher rate constant
(Higher rate constant, faster the rate)
What is the Arrhenius equation
This links the rate constant (k) with activation energy (Ea) and temperature (T)
How does activation energy and temperature effect k in the Arrhenius equation
As the activation energy increases, k gets smaller
Means that a large Ea will have a slow rate (as less successful collisions)
Shows that as the temperature rises, k increases
Putting Arrhenius equation into logarithmic form
How to create Arrhenius plot from logarithmic form of Arrhenius equation
Plot lnk against 1/T
This will produce a graph with a gradient of -Ea/R and a y intercept of lnA
What is Le Chatelier’s principle
If there’s a change in concentration, pressure or temperature, the equilibrium will move to help counteract the change
What does the size of the Kc say about the position
The larger the value of Kc, the further to the right the equilibrium lies and the more products there will be relative to reactants
The smaller the value of Kc, the further to the left the equilibrium lies and the more reactants there will be relative to products
General rule for what happens to an equilibrium when you change the temperature
If changing the temperature causes less product to form, the equilibrium moves to the left, and the equilibrium constant decreases
If changing the temperature causes more product to form, the equilibrium moves to the right, and the equilibrium constant increases
What does increasing concentration do to the equilibrium constant
The value of the equilibrium constant is fixed at a given temperature
So if the concentration of one thing in the equilibrium mixture changes then the concentration of the others must change to keep the value of Kc the same
Kc doesn’t change
What does changing the pressure do to the equilibrium constant
Increasing the pressure shifts the equilibrium to the side with fewer gas molecules - reduces the pressure
Decreasing the pressure shifts the equilibrium to the side with more gas molecules - this raises the pressure again
Kp (Kc) stays the same no matter what you do to the pressure
What do concentrations, pressure and temperature do to the values of Kc or Kp
Pressure and concentration do not affect the values of Kc or Kp
They do change the amounts of products and reactants present at equilibrium
Changes in temperature not only alter the amounts of product and reactants present at equilibrium but also change the value of equilibrium constant
What do catalysts do to the position of equilibrium and Kc and Kp
Catalysts have no effect on the position of equilibrium of the value of Kc/Kp
Can’t increase yield but do mean that equilibrium is approached faster
How to find the half life of a first order reaction
Means that the half life of a first order reaction can be read off its concentration-time graph by seeing by seeing how long it takes to halve reactant concentration
Find rate constant from the half life