rate equations and gas equillibria

Question 1

how do we work out the rate of a reaction

Answer 1

if you want to find the rate of a reaction, you need to be able to follow the reaction as it is occurring

You can follow a reaction all the way through from start to end by recording the amount of product (or reactant) you have at a regular time intervals

This is called CONTINUOUS MONITORING

Question 2

whar are the different methods of continuous monitoring

Answer 2

• gas volume
• change in pH
• colour change
• loss of mass

Question 3

gas volume

Answer 3

if gas is given off, you could collect it in a gas syringe and record how much you have at regular intervals

• the rate of the equation is in terms of concentration per unit per time by drawing a concentration-time graph

you should know the concentration of the reactants at the start of the equation

Question 4

ph change

Answer 4

if the reaction produces or uses up H+ ions, the pH of the solution will change

So you could measure the pH of the solution at regular intervals and calculate the concentration of H+

Question 5

colour change

Answer 5

sometimes you can track the colour change of a reaction using a colourimeter

1. plot a calibration curve of known concentrations against absorption
2. During the experiment, take a small sample from your reaction solution at regular intervals and read absorbance
3. use your calibration curve to convert absorbance at each time point into a concentration

the more concentrated a solution the higher the absorbance is

Question 6

what is a colourimeter

Answer 6

a colourimeter measures absorbance (the amount of light of a particular wavelength absorbed) of the solution

Question 7

loss of mass

Answer 7

if gas is given off, the system will lose mass

you can measure this at regular intervals with a balance

Question 8

how do you work out the rate from a straight line graph

Answer 8

gradient = change in Y / change in X

Question 9

order effect of 0

Answer 9

if the concentration of reactant changes and the rate remains the same, the order of reaction with respect to e.g. A is 0 ( or 0 order with respect to A)

So if A doubles, the rate will stay the same. If A triples, the rate will stay the same

Question 10

order effect of 1

Answer 10

if the rate is proportional to [A], then order or reaction with respect to A is 1 (or first order with respect to A).

So if [A] doubles the rate will double and so on

Question 11

order effect of 2

Answer 11

If the rate is proportional to [A]² , then the order of reaction with respect to A is 2 ( or second order with respect to A)

Question 12

What is the units of rate

Answer 12

mol dm^-3 s^-1

Question 13

what is the rate constant

Answer 13

k is the rate constant - it is a constant that links the concentration of reactants to the rate of reaction

the bigger the rate constant, the faster the reaction

if you increase the temp, the rate constant increase too

Question 14

what is the initial rates method

Answer 14

you can use a process called the initial rates method to work out the order of reaction for each reactant:

1. carry out the reaction and monitor its progress - you need to collect enough data to work out an initial rate. Use this data to draw a concentration-time graph
2. repeat the experiment using a different initial concentration of one of the reactants. Keep the concentrations of other reactants and other conditions the same. Draw another concentration-time graph
3. now look at how the different initial concentrations affect the initial rate - use this to work out the order for the reactants

Question 15

what are clock reactions

Answer 15

clock reactions are an example of an initial rates method. They involve measuring how long it takes for a certain amount of product to form as you vary the concentrations of one or the reactants

As part of a clock reaction, there will be a sudden increase in the concentration of a certain product as a limiting reactant is used up

Question 16

what is an important feature of a clock reaction

Answer 16

there is usually an easily observable endpoint such as a colour change to tell when the desired amount of product is formed

Question 17

the quicker the clock reaction finsihes

Answer 17

the faster the initial rate of the reaction

Question 18

what are the following assumptions

Answer 18

• the concentration of each reactant does not change significantly over the time period of your clock reaction
• the temperature stays constant
• when endpoint is seen the reaction has not proceeded too far

Question 19

what is the iodine clock reaction

Answer 19

in an iodine clock reaction the reaction that you are monitoring is this:

H₂O+ 2I^- + 2H⁺ →2H₂O +I₂

A small amount of sodium thiosulfate solution and starch are added to an excess of hydrogen peroxide and iodide ions in the acid solution

(starch is used as an indicator - it turns blue-black in the presence of iodine)

the sodium thiosulfate is added to the reaction mixture reacts instantaneously with the iodine that is formed

2S₂O₃^2- + I₂→ 2I^- + S₄O₆^2-

To begin with, 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 the 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 concentrations while keeping the others constant will give different times for the colour change

Question 20

describe the different rate graphs

Answer 20

the blue line represents the lines for concentration against time

Question 21

what is Arrhenius equation

Answer 21

Question 22

what does each letter in the Arrhenius equatoin stand for

Answer 22

k = rate

EA = activation energy (J mol^-1)

T= temperature(K)

R = gas constant (8.31J K^-1mol^-1)

A= the Arrenhius constant (another constant)

Question 23

what is the Arrhenius equation useful for

Answer 23

it is a useful tool for helping us to work out what happens to the rate constant if various conditions, such as the temperature or activation energy change

As the activation energy, EA gets bigger, k gets smaller

this therefore mean slower rate

Question 24

how do we work out the activation energy from a graph

Answer 24

in this graph, the y-intercept is equal to A and the gradient is -EA/R

Question 26

what happens in a mixture of gases

Answer 26

in a mixture of gases, each individual gas exerts its own pressure - this is called its partial pressure

the total pressure pf a gas mixture is the sum of all the partial pressures of the individual gases

Question 27

what is the unit for partial pressure

Answer 27

kPa

Question 28

what is a mol fraction

Answer 28

a mole fraction is just the proportion of a gas mixture that is made up of a particular gas.

mole fraction of a gas in a mixture = number of moles of gas/ total number of moles of gas in the mixture

Question 29

how do we work out the partial pressure of a gas in a mixture

Answer 29

mole fraction of a gas x total pressure of the mixture

Question 30

what is the gas equilibrium constant

Answer 30

Kp is the equilibrium constant for a reversible reaction where some or all of the reactants and products are gases

the expression for the constant is like the one for Kc but you use partial pressures instead of concentration

Kp= partial pressure of products/ partial pressure of rectants

Question 31

what effect does temp have on Kp

Answer 31

temp can increase or decrease Kp depending on what side equilibrium is pushed (Kp is lower if fewer products is formed)

Question 32

what effect does pressure have on Kp

Answer 32

if you increase the pressure, the equilibrium shifts to the side with fewer moles of gas, and if you decrease the pressure, equilibrium shifts to the side with most moles

However, these changes do not effect Kp.

Instead, if you change the pressure, the position of equilibrium shifts in such a way that the partial pressure of reactants and products at the new equilibrium position keep Kp constant

Question 33

what effect do catalysts have on Kp

Answer 33

catalysts have no effect on the position of equilibrium it in the value of Kp

This is because a catalyst will increase the rate of both the forward and backwards reactions by the same amount

As a result, the equilibrium position will be the same as the uncatalysed reaction, but equilibrium will be reached faster

So catalysts can’t increase the amount of product formed, but they do increase the time taken to each equilibrium