Equilibrium.

Question 1

What does the term dynamic in dynamic equiibrium mean.

Answer 1

The term dynamic means both forward and backward reactions are occurring simultaneously.

Question 2

State Le Chatelier’s Principe.

Give the little bit you sometimes forget.

Answer 2

Le Chatelier’s principle states that if an external condition is changed the equilibrium will shift to oppose the change (and try to reverse it).

It is the equilibrium that will shift not just “the system”.
use equilibrium system to be safe.

Question 3

What are the two features of a dynamic equilibrium.

Answer 3

1. Forward and backward reactions are occurring at
   equal rates.
2. The concentrations of reactants and products stays constant.

Question 4

Typical Exam question: What effect would increasing temperature have on the yield of ammonia?
N2 +3H2—> 2NH3. Delta H=-ve exo

Answer 4

Exam level answer : must include capital points…
If temperature is increased the EQUILIBRIUM WILL SHIFT TON OPOSE this and move in the ENDOTHERMIC BAACKWARDS direction to try to DECREASE TEMPERATURE. The position of equilibrium will SHIFT TOWORDS THE LEFT, giving a LOWER YIELD OF AMMONIA.

Question 5

What does the term “position of equilibrium” actually mean.

Answer 5

We use the expression ‘position of equilibrium’ to describe the composition of the equilibrium mixture.
If the position of equilibrium favours the reactants (also described as “towards the left”) then the equilibrium mixture will contain mostly reactants.

Question 6

What is the general affect of increasing temperature on the position of equilibrium.

Answer 6

If temperature is increased the equilibrium will shift to oppose this and move in the endothermic direction to try to reduce the temperature by absorbing heat.

Question 7

What is the general affect of decreasing temperature on the position of equilibrium.

Answer 7

If temperature is decreased the equilibrium will shift to oppose this and move in the exothermic direction to try to increase the temperature by giving out heat.

Question 8

What is the general affect of increasing pressure on an equilibrium position.

Answer 8

Increasing pressure will cause the equilibrium to shift towards the side with fewer moles of gas to oppose the change and thereby reduce the pressure.

Question 9

What is the general affect of decreasing pressure on an equilibrium position.

Answer 9

Decreasing pressure will cause the equilibrium to shift towards the side with more moles of gas to oppose the change and thereby increase the pressure.

Question 10

Study the following reversible reaction. What will the affect be of increasing pressure on the positron of equilibrium.
H2 + Cl —> 2HCl

Answer 10

If the number of moles of gas is the same on both sides of the equation then changing pressure will have no effect on the position of equilibrium

Question 11

Typical Exam question: What effect would increasing pressure have on the yield of methanol?
CO +2H2 ⇌ CH3OH

Answer 11

If pressure is increased the EQUILIBRIUM WILL SHIFT TO OPPOSE this and move towards the side with FEWER MOLES OF GAS TO TRY TO REDUCE THE PRESSURE . The position of equilibrium will SHIFT TOWARDS THE RIGHT BECAUSE there are 3 moles of gas on the left but only 1 mole of gas on the right, giving a HIGHER YIELD OF METHANOL.
(Bits on capital are needed for all marks)

Question 12

All of the reversible reactions that you must need to know are exothermic and so why are they all not done I. the freezing cold to increase the yield of desired product.

Answer 12

Low temperatures may give a higher yield of product but will also result in slow rates of reaction. Often a compromise temperature is used that gives a reasonable yield and rate.

Question 13

CO +2H2 ⇌ CHOH
Consider the above reaction used to produce methanol in industry why is it not done at incredibly high pressure to increase the yield of methanol.

Answer 13

Increasing pressure may give a higher yield of product and will produce a faster rate. Industrially high pressures are expensive to produce ( high electrical energy costs for pumping the gases to make a high pressure) and the
equipment is expensive (to contain the high pressure
s).
In all cases high pressure leads to too high energy costs for pumps to produce the pressure and too high equipment costs to have equipment that can withstand high pressures.

Question 14

Why does a catalyst not affect the position of equilibrium and why if not is it used In industry.

Answer 14

It does not effect the position of equilibrium because it speeds up the rates of the forward and backward reactions by the same amount.
In all cases catalysts speeds up the rate, allowing a lower temp to be used (and hence lower energy costs), but have no effect on position of equilibrium

Question 15

How can the overall yield of all industrial processes be improved.

Answer 15

Recycling unreacted reactants back into the reactor can improve the overall yields of all these processes.

Question 16

What are the uses of both ethanol and methanol.

Answer 16

Both methanol and ethanol can be used as fuels.

Question 17

CO +2H2 ⇌ CH OH
How could this reaction be made carbon neutral, and define carbon neutral in your answer. What would have to happen for it top really be carbon neutral.

Answer 17

The term carbon neutral refers to “an activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere”.
If the carbon monoxide used to make methanol in the above reaction was extracted from the atmosphere then it could be classed as carbon neutral.
It would only be carbon neutral, however, if the energy required to carry out the reaction was not made by combustion of fossil fuels .

Question 18

Give the reaction and conditions for the hydration of ethene. Give also the conditions used.

Answer 18

CH=CH +H2O ⇌ CHCHOH H=-ve

T= 300oC, P= 70 atm, catalyst = conc H3PO4.

Question 19

State why , with the hydration of ethene , excess steam and really high pressures are not used.

Answer 19

High pressure also, as well as too high energy costs, leads to unwanted polymerisation of ethene to poly(ethene).
Excess steam would dilute the phosphoric acid catalyst.

Question 20

Give the equation for the production of methanol and give also the conditions that are used in industry.

Answer 20

CO +2H 2⇌ CH3 OH H=-ve exo

T= 400oC, P= 50 atm, catalyst = chromium and zinc oxides

Question 21

Calculate the units for Kc for the following reactions.

1) N2 (g)+3H2 (g) 2NH3 (g)
2) H2 (g) +Cl2 (g) 2HCl (g)

Answer 21

1) Unit = mol-2 dm+6

2) no unit

Question 22

What types of condition change will affect the value of Kc.

Why out off curiosity. (Don’t need to know)

Answer 22

Kc only changes with temperature. It does not change if pressure or concentration is altered.
A catalyst also has no effect on Kc
Both the equilibrium position and the value of Kc will change when you alter temperature but or all other condition (not a catalyst) only the position of equilibrium will change.

With concentration and pressure you changed them in the first place for the equilibrium to shift so the effect cancels. With temp you get a reaction without provoking the system since the temp is not in the Kc expression so can’t be compensated for.

Question 23

What does the value of Kc say about the position of equilibrium.

Answer 23

Since Kc= Products

Reactants

Question 24

What does the value of Kc say about the position of equilibrium. What value is key here.

Answer 24

Since Kc= Products
Reactants
The larger the Kc the greater the amount of products.
If Kc is small we say the equilibrium favours the reactants. The number 1 is the key value here and if Kc is less than 1 the reactants predominate and vice versa.

Reactions where Kc is equal to 10 to the 10 are considered to have gone to completion and reactions that have a Kc of 10 to the -10 are considered to have not taken place at all.

Question 25

What two reversible reactions that are in the gas phase are key reactions used in industry and why do we not consider them in terms of Kc like we would for the hydration of ethene and the production of methanol.

Answer 25

The production of methanol and also ammonia by the Haber process are all in the gas phase. They do obey the equilibrium laws however they are all in the gas phase and so their concentrations are expressed as partial pressures rather than concentrations which is a year 2 topic. This is denoted by Kp not Kc.

Question 26

Example 3 from notes.
For the following equilibrium
H2 (g) +Cl2 (g) 2HCl (g)
In a container o fvolume 600cm3 there were initially 0.500mol of H2 and 0.600 molof Cl2. At equilibrium there were 0.200 mol of HCl. Calculate Kc

Answer 26

= 0.196 no unit

This is a standard Kc calculation simply set up your table and then find equilibrium moles.

Notice how the equation is equimolar and so you can just bang moles straight in.

Question 27

Example 4 from notes.
For the following equilibrium
N2 (g)+3H2 (g) 2NH3 (g)
Initially there were 1.50 moles of N2 and 4.00 mole of H2, in a 1.50 dm3 container. At equilibrium 30% of the Nitrogen had reacted. Calculate Kc.

Answer 27

= 0.0927 mol-2 dm+6

Here you do need to convert moles Into conc before you stick in Kc expression.

You know that 30% of the nitrogen had reacted = 0.3 x1.5 = 0.45 moles reacted.
Using the balanced equation 3 x 0.45 moles of H2 must have reacted and 2x 0.45 moles of NH3 must have formed.
Make this concentrations by dividing each by 1.5 which is the volume of the vessel.

Question 28

Example 5.
For the following equilibrium H2 (g) + Br2 (l) 2HBr (g)
0.200 mol of H2 and 0.200 mol of Br2 are mixed and allowed to reach equilibrium. If Kc = 0.210 calculate the equilibrium amounts, in mol, of each substance.

What do you need to remember about this method.

Answer 28

At equilibrium
Make x = moles of H2 that have reacted at equilibrium
V = volume of container notice this particual equation is equimolar so V here will actually cancel which is handy.

Moles of H2 = 0.2000- 0.0373= 0.163 mol
Moles of Br= 0.2000- 0.0373 2 = 0.163mol
Moles of HBr = 2x 0.0373 = 0.0746mol.

method : use 2X and X not reactions or decimals. Focos in terms of what reacted not what increased as X

Question 29

What things dip you need top consider for an equilibrium table calculation for Kc to get the correct answer every time. State also what the general formula is for the moles of reactant and product at equilibrium in layman’s terms.

Answer 29

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Question 30

H2 (g) +Cl2 (g)—-> 2HCl (g)
Study the following equation why would you consider the mole ratio when going through your change column (delta column) . Explain this in regards to if you know 0.2 moles of HCl is formed.

Answer 30

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Question 31

If Kc has no units what is the significance of this to your equation and why is this the case.

Answer 31

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Question 32

In this equilibrium which is exothermic in the forward direction N2 (g)+3H2 (g)—> 2NH3 (g)
Predict what will happen if the temperature is increased.
to Kc and the position of equilibrium.

Answer 32

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Question 33

In this equilibrium which has fewer moles of gas on the product sideL: N2 (g)+3H2—> (g) 2NH3 (g)
predict what will happen if the pressure is increased to the equilibrium position and value of Kc.
State also the affect a catalyst would have on these 2 things.

Answer 33

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Question 34

Give a common reaction for working out the equilibrium constant Kc.

Answer 34

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Question 35

Give , in detail , the two part method that is used for calculating the equilibrium constant for the reaction:
CH3CO2H + CH3CH2OH CH3CO2CH2CH3 + H2O Ethanoic acid Ethanol Ethyl Ethanoate

List some of the possible calculations that can be done from this 2 part method.

Answer 35

Method
Part 1
Preparing the equilibrium mixture
1 Use burettes to prepare a mixture in boiling tube of carboxylic acid, alcohol, and dilute sulfuric acid. 2 Swirl and bung tube. Leave the mixture to reach equilibrium for one week.

Part2
Titrating the equilibrium mixture
1 Rinse a 250 cm3 volumetric flask with distilled water.
Use a funnel to transfer the contents of the boiling tube into the flask. Rinse the boiling tube with water and add the washings to the volumetric flask.
2 Use distilled water to make up the solution in the volumetric flask to exactly 250 cm3.
Stopper the flask, then invert and shake the contents thoroughly.
3 Use the pipette to transfer 25.0 cm3 of the diluted equilibrium mixture to a 250 cm3 conical flask.
4 Add 3 or 4 drops of phenolphthalein indicator to the conical flask.
5 Set up the burette with sodium hydroxide solution..
6 Add the sodium hydroxide solution from the burette until the mixture in the conical flask just turns pink. Record this burette reading in your table.
7 Repeat the titration until you obtain a minimum of two concordant titres.

Possible calculations include:

1) Working out initial amount of moles of reactants
2) Working out equilibrium amount of moles of acid present from the titre results.
3) Working out equilibrium amount of moles of other substances.
4) Finally it is possible to calculate the value of Kc from the experiment.

Question 36

Need to look into all calls that come with this practical.

Answer 36

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