CI fghk: Equilibrium; industrial sustainability

Question 1

Explain what is meant by “dynamic equilibrium”.

Answer 1

• Closed system
• Forward + reverse reactions occur at equal rates
• Concentrations of products + reactants are constant (but not equal)

Question 2

How is equilibrium concentration represented symbolically?

Answer 2

[…]_eq

Question 3

What formula is used to calculate the equilibrium constant, K_c?

Answer 3

product of conc. of products

product of conc. of reactants

Raise each concentration to the power of the moles of that substance in the reaction equation.

Question 4

What must be quoted alongside K_c?

Answer 4

Temperature, as K_c is temperature-dependent.

Question 5

Calculate the value and unit of K_c for the oxidation of ammonia at 890 K, given the following equilibrium concentrations in mol dm^-3:

[NH₃] = 1.230

[O₂] = 0.432

[NO] = 0.752

[H₂O] = 1.010

Answer 5

4NH_{3 (g)} + 5O_{2 (g)} ⇌ 4NO_(g) + 6H₂O_(g)

Use equation to work out units

K_c = 9.86 mol dm^-3

Question 6

How do changes in concentration or total pressure affect the magnitude of the equilibrium constant, assuming a constant temperature?

Answer 6

No effect

(Equilibrium position (composition of mixture) changes in order to restore K_c)

Question 7

Explain the effect, on the magnitude of the equilibrium constant, of raising the temperature of a reversible reaction which is:

• exothermic
• endothermic

Answer 7

• Exothermic: favours back reaction; equilibrium position moves towards reactants → K_c decreases
• Endothermic: favours forward reaction; equilibrium position moves towards products → K_c increases

Question 8

What effect does adding a catalyst have on a reversible reaction?

Answer 8

• No effect on equilibrium position (composition of mixture)
• No effect on K_c (relative rates)
• Increases rate at which equilibrium is attained
• Increases overall rate (relevant if temperature is lowered e.g. for cost, or to favour endothermic forward reaction)

Question 9

Explain how the value of K_c can be found experimentally.

Answer 9

• Record initial reactant concentrations
• Allow mixture to reach equilibrium
• Measure equilibrium concentration of a reactant or product
• Calculate equilibrium concentrations of other reactants + products using above + equation
• Write expression for K_c, then calculate value using equilibrium concentrations. State with unit + temperature

Question 10

Suggest 3 methods of analysis which could be used to determine the value of an equilibrium constant.

Answer 10

• Titration (redox / acid-base)
• Colorimetry
• pH measurement

Question 11

The hydrolysis of ethyl ethanoate at 298 K forms an equilibrium system:

CH₃COOC₂H₅ + H₂O ⇌ CH₃COOH + C₂H₅OH

The ester, which is of known concentration, and water are mixed and allowed to reach equilibrium.

1. How would you find the concentration of ethanoic acid in the equilibrium mixture?
2. How would you find an expression for K_c?

Answer 11

1. Titrate a known volume of equilibrium mixture with standardised NaOH solution

2.

• Assume [CH₃COOH]_eq = [CH₃CH₂OH]_eq = x
• [CH₃COOCH₂CH₃]_eq and [H₂O]_eq must respecively equal […]_initial - x
• Let initial concentrations respectively be y and z
• K_c = x² / (y - x)(z - x) at 298 K (no unit)

Question 12

Nitrogen monoxide takes part in the following equilibrium.

2NO + O₂ ⇌ 2NO₂

0.50 mol of NO and 0.50 mol of O₂ are mixed in a flask of volume 5.0 dm³ and allowed to reach equilibrium.

At equilibrium at a certain temperature, the amount of NO₂ present in the flask is 0.10 mol.

Calculate K_c for the reaction at this temperature and give its units. [5]

Answer 12

Question 13

Ammonia is made by reacting nitrogen and hydrogen industrially.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ∆_rH = -100 kJ mol^-1

Suggest why this reaction cannot be carried out in the laboratory.

Answer 13

A high pressure is required.

Question 14

Answer 14

Score full marks by discussing both rate + yield

Pressure:

• Lower pressure gives greater yield, since reverse reaction produces fewer molecules
• But higher pressure gives more frequent successful collisions, so higher rate
• Pressure chosen is a suitable compromise between rate + yield

Temperature:

• Lower temperature would give greater yield, since forward reaction is exothermic
• But higher temperature increases rate, since a greater frequency of collisions have E ≥ E_a
• 500K is a suitable compromise between rate + yield

Catalyst:

• Catalyst lowers E_a, so more frequent successful collisions
• Increases rate at which equilibrium is achieved (or overall rate of both forward + reverse reactions)
• No effect on equilibrium position / relative rates
• But allows higher rate at lower temperature, which is better for yield

Question 15

Compare raw materials with feedstock.

Answer 15

Raw materials: unprepared, untreated materials from which feedstocks are made

Feedstock: reactants entering a chemical process

Question 16

Compare co-products with by-products.

Answer 16

Co-products: useful, from intended reaction.

By-products: not useful, from side reactions.

Question 17

List the different sources of cost in the industrial manufacture of chemicals.

Answer 17

Costs relating to:

• Plant (labour, land, sales, loss in property’s value)
• Raw materials
• Energy
• Waste disposal
• Distribution

To remember these, think about the manufacturing process chronologically.

​(Fixed costs are those associated with plant; variable costs are those relating directly to unit of production)

Question 18

What factors are considered in maximising the efficiency and economy of a given chemical process?

Answer 18

• Balancing rate & equilibrium position (temperature, pressure)
• Recycling unreacted feedstock
• Making use of co-products
• Redirecting energy which would otherwise dissipate: heat exchangers (reactant to product / cross-process), water, steam

Question asks about a given synthetic pathway. So minimising number of steps + maximising atom economy are not valid answers

Question 19

Compare “hazard” with “risk”.

Answer 19

Hazard: potential of a dangerous substance to cause harm.

Risk: likelihood that a hazardous substance will cause harm, under the conditions of its use.

Question 20

What is a principle of green chemistry?

1. Heating a reaction to speed it up
2. Improving the atom economy of a process
3. Disposing of waste efficiently
4. Using organic solvents

Answer 20

2