O hn: Solubility product; greenhouse effect

Question 1

What is meant by “solubility product”?

Answer 1

Represents the conditions for equilibrium between a sparingly soluble solid and a saturated solution of its ions.

Question 2

How solubility product represented symbolically?

Answer 2

K_sp

Question 3

What is meant by “saturated solution”?

Answer 3

One at its maximum concentration; no further dissolving can occur.

Question 4

A sparingly soluble ionic solid in contact with a saturated solution of its ions is at equilibrium.

• Write the equilibrium for calcium carbonate
• Derive an expression for K_sp

Answer 4

CaCO_{3 (s)} ⇌ Ca²⁺_(aq) + CO₃^2-_(aq)

K_c = [Ca²⁺] [CO₃^2-] / [CaCO₃]

Since solution is saturated (at this temperature), adding more solid does not cause more ions to form so doesn’t affect equilibrium. So:

K_sp = [Ca²⁺] [CO₃^2-]

Question 5

A sparingly soluble ionic solid is in contact with a saturated solution of its ions:

XY ⇌ X⁺ + Y^-

Predict what will happen when:

• [X⁺] [Y^-] > K_sp
• [X⁺] [Y^-] ⩽ K_sp

Answer 5

• Eq shifts to left; XY precipitates out of solution
• Eq shifts to right or stays still; ions remain in solution (and could increase in concentration)

Question 6

A sparingly soluble ionic solid is in contact with a saturated solution of its ions.

Explain what must be true, regarding K_sp, for the solid to precipitate out of solution.

Answer 6

XY ⇌ X⁺ + Y^-

• Eq position has shifted to left (since solid has precipitated out of solution)
• Product of concentrations of dissolved ions has increased from that at original equilibrium
• So K_sp < [X⁺] [Y^-]

Question 7

A sparingly soluble ionic solid is in contact with a saturated solution of its ions.

What must be true regarding K_sp for the ions to remain in solution?

Answer 7

XY ⇌ X⁺ + Y^-

K_sp = [X⁺] [Y^-]

• Eq position has not moved or has shifted to right (since ions remained in solution)
• Product of concentrations of dissolved ions has stayed the same or decreased from that at original equilibrium
• K_sp ⩾ [X⁺] [Y^-]

Question 8

Will a precipiate form from a solution containing calcium and carbonate ions both at a concentration of 1 x 10^-5 mol dm^-3? The solubility product of calcium carbonate is 5 x 10^-9 mol² dm^-6.

Answer 8

[Ca²⁺] [CO₃^2-] = 1 x 10^-10 mol dm^-3

1 x 10^-10 < 5 x 10^-9

[Ca²⁺] [CO₃^2-] < K_sp

So a precipitate does not form.

Question 9

Compound F dissolves to form a solution of Na⁺ and L^-.

Explain why adding NaCl to a solution of F causes the salt to precipitate out.

Answer 9

• [Na⁺] increases such that [Na⁺] [L^-] > K_sp
• K_sp is a constant
• To restore K_sp, eqilibrium shifts to left to decrease [Na⁺]
• More solid / ppt formed

Question 10

Compound F dissolves to form a solution of Na⁺ and L^-.

The solubility in water of compound F is 24.0 g dm^-3 at 24 ^oC. The molar mass of F is 222 g mol^-1.

Calculate the minimum mass, in g, of solid NaCl needed to form a precipitate when added to 500 cm³ of a 1.00 x 10^-2 mol dm^-3 solution of F at 24 ^oC.

Assume you can neglect the [Na⁺_(aq)] from salt F in comparison with that from NaCl.

Answer 10

F ⇌ Na⁺ + F^-

Work out K_sp for a hypothetical saturated solution

Solubility of F in mol dm^-3 = 24/222 = [Na⁺] = [L^-]

K_sp = [Na⁺] [F^-] = (24/222)² = 0.01167 mol² dm^-6

Use K_sp to work out [Na⁺] in this non-saturated circumstance

[L^-] = [F] = 1 x 10^-2 mol dm^-3

K_sp = 0.01167 = [Na⁺] x 1 x 10^-2

[Na⁺] = 0.1167 / (1 x 10^-2) = 1.169 mol dm^-3

Mol NaCl = conc x volume = 1.169 x 500 x 10^-3 = 0.5844 mol

Mass NaCl = mol x Mr = 0.5844 x 58.5 = 34.2 g

Question 11

Compound F dissolves to form a solution of Na⁺ and L^-.

The solubility in water of compound F is 24.0 g dm^-3 at 24 ^oC. The molar mass of F is 222 g mol^-1.

A student calculates the minimum mass, in g, of solid NaCl needed to form a precipitate when added to 500 cm³ of a 1.00 x 10^-2 mol dm^-3 solution of F at 24 ^oC.

They neglect the [Na⁺(aq)] from salt F in comparison with that from NaCl. Name one other assumption they make.

Answer 11

Volume of solution doesn’t change when NaCl is added.

(Temperature wouldn’t directly affect the calculation, but would cause a change in volume)

Question 12

The maximum solubility of magnesium hydroxide is 1.71 x 10^-4 mol dm^-3.

What is the value of the solubility product?

Answer 12

Mg(OH₂) ⇌ Mg²⁺ + 2OH^-

[OH^-] = 2 x 1.71 x 10^-4

K_sp = [Mg²⁺] [OH^-]²

= 2.00 x 10^-11

Question 13

Name 3 greenhouse gases.

Answer 13

CO₂, CH₄, H₂O

CFCs are also greenhouse gases. They produce Cl radicals, which catalyse the decomposition of ozone.

Question 14

What is the IR window?

Answer 14

The region of the IR spectrum where there is little to no absorption by atmospheric gases, save small (historically negligable) amounts by CO₂ and CH₄. Mainly a gap in the absorption spectrum of water vapour.

Question 15

Describe the greenhouse effect and explain the mechanisms behind it.

Answer 15

• Solar radition, mainly visible + UV, reaches Earth’s surface
• Earth absorbs some of this (rest is reflected), heats up + radiates IR
• Tropospheric greenhouse gases (e.g. CO₂ + CH₄) absorb some of this IR, in the IR window
• Vibrational energy of their bonds increases
  
  • They re-emit some absorbed IR in all directions, of which some heats up Earth (rest escapes into space)
  • They transfer some of their vibrational energy to the KE of other molecules during collisions, so atmosphere heats up

Greenhouse gases act like a blanket, trapping IR. Atmospheric temperature would be constant if Earth absorbed + emitted radiation at equal rates.

Question 16

Explain why the greenhouse effect, as a result of human activity, is now causing global heating.

Answer 16

• Greenhouse effect is where greenhouse gases absorb IR emitted by earth, heating atmosphere
• Increased concentrations of greenhouse gases (particularly due to CO₂ and CH₄ emissions)
• Higher % absorption of IR radiated by Earth
• Enhanced greenhouse effect
• Earth absorbs more radiation than it emits, so heats up

​Earth warms, meaning more water vapour and possibly more absorption. Not known whether this is counteracted by more cloud + more reflection

Question 17

Which statement about the greenhouse effect is true?

1. Absorption of infrared radiation causes homolytic fission and creates radicals.
2. Oxygen molecules do not absorb infrared radiation.
3. Greenhouse gases reflect infrared radiation back to the Earth’s surface, trapping the heat.
4. Solar energy reaches the Earth mainly as infrared radiation.

Answer 17

2

• 1 should say “UV”, not “infrared”*
• 3 should say “re-emit”, not “reflect”*
• 4 should say “UV + visible”, not “infrared”*