Mod 5 EQUILIBRIUM AND ACID REACTIONS: Solution equilibria

Question 1

Identify the structure of ionic compounds, and what force holds them together

Answer 1

Lattice structure, held by electrostatic forces between cations and anions

Question 2

List and describe the steps in dissolution

Answer 2

The steps of dissolution describe the process where a solute dissolves in a solvent, resulting in a homogeneous solution.
1. Separation of water molecules (endothermic): energy is absorbed in order to overcome intermolecular forces present between molecules.

2. Dissociation of solutes into ions (endothermic): energy is absorbed to overcome electrostatic forces present in the lattice structure of ions. If the force between a polar water molecule and an ion is greater than the forces within the ionic lattice, the ion will move away from the crystal structure.
3. Hydration of ions (exothermic): Ions of the lattice will be surrounded by water. The oxygen part of the water will surround cations due to negative-positive charge attraction, and the hydrogen part of water will surround anions. As more and more ions are removed from the lattice, ion concentration increases, leading to the interaction between ions in the lattice and in the solution; some ions can be reclaimed back.

Question 3

What is the solubility product

Answer 3

Ksp = C+ represents the solubility product constant for a sparingly soluble ionic compound

Where…
[C+] = Concentration of cation in a saturated solution

Question 4

How did Aboriginal and Torres Strait Islander Peoples removed toxicity from foods

Answer 4

• Leaching
• Pounding and grinding

They used techniques such as pounding and grinding to maximise the surface area of seeds with toxins. For instance, the cycad fruit consists of a primary toxin, known as cycasin. And since this toxin is highly soluble in water, they also utilised leaching with the assistance of flowing water, preventing equilibrium from being established. This ensures dissolved toxins are washed away with running water.

Question 5

All nitrates are soluble?

Answer 5

True.

Question 6

All acetates are soluble?

Answer 6

True

Question 7

All salts of Group 1 metals are soluble?

Answer 7

True

Question 8

All ammonium salts are soluble?

Answer 8

True

Question 9

All Group 7 ions are soluble, except those of mercury, silver, and lead?

Answer 9

True

Question 10

All sulfates are soluble, except those of mercury, silver, lead, barium, calcium, and strontium?

Answer 10

True

Question 11

All carbonates are insoluble, except those of Group 1 metals and ammonium?

Answer 11

True

Question 12

All hydroxides are insoluble, except those of Group 1 metals and ammonium?

Answer 12

True

Question 13

All phosphates are insoluble, except those of Group 1 metals and ammonium?

Answer 13

True

Question 14

All salts of Group 7 ions are insoluble?

Answer 14

False (Group 7 ions are generally soluble except with mercury, silver, and lead)

Question 15

All sulfates are insoluble?

Answer 15

False (Sulfates are generally soluble except with mercury, silver, lead, barium, calcium, and strontium)

Question 16

All carbonates are soluble?

Answer 16

False (Carbonates are generally insoluble except with Group 1 metals and ammonium)

Question 17

All hydroxides are soluble?

Answer 17

False (Hydroxides are generally insoluble except with Group 1 metals and ammonium)

Question 18

All phosphates are soluble?

Answer 18

False (Phosphates are generally insoluble except with Group 1 metals and ammonium)

Question 19

Sulfates of barium, calcium, and strontium are soluble?

Answer 19

False (Sulfates of barium, calcium, and strontium are insoluble)

Question 20

Acetates are generally insoluble?

Answer 20

False (Acetates are generally soluble)

Question 21

Ammonium salts are generally insoluble?

Answer 21

False (Ammonium salts are generally soluble)

Question 22

Silver chloride is soluble?

Answer 22

False

Question 23

Lead bromide is soluble?

Answer 23

False

Question 24

Ammonium carbonate is soluble?

Answer 24

True

Question 25

Calcium carbonate is soluble?

Answer 25

False

Question 26

Potassium phosphate is soluble?

Answer 26

True

Question 27

Is the reaction:
KCl(aq) + AgNO₃(aq) ⇋ AgCl(s) + KNO₃(aq)
soluble or insoluble?

Answer 27

• NO3- is always soluble; spectator ion
• K+ is group 1 metal and therefore always soluble; spectator ion
• All group 7 ions are soluble BUT silver is an exception; precipitate

Therefore, KNO₃ is soluble, but AgCl precipitates out, making part of the reaction insoluble.

Question 28

Is the reaction:
2KI(aq) + Pb(NO3)2(aq) ⇋ 2KNO3(aq) + PbI2(s)
soluble or insoluble?

Answer 28

• NO3- is always soluble; spectator ion
• K+ is group 1 metal and therefore always soluble; spectator ion
• All group 7 ions are soluble except lead; precipitate

The reaction is partially insoluble because of the formation of the insoluble precipitate Pbl2.

Question 29

Is the reaction: Na2SO4(aq) + Ba(NO3)2(aq) ⇋ 2NaNO3(aq) + BaSO4(s)
soluble or insoluble?

Answer 29

• Na+ is group 1 metal; soluble; spectator ion
• NO3- are always soluble; spectator ion
• Sulphates are soluble with barium being an exception; precipitates

The reaction is partially insoluble due to the formation of the insoluble precipitate BaSO4.

Question 30

Identify what it means when value of Ksp is high

Answer 30

Substance is more soluble

Question 31

Identify what it means when value of Ksp is low

Answer 31

Substance is less soluble

Question 32

Barium carbonate’s Ksp of 0.00000000258.
Determine the concentration of [Ba2+] at saturation point, with how much solid barium carbonate would be needed in 100mL of water to reach this point.

Answer 32

BaCO3(s) ⇋ Ba2+(aq) + CO32-(aq)

Ksp = [Ba2+] [CO32-]
Since molar ratio is 1:1, let concentration of [Ba2+] be “x”
Ksp = xx
2.5810^-9 = x^2
x = 0.0000508mol/L (conc of barium ions)

C = n/v
n = Cv
n = (5.08 * 10-5) * (0.1)
n = 5.08 * 10-6 (mol)

n(Ba2+) = n(BaCO3)
m(BaCO3) = n * MM
m (BaCO3) = (5.08 * 10-6 ) * (197.35) = 1.00 (mg)

Question 33

What is the ion product (Q)

Answer 33

“Q” is a measure of the concentration of ions in a solution at any given point in time, similar to Ksp for a solution at equilibrium.

Question 34

How does the ion product (Q) relate to Ksp in determining if a precipitate will form?

Answer 34

If Q = Ksp: Saturation is reached; no precipitate will form.

If Q < Ksp: Saturation has not been reached; no precipitate will form.

If Q > Ksp: The solution has exceeded saturation; a precipitate will form.

Question 35

List the steps to predict a precipitate?

Answer 35

Step 1: Write a chemical equation and predict a precipitate using solubility rules

Step 2: Find the limiting reagent using moles

Step 3: Write equilibrium for dissociation

Step 4: Calculate the concentration of potential precipitate and its ions using limiting reagent

Step 5: Deduce Qsp

Step 6: Compare Qsp to Ksp referring to the data sheet

Question 36

Following the steps, will lead(II) chloride precipitate when 50 mL of 0.10 M Pb(NO₃)₂ is mixed with 50 mL of 0.10 M NaCl? Write the balanced chemical equation.

Answer 36

1. Pb(NO3)2(aq) + 2NaCl(aq) → 2NaNO3(aq) + PbCl2(s)
2. n(Pb(NO3)2) = 0.10mol/L * 0.05L = 0.005 (mol)
   n(NaCl) = 0.10mol/L * 0.05L = 0.005 (mol)

Pb(NO3)2 : NaCl
1 : 2
0.005mol : 0.005mol
1 : 1
0.005mol : 0.0025mol
∴ limiting is NaCl

3. PbCl2(s) ⇋ Pb2+(aq) + 2Cl-(aq)
   Qsp = [Pb2+] [Cl-]2
4. n(PbCl2) = 0.005mol / 2 = 0.0025 (mol)
   n(Pb2+) = 0.0025 (mol) because of 1:1 molar ratio
   n(Cl-) = 0.0025 * 2 = 0.005 (mol)

0.05L + 0.05L = 0.1L
[Pb2+] = 0.0025/0.1 = 0.025 (mol/L)
[Cl-] = 0.005/0.1 = 0.05 (mol/L)

5.
Qsp = [Pb2+] [Cl-]2
Qsp = 0.025 * 0.052 = 6.25 * 10-5

6.
Qsp = 6.25 * 10-5
Ksp(PbCl2) = 1.70 * 10-5

Qsp > Ksp
∴ the solution has exceeded the saturation point, a precipitate will form.