Measuring enthalpy change of solution

Question 1

What is standard enthalpy change of a solution?

Answer 1

The enthalpy change that takes place when 1 mole of compound is completely dissolved in water under standard conditions

Question 2

What is the equation for enthalpy change of solution?

Answer 2

MX(s) + aq -> M⁺(aq) + X⁻(aq)

Question 3

How do you find enthalpy change of solution by experiment?

Answer 3

1. Add a known mass of ionic solid is enough water to dissolve it completely
2. Measure initial temperature of the water and the final temperature
3. When mass of solution is known, enthalpy change can be calculated

Question 4

How do ionic substances dissolve?

Answer 4

1.Ionic lattice breaks up to release ions
2. Ions are attracted to, and bond with, the water molecules, forming the solution

Question 5

How do ionic solids becomes aqueous?

Answer 5

Solid to gaseous ions -> gaseous to aqueous ions

Question 6

How do ionic solids becomes aqueous?

Answer 6

Solid to gaseous ions -> gaseous to aqueous ions

Question 7

What are the enthalpy changes when an ionic substance dissolves?

Answer 7

1. Solid dissolves into water to form aqueous solution (enthalpy change of solution)
2. Ionic solid breaks up into gaseous ions (reverse of lattice enthalpy)
3. Gaseous ions turning into aqueous ions (enthalpy change of hydration)

Question 8

What is the value of enthalpy change of solution?

Answer 8

Positive or negative depending on the relative sizes of lattice enthalpy and enthalpies of hydration

Question 9

What is enthalpy change of hydration?

Answer 9

The enthalpy change that takes place when 1 mole of isolated gaseous ions is dissolved in water, forming 1 mole of aqueous ions under standard conditions

Question 10

What are the equations for enthalpy change of hydration?

Answer 10

M⁺(g) + aq -> M⁺(aq)
X⁻(g) + aq -> X⁻(aq)

Question 11

What is the value of enthalpy change of hydration?

Answer 11

Always negative because the gaseous ions are forming bonds with the water molecules

Question 12

What is on the highest level of a Born-haber cycle?

Answer 12

Gaseous ions have more energy than other states

Question 13

Where is the solid ionic compound on a Born-Haber cycle?

Answer 13

Lower enthalpy than gaseous ions

Question 14

Where are aqueous ions on a Born-Haber cycle?

Answer 14

A lower enthalpy than gaseous ions

Question 15

What is Hess’ law?

Answer 15

ΔsolH = ΔleH + Σ ΔhydH

Question 16

What is the trend in enthalpy of hydration?

Answer 16

The more exothermic the enthalpy of hydration, the stronger the bonds formed between the ion and water molecules

Question 17

What is the effect of charge density on enthalpy of hydration?

Answer 17

The bonds between the ions and water are electrostatic so the higher the charge density of an ion, the stronger the bond

Question 18

What is the effect of ionic radius on enthalpy off hydration?

Answer 18

Ions with a smaller ionic radii (same charge) or more highly charged ions (smaller size) will have a more negative enthalpy of hydration