T-2: Enthalpy

Question 1

Energy required to break a chemical bond

Answer 1

Bond Energy

Question 2

Standard states

Answer 2

Phase at 25ºC, 1 atm for gases and 1 M for solutions.

Question 3

Energy change that occurs when you make one mole of a compound from its constituent elements at standard states

Answer 3

Standard Enthalpy of Formation

Question 4

The equation to find ΔHº from standard enthalpies of formation

Answer 4

ΔHº = Σ(nΔHº_f,prod) - Σ(nΔHº_{f, react})

Question 5

The equation to find ΔH from bond energies.

Answer 5

ΔH = Σ(nB.E._broken) - Σ(nB.E._formed)

Question 6

The amount of energy required to break a chemical bond.

Answer 6

Bond energy

Question 7

Bond Energy

Answer 7

Energy required to break a chemical bond

Question 8

Phase at 25ºC, 1 atm for gases and 1 M for solutions.

Answer 8

Standard states

Question 9

Standard Enthalpy of Formation

Answer 9

Energy change that occurs when you make one mole of a compound from its constituent elements at standard states

Question 10

ΔHº = Σ(nΔHº_f,prod) - Σ(nΔHº_{f, react})

Answer 10

The equation to find ΔHº from standard enthalpies of formation

Question 11

ΔH = Σ(nB.E._broken) - Σ(nB.E._formed)

Answer 11

The equation to find ΔH from bond energies.

Question 12

Bond energy

Answer 12

The amount of energy required to break a chemical bond.

Question 13

Exothermic or Endothermic

Breaking a covalent bond

Answer 13

Endothermic

Question 14

Exothermic or Endothermic

Making a covalent bond

Answer 14

exothermic

Question 15

Exothermic or Endothermic

F₂(g) → 2F(g)

Answer 15

endothermic - breaking a bond

Question 16

Exothermic or Endothermic

2Br(g) → Br₂(g)

Answer 16

exothermic - making a bond

Question 17

Exothermic or Endothermic

Li⁺(g) + Cl^-(g) → LiCl(s)

Answer 17

exothermic - making a bond

Question 18

Exothermic or Endothermic

NaF(s) → Na⁺(g) + F^-(g)

Answer 18

endothermic - breaking a bond

Question 19

Write out reaction equation for standard enthalpy of formation for HCl.

Answer 19

1/2H₂(g) + 1/2Cl₂(g) → HCl(g)

Question 20

Write out reaction equation for standard enthalpy of formation for CO₂.

Answer 20

C(s) + O₂(g) → CO₂(g)

Question 21

S₈(s) + 8 O₂(g) →8 SO₂ (g)

Write this reaction as a enthalpy of formation for SO₂

Answer 21

1/8 S₈(s) + O₂(g) → SO₂(g)

Question 22

The ΔHº for the following reaction is…

1/2H₂(g) + 1/2Cl₂(g) → HCl(g) ΔHº = -92 kJ/mol

What is the ΔHº for this reaction?

H₂(g) + Cl₂(g) → 2 HCl(g)

Answer 22

ΔHº = - 184 kJ/mol

• multiply reaction by 2 so multiple value by 2

Question 23

The ΔHº for the following reaction is…

1/2H₂(g) + 1/2Cl₂(g) → HCl(g) ΔHº = -92 kJ/mol

What is the ΔHº for this reaction?

HCl(g) → 1/2H₂(g) + 1/2Cl₂(g)

Answer 23

ΔHº = 92 kJ/mol

• reverse reaction so reverse sign

Question 24

The ΔHº for the following reaction is…

1/2H₂(g) + 1/2Cl₂(g) → HCl(g) ΔHº = -92 kJ/mol

What is the ΔHº for this reaction?

2 HCl(g) → H₂(g) + Cl₂(g)

Answer 24

ΔHº = 184 kJ/mol

• reversed the reaction so reverse the sign
• multiplied reaction by 2 so multiple value by 2

Question 25

The bond energy for O₂ (O=O) is 498 kJ/mol. What does that mean?

Answer 25

It takes 498 kJ of energy to break 1 mole of O₂ molecules into individual oxygen atoms.

Question 26

The bond energy for C-H is 439 kJ/mol. How much energy is released when 4 moles of hydrogen atoms bind to 1 mole of carbon atoms?

Answer 26

(439)4 = 1756 kJ are liberated for each mole of CH₄ produced in this way.

Question 27

It takes 872 kJ to break apart 2 moles of H₂ molecules to form 4 moles of H atoms. 1756 kJ of energy are liberated when 4 moles of hydrogen atoms bind to 1 mole of carbon atoms. How much energy is liberated/absorbed when 2 mole of H₂ molucules combine with 1 mole of C atoms?

Answer 27

ΔH = sum of bonds broken - sum of bonds formed so ΔH = 872 kJ - 1756 kJ = - 884 kJ - Exothermic