Thermochemistry pt 2

Multiple Choice

• Enthalpy (ΔH) = heat absorbed or released at constant pressure

• Entropy (ΔS) = measure of molecular disorder

How are they different, and how can they be related?(talk with instructor)

• ΔH tracks energy flow as heat; ΔS measures disorder. Adding heat (ΔH↑) often increases molecular motion, raising disorder (ΔS↑)

• ΔH and ΔS both measure temperature changes, but ΔS is faster.

• They’re identical—both track energy released in reactions

Multiple Choice

The diagram shows two paths for an apple falling to the ground:

1. Direct Path: Falls straight from the table.

2. Indirect Path: Lifted higher first, then dropped.

How does this demonstrate Hess's Law?

• Both paths release the same net energy. Like Hess's Law, the energy change depends only on start/end points, not the path.

• Only the direct path follows Hess's Law because it's simpler

• The indirect path releases more energy due to the extra lift

• Hess's Law only applies to chemical reactions, not physical systems.

Multiple Choice

Examine the diagram. What is the FIRST step in this reaction?"

• Water molecules break(dissociate) into H₂ and O₂ gas, absorbing 483.6 kJ/mol

• Carbon bonds with hydrogen to form methane (CH₄)

• Oxygen gas spontaneously decomposes into atoms

• CO₂ forms immediately, releasing heat

Multiple Choice

What happens to the oxygen (O₂) produced in the first step?

• It escapes as a gas product

• It reacts with carbon to form CO₂, releasing 393.5 kJ/mol

• It converts back into water

• It dissolves into the solution

Multiple Choice

When we add these two steps together, what happens to the O₂ molecules?

• They double in quantity to form O₄

• They remain as a final product

• They cancel out because O₂ appears on both sides of the combined equation

• They all become part of the CO₂ product

Multiple Choice

The overall ΔH is +90.1 kJ/mol. This means the reaction is:

(pg 375)

• Spontaneous - happens without energy input

• Isothermic - no energy change

• Exothermic - net energy release

• Endothermic - more energy absorbed than released

Categorize

Options (18)

Gas (g)

Gas (g

Liquid (l)

Colorless

Odorless

Flammable

Used in fuel cells

Lightest element

Colorless

Odorless

Essential for respiration

Supports combustion

Found in the atmosphere

Colorless

Odorless

Essential for life

Used in hydration

Has a high specific heat capacity

What are the correct phases of H₂,O₂, and H₂O at standard state (25°C, 101.3 kPa) (review pg 383)

H₂

O₂

H₂O

Multiple Choice

How do you calculate the standard enthalpy (ΔH°ᵣₓₙ) for:
2SO₂(g) + O₂(g) → 2SO₃(g)? (read pg 384)

• Add all formation enthalpies: ΔH°f(SO₂) + ΔH°f(O₂) + ΔH°f(SO₃)

• [2×ΔH°f(SO₃)] − [2×ΔH°f(SO₂) + ΔH°f(O₂)]

• Use only ΔH°f(SO₃) since it’s the main product

• [2×ΔH°f(SO₂)] − [2×ΔH°f(SO₃)]

Multiple Choice

Which actions REQUIRE energy when forming a solution?"

• Breaking ionic bonds in solute AND intermolecular forces in solvent

• Only breaking ionic bonds in the solute

• Forming the solvation shell

• Stirring the mixture

Multiple Choice

What actions release energy? (pg 386)

• Separating solvent molecules

• Breaking ionic bonds in the solute

• Forming the solvation shell

• Increasing the system's disorder

Multiple Choice

How can you use standard enthalpies of formation (ΔH°f) to determine ΔH°ᵣₓₙ for a reaction?

• Only use the formation energy of the first product formed

• Add together the formation energies of all chemicals in the reaction

• Subtract the total formation energy of the starting materials from the total formation energy of the products

• Average the formation energies of the starting materials and products