Thermochemistry & Thermodynamics Review

Question-and-answer flashcards covering key concepts from thermochemistry, bond energy, heat/work, enthalpy, and the three laws of thermodynamics.

What does thermochemistry study?

The energy changes—especially heat—during chemical reactions and physical transformations.

How is energy defined in thermochemistry?

The capacity to do work or transfer heat.

Name the two main forms of energy discussed in thermochemistry.

Kinetic energy and potential energy.

In molecules, what motions contribute to kinetic energy?

Translational, rotational, and vibrational motion.

How is potential energy stored in chemical systems?

In chemical bonds; it decreases when bonds form and increases when bonds break.

Does breaking a chemical bond absorb or release energy?

It absorbs energy, raising the potential energy of the system.

Give the symbolic equation that represents breaking an O–H bond in water.

H₂O → H⁺ + OH⁻

Does forming a chemical bond absorb or release energy?

It releases energy, lowering the system’s potential energy.

Give the symbolic equation that represents forming an O–H bond in water.

H⁺ + OH⁻ → H₂O

What is internal energy (U)?

The total energy (kinetic + potential) of a system.

What is heat (q) in the context of energy transfer?

The flow of energy due to a temperature difference, always from hot to cold.

What is work (w) in thermochemistry?

Energy transfer that results from a force acting over a distance.

Write the formula that relates change in internal energy to heat and work.

ΔU = q + w

According to sign conventions, what does q > 0 indicate?

Heat is absorbed by the system (endothermic process).

According to sign conventions, what does w < 0 indicate for a gas?

Work is done by the system on the surroundings, as in gas expansion.

Provide the equation for pressure–volume work in a gas reaction.

w = −PΔV

Define enthalpy (H).

A thermodynamic property representing the heat content of a system at constant pressure (H = U + PV).

How is the enthalpy change (ΔH) of a reaction at constant pressure calculated?

ΔH = Hproducts − Hreactants

What sign of ΔH characterizes an exothermic reaction?

ΔH < 0 (heat is released).

Give an example of an exothermic reaction and its ΔH value.

Combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH = −890 kJ.

What sign of ΔH characterizes an endothermic reaction?

ΔH > 0 (heat is absorbed).

Give an example of an endothermic process noted in the lecture.

Photosynthesis: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂.

State the First Law of Thermodynamics.

Energy is conserved; ΔU = q + w, and energy cannot be created or destroyed, only transferred or converted.

State the Second Law of Thermodynamics in terms of entropy.

For any spontaneous process, the total entropy of the universe increases (ΔS_universe > 0).

Provide an everyday example illustrating the Second Law.

Ice melting at room temperature: the organized solid becomes a more disordered liquid, increasing entropy.

State the Third Law of Thermodynamics.

The entropy of a perfect crystal at absolute zero (0 K) is zero.