AP Chemistry Unit 6 Energy Changes - Endothermic and Exothermic Processes

A comprehensive set of practice flashcards covering heat, work, sign conventions, enthalpy, phase changes, energy diagrams, and common thermochemistry scenarios from Unit 6.

What is heat (q) in thermodynamics?

The energy transferred into or out of a system due to a temperature difference.

What are the common units for heat in chemistry?

Joules (J) or kilojoules (kJ).

What is the sign of q if heat is transferred from a system to its surroundings?

Negative (q < 0).

What is the sign of q if heat is transferred from the surroundings to the system?

Positive (q > 0).

What is work in the context of chemical thermodynamics, and how is it treated with a piston?

Work is energy required to move against a force; for expansion against the surroundings, w is negative; for compression by the surroundings, w is positive.

What is the sign of work when the system expands and pushes on the surroundings?

Negative.

What is the sign of work when the surroundings compress the system?

Positive.

Define system, surroundings, and universe in thermodynamics.

System: reacting species; surroundings: everything else; universe: system + surroundings.

What is ΔE in thermodynamics, in terms of heat and work?

ΔE = q + w (change in the internal energy).

How is qsys related to qsurr when no other energy transfer occurs?

qsys = - qsurr (the heat gained by the system equals the heat lost by the surroundings).

What is the difference between exothermic and endothermic reactions in terms of enthalpy change (ΔH)?

Exothermic: ΔH < 0 (heat released to surroundings). Endothermic: ΔH > 0 (heat absorbed from surroundings).

At constant pressure, how is q related to ΔH?

q = ΔH for processes at constant pressure (assuming only pressure-volume work is present).

Which phase changes are endothermic and which are exothermic?

Endothermic: melting, vaporization, sublimation. Exothermic: freezing, condensation, deposition.

Is breaking bonds endothermic or exothermic? What about forming bonds?

Breaking bonds is endothermic (requires energy); forming bonds is exothermic (releases energy).

What is activation energy (Ea)?

The minimum energy required for reactants to reach the transition state and proceed to products.

In an energy profile diagram, what does Ea represent?

Activation energy for the forward reaction.

What does ΔH_rxn represent in a reaction pathway diagram?

The enthalpy change of the reaction; energy difference between products and reactants (positive for endothermic, negative for exothermic).

How can you tell if a reaction is endothermic or exothermic from an energy diagram when the products are higher or lower in energy than reactants?

If products are higher than reactants, endothermic; if lower, exothermic.

What is the Born-Haber cycle?

A sequence of steps (sublimation, ionization, dissociation, electron affinity, lattice energy) that sums to the heat of formation of an ionic solid.

What are the three main energetic steps involved in dissolving an ionic solid in water?

Breaking the lattice (endothermic), separating ions and hydration interactions (enthalpy changes can be either endo or exo depending on magnitudes); overall dissolution enthalpy depends on lattice energy vs. hydration energies.

What is the dissolution enthalpy sign for ammonium nitrate in water used in cold packs?

Endothermic; absorbs heat from surroundings, causing the surroundings to feel cold (q_sys > 0).

What does a heating curve versus a cooling curve illustrate about energy changes during phase transitions?

Heating curve shows endothermic steps (melting, boiling) as energy is absorbed; cooling curve shows exothermic steps (freezing, condensation) as energy is released.

How do you interpret a temperature increase in a reaction or dissolution experiment?

Indicates heat transfer from system to surroundings (exothermic) or a release of energy into the surroundings; context determines the sign of q and heat flow direction.

What happens to the system and surroundings during an endothermic process?

System absorbs heat from surroundings; surroundings feel cooler as heat leaves the surroundings.

Why is the thermometer considered part of the surroundings in heat flow analysis?

Because it measures the temperature of the surroundings, which change as heat flows between system and surroundings.

If a process has q = 60 J and w = -50 J, what is ΔE?

ΔE = q + w = 60 J + (-50 J) = 10 J; the system gains energy.