Chapter 6 Thermochemistry: Energy Changes in Reactions

Vocabulary flashcards covering key terms from the Thermochemistry notes.

Thermodynamics

The study of energy and its transformations.

Thermochemistry

The study of the relation between chemical reactions and changes in energy.

Thermochemical equation

A chemical equation that shows the energy change accompanying a reaction.

Thermal equilibrium

A condition in which temperature is uniform throughout a material and no energy flows.

Heat

The energy transferred between objects due to a difference in temperature.

Work

Energy transferred to move an object; w = F × d; in thermodynamics, often w = –PΔV (pressure-volume work).

Potential energy (PE)

Energy stored due to position; PE = m g h.

Kinetic energy (KE)

Energy of motion; KE = 1/2 m v^2.

State function

A property that depends only on the current state, not how the system arrived there.

Total energy

The sum of potential and kinetic energy: PE + KE.

Law of conservation of energy

Energy cannot be created or destroyed; it can be transformed from one form to another.

Thermal energy

Kinetic energy of atoms, ions, and molecules.

Molecular KE relation to temperature

As temperature increases, molecular motion (kinetic energy) increases.

Electrostatic potential energy

Potential energy due to electrostatic interactions between charged particles; depends on distance and charges.

Isolated system

Exchanges no energy or matter with surroundings.

Closed system

Exchanges energy but not matter with surroundings.

Open system

Exchanges both energy and matter with surroundings.

Surroundings

Everything outside the system in a thermochemical study.

System

The part of the universe under study in a thermochemical analysis.

Exothermic

Process where energy flows out of the system (q < 0).

Endothermic

Process where energy flows into the system (q > 0).

q (heat)

Amount of heat transferred during a process.

Sublimation

Phase change from solid to gas.

Fusion (melting)

Phase change from solid to liquid.

Vaporization

Phase change from liquid to gas.

Condensation

Phase change from gas to liquid.

Deposition

Phase change from gas to solid.

Solidification

Phase change from liquid to solid.

Internal energy (E)

Sum of kinetic and potential energy of all components; a state function.

∆E

Change in internal energy; ∆E = Efinal − Einitial.

Translational motion

Molecules moving in straight-line trajectories.

Rotational motion

Molecules rotating about axes.

Vibrational motion

Molecular bond stretching and bending motions.

∆E = q + w

Change in internal energy equals heat plus work.

Work = –P∆V

Pressure-volume work; work done by the system is negative when it expands.

Calorie (cal)

Amount of heat needed to raise 1 g of water by 1°C; 1 cal = 4.184 J.

Joule (J)

SI unit of energy; 1 J = 1 kg·m^2·s^-2.

Enthalpy (H)

H = E + PV; energy content at constant pressure.

∆H

Enthalpy change; ∆H = ∆E + P∆V; also ∆H = qP (heat at constant pressure).

qP

Heat at constant pressure; equals ∆H.

ΔHvap

Molar heat of vaporization; energy to convert 1 mole of liquid at boiling point to vapor.

ΔHfus

Molar heat of fusion; energy to convert 1 mole of solid at melting point to liquid.

Standard enthalpy of formation (∆H°f)

Enthalpy change when 1 mole of a substance forms from its elements in standard states.

Hess's Law

The enthalpy change of a reaction equals the sum of enthalpy changes of its steps.

Standard enthalpy of reaction (∆H°rxn)

Enthalpy change for a reaction under standard conditions; calculable from formation enthalpies.

Standard state

Most stable form of a substance under standard conditions (1 bar, usually 25°C).

Bomb calorimeter

Constant-volume device to measure energy released during combustion; qcal = CcalΔT.

Calorimeter

Device to measure heat absorbed or released by a process; in a closed system, qsystem = −qcal.

qcal

Heat gained by the calorimeter (heat released by the system is its negative).

Fuel value

Energy released during complete combustion per unit mass or volume of a substance.

Alkanes

Hydrocarbons with only single C–C bonds; formula CnH2n+2; include methane, ethane, propane, butane, etc.

Methane

CH4; simplest alkane.

Ethane

C2H6.

Propane

C3H8.

Butane

C4H10.

Heating curve

Graph showing how temperature changes as heat is added, including phase changes.

Molar heat capacity (cP)

Energy required to raise the temperature of 1 mole of a substance by 1°C (J/(mol·°C)).