Chem 1 chapter 7 - Thermochemistry

Internal energy (E)

The total energy contained within a system

Work (w)

Energy transferred when an object is moved by a force; in chemistry

Heat (q)

Energy transferred between a system and its surroundings due to a temperature difference.

First law of thermodynamics

Energy cannot be created or destroyed

System

The specific part of the universe being studied

Surroundings

Everything outside the system that can exchange energy or matter with it.

Endothermic

A process that absorbs heat from the surroundings (q > 0).

Exothermic

A process that releases heat to the surroundings (q < 0).

Endergonic

A process that requires or consumes free energy (ΔG > 0); nonspontaneous.

Exergonic

A process that releases free energy (ΔG < 0); spontaneous.

State functions

Properties that depend only on the current state of the system

Enthalpy (H)

The heat content of a system at constant pressure; H = E + PV.

Enthalpy of reaction (ΔHrxn)

The heat change that occurs during a chemical reaction at constant pressure.

Calorimetry

The experimental measurement of heat flow in or out of a system during a physical or chemical process.

Calorimeter (bomb or coffee cup)

A device used to measure heat changes; a bomb calorimeter operates at constant volume

Heat capacity

The amount of heat required to raise the temperature of an object by 1°C (or 1 K).

Specific heat

The amount of heat required to raise the temperature of 1 gram of a substance by 1°C.

Molar heat capacity

The amount of heat required to raise the temperature of 1 mole of a substance by 1°C.

Hess’s law

The total enthalpy change for a reaction is the same regardless of the number of steps or pathway taken.

Enthalpy of formation (ΔHf)

The enthalpy change when one mole of a compound forms from its elements in their standard states.

Enthalpy of combustion (ΔHc)

The enthalpy change when one mole of a substance reacts completely with oxygen to form products (usually CO₂ and H₂O).

Sign conventions for q (heat)

Positive (+) when heat is absorbed by the system (endothermic)

Sign conventions for w (work)

Positive (+) when work is done on the system (energy enters)

Sign conventions for ΔE (change in internal energy)

Positive (+) when the system gains energy (from heat or work)

Formula to calculate heat (q)

q=m×C×ΔT

Formula for heat in calorimetry (constant pressure)

𝑞=𝐶𝑝×Δ𝑇

Formula for heat in calorimetry (constant pressure)

𝑞=𝐶𝑝×Δ𝑇q=Cp

×ΔT for coffee-cup calorimeters

q=−q surroundings(heat lost by system = heat gained by surroundings).

Formula for work (w)

w=−PΔV

Formula for change in internal energy (ΔE)

ΔE=q+w.
​
heat at constant pressure

Relationship between ΔH and q

At constant pressure - the heat flow (qₚ) equals the enthalpy change of the system (ΔH).