Thermochemistry Study Guide

Energy

the ability to do work or produce heat

Work

causing movement against an opposing force

Heat (q)

energy transfer due to temperature differences

There are two broad categories:

-Kinetic energy (KE)

-Potential energy (PE)

Kinetic Energy

energy due to motion

Example: A moving baseball or flowing water

KE=1/2. mv2

Potential energy 

stored energy due to position or composition

Example: Water at the top of a dam, or energy stored in chemical bonds

Law of Conservation of Energy

• Energy cannot be created or destroyed — it can only change form.

• Total energy of the universe is constant.

  • Example: When gasoline burns, chemical potential energy → heat + work

System

The part of the universe you’re studying (ex: a reaction in a beaker)

Surroundings

Everything else (the beaker, air, your hand)

Universe =

system + surroundings

State function (state property)

Property that does not depend in any way on the system’s past or future

Exothermic

Energy is released (usually as heat)

System → Surroundings

Burning wood, freezing water

(Feels hot)

Endothermic

Energy is absorbed

Surroundings → System

Melting ice, photosynthesis

(Feels cold)

internal Energy (E)

The total energy (PE + KE) of a system.

• Changes in internal energy:

  ΔE=q+w

Work (w)

When a gas expands or compresses:

w=−PΔV

Expansion (ΔV > 0)

system does work on surroundings → w<0w < 0w<0

Compression (ΔV < 0)

surroundings do work on system → w>0w > 0w>0

What is Enthalpy (H)?

• The total heat content of a system at constant pressure.

• Defined as:

  H=E+PV

The change in enthalpy (ΔH) represents the heat flow at constant pressure:

ΔH=qp

• ΔH>0ΔH > 0ΔH>0: endothermic

• ΔH<0ΔH < 0ΔH<0: exothermic

Calorimetry

measures the heat absorbed or released during a reaction.

Constant-Pressure Calorimetry (Coffee-Cup)

• Used for reactions in solution (like neutralization).

• Formula:

  q=smΔTq = smΔT

  • sss: specific heat capacity (J/g·°C)

  • mmm: mass (g)

  • ΔT=Tfinal−TinitialΔT

Constant-Volume Calorimetry (Bomb Calorimeter)

• sed for combustion reactions.

• No volume change (so w=0)

  ΔE=qv=CcalΔTΔE

• Ccal​: heat capacity of the entire calorimeter (J/°C).

Hess’s Law:

The total enthalpy change for a reaction is the same, no matter how many steps it takes.
Energy changes are additive.

ΔHtotal​=ΣΔHsteps

Rules for Manipulating Equations

• If you reverse a reaction → change the sign of ΔH.

• If you multiply a reaction → multiply ΔH by the same factor.

• If you add reactions → add their ΔH values.

Standard Enthalpy of Formation (ΔH°f)

The heat change when 1 mole of a compound forms from its elements in their standard states (1 atm, 25°C)ΔH°rxn​=Σnp​ΔH°f​(products)−Σnr​ΔH°f​(reactants)

Combustion

• reaction of a fuel with oxygen → CO₂ + H₂O + energy (heat/light).

• Always exothermic (ΔH < 0).

Enthalpy of Combustion

the heat released when 1 mol of a substance burns completely in O₂