# Unit 6 Thermodynamics

## 6.9 Hess’s Law

Hess’s Law: if you add chemical equations to get an overall equation, then you can also add the heat changes (ΔH) to get the overall heat change If two identical substances are on opposite sides of the arrow, they will cancel (reduce) If two identical substances are...

## 6.8 Enthalpies of Formation

Standard State Standard enthalpy (ΔH⁰): the enthalpy change at standard conditions           ○ ⁰ = recorded in standard state Standard State conditions:          ○ For a gas: pressure is 1 atm          ○ For a pure substance in a condensed state (liquid or solid) =...

## 6.7 Bond Enthalpies (Energies)

It is the energy stored in a chemical bond → gives information about the strength of a bonding interaction (sigma = sum, D = bond)              ○ Key Word: Bond enthalpy             ○ If not sure what bond enthalpies to use, draw basic lewis...

## 6.6 Introduction to Enthalpy of Reaction

Enthalpy (H) = heat          ○ Is an extensive property and state function                ○ E is the internal energy of the system, P is the pressure of the system, and V is the volume of the system                ○ Units of ΔH = kJ                   ○ Key Word:...

## 6.5 Energy of Phase Changes

→ endothermic (attractive forces are being broken; final answer [ΔH or q] = +); ← exothermic (attractive forces are forming; final answer [ΔH or q] = -)             ○ Heat of fusion will be positive for melting and negative for freezing; heat of vaporization will be...

## 6.4 Heat Capacity and Calorimetry

Heat capacity (C): heat absorbed per degree (J/C or J/K)            ○ Extensive property: depend on amount of substance                                                      Specific heat capacity (cp): heat capacity per gram (J/C·g or J/K)           ○ Amount of heat...

## 6.3 Heat Transfer and Thermal Equilibrium

Temperature: average kinetic energy of a substance             ○ Reflects random motions of particles             ○ A change in temperature indicates an energy change                                                                      Heat: involves the transfer of...