Thermodynamics

Energy is neither created or destroyed during chemical or physical changes, but it is transformed from one form to another. Euniverse = 0

Mechanical Gravitational Thermal Electrostatic Electrical Chemical TYPES of ENERGY Kinetic Potential Mechanical Gravitational Thermal Electrostatic Electrical Chemical Radiant Energy Conversion Examples: 1. dropping a rock 2. using a flashlight 3. driving a car

Endo: heat added to system Exo: heat released by system SYSTEMS and SURROUNDINGS System: The thing under study Surroundings: Everything else in the universe Energy transfer between system and surroundings: Endo: heat added to system Exo: heat released by system

HEAT: What happens to thermal (heat) energy? Three possibilities: Warms another object Causes a change of state Is used in an endothermic reaction

Temperature Changes from Heat Exchange Example 1: 5 g wood at 0 oC + 5 g wood at 100 oC Example 2: 10 g wood at 0 oC + 5 g wood at 100 oC Example 3: 5 g copper at 0 oC + 5 g copper at 100 oC Example 4: 5 g wood at 0 oC + 5 g copper at 100 oC Clicker Choices: 1: 0 oC 2: 33 oC 3: 50 oC 4. 67 oC 5: 100 oC 6: other

What happens to thermal (heat) energy? When objects of different temperature meet: Warmer object cools Cooler object warms Thermal energy is transferred qwarmer = -qcooler

Quantitative: Calculating Heat Exchange: Specific Heat Capacity

Specific Heat Capacity The energy required to heat one gram of a substance by 1 oC. Usefulness: #J transferred = S.H. x #g x T How much energy is used to heat 250 g water from 17 oC to 100 oC?

What happens to thermal (heat) energy? When objects of different temperature meet: Warmer object cools Cooler object warms Thermal energy is transferred qwarmer = -qcooler specific heat x mass x T = specific heat x mass x T warmer object cooler object

Heat transfer between substances:

Conceptually Easy Example with Annoying Algebra: If we mix 250 g H2O at 95 oC with 50 g H2O at 5 oC, what will the final temperature be?

Thermal Energy and Phase Changes First: What happens?

Thermal Energy and Phase Changes First: What happens?

Thermal Energy and Phase Changes First: What happens?

But what’s really happening? Warming: Molecules move more rapidly Kinetic Energy increases Temperature increases Melting/Boiling: Molecules do NOT move more rapidly Temperature remains constant Intermolecular bonds are broken Chemical potential energy (enthalpy) increases

Energy and Phase Changes: Quantitative Treatment Melting: Heat of Fusion (DHfus) for Water: 333 J/g Boiling: Heat of Vaporization (DHvap) for Water: 2256 J/g

Total Quantitative Analysis Convert 40.0 g of ice at –30 oC to steam at 125 oC Warm ice: (Specific heat = 2.06 J/g-oC) Melt ice: Warm water (s.h. = 4.18 J/g-oC)

Total Quantitative Analysis Convert 40.0 g of ice at –30 oC to steam at 125 oC Boil water: Warm steam (s.h. = 1.92 J/g-oC)

Enthalpy Change and Chemical Reactions DH = energy needed to break bonds – energy released forming bonds Example: formation of water: DH = [498 + (2 x 436)] – [4 x 436] kJ = -374 kJ

Enthalpy Change and Chemical Reactions DH is usually more complicated, due to solvent and solid interactions. So, we measure DH experimentally. Calorimetry Run reaction in a way that the heat exchanged can be measured. Use a “calorimeter.”

Calorimetry Experiment N2H4 + 3 O2  2 NO2 + 2 H2O Energy released = E absorbed by water + E absorbed by calorimeter Ewater = Ecalorimeter = Total E = H = energy/moles = 0.500 g N2H4 600 g water 420 J/oC

Hess’s Law If reactions can be “added” so can their H values.