Thermochemistry Thermo = heat Chemistry = study of matter

Thermochemistry Study of heat changes and energy that accompany chemical reactions and phase changes

Review of Energy Capacity to do work or to create heat and or generate electricity Types: Chemical Nuclear Thermal Radiant (light) Electrical Mechanical

Law of conservation of Energy Energy can be converted from one form to another but it cannot be created or destroyed

Forms of energy: Potential –Stored energy Kinetic –Energy of motion

Chemical potential energy Energy stored within the structural units of chemical compounds

Heat is energy (Q) flows from an area of high heat to an area of low heat Can be measured in calories, Calories, or Joules 1000 cal = 1 Cal = J

Heat energy (Q) System: part of the universe being studied Surroundings: everything outside of the system Universe: system + surroundings

Heat energy (Q) Endothermic: feels cold to touch temperature decreases heat absorbed by system Q = +

Heat energy (Q) Exothermic: feels warm to touch temperature increases heat released by system Q = -

Specific heat capacity C or C p The amount of energy required to raise the temperature of 1 gram of a substance 1ºC Water= 4.18 J/g 0 C Glass=.50 J/g 0 C

Specific heat capacity Measure of a substance’s resistance to temperature change High=resistant (water) Low=easy (metals, glass, etc.)

Equation Q = m C ∆T - Q is the heat absorbed or released in Joules -m is mass in grams -C is the specific heat in J/g/ºC -∆T is the temperature change in ºC

mass Temperature change (final temp minus initial temp) Specific heat capacity in J/g- o C (Q) = m C (  T)

When 1982 g of water underwent a temperature change from o C to o C, how much energy in kJ did the water absorb? The specific heat of water is J/(g- o C). Show all work!

How much energy in J is required to raise the temperature of g of copper from 22.8 o C to o C? The specific heat of copper is J/g/ o C. Show all work!

Measuring heat exchange Calorimeter- insulated device used for measuring the amount of heat absorbed or released during a reaction

Calorimeter

In a closed system: Heat loss = Heat gain Q lost = Q gained m C ∆ T = m C ∆ T

Enthalpy (∆H) heat content of a system at constant pressure Can measure changes in enthalpy during a reaction

Enthalpy heat of reaction ∆ H rxn ∆ H rxn = H final - H initial Or ∆ H rxn = H products - H reactants

Thermochemical Equations Used to calculate and express heat changes. The physical states of the reactants and products in this equation is important! Example: CH 4(g) + 2O 2(g) → CO 2(g) + 2H 2 O (l) ∆H = kJ

Exothermic rxn: ∆ H rxn is < 0 ∆ H rxn is negative

Endothermic rxn: ∆ H rxn is > 0 ∆ H rxn is positive