Thermochemistry AP Chem Ch. 6

Thermochemistry Thermochemistry – the study of heat changes that accompany chemical reactions and phase changes Universe = System + Surroundings Endothermic Reaction – one in which energy (heat) is absorbed into the system Exothermic Reaction – one in which energy (heat) is released from the system

Energy the SI unit for heat is the Joule (J) Energy – the ability to do work or produce heat the SI unit for heat is the Joule (J) Heat CANNOT be measured directly; it is calculated “q” = quantity of heat

In the lab… Calorimeter – an insulated device used for measuring the temperature change during a chemical process

Useful Conversion Factors 1 cal = 4.184 J 1 Cal = 1000 cal 101.3 J = 1 L•atm 1 J = 1 kg•m2 / s2

Specific Heat Specific Heat – energy required to raise the temperature of one gram of any substance one degree Celsius. Each substance has its own specific heat The higher the specific heat, the longer it will take to raise the temp. water has a very high specific heat 4.184 J/g°C

q = m c ΔT q = heat absorbed or released c = specific heat value m = mass or moles of substance ΔT = change in temperature Use this when there is NO change in state **Make sure units will cancel (usually “c” will determine what units to use for q, m, DT)**

Heat Lost = Heat Gained When 2 substances at different temperatures come into contact with each other, heat is transferred from the warmer to the cooler substance until both substances are at the same temperature

Enthalpy Exothermic (lose heat) = -ΔHrxn Enthalpy (H) – measure of the amount of energy aborbed/released by a reaction Changes in enthalpy (ΔH rxn )can be calculated for specific chemical reactions Exothermic (lose heat) = -ΔHrxn Endothermic (gains heat) = +ΔHrxn

Enthalpy (cont’d) ΔH rxn = H products – H reactants DH = q only at constant pressure ΔH° = standard enthalpy change ° = occurs under standard conditions standard conditions = 1atm, 298K (25°C)

Thermochemical Equations A balanced chemical equation that includes all reactants, products, and the change in energy (ΔH) Use energy in stoichiometric calculations

Thermochemical Equations Can be written 2 ways: a. 4Fe + 3O2 2Fe2O3 + 1625kJ or 4Fe + 3O2 2Fe2O3 ΔH = -1625kJ b. NH4NO3 +27kJ  NH4+ + NO3- NH4NO3  NH4+ + NO3- ΔH = 27kJ

DHo = bond energies broken – bond energies formed Bond Energy Bond energy = energy required to BREAK a bond Forming bonds = energy is released Breaking bonds = energy is absorbed DHo = bond energies broken – bond energies formed

Heat in Changes of State Molar Heat Values (table 11.5 on handout) ΔH vap = - ΔH cond ΔH fus = - ΔH solid

Heat in Changes of State q = mDH m = mole or mass DH = molar heat value **units for m depend on units of DH**

Heating/Cooling Curve

Changes in Temp & State Use Heating/Cooling Curve Use q=mcDt and q=mDH to calculate total heat

Heat of Formation Heat of formation-change in enthalpy that takes place when a compound is formed from it’s elements ΔH°f = standard heat of formation = the change in enthalpy that accompanies the formation of 1 mole of compound with all substances in their standard states Table 11.6 in handout all in standard states: 1atm, 298K (25°C)

ΔH°rxn = Σ ΔH°f (products) - Σ ΔH°f (reactants) Heat of Formation ΔH°rxn = Σ ΔH°f (products) - Σ ΔH°f (reactants) **ΔH°f of a pure element in it’s standard state is 0.0 kJ**

Hess’s Law Add two or more thermochemical equations together to obtain a desired thermochemical equation Multiply DH by same coefficient Divide DH by same coefficient Change sign of DH Multiply rxn by a coefficient Divide rxn by a coefficient Reverse rxn