Thermochemistry AP Chem Ch 6 Thermochemistry the study

Thermochemistry AP Chem Ch. 6

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

Energy l Energy – the ability to do work or produce heat l the SI unit for heat is the Joule (J) l Heat CANNOT be measured directly; it is calculated l “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 • m 2 / s 2

Specific Heat l l Specific Heat – energy required to raise the temperature of one gram of any substance one degree Celsius. Each substance has its own specific heat l The higher the specific heat, the longer it will take to raise the temp. l 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 l 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 l l 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 l DH = q only at constant pressure l ΔH° = standard enthalpy change l l ° = occurs under standard conditions l standard conditions = 1 atm, 298 K (25°C)

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

Thermochemical Equations Can be written 2 ways: a. 4 Fe + 3 O 2 2 Fe 2 O 3 + 1625 k. J or 4 Fe + 3 O 2 2 Fe 2 O 3 ΔH = -1625 k. J b. NH 4 NO 3 +27 k. J NH 4+ + NO 3 or NH 4 NO 3 NH 4+ + NO 3 - ΔH = 27 k. J

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

Heat in Changes of State l Molar Heat Values (table 11. 5 on handout) l ΔH vap = - ΔH cond l ΔH fus = - ΔH solid

Heat in Changes of State q = m. DH 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 l Use q=mc. Dt and q=m. DH to calculate total heat l

Heat of Formation l l 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 l l Table 11. 6 in handout all in standard states: 1 atm, 298 K (25°C)

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 k. J**

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