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Thermochemistry AP Chem Ch. 6. Thermochemistry Thermochemistry – the study of heat changes that accompany chemical reactions and phase changes Universe.

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Presentation on theme: "Thermochemistry AP Chem Ch. 6. Thermochemistry Thermochemistry – the study of heat changes that accompany chemical reactions and phase changes Universe."— Presentation transcript:

1 Thermochemistry AP Chem Ch. 6

2 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

3 Energy 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

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

5 Useful Conversion Factors 1 cal = J 1 Cal = 1000 cal J = 1 Latm 1 J = 1 kgm 2 / s 2

6 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 J/g°C

7 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, T)**

8 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

9 Enthalpy 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) = -ΔH rxn Endothermic (gains heat) = +ΔH rxn

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

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

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

13 Bond Energy Bond energy = energy required to BREAK a bond Forming bonds = energy is released Breaking bonds = energy is absorbed H o = bond energies broken – bond energies formed

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

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

16 Heating/Cooling Curve

17 Changes in Temp & State Use Heating/Cooling Curve Use q=mc t and q=m H to calculate total heat

18 Heat of Formation Heat of formation-change in enthalpy that takes place when a compound is formed from its 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)

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

20 Hesss Law Multiply rxn by a coefficient Divide rxn by a coefficient Reverse rxn Multiply H by same coefficient Divide H by same coefficient Change sign of H Add two or more thermochemical equations together to obtain a desired thermochemical equation


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