chapter 5 thermochemistry. terms i thermochemistry –study of – kinetic energy –energy of...
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Chapter 5
Thermochemistry
Terms I
• Thermochemistry– Study of –
• Kinetic Energy– Energy of
• Potential Energy– Energy of
Units of E
• JOULE
• Calorie vs. calorie– 1 Cal = – 1 cal is the amount of heat necessary to
raise the temperature of 1 g of water 1oC
• 1 cal = J
Terms II
• Force– A
• Work– Energy used to move an object– W =
• Energy – Capacity to do
First Law of Thermodynamics
• Energy
• Δ E =
Equation II
• Δ E = • q = heat in or heat out of system
• w = work done on or by the system
Practice
• Calculate the change in the internal energy of the system for a process in which the system absorbs 140 J of heat from the surroundings and does 85 J of work on the surroundings.
Endothermic Reaction
• Process when a system
• Gets
• ΔH >
Exothermic Reaction
• Process when a system
• Gets
• ΔH <
State Function
• Describes the condition of a system
• Does not matter how the system got there
• For example…
ENTHALPY
• Heat… –
• Represented by
• Is a
Enthalpy Equation
• ΔH = Hfinal – Hinitial = qp
• P indicates constant pressure
Enthalpy of Reaction
• ΔHrxn = H(products) – H(reactants)
• 2H2O (g) 2H2 (g) + O2 (g) ΔH =
• 2H2 (g) + O2(g) 2H2O(g) ΔH =
Return
Rules for Enthalpy and RXNS
1. Enthalpy is an extensive property
2. Enthalpy for a reaction
3. Enthalpy change depends on state of reactants and products
Practice
• 2Mg(s) + O2(g) 2MgO(s) ΔH=-1204 kJ
• Is this • How much heat is transferred when 2.40 g of
Mg are reacted?• How many grams of MgO are produced
during an enthalpy change of 96.0 kJ?
Continued
• 2Mg(s) + O2(g) 2MgO(s) ΔH=-1204 kJ
• How many kJ of heat are absorbed when 7.50 g of MgO(s) are decomposed into magnesium and oxygen at constant pressure?
Calorimetry
• The study of
• Heat capacity (C):– Temperature change of an object as it
– Amount of heat required to
– Greater heat capacity means…
Calorimetry
• Molar heat capacity (Cmolar)
• Specific heat capacity (Cp)
–
• Equation:
Calorimetry
• What is the heat required to raise 400.00 g of water by 34.50oC?
• What is the heat lost when 200.00 g of iron changes from 115.50 oC to 22.00 oC?
Calorimetry
• Large beds of rocks are used in some solar heated homes to store heat. Assume that the specific heat of rock is 0.082 J/g-K. Calculate the quantity of heat absorbed by 50.0 kg of rocks if their temperature increases by 12.0 oC
Flashback…
• Law of conservation of energy (First law of thermodynamics):– –
Calorimetry
• You have heated a 55.00 g piece of iron, Cp = 0.385 J/g-K, to 200.00 oC. You then put the iron into water in a calorimeter. There are 300.00 g of water at 22.00 oC. What is the final temperature of the mixture?
Calorimetry
• In the calorimetry lab you will be mixing an acid and a base and studying the temperature changes.
• You mix 35.00 mL each of 1 M HCl and 1 M NaOH in a calorimeter. The temperature increases from 21.0 to 27.5 oC. What is the enthalpy change for the reaction in kJ/mol HCl?
Hess’s Law
Enthalpy of Formation
• Energy change for the formation
• Symbolized by Hf
• Tables are for standard conditions– 1 atm– 25oC
• Appendix C
Enthalpy of Formation
• Units for Hof are in
• Magnitude depends on state
• 2H2(g) + O2(g) 2 H2O(l) Hof = -285.8
• 2H2(g) + O2(g) 2 H2O(g) Hof = -241.8
Enthalpy of Formation
• By definition: the standard enthalpy of formation of the most stable form of ANY element is 0 kJ/mol
Hof C(s) = 0
Hof H2(g) = 0
Enthalpy of RXN
Horxn = Σn Ho
f (products) - Σm Hof
(reactants)
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