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)