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 1 o C 1 cal = J

Terms II Force –A–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 = H final – H initial = q p P indicates constant pressure

Enthalpy of Reaction ΔH rxn = H(products) – H(reactants) 2H 2 O (g)  2H 2 (g) + O 2 (g) ΔH = 2H 2 (g) + O 2(g)  2H 2 O (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) + O 2(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) + O 2(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 (C molar ) Specific heat capacity (C p ) – Equation:

Calorimetry What is the heat required to raise g of water by o C? What is the heat lost when g of iron changes from o C to o C?

Calorimetry Large beds of rocks are used in some solar heated homes to store heat. Assume that the specific heat of rock is J/g-K. Calculate the quantity of heat absorbed by 50.0 kg of rocks if their temperature increases by 12.0 o C

Flashback… Law of conservation of energy (First law of thermodynamics): – –

Calorimetry You have heated a g piece of iron, C p = J/g-K, to o C. You then put the iron into water in a calorimeter. There are g of water at o C. 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 mL each of 1 M HCl and 1 M NaOH in a calorimeter. The temperature increases from 21.0 to 27.5 o C. 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  H f Tables are for standard conditions –1 atm –25 o C Appendix C

Enthalpy of Formation Units for  H o f are in Magnitude depends on state 2H 2(g) + O 2(g)  2 H 2 O (l)  H o f = H 2(g) + O 2(g)  2 H 2 O (g)  H o f =

Enthalpy of Formation By definition: the standard enthalpy of formation of the most stable form of ANY element is 0 kJ/mol  H o f C (s) = 0  H o f H 2(g) = 0

Enthalpy of RXN  H o rxn = Σn  H o f (products) - Σm  H o f (reactants)