Chemistry and Energy

How are they related?

Energy Encountered Daily

Is Heat Used or Released? Endothermic reactions used heat

from the surroundings Sweating Refrigeration

Exothermic heat releases heat to the surroundings Hot hands Combustion Exercise

Endothermic Reactions

Decrease in kinetic energy decrease in temperature heat will transfer from the environment to the system resulting in a cooler environment

Absorbs heat from its surrounding. The system gains heat Positive value for q H = q = 0 Hproducts Hreactants

Exothermic Reactions

Increase in kinetic energy increase in temperature of system heat released to the environment resulting in a hotter environment

Releases heat to its surroundings The system loses heat Negative value for q H = q = 0 Hproducts Hreactants

Enthalpy

Heat content for systems at constant pressure

Symbol is H Terms heat and enthalpy are used

interchangeably for this course H = q = m C T

Heat moves from ________ to ___________.

Law of Conservation of Energy

Energy is not lost or gained in a chemical reaction

 In a chemical reaction potential energy is transferred to kinetic energy

Thermochemical Equations

An equation that includes the heat change

Example: write the thermochemical equation for this reaction CaO(s) + H2O(l) Ca(OH)2(s) H = -

65.2 kJ

CaO(s) + H2O(l) Ca(OH)2(s) + 65.2 kJ

Stoichiometry and Thermochemistry

Tin metal can be extracted from its oxide according to the following reaction:

SnO2(s) + 4NO2(g) + 2H2O(l) + 192 kJ Sn(s) + 4HNO3(aq)

How much energy will be required to extract 59.5 grams of tin?

How to solve

1. Use your stoichiometry2. Treat heat as a reactant or

productSnO2(s) + 4NO2(g) + 2H2O(l) +

192 kJ Sn(s) + 4HNO3(aq)59.5 g Sn 1 mol Sn 192 kJ 1 g Sn 1 mol Sn

If an Object feels hot, it means it is undergoing a change with a H that is:

a. positiveb. negativec. whether the object feels hot

or not is unrelated to its Hd. I don’t know  

If the object feels hot, it means it is undergoing:

a. an exothermic reactionb. an endothermic reactionc. whether it feels hot or not is

unrelated to whether it is undergoing an exothermic or an endothermic change

How does ice melt?

Molar Heat of Fusion

Heat absorbed by one mole of a substance during melting

Constant temperature Hfus H2O(s) H2O(l) H = 6.01

kJ/mol

Molar Heat of Solidification Heat lost when 1 mole of a liquid

solidifies Temperature is constant Hsolid Hfus = -Hsolid

H2O(l) H2O(s) H = -6.01 kJ/mol

Molar Heat of Vaporization Heat needed to vaporize 1 mole

of a liquid Hvap H2O(l) H2O(g) Hvap = 40.7

kJ/mol

Molar Heat of Condensation Heat released when 1 mole of

vapor condenses Hcond

H2O(g) H2O(s) Hcond = -40.7 kJ/mol

Hvap = -Hcond

Phase Change Diagram for Water

Phase Change Diagram

The House that Heats Itself http://www.sciencefriday.com/videos/

watch/10007

Calorimetry

Method used to determine the heat involved in a physical or chemical change.

Relies on the law of conservation of energy

Calorimeter

Simple Calorimeter

Calorimetry Math Heat gained by the water = q Heat lost by the system = -q

mCT = q T = Tf –Ti , m = mass, C = specific heat

q gained by water = q lost by system q water = - q system  mCT = -mCT(mass H2O)(spec. heat H2O)(T H2O) = -

(mass sys)(spec. heat sys)(T sys)