chemistry and energy

How are they related?

Energy Encountered Daily

What is Energy?

Defined as the ability to do work or create heat.

Many types of energy Thermal Light Gravitational Kinetic Potential

Light Energy Review

How is light energy produced? Electrons release light energy when

they fall from a high energy level to a lower energy.

We’re now going to talk about energy released or used in a chemical reaction. Heat energy

Thermochemistry

The study of heat used or released in a chemical reaction.

Let’s investigate heat as it compares to temperature using the Heat vs. Temperature Handout

Specific Heat Calculations q = mCΔT

q = heat (J or cal or Cal)4.184 cal = 1 Joule1000 cal = 1 Cal (dietary calorie)

m = mass (g)C = specific heat (J/g oC or cal/g ooC)ΔT = change in temperature (o C or

K) = Tf - Ti

Specific Heat

Specific heat of water = 1 cal /g o C or = 4.184 J / goC

Specific heat of most metals = < 1 J / goC

Do metals heat slowly or quickly compared to water?

Do metals stay warm longer or shorter than water?

Practice Problem

How much energy is required to heat 120.0 g of water from 2.0 oC to 24.0oC?

q = mCΔTm= 120.0 gC = 4.184 J/goCΔT= (24.0 – 2.0)oC = 22.0oC

q = (120.0g)(4.184 J/goC)(22.0oC) =

Practice Problem

How much heat (in kJ) is given off when 85.0 g of lead cools from 200.0oC to 10.0 oC? (Specific heat of lead = 0.129 J/g oC)

q = mCΔTm = 85.0 gC = 0.129 J/g oCΔT = (10.0 – 200.0)oC = - 190.0oC

q = (85.0 g)(0.129 J/g oC)(- 190.0oC) = -

How Do Chemical Reactions Create Heat energy? Consider the combustion of gasoline

(octane)2 C8H18 +25 O2 16 CO2 +18 H2O

Potential Energy: Stored energy Potential energy is stored in the bonds of the

reactant s and the products When bonds are broken, the energy is

available When produce bonds form, some energy is

used in these bonds The excess energy is released as heat

Kinetic Energy

Directly related to temperature

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)

Standard Heat of Reaction Heat change for the equation as it is

writtenH = Hf(products) - Hf(reactants)

Standard Heats of Formation (Hf) Change in enthalpy when 1 mole

of the compound is formed from its elements in their standard states at 25oC and 101.3 kPa

Hess’s Law

 A way to calculate the heat of a reaction that may be too slow or too fast to collect data from.

Add together several reactions that will result in the desired reaction. Add the ΔH for these reactions in the same way.

Htotal = Hproducts - Hreactants