Thermochemistry

Thermochemistry

Dr. Ron Rusay

Energy Joules (J) / calorie (cal) : (4.184 J = 1 cal)

Can be defined as the capacity to do work.

Chemical energy is defined as heat.

Name five other types of Energy.

Two Types of Energy

Potential: due to an object’s position or material’s composition - which can be converted to work

Kinetic: due to motion of an object

KE = 1/2 mv2

(m = mass, v = velocity)

Law of Conservation of Energy

Different forms of energy can be inter-converted but can neither be created nor destroyed.

(Euniverse is constant)

Describe three inter-conversions of energy.

Temperature v. Energy

Temperature reflects random motions of particles; i.e. the kinetic energy of a system.

Heat involves a transfer of energy between 2 objects due to different energies and temperature differences. Always: HOT cold

Heat (Energy) Loss

Energy: A State Function

Depends only on the state of the system - not the path of how it arrived at that state.

It is independent of pathway.

System and Surroundings

System: That on which we focus attention

Surroundings: Everything else in the universe

Universe = System + Surroundings

Defining Energy ChangeExo- and Endo- thermic

(Exergonic and Endergonic)

Two types of energy change :

Exothermic: Heat flows out of the system (to the surroundings).…negative sign

Endothermic: Heat flows into the system (from the surroundings).…positive sign

First of Three Laws of Thermodynamics

First Law of Thermodynamics: The energy of the universe is constant or “energy is conserved”.

Heat Capacity(Specific Heat)C = heat absorbedincrease in temperature = JC or JKϒ O

P

http://chemconnections.org/general/chem120/Flash/specific_heat_s.swf

Terminology

Specific heat capacityheat capacity per gram = J/°C g or J/K g

Molar heat capacityheat capacity per mole = J/°C mol or J/K mol

Calorimeters

http://chemconnections.org/general/chem120/Flash/calorimetry_s.html

QUESTION

Heat CapacitiesSubstanceSpecific HeatJ / oC . gH2O (l) 4.18H2O (s) 2.03Al (s) 0.89Fe (s) 0.45

QUESTION

Specific HeatH2O (l)

J / oC . g4.18

Why can you burn the top of your mouth with hot pizza and not the bottom?

(The top & bottom are at the same temperature!)

http://www.dailymotion.com/video/x3hfwx_the-science-of-pizza_people

Why can you burn the top of your mouth with hot pizza and not the bottom? (The top & bottom are at the same temperature.)

(Cp) on body fat. In obese mice (fat content 52.76% body wt) the heat capacity was 2.65 kJ kg-1 K-1 and in lean mice (fat content 7.55% body wt) the heat capacity was 3.66 kJ kg-1 K-1.

Specific Heat Interactive

Energy diagrams

E

Process PathIf a system has 2,000 kJ put into it

and the system does work of 1500 kJ on the surroundings, what is ?

w = - 1500 kJ q = +2000 kJ

= q + w= +2000kJ + (-1500kJ)= +500kJ

Can the system do 3000 kJ of work on the surroundings?

Energy diagrams

E

Process Path

w = - 3000 kJ q = +2000 kJ

Can the system do 3000 kJ of work on the surroundings?

QUESTION

QUESTION

EnergyThe Gas Combustion Engine

http://chemconnections.org/general/movies/html-swf/workversusenergyflow.htm

Enthalpy diagramsqp = H

H

Process PathIf 50.0 g of ice @ -25 oC warms

to 25 oC what is H of the process?

H2O (s) @- 25 oC

H = ?

H2O (l) @ 25 oC

http://chemconnections.org/general/movies/HeatingCurves.swf

Hfusion= (+)

Hsolidification= (-)

Hvaporization= (+)

Hcondensation= (-)

Hsublimation= (+)

Hdeposition= (-)

cooling

heatingcoolingheating

b.p.oC

f.p.oC

m.p.oC

H = J or kJ cal or kcal

Enthalpies H?

s l g

H

Process PathIf 50.0 g of ice @ -25 oC warms

to 25 oC what is H of the process?

H2 O (s) @ 0 oC

H = ?

H2O (l) @ 25 oC

H = Cp ice x mass x TH2 O (s) @- 25 oC

H2 O (l) @ 0 oCHfusion = 6.009 kJ/mol

H = Cp liq x mass x T

SubstanceSpecific HeatJ / oC . gH2O (l) 4.18H2O (s) 2.03Al (s) 0.89Fe (s) 0.45 H = Hice +Hfusion +Hliq

QUESTION

“Heat of Reaction” Change in Enthalpy

The heat of any reaction can be calculated from enthalpies of formation of reactants and products.(“Hess’s Law”)

Hrxn° = npHf(products)

nrHf(reactants)

http://chemconnections.org/general/movies/hesslaw.mov

QUESTION

A) 4675 kJ B) -1545 kJ C) -290 kJ

D) -1720 kJ

Ex. Combustion of MethaneCH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

Hrxn° = [1Hf(c) + 2Hf(d)]

[1Hf(a) + 2Hf(b)]

Hrxn°= [-394kJ+(-572kJ)]-[-75kJ+0kJ]= -891kJ

-

QUESTION

Exo- and Endo- thermic(Exergonic and Endergonic)

Heat exchange accompanies chemical reactions.

Exothermic: Heat flows out of the system (to the surroundings).…negative sign

Endothermic: Heat flows into the system (from the surroundings).…positive sign

QUESTION

(Cp) of a body: In obese mice (fat content 52.76% body wt) the heat capacity was 2.65 kJ kg -1 K-1 and in

lean mice (fat content 7.55% body wt) the heat capacity was 3.66 kJ kg-1 K-1.

Remember from earlier slides:http://chemconnections.org/general/chem120/Flash/specific_heat_s.swf

http://chemconnections.org/general/chem120/Flash/heat_transfer_s.swf

How does the relative (Cp) of the body of an obese person compared to that of a lean person relate to their relative rise in body temperatures if both persons

exercise at exactly the same level?

“Heat of Reaction” Change in Enthalpy

The heat of any reaction can be calculated from enthalpies of formation of reactants and products.(“Hess’s Law”)

Hrxn° = npHf(products)

nrHf(reactants)

http://chemconnections.org/general/chem120/Flash/hess_law_s.swf

HCl(aq) + NaOH (aq) NaCl (aq) + H2O(l)

HCl(aq) + NaOH (s) NaCl (aq) + H2O(l)

HNO3(aq) + NaOH (aq) NaNO3 (aq) + H2O(l)

NaOH (s) + H2O(l) NaOH (aq)

Thermochemistry LabNeutralization Reactions / Hess’s law

Calorimetry (Interactive)

Thermochemistry LabNeutralization Reactions / Hess’s law

Tinitial

Tfinal

QUESTION

Report TableVol. Solution (mL)

Density Solution (g/mL)

Mass Solution (g)

Specific Heat Solution (J/g oC)

Tf (oC) Ti (oC) Δ T (oC)

Δ Hrxn

(J)Δ Hrxn

(kJ/mol)

Results/Conclusions: answer questions #3,4,& 6 pg. 41 (DVC Lab

Manual)

QUESTION

QUESTION

Exothermic Reaction

http://www.youtube.com/watch?v=rdCsbZf1_Ng

Heats of Combustion

Combustion of octane releases 5,470 kJ per mole of octane (Hcomb = -5,470 kJ/mol)

How many gallons of water can be boiled by burning 1 gallon of gasoline? (Assume the water is at 25oC)

How many grams of fat have the equivalent combustion energy as 1 gallon of gasoline?

How many pounds of CO2 are added to atmosphere from burning 1 gallon of gasoline? (This question relates to the Greenhouse Gas Workshop.)

Endothermic Reaction

QUESTION

http://chemconnections.org/general/chem120/Flash/calorimetry_s.swf

Activation Energy (Ea) & Chemical Reactions

CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (g)

N2 (g) + O2 (g) 2 NO g)

NOTE: Ea depends on pathway.

Hess’s Law Continued

Reactants Products H = + (endothermic); H = - (exothermic) The change in enthalpy is the same whether the reaction takes place in one

step or a series of steps.

Calculations via Hess’s Law

1. If a reaction is reversed, H is also reversed. N2(g) + O2(g) 2 NO(g) H = 180 kJ 2 NO(g) N2(g) + O2(g) H = 180 kJ

2. If the coefficients of a reaction are multiplied by an integer, H is multiplied by that same integer. 6 NO(g) 3 N2(g) + 3 O2(g) H = 540 kJ

QUESTION

Enthalpies of formation data are not always experimentally easy to obtain. However, enthalpies of combustion data are readily

available. Calculate the enthalpy of formation of methane from the combustion data provided.

C(graphite) + O2(g) → CO2(g) H0comb = −393.5 kJ

H2(g) + ½ O2(g) → H2O(l) H0comb = −285.8 kJ

CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l) H0comb = −890.3 kJ

A) −19.4 kJ/mol B) −74.8 kJ/mol

C) −221.9 kJ/mol D) −296.0 kJ/mol

QUESTION