energy, heat, and work… oh my…

Energy, Heat, and Work… Oh My…

Energy is

Potential Energythe energy due to the position of an object.

Kinetic Energythe energy due to the movement of an object.

Total Energy =

Two types of energy to be concerned with:

Internal Energy =

A system of oppositely charged particles. The potential energy gained when the charges are separated is converted to kinetic energy as the attraction pulls these charges together.


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A system of fuel and exhaust. A fuel is higher in chemical potential energy than the exhaust. As the fuel burns, some of its potential energy is converted to the kinetic energy of the moving car.



joule (J) is the amount of energy needed to move a 1-kg mass a distance of 1 meter 1 J = 1 N∙m = 1 kg∙m2/s2

calorie (cal) is the amount of energy needed to raise the temperature of one gram of water 1°C kcal = energy needed to raise 1000 g of water 1°C food Calories = kcals

Energy Conversion Factors

1 calorie (cal) = 4.184 joules (J)

1 Calorie (Cal) = 1000 cal = 1 kcal = 4184 J

1 kilowatt-hour (kWh) = 3.60 x 106 J

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Energy, Heat, and Work… Oh My…nanojoule (nJ) 160 nJ is about the kinetic energy of a mosquito

megajoule (MJ)Kinetic energy of a 1-ton car moving at 100 mi/hr 3.6 MJ = 1 kilowatt-hour

gigajoule (GJ) 6 GJ = chemical energy in 1 barrel of oil when burned

exajoule (EJ = 1x1018 J) 1.41 EJ = 2011 earthquake in Japan 52 EJ = Energy release per day of the average hurricane 94 EJ = annual energy consumption in the US


Heat – The transfer of thermal energy as a result of a temperature difference

Work – Force acting through a distance Lifting a book Pushing a desk Inflating a balloonBreathing

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Heat Capacity

When a system absorbs heat, its temperature increases

The increase in temperature is directly proportional to the amount of heat absorbed

The proportionality constant is called the heat capacity, C units of C are J/°C or J/K

Specific Heat Capacity

The specific heat capacity is the amount of heat energy required to raise the temperature of one gram of a substance 1°C units are J/(g∙°C)

The molar heat capacity is the amount of heat energy required to raise the temperature of one mole of a substance 1°C

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J

g C

q m C ΔT

cal

g C

q Tm C Δ q m ΔTC q Tm ΔC q m C ΔT


The amount of energy (J or cal) that is required to raise 1 g of a substance by 1 °C

Specific heat for water = or1.00

1 1 2

H O

calg C

4.184

1 1 2

H O

J

g C

Specific heat for silver = or0.0566

1 1

Agcal

g C

0.237

1 1

Ag

J

g C


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A SMALLER specific heat means a LARGER temperature change given the same amount of energy.

Temperature change when 51000 calories are added to 1000.0 grams of each:

1.00

1 1 2

H O

calg C

0.0566

1 1

Agcal

g C


When 6.789 kilojoules was removed from a piece of gold, its temperature fell from 89.7 °C to 23.0 °C. What was the mass of the gold? (S.H. = 0.0305 cal/g°C)


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Law of Conservation of Energy“Energy can be neither created nor destroyed”

Energy CAN be transformed from one type to anotherEnergy CAN be transferred between system Transfer can take place through WORK or HEAT


Energy can be transferred between systemsSystem – The “stuff” in which changes in energy are

being studied Stuff = material or process

Surroundings – Everything with which the system can exchange energy

System + Surroundings =______________________

(energy of the universe must remain constant)

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Energy can be transferred between systems System –

Surroundings –

Energy can be transferred between a system and its surroundings


Law of Conservation of Energy “Energy can be neither created nor destroyed”

Etotal @ start = Etotal @ end

Energy can be transferred between systems

If energy is lost by the system, what must happen?

If energy is gained by the system, what must happen?

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Energy can be transferred between a system and its surroundings

All energy lost by the system is

All energy gained by the system is


When energy flows out of a system, it must all flow into the surroundings

When energy flows out of a system, Esystem is

When energy flows into the surroundings, Esurroundings is

Therefore:

SurroundingsE +

SystemE ─

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When energy flows into a system, it must all come from the surroundings

When energy flows into a system, Esystem is

When energy flows out of the surroundings, Esurroundings is

Therefore:

SurroundingsE ─

SystemE +

A student drops some magnesium turnings into a styrofoam coffee cup containing HCl(aq)

and uses a thermometer to monitor the reaction. Assume that the cup is a perfect insulator.

1) What is the system?

2) What are the surroundings?


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Changes in Energy

Change in ENERGY isE final – E initial

ORE = E final – E initial

Conditions Erxn sign

E prod > E react

OREreaction = E products – E reactants

E react > E prod


Surroundings

SystemC + O2 → CO2

does this reaction consume or produce energy?

C(s) + O2(g) CO2(g)

does the reaction lead to more or less stability?

what has more internal energy, products or reactants


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Surroundings

SystemCO2 → C + O2

does this reaction consume or produce energy?

CO2(g) C(s) + O2(g)

does the reaction lead to more or less stability?

what has more internal energy, products or reactants


C(s) + O2(g) CO2(g)

the reactants have more energy, which is released during the reaction, leading to greater stability

E react > E prod – (Erxn < 0)

Erxn = (─)energy released

rxnarrow

Incr

easi

ng

Inte

rnal

Ene

rgy

C(s) + O2(g)

CO2(g)

less stable (always)

more stable (always)

CO2(g) C(s) + O2(g)

the products have more energy, which is absorbed during the reaction, leading to less stability

E prod > E react + (Erxn > 0)

Erxn = (+)energy absorbed

rxn

arro

w

Incr

easi

ng

Inte

rnal

Ene

rgy

less stable (always)

more stable (always)

C(s) + O2(g)

CO2(g)


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ColorimetryWhen two objects at different temperatures are placed in contact, heat flows

from the material at the higher temperature to the material at the lower temperature

Heat flows until both materials reach the same final temperature Thermal equilibrium

The amount of heat energy lost by the hot material equals the amount of heat gained by the cold material

There are only 4 equations in calorimetry!!

GOOD NEWS!

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A piece of copper at 100.0 °C is put into a perfectly insulating coffee cup calorimeter containing 160.0 g of water at 22.4 °C. If the final temperature in the calorimeter is 35.2 °C, What is the mass of the copper? C(water) = 4.184 J/g°C, C(Cu) = 0.385 J/g°C

define your universe: system and surroundings (gives your q = –q equation)

system: surrounding: surrounding:

determine your q equations: can you take temp before and after? Is there a calorimeter?

can you take temp of water before and after?

can you take temp of Cu before and after?

is there a calorimeter?

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can you take temp of Cu before and after?

A piece of copper at 100.0 °C is put into a coffee cup calorimeter (C(cal) = 25 J/K) containing 160.0 g of water at 22.4 °C. If the final temperature in the calorimeter is 35.2 °C, What is the mass of the copper? C(water) = 4.184 J/g°C, C(Cu) = 0.385 J/g°C


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can you take temp of oil before and after?


75.0 mL of olive oil at 25.0 °C is poured into a perfectly insulating coffee cup calorimeter containing 50.0 g of water at 33.4 °C. What is the final temperature of the mixture? C(water) = 4.184 J/g°C, C(oil) = 1.97 J/g°C, d(oil) = 0.916 g/mL

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System and surrounding exchange energy through: heat (thermal) energy:

work:

work and heat are NOT state functions!!

q (heat) system gains heat ( ) system loses heat ( )

w (work) system gains energy from work done on it ( ) system loses energy doing work ( )

E system gains energy ( ) system loses energy ( )


If the burning of the fuel in a potato cannon performs 855 J of work on the potato and produces 1422 J of heat, what is E for the burning of the fuel?

What is the system being investigated?

What are the surroundings?

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If the burning of the fuel in a potato cannon performs 855 J of work on the potato and produces 1422 J of heat, what is E for the burning of the fuel?



Reacting 50 mL of H2(g) with 50 mL of C2H4(g) produces 50 mL of C2H6(g) at 1.5 atm. If the reaction produces 3.1 x 102 J of heat and the decrease in volume requires 7.6 J of work, what is the change in internal energy of the gases?



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When gasoline burns in a car engine, the heat released causes the products CO2 and H2O to expand, which pushes the pistons outward. Excess heat is removed by the car’s radiator. If the expanding gases do 451 J of work on the pistons and the system releases 325 J to the surroundings as heat, calculate the change in energy (E) in J.

What is the system?


Where is the heat going, relative to the system?

Where is the work going, relative to the system?

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Work – force acting through a distancePressure – force per unit area

Expandable sealed container

when expanding:

when contracting:

At equilibrium:

When expanding:

When contacting:

E = q + w To determine E, both heat and work must be measured.

The most common chemical work is the work done when the

Enthalpy


The most common chemical work is the work done when the volume of a system changes in the presence of an external pressure.

forceinside forceoutside



Rigid sealed container

Enthalpy

Work – force acting through a distancePressure – force per unit area

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Energy is a “State Function” Path independent – does not matter HOW it got there

Matters only on the state of the system

Change in elevation isElv final – Elv initial

OR Elv = Elv final – Elv initial

Change in ENERGY is

OR



The most common chemical work is PV work the work done when the volume of a system changes in the presence of an external pressure.

Enthalpy (H) is defined as E + PV so

If a system remains at constant pressure and its volume does not change much, then

H = E + PV E = q + w

PV = – wH = q + w + PV

H = q + w – w H = qH = qp

H = qp ≈ E

Enthalpy

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Enthalpy (H)

Even though q and w for the two paths are different, the total E is the same for both.

Enthalpy (H) H is the change in heat for a system at constant pressure.

qP = E + PV = H

H ≈ E

for reactions that do not involve gases

for reactions in which the total amount (mol) of gas does not change

for reactions in which qP is much larger than PV, even if the total moles of gas does change.

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Enthalpy (H)Enthalpy diagrams for exothermic and endothermic processes.

CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g) H2O(s) → H2O(l)

Enthalpy (H)H(–)

H(+)

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Enthalpy (H)Thermochemical equations:

Same as regular balanced equations, but include enthalpy!

Balanced chemical equation for combustion of propane:

C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g)

Balanced thermochemical equation for combustion of propane:

C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g)

or, more typically

C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g)

Enthalpy (H)Thermochemical equations:

Same as regular balanced equations, but include enthalpy!

Balanced chemical equation for decomposition of aluminum oxide:

2 Al2O3(s) 4 Al(s) + 3 O2(g)

Balanced thermochemical equation for decomposition of aluminum oxide:

2 Al2O3(s) 4 Al(s) + 3 O2(g)

or, more typically

2 Al2O3(s) 4 Al(s) + 3 O2(g)

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Enthalpy (H)For each of the following, determine the sign of H, state whether the reaction is exothermic or endothermic, and draw an enthalpy diagram.

b) H2(g) + ½ O2(g) H2O(l) + 285.5 kJ

a) 40.7 kJ + H2O(l) H2O(g)

Ene

rgy

c) 41.38 kJ + KClO3(s) KClO3(aq)

Ene

rgy

Ene

rgy

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½ H2(g) + 2 C(s, graphite) + ½ N2(g) 2 HCN(g) Hf = +135 kJ/mol

2 H2(g) + ½ O2(g) 2 H2O(l) Hf = 285.840 kJ/mol

H2(g) + O2(g) H2O2(l) Hf = 187.80 kJ/mol

Formation ReactionsFormation reactions are…Formation reactions are… reactions for the formation of a compound…

Must always form

All REACTANTS

Write the formation reaction for potassium chlorate

Takes advantage of the state functionness of energy

Used to solve for the heat of reaction in an unknown reaction

Let’s say you want to know the heat of reaction for this reaction:

CO(g) + NO(g) CO2(g) + ½ N2(g) H = ??

given only the following:

CO(g) + ½ O2(g) CO2(g) H = 283.0 kJ

N2(g) + O2(g) 2 NO(g) H = 180.6 kJ

Your job? Manipulate the given equations until they look like the unknown equation.

Allowed manipulations:

Hess’s Law

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Hess’s LawYour job? Manipulate the given equations until they look like the unknown equation.

Reversing a reaction changes the sign on H

Multiplying a reaction by a coefficient multiplies H also

CO(g) + ½ O2(g) CO2(g) H = 283.0 kJ

N2(g) + O2(g) 2 NO(g) H = 180.6 kJ

Target: CO(g) + NO(g) CO2(g) + ½ N2(g) H = ??

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Given the reactions and enthalpies below, find H for the reaction

H2(g) + ½ O2(g) H2O(l) H = 285.8 kJ

N2H4(l) + O2(g) N2(g) + 2 H2O(l) H = 622.3 kJ

H2(g) + O2(g) H2O2(l) H = 187.8 kJ

target: N2H4(l) + 2 H2O2(l) N2(g) + 4 H2O(l) Hrxn = ??

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C(graphite) + ½ O2(g) CO(g) H = 110.5 kJ

CO(g) + ½ O2(g) CO2(g) H = 283.0 kJ

H2(g) + ½ O2(g) H2O(l) H = 285.8 kJ

C(graphite) + 2 H2(g) CH4(g) H = 74.81 kJ

target: 4 CO(g) + 8 H2(g) 3 CH4(g) + CO2(g) + 2 H2O(l) Hrxn = ??

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33 (l) 2(g) 2(g) 2 (l)2CH OH + O CO + 2 H O H 726.6 kJ/mol

1(s, graphite) 2(g) (g)2C + O CO H 110.5 kJ/mol

12(g) 2(g) 2 (l)2H + O H O H 285.8 kJ/mol

(s, graphite) 2(g) 2(g)C + O CO H 393.5 kJ/mol

( () g 3) (l)g ?2 Ht CO H arget: + HO C ?H ??

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target: CO(g) + FeO(s) Fe(s) + CO2(g) Hrxn = ??

Calculate the reaction enthalpy for the reduction of iron (II) oxide by carbon monoxide :

3 Fe2O3(s) + CO(g) 2 Fe3O4(s) + CO2(g) H = 47.2 kJ

Fe2O3(s) + 3 CO(g) 2 Fe(s) + 3 CO2(g) H = 24.7 kJ

Fe3O4(s) + CO(g) 3 FeO(s) + CO2(g) H = 35.9 kJ

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target: C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g)

Hess’s Law Part I II

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Heat of Reaction for Heats of FormationJust like in the enthalpy diagrams, but with less diagram

C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g)

2 ( ),241.826

gf H OH

kJ

mol2( ),393.5

gf COH

kJ

mol3 8( ),105

gf C HH

kJ

mol 2( ),0

gf OH kJ

mol

formation, produ cs formation, rea tantsctrxn HH H

formation, productsH

formation, reactantsH

N2O(g) + 2 O2(g) N2O5(s)

2 5( ),43.1

sf N OH

kJ

mol2 ( ),82.05

gf N OH

kJ

mol 2( ),0

gf OH kJ

mol

Heat of Reaction for Heats of FormationJust like in the enthalpy diagrams, but with less diagram

formation, produ cs formation, rea tantsctrxn HH H

formation, productsH

formation, reactantsH

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Enthalpy (H)

C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g) Hrxn = –2202.0 kJ

If it takes 7.20x103 kJ to perfectly cook a steak, how many grams of propane are necessary (assuming perfect heat transfer)?

What mass of carbon dioxide is produced when enough propane is burned to release 8.88x103 kJ?

There are only 4 equations in calorimetry!!

system surroundingq q required anytime an exchange of energy occurs

Δq = mC T used when the substance remains unchanged through a processthe temperature of the substance can be taken both before AND after?

Δq = C T (typically) used for calorimeters

GOOD NEWS!

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system: surrounding:



can you take temp of C3H8 before and after?


What mass of propane would need to be burned to raise 2500 grams of water from 25.0 °C to 100.0 °C? (MMpropane = 44.11 g/mol)

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1 C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g) Hrxn = –2202.0 kJ

What mass of propane would need to be burned to raise 2500 grams of water from 25.0 °C to 100.0 °C? (MMpropane = 44.11 g/mol)

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1.00 grams of solid KClO3 (122.55 g/mol) is dropped into 25.00 grams of water at 25.0 °C. If the heat of dissolving for KClO3 is +41.38 kJ/mol, what is the final temperature of the water?


system: dissolving surrounding:



can you take temp of KClO3(s) before and after?


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1.00 grams of solid KClO3 (122.55 g/mol) is dropped into 25.00 grams of water at 25.0 °C. If the heat of dissolving for KClO3 is +41.38 kJ/mol, what is the final temperature of the water?

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When 2.10 grams of solid KOH (56.11 g/mol) is dropped into a calorimeter (79 J/°C) containing 55.00 grams of water at 22.5 °C, the water raises to 31.9 °C. What is the enthalpy of dissolving for KOH in kJ/mol?


system: dissolving KOH surrounding:



can you take temp of KOH(s) before and after?


If the actual enthalpy of dissolving for KOH is 56 kJ/mol, what was the percent error in this measurement?

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A 412g sample of silver (CAg = 0.233 J/g°C) needs to be heated from 17.4°C to 147°C. It is heated by burning octane beneath it. The reaction for burning octane is shown below.

2 C8H18(l) + 25 O2(g) 16 CO2(g) + 18 H2O(l) ΔH = –10942.6 kJ

What volume of CO2 at 662 mmHg and 22.4 °C, would be produced if enough octane were burned to heat the silver?

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energy, heat, and work… oh my…

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