Chemistry and Heat

That’s hot stuff.

Energy is the capacity to do work.

Energy is measured in Joules

1 Joule of energy can raise 1 N of weight exactly 1 meter

1 J=1N•m

(PS: your diet offers ~5-10 million J/day

Energy is the capacity to do work.

Forms include:

• Kinetic energy

• Gravitational potential energy

• Elastic potential energy

• Electrical energy

• Chemical potential energy

• Heat

Energy is the capacity to do work.

Forms include:

• Kinetic energy

• Gravitational potential energy

• Elastic potential energy

• Electrical energy

• Chemical potential energy

• Heat …of the greatest interest to a

chemist

Exothermic process

System

Surroundings

An exothermic process releases energy

Energy

Endothermic process

System

Surroundings

Energy

An endothermic process absorbs energy

If you add heat to matter, it may…

a)

b)

c)

d)

If you add heat to matter, it may…

a) warm up.

b) melt

c) boil

d) expand (tough to calculate, don’t bother)

Let’s try to warm up a cup of cold coffee.

Step 1: Add heat.

Let’s try to warm up a cup of cold coffee.

Step 1: Add heat.

Well, that was easy.

Let’s try to warm up a cup of cold coffee.

What if you add half as much heat?

Let’s try to warm up a cup of cold coffee.

What if you add half as much heat?

a)

b)

c)

Let’s try to warm up a cup of cold coffee.

What if you add half as much heat?

a) Raise the temperature only half as much.

b)

c)

Let’s try to warm up a cup of cold coffee.

What if you add half as much heat?

a) Raise the temperature only half as much.

b) Use half as much coffee (and cup)

c)

Let’s try to warm up a cup of cold coffee.

What if you add half as much heat?

a) Raise the temperature only half as much.

b) Use half as much coffee (and cup)

c) Use a different substance

The effect of heat, q!

• When something warms up:

The heat, q, depends on:

• The mass of the sample (m)

• The change in temperature (T)

• The nature of the sample (C)

The effect of heat (q)

• When something warms up:

The heat, q, depends on:

• The mass of the sample (m)

• The change in temperature (T)

• The nature of the sample (C)

C is the specific heat capacity for a given substance. Its units are (J/goC)

If you add heat to a sample, it may…

a) warm up. q=mCT

b) melt

c) boil

d) expand (tough to calculate, don’t bother)

q=mCT

• q – heat, in Joules

• m –mass, in grams

• C –specific heat capacity, in J/goC

• T—change in temperature (Tfinal-Tinitial)

Cwater=4.184 J/goC

• Cwater =4.2 J/goC

• Cethanol =2.4 J/goC

• Cice =2.1 J/goC

• CAl =.90 J/goC

• CFe =.46 J/goC

• Cglass =.50 J/goC

• CAg =.24 J/goC

How much heat?

• How much heat does it take to raise 50.g water from 15oC to 80.oC?

• q=mCT

How much heat?

• How much heat does it take to raise 50.g water from 15oC to 80.oC?

• q=mCT = 50.g x 4.18 J/goC x (80.oC-15oC)

How much heat?

• How much heat does it take to raise 50.g water from 15oC to 80.oC?

• q=mCT = 50.g x 4.18 J/goC x (80.oC-15oC) = 50.g x 4.18 J/goC x (65oC)

How much heat?

• How much heat does it take to raise 50.g water from 15oC to 80.oC?

• q=mCT = 50.g x 4.18 J/goC x (80.oC-15oC) = 50.g x 4.18 J/goC x (65oC)

=14000 J (14 kJ)

What is the change in temperature?

• If you add 1550 J to 12 g water, how much will it heat up?

T =q/mC

What is the change in temperature?

• If you add 1550 J to 12 g water, how much will it heat up?

T =q/mC1550 J / (12 g x 4.18 J/goC )

What is the change in temperature?

• If you add 1550 J to 12 g water, how much will it heat up?

T =q/mC1550 J / (12 g x 4.18 J/goC )

= 31oC

What is the change in temperature?

• If you add 1550 J to 12 g water, how much will it heat up?

T =q/mC1550 J / (12 g x 4.18 J/goC )

= 31oC

If the temperature starts at 25oC, it will heat up to …

What is the change in temperature?

• If you add 1550 J to 12 g water, how much will it heat up?

T =q/mC1550 J / (12 g x 4.18 J/goC )

= 31oC

If the temperature starts at 25oC, it will heat up to 56oC

Calorimetry

• --the measurement of heat.

Calorimetry

• --the measurement of heat.

• If one thing gains heat…

Calorimetry

• --the measurement of heat.

• If one thing gains heat…

…something else lost it.

• If 75 g of a metal at 96oC is placed in 58 g of water at 21oC and the final temperature reaches 35oC, what is the specific heat capacity of the metal?

Step 1

• How much heat did the water gain?

Step 1

• How much heat did the water gain?

q=mCT

Mass of water, in grams

Specific heat of water, 4.18 J/goC

Change in the temperature of water, in oC

Step 2

• How much heat did the metal lose?

Step 2

• How much heat did the metal lose?

• Heat lost = - heat gained

• qlost=-qgained

Step 3

• What is the specific heat capacity of the metal?

Step 3

• What is the specific heat capacity of the metal?

C=q/mT

Mass of metal, in grams

Specific heat of metal, in J/goC

Change in the temperature of metal, in oC

Heat lost by metal

• If 75 g of a metal at 96oC is placed in 58 g of water at 21oC and the final temperature reaches 35oC, what is the specific heat capacity of the metal?

.74 J/goC

Thermochemistry

• 2H2(g)+O2(g) 2H2O(g)+ 443,000 J

• (read it.)

Thermochemistry

• 2H2(g)+O2(g) 2H2O(g)+ 443,000 J

• Two moles of hydrogen gas reacts with one mole of oxygen gas to form two moles of water vapor, releasing 443 kJ of heat.

Chemical Energy

• Chemical energy (enthalpy) is stored in bonds.

Chemical Energy

• Chemical energy (enthalpy) is stored in bonds.

• Forming bonds releases energy

• Breaking bonds requires energy

Chemical Energy

• Chemical energy (enthalpy) is stored in bonds.

• Forming bonds is exothermic

• Breaking bonds is endothermic

Chemical Energy

• Chemical energy (enthalpy) is stored in bonds.

• Exothermic reactions have a negative change in enthalpy

• Endothermic reactions have a positive change in enthalpy

Chemical Energy

• Chemical energy (enthalpy) is stored in bonds.

• Exothermic reactions have a negative change in enthalpy

• Endothermic reactions have a positive change in enthalpy

The substances gain energy!

The substances lose energy!

Thermochemistry

2H2 +O22H2O

Breaking these bonds requires energy

Thermochemistry

2H2 +O22H2O

Breaking these bonds requires energy

Thermochemistry

2H2 +O22H2O

Breaking these bonds requires energy

Forming these bonds releases a lot more energy

The mass to heat problem

gkJ

g

1 mol kJ

mol

The heat of reaction, Hrxn

How about some stoichiometry?

• The oxidation of carbon releases 394 kJ/mol.

• How much heat is produced from the oxidation of 15 g C?

How about some stoichiometry?

• The oxidation of carbon releases 394 kJ/mol.

• How much heat is produced from the oxidation of 15 g C?

490 kJ

Special reactions:• Formation

• Combustion

• Fusion

• Vaporization

• Dissolution

Special reactions:• Formation —forming of 1 mole of a

compound from its elements in their normal state

• Combustion —burning 1 mole of a substance in oxygen

• Fusion —freezing 1 mole of a substance at its melting point

• Vaporization —boiling 1 mole of a substance at its boiling point

• Dissolution —dissolving 1 mole of a substance in water

Special Heats

The Heat of Formation

• Hformation of Mg(OH)2 (s) =-925 kJ/mol

says

• Mg(s)+O2(g)+H2(g)Mg(OH)2(s)+ 925 kJ

Special Heats

Hformation of CO2 (g) =-393.5 kJ/mol

?

2Na(s)+C(s)+1½O2(g)Na2CO3(s)+1131 kJ

?

Special Heats

Hformation of a pure element in its standard state is 0 kJ.

(Ex. It takes no energy to make oxygen gas from oxygen gas.)

Special Heats

The Heat of Combustion

• Hcombustion of N2 (g) =90.4 kJ/mol

says

• N2(g)+O2(g) + 90.4 kJ 2NO(g)

Thermochemistry

N2 +O22NO

Breaking these bonds requires more energy…

…than forming these bonds releases

H=

+ 90.4 kJ/mol

Special Heats

Hcombustion of H2 (g) =-286 kJ/mol

?

CH3OH+1½O2(g)CO2(g)+H2O(l) +726 kJ

?

Special Heats

The Heat of Fusion

• Hfusion of Fe =13.8 kJ/mol

says

• Fe(s)+ 13.8 kJ Fe(l) at 1536oC

Special Heats

Hfusion of C6H6 =9.87 kJ/mol

?

CH3OH(s) + 3160 J CH3OH(l) at -98oC

?

Special Heats

The Heat of Vaporization

• Hvaporization of CS2 =28 kJ/mol

says

• CS2(l)+ 28 kJ CS2(g) at 46.3oC

Special Heats

Hvaporization of CH4 =8.2 kJ/mol

?

O2(l) + 6820 J O2(g) at -183oC

?

Special Heats

The Heat of Solution

• Hsolution of (NH2)2CO =-14.0 kJ/mol

says

• (NH2)2CO(s) (NH2)2CO (aq) + 14.0 kJ

Special Heats

Hsolution of NH3 (g) = -34 kJ/mol

?

H2SO4(l) H2SO4 (aq) + 909 kJ

?

Special Heats

• Formation

• Combustion

• Fusion

• Vaporization

• Dissolution

Hess’s Law

• If:

• 2C + O22CO+ 221 kJ and

• 2CO + O22CO2 + 566 kJ

Hess’s Law

• If

• 2C + O22CO+ 221 kJ and

• 2CO + O22CO2 + 566 kJ

Hess’s Law

• If

• 2C + O22CO+ 221 kJ and

• 2CO + O22CO2 + 566 kJ

…then

• 2C + 2O22CO2 + 787 kJ

Hess’s Law

• If

• 2C + O22CO+ 221 kJ and

• 2CO + O22CO2 + 566 kJ

…then

• 2C + 2O22CO2 + 787 kJ

Hess’s Law is usually used with heats of formation

Hess’s Law

• CH4 + 2O2 CO2 + 2 H2O

Hess’s Law is usually used with heats of formation

Hess’s Law

• CH4 + 2O2 CO2 + 2 H2O

• CH4 + 74.86 kJ C + 2 H2

• C + O2 CO2 +393.5 kJ

• 2H2 + O2 2 H2O + 571.6 kJ

Hess’s Law is usually used with heats of formation

Hess’s Law

• CH4 + 2O2 CO2 + 2 H2O

• CH4 + 74.86 kJ C + 2 H2

• C + O2 CO2 +393.5 kJ

• 2H2 + O2 2 H2O + 571.6 kJ

Hess’s Law is usually used with heats of formation

Hess’s Law

• CH4 + 2O2 CO2 + 2 H2O

• CH4 + 74.86 kJ C + 2 H2

• C + O2 CO2 +393.5 kJ

• 2H2 + O2 2 H2O + 571.6 kJ

• CH4 + 2O2 CO2 + 2 H2O + 890.2 kJ

Hess’s Law is usually used with heats of formation

Hess’s Law

• CH4 + 2O2 CO2 + 2 H2O

• Hrxn= Hform,products-Hform,reactants

Hess’s Law is usually used with heats of formation

Hess’s Law

• CH4 + 2O2 CO2 + 2 H2O

• Hrxn= Hform,products-Hform,reactants

=(Hform,CO2+2Hform,H2O)-Hform,CH4

=(-393.5 kJ + 2x-285.8 kJ)-(-74.86 kJ)

= -890.2 kJHess’s Law is usually used with heats of formation

Hess’s Law

• Using table 11.6, What is Hrxn for…

1) 2H2O2 2H2O + O2

2) CaCO3 CaO + CO2

3) 2H2S + 3O22H2O + 2SO2

Hess’s Law is usually used with heats of formation

Heating and cooling curve— naphthalenenaphthalene in a water bath

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What is going on?

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Notice the rate of cooling (from 75oC)

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