Thermochemistry

• When energy is changed from one form to another, all of the energy can be accounted for. This is called the Law of Conservation of Energy.

• Heat is energy. If heat is released by a chemical system, an equal amount of heat will be absorbed by the surroundings

• (For example) - When your body breaks down sugar completely, the body releases the same amount of heat as compared to burning the same amount of sugar in a flame.

• Heat is transferred due to a temperature difference. Faster molecules speed up the slower molecules. This slows the faster molecules.

• In an endothermic process, the system absorbs heat from the surroundings. A process that absorbs heat is a(n) endothermic process.

• In an exothermic process, the system gives off heat to the surroundings.

• If you were to touch the flask in which an endothermic reaction was occurring, the flask would probably feel warmer than before the reaction started.

• In an exothermic reaction, the energy stored in the chemical bonds of the reactants is greater than the energy stored in the bonds of the products.

• As perspiration evaporates from your skin, your body is cooled.

• With respect to your body, this process is said to be exothermic.

• A calorie is the quantity of heat that raises the temperature of 1 gram of pure water by 1C.

• 1 Calorie = 4.18 kJ of heat

• How many kJ of energy can be released by a banana containing 150 Cal?

(1 Calorie = 4.18 kJ)

kJCal

kJCal 630

1

18.4150

• An object's heat capacity is the amount of heat required to change the temperature of an object by exactly 1°C.

• The specific heat capacity or specific heat is the heat divided by the mass and temperature change.

• Where c = specific heat Q = heat m = mass and T = change of temperature

• The temperature of a 10. g sample of metal changed from 25°C to 50°C when it absorbed 500. J of heat. What is the specific heat of this sample?

• c = ?

• m = 10. g T = 50°C-25°C = 25°C

• Q = 500. J Tm

Qc

Cg

J

Cg

J

Tm

Qc

0.2

25.10

.500

• Determine the specific heat of a material if an 18 g sample absorbed 75 J as it was heated from 15°C to 40.°C.

• c = ?

• m = 18 g T = 40.°C-15°C = 25°C

• Q = 75 J

Cg

J

Cg

J

Tm

Qc

17.0

2518

75

• What is the specific heat of a substance if 2000. cal are required to raise the temperature of a 300. g sample by 20.C?

• c = ?

• Q = 2000. cal

• m = 300. g T = 20.C

Cg

cal

Cg

cal

Tm

Qc

33.0

.20.300

.2000

• What is the amount of heat required to raise the temperature of 200.0 g of aluminum by 10.C?

• (specific heat of aluminum = 0.21 cal/gC)

• Q = ? • m = 200. g • c = 0.21 cal/gC T = 10.C

• Q = mcT

• Q = (200. g)(0.21 cal/gC)( 10.C) = 420 cal

• Water has the highest specific heat of all common substances.

C°g

cal00.1=c

• The symbol H stands for the heat of reaction for a chemical reaction.

• This is also called the change in enthalpy. H = Q

• The heat content of a system is equal to the enthalpy only for a system that is at constant pressure.

• Calorimetry depends on the law of conservation of energy.

• If 27.0 mL of water containing HCl is mixed with 28.0 mL of water containing NaOH in a calorimeter such that the initial temperature of each solution was 24.0°C and the final temperature of the mixture is 33.0°C, how much heat (in kJ) is released in the reaction? Assume that the densities of the solutions are 1.00 g/mL.

H = ?

• m = 27.0 g + 28.0 g = 55.0 g T = 33.0°C - 24.0°C = 9.0°C

• c = 0.00418 kJ/g°C

kJ1.2C0.9Cg

kJ00418.0g0.55TmcH

• A lead mass is heated and placed in a foam cup calorimeter containing 40.0 mL of water at 17.0°C. The water reaches a temperature of 20.0°C. How many joules of heat were released by the lead?

H = ?

• m = 40.0 g T = 20.0°C - 17.0°C = 3.0°C• c = 4.18 J/g°C

J502C0.3Cg

J18.4g0.40TmcH

• Fusion = solid liquid

• Solidification = liquid solid

• Molar heat of fusion –

–heat required to melt a mole = Hfusion

• Molar heat of solidification –

–heat required to freeze a mole =Hsolid

• Hfusion = -Hsolid

• Vaporization is liquid vapor

• Condensation is vapor liquid

• Molar heat of vaporization – – heat required to vaporize a mole = Hvap

• Molar heat of condensation – – heat required to condense a mole = Hcond

• Hvap = -Hcond

• Given the equation

I2(s) + 62.4 kJ I2(g) H = +62.4 kJ

• How much heat, in kJ, is released when 108 g of water at O°C freezes to ice at O°C if Hsolid

for water = -6.01 kJ/mol?

kJ1.36OHmol1

kJ01.6

OHg18

OHmol1g108

22

2

• How much heat is released in the condensation of 27.0 g of steam at 100°C to water at 100°C if

Hcond for water = -40.7 kJ/mol?

kJ0.61OHmol1

kJ7.40

OHg18

OHmol1g0.27

22

2

• How many grams of ice at 0°C can be melted into water at 0°C by the addition of 75.0 kJ of heat?

Hfus for water = 6.01 kJ/mol

g225kJ01.6

OHmol1

OHmol1

OHg18kJ0.75 2

2

2

• The heat of solution is the amount of heat absorbed or released when a solid dissolves.

• If the molar heat of solution of NaOH is -445.1 kJ/mol, how much heat (in kJ) will be released if 80.00 g of NaOH are dissolved in water?

kJ2.890NaOHmol1

kJ1.445

NaOHg.40

NaOHmol1NaOHg00.80

• Hess's law makes it possible to calculate H for complicated chemical reactions.

• Hess’s Law – If you add 2 or more thermochemical equations to give a final equation, then you also ADD the heats of reactions to give the final heat of reaction.

• Given the equation: • C2H4 (g) + 3O2(g) + 2H2O(l) + 1411 kJ

• How much heat is released when 8.00 g of O2 react?

kJ118NaOHmol3

kJ1411

Og.32

Omol1Og00.8

2

22

• What is the heat of reaction (H) for the combustion (with O2) of benzene, C6H6 to form carbon dioxide and water? Write the balanced equation for the reaction.

• Standard heats of formation:

• C6H6 = +48.50 kJ

• O2 = 0.0 kJ

• CO2 = -393.5 kJ

• H20 = -285.8 kJ

• 2 C6H6 + 15 O2 12 CO2 + 6 H2O

H + (2) (48.50 kJ) + (15)(0.0 kJ)

= (12) (-393.5 kJ) + (6) (-285.8 kJ)

H = -3266.9 kJ