Chapter 7

Thermochemistry

7.1 TerminologySystem- the part you are investigating

Surroundings- the rest of the universe

Open System: freely exchanges matter and energyClosed System: exchanges energy but not matterIsolated System: does not interact with surroundings

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Energy

The ability to do work or transfer heat.–Work: Energy used to cause an object

that has mass to move.–Heat: Energy used to cause the

temperature of an object to rise.

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Work

• Energy used to move an object over some distance.

• w = F d,where w is work, F is the force, and d is the distance over which the force is exerted.

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Kinetic EnergyEnergy an object possesses by virtue of its motion.

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KE = mv2 Kinetic energy increases as the speed and mass of

an object increases.

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Potential EnergyEnergy an object possesses by virtue of its position or chemical composition.

PE= m x g x h m=mass=kgg=gravitational constant=9.8m/s2

h= height (or distance)

7.2 HeatEnergy transferred between a system and its surroundings as a result of temperature difference

Heat flows from a warm body to a cooler body.

The quantity of heat, q, required to change a substances temperature is dependent on:– Temperature change– Quantity of substance– Nature of substance

calorie: heat to change 1 g of water 1°C1 cal = 4.184 J

Specific Heat Capacity: heat to change 1 g of a substance 1°C

Example 1A

How much heat in kJ is required to raise the temperature of 237 g of water from 4.0°C to 37.0°C?

q = mCΔT qsystem +qsurroundings = 0

Example 2A

• When 1.00 kg of lead (c = 0.13J/g°C) at 100.0°C is added to a quantity of water and the final temperature of the lead/water is 32.5°C. What is the mass of water present?

Specific Heat values given on page 247 of textbook.

7.3 Calorimetry

Chemical Energy: the energy associated with chemical bonds and intermolecular forcesChemical Reaction: breaking and forming bondsHeat of Reaction: heat exchange during a chemical reaction

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Exchange of Heat

• When heat is absorbed by the system from the surroundings, the process is endothermic.

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Exchange of Heat

• When heat is absorbed by the system from the surroundings, the process is endothermic.

• When heat is released by the system to the surroundings, the process is exothermic.

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Bomb Calorimetry

Reactions can be carried out in a sealed “bomb” and measure the heat absorbed by the water.

Example 3A

• Vanillin in a natural constituent of vanilla. It is also manufactured for use in artificial vanilla flavor. The combustion of 1.013 g of vanillin, C8H8O3, in a bomb calorimeter causes the temperature to raise from 28.89°C to 30.09°C. What is the heat of combustion of vanillin in kJ/mol?

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Constant Pressure Calorimetry

By carrying out a reaction in aqueous solution in a simple calorimeter such as this one, one can indirectly measure the heat change for the system by measuring the heat change for the water in the calorimeter.

Example 4A

Two solutions, 100.0 mL of 1.00M AgNO3(aq) and 100.0 mL of 1.00M NaCl (aq), both initially at 22.4°C are added to a Styrofoam cup allowed to react. The temperature rises to 30.2°C. Determine the qrxn per mole of AgCl(s) in the reaction

Ag+(aq) + Cl-

(aq) AgCl(s)

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7.4 WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston.

w = −PV

Example 5A

How much work, in Joules, is involved when 0.255 mol N2 at constant temperature of 23°C is allowed to expand by 1.50L in volume against an external pressure of 0.750 atm?

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7.5 First Law of Thermodynamics• Energy is neither created nor destroyed.• In other words, the total energy of the universe is a constant; if the system

loses energy, it must be gained by the surroundings, and vice versa.

qsystem = -qsurroundings

U = q + w

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E, q, w, and Their Signs

q + (system gains heat) - (system loses heat)

w + (work done on system)

- ( work done by system)

U + (net gain of energy by system)

- ( net loss of energy by system)

Example 6A

In compressing a gas, 355J of work is done on the system. At the same time 185 J of heat escapes from the system. What is the U for the system?

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7.6 Enthalpies of Reaction

This quantity, H, is called the enthalpy of reaction, or the heat of reaction.

Example 7

How much heat is associated with the complete combustion of 1.00kg of sucrose?ΔH = -5.65 x 103 kj/mol

Example 7A

What mass of sucrose must be burned to produce 1,000 kJ of heat? ΔH = -5.65 x 103 kj/mol

Changing States

Example 8A

What is the enthalpy change when a cube of ice 2.00 cm on edge is brought from -10.0°C to a final temperature of 23.2°C? For ice, use d = 0.917 g/mL, a specific heat of 2.01 J/g°C and enthalpy of fusion of 6.01kJ/mol.

7.7 Hess’s Law

Change in enthalpy is the same if the reaction took one step or many steps•If you can add balanced equations, you can add ΔH •If you alter the equation, alter ΔH – Double reactant/products, double ΔH – Reverse equation, change sign of ΔH – Cancel items if they are in reactants and products

Hess’s Law Example

7.8 Heat of Formation

ΔHf°: enthaply of formation

•Defined as heat change that results when one mole of compound is formed from its constituent elements•Pure element ΔHf°= 0

Example 10A

The standard enthalpy of formation for the amino acid leucine is -637.3 kJ/mol C6H13O2N (s). Write the chemical equation to which this value applies.

Finding ΔHrxn

ΔHrxn = Σ Hf°(products) - Σ Hf°(reactants)

Values in back of textbook

Example 11A

Use data table 7-2 to calculate the enthalpy for the combustion of ethanol, CH3CH2OH (l), at 298.15K.

Example 12A

Determine the standard enthalpy for the formation of glucose C6H12O6.

ΔHrxn = 2803 kJ

Example 13A

Given that Hf°[AgI(s)] = -61.84 kJ/mol, what is the standard enthanpy change for the precipitation of silver iodide?

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Calculation of H

• Imagine this as occurringin 3 steps:

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

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

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Calculation of H

• Imagine this as occurringin 3 steps:

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

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

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Calculation of H

• Imagine this as occurringin 3 steps:

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

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

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

C3H8 (g) 3 C(graphite) + 4 H2 (g)

3 C(graphite) + 3 O2 (g) 3 CO2 (g)

4 H2 (g) + 2 O2 (g) 4 H2O (l)

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

Calculation of H

• The sum of these equations is:

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Calculation of H

We can use Hess’s law in this way:

H = nHf(products) - mHf(reactants)

where n and m are the stoichiometric coefficients.

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

Calculation of H

H = [3(-393.5 kJ) + 4(-285.8 kJ)] - [1(-103.85 kJ) + 5(0 kJ)]= [(-1180.5 kJ) + (-1143.2 kJ)] - [(-103.85 kJ) + (0 kJ)]= (-2323.7 kJ) - (-103.85 kJ)= -2219.9 kJ