Ch05_Lecture PPT-Part 2

Chapter 5

ThermochemistryPart-2

John D. BookstaverSt. Charles Community College

Cottleville, MO

Lecture Presentation

© 2012 Pearson Education, Inc.

Thermochemistry


Bomb Calorimetry

• Because the volume in the bomb calorimeter is constant, what is measured is really the change in internal energy, E, not H.

• For most reactions, the difference is very small.

Thermochemistry

Measuring qrxn Using a Bomb Calorimeter

A 0.5865-g sample of lactic acid (HC3H5O3) is burned in a calorimeter whose heat capacity is 4.812 kJ/C.The temperature increases from 23.10 C to 24.95 C. Calculate the heat of combustion of lactic acid (a) per gram and (b) per mole.

Answer: (a) –15.2 kJ/g, (b) –1370 kJ/mol

Thermochemistry

Specific Heat Problem

A 45.9 g sample of a metal is heated to 95.2°C and then placed in a calorimeter containing 120.0 g of water (c = 4.18 J/g°C) at 21.6°C. The final temperature of the water is 24.5°C. Which metal was used?A) Aluminum (c = 0.89 J/g°C)

B) Iron (c = 0.45 J/g°C)

C) Copper (c = 0.20 J/g°C)

D) Lead (c = 0.14 J/g°C)

E)none of these

Answer B

Thermochemistry

Enthalpy Problem

The value of ΔH° for the reaction below is -1107 kJ:

2Ba (s) + O2 (g) → 2BaO (s)

How many kJ of heat are released when 15.75 g of Ba (s) reacts completely with oxygen to form BaO (s)?

A) 20.8

B) 63.5

C) 114

D) 70.3

E) 35.1

Answer: B


Thermochemistry


Hess’s Law

H is well known for many reactions, and it is inconvenient to measure H for every reaction in which we are interested.

• However, we can estimate H using published H values and the properties of enthalpy.

Thermochemistry


Hess’s Law

Hess’s law states that “If a reaction is carried out in a series of steps, H for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps.”

Thermochemistry


Hess’s Law

Because H is a state function, the total enthalpy change depends only on the initial state of the reactants and the final state of the products.

Thermochemistry

Hess’s Law Practice Problem -1

Consider the following processes:

Calculate H for: B E + 2C

Answer: –175 kJ/mol


H (kJ/mol)

3B 2C + D –125.

(1/2)A B 150

E + A D 350

Thermochemistry

Hess’s Law Practice Problem -2

Answer: –108.7 kJ/mol

H (kJ/mol)

2ClF + O2 Cl2O + F2O 167.4

2ClF3 + 2O2 Cl2O + 3F2O 341.4

2F2 + O2 2F2O –43.4

At 25°C, the following heats of reaction are known:At the same temperature, calculate H for the reaction:

ClF + F2 ClF3

Thermochemistry

Hess’s Law Practice Problem-3

Calculate the standard enthalpy change, ΔHo, for the formation of 1 mol of strontium carbonate (the material that gives the red color in fireworks) from its elements.

Answer: – 1220 kJ


Thermochemistry

Hess’s Law Practice Problem- 4

The combination of carbon and steam produces a mixture called coal gas, which can be used as a fuel or as a starting material for other reactions. If we assume carbon can be represented by graphite, the equation for the production of coal gas is:

Answer: + 15.3 kJ


Thermochemistry

Hess’s Law Practice Problem- 5

Challenging Problem!!

One reaction involved in the conversion of iron ore to the metal is :

Answer: -11.0kJ


Thermochemistry


Enthalpies of Formation

An enthalpy of formation, Hf, is defined as the enthalpy change for the reaction in which a compound is made from its constituent elements in their elemental forms.

Thermochemistry


Standard Enthalpies of Formation

Standard enthalpies of formation, Hf°, are measured under standard conditions (25 °C and 1.00 atm pressure).

Thermochemistry

Conventional Definition of Standard States

Definitions of Standard State:

•–For a gas: Pressure = 1 atm.

•–For solutions: Concentration = 1 M (mol/L).

•–For liquids and solids: pure liquid or solid

•–For elements: The form in which the element exists under conditions of 1 atm and 298 K. (i.e. 25C)


Thermochemistry

Conventional Definition of Standard States

Sample Standard States of Elements:

-–Hydrogen: H2(g) (not atomic H)

–Oxygen: O2(g)

–Carbon: C (gr). Graphite, solid, as opposed to diamond

Standard state is denoted using the superscript “°”.

–Example: ΔH rxn is a reaction carried out under standard state


Thermochemistry


Calculation of H

• Imagine this as occurring

in three steps:

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

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

Thermochemistry


Calculation of H


in three steps:


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

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

Thermochemistry


Calculation of H


in three steps:



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

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

Thermochemistry




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

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

Calculation of H

• The sum of these equations is

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

Thermochemistry


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.

Thermochemistry


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

Calculation of H

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

Thermochemistry

Key Concepts when doing Enthalpy Calculations• When a reaction is reversed, the magnitude of H

remains the same, but the sign changes.• When the balanced equation for a reaction is

multiplied by an integer, the value of H for that reaction must be multiplied by the same integer.

• The change in enthalpy for a given reaction can be calculated from the enthalpies of formation of the reactants and products:

Hreaction = npHf, (products) – nr Hf, (reactants)

• Elements in their standard states are not included in the H reaction calculations, that is Hf for an element in the standard state is zero.

Thermochemistry

Standard Enthalpies of Formation Problem - 1

For which one of the following reactions is ΔH°rxn equal to the heat of formation of the product?

A) N2 (g) + 3H2 (g) → 2NH3 (g)

B) (1/2) N2 (g) + O2 (g) → NO2 (g)

C) 6C (s) + 6H (g) → C6H6 (l)

D) P (g) + 4H (g) + Br (g) → PH4Br (l)

E) 12C (g) + 11H2 (g) + 11O (g) → C6H22O11 (g)

Answer: B© 2012 Pearson Education, Inc.

Thermochemistry


For which one of the following reactions is the value of ΔH°rxn equal to ΔHf° for the product?

A) 2Ca (s) + O2 (g) → 2CaO (s)

B) C2H2 (g) + H2 (g) → C2H4 (g)

C) 2C (graphite) + O2 (g) → 2CO (g)

D) 3Mg (s) + N2 (g) → Mg3N2 (s)

E) C (diamond) + O2 (g) → CO2 (g)

Answer: D


Thermochemistry


The value of ΔH° for the following reaction is -3351 kJ:

2Al (s) + 3O2 (g) → 2 Al2O3 (s)

The value of ΔHf ° for Al2O3 (s) is __________ kJ.

Answer: -1676


Thermochemistry


Given the data in the table below, ΔH°rxn for the reaction

4NH3 (g) + 5 O2 (g) → 4NO (g) + 6 H2O (l)

is __________ kJ.

Answer: -1172 © 2012 Pearson Education, Inc.

Thermochemistry


Given the data in the table below, ΔH°rxn for the reaction

2 Ag2S (s) + O2 (g) → 2 Ag2O (s) + 2S (s)

is __________ kJ.

Answer: +3.2 © 2012 Pearson Education, Inc.

Thermochemistry


Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fats.

Thermochemistry


Energy in Fuels

The vast majority of the energy consumed in this country comes from fossil fuels.

Ch05_Lecture PPT-Part 2

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