Chapter 6 Thermochemistry. Copyright © Houghton Mifflin Company. All rights reserved.6 | 2 Contents and Concepts Understanding Heats of Reaction The first

Chapter 6Thermochemistry

Copyright © Houghton Mifflin Company. All rights reserved. 6 | 2

Contents and Concepts

Understanding Heats of ReactionThe first part of the chapter lays the groundworkfor understanding what we mean by heats of reaction.

1. Energy and Its Units2. Heat of Reaction3. Enthalpy and Enthalpy Changes4. Thermochemical Equations5. Applying Stoichiometry to Heats of Reaction6. Measuring Heats of Reaction


Using Heats of Reaction

Now that we understand the basic properties of heats of reaction and how to measure them, we can explore how to use them.

7. Hess’s Law

8. Standard Enthalpies of Formation

9. Fuels—Foods, Commercial Fuels, and Rocket Fuels


Thermodynamics

The science of the relationship between heat and other forms of energy

Thermochemistry

An area of thermodynamics that concerns the study of the heat absorbed or evolved by a chemical reaction


Energy

The potential or capacity to move matter

One form of energy can be converted to another form of energy: electromagnetic, mechanical, electrical, or chemical.

Next, we’ll study kinetic energy, potential energy, and internal energy.


Kinetic Energy, EK

The energy associated with an object by virtue of its motion.

m = mass (kg)

v = velocity (m/s)

2mvE2

1K


The SI unit of energy is the joule, J, pronounced “jewel.”

The calorie is a non-SI unit of energy commonly used by chemists. It was originally defined as the amount of energy required to raise the temperature of one gram of water by one degree Celsius. The exact definition is given by the equation:

2

2

s

mkgJ

(exact)J4.184cal1

?


A person weighing 75.0 kg (165 lbs) runs a course at 1.78 m/s (4.00 mph). What is the person’s kinetic energy?

2

K s

m1.78kg)(75.0

2

1

E

m = 75.0 kgV = 1.78 m/s

EK = ½ mv2

figures)tsignifican(3

J119s

mkg119

2

2

K

E


Potential Energy, EP

The energy an object has by virtue of its position in a field of force, such as gravitaitonal, electric or magnetic field.

Gravitational potential energy is given by the equation

m = mass (kg)

g = gravitational constant (9.80 m/s2)

h = height (m)

mghE P


Internal Energy, U

The sum of the kinetic and potential energies of the particles making up a substance

Total Energy

Etot = EK + EP + U


Law of Conservation of Energy

Energy may be converted from one form to another, but the total quantity of energy remains constant.


Thermodynamic System

The substance under study in which a change occurs is called the thermodynamic system (or just system).

Thermodynamic Surroundings

Everything else in the vicinity is called the thermodynamic surroundings (or just the surroundings).


Heat, q

The energy that flows into or out of a system because of a difference in temperature between the thermodynamic system and its surroundings

Heat flows spontaneously from a region of higher temperature to a region of lower temperature.

• q is defined as positive if heat is absorbed by the system (heat is added to the system)

• q is defined as negative if heat is evolved by a system (heat is subtracted from the system)


Heat of Reaction

The value of q required to return a system to the given temperature at the completion of the reaction (at a given temperature)


Endothermic Process

A chemical reaction or process in which heat is absorbed by the system (q is positive). The reaction vessel will feel cool.

Exothermic Process

A chemical reaction or process in which heat is evolved by the system (q is negative). The reaction vessel will feel warm.


In an endothermic reaction:

The reaction vessel cools.

Heat is absorbed.

Energy is added to the system.

q is positive.

In an exothermic reaction:

The reaction vessel warms.

Heat is evolved.

Energy is subtracted from the system.

q is negative.


Enthalpy, H

An extensive property of a substance that can be used to obtain the heat absorbed or evolved in a chemical reaction

Extensive Property

A property that depends on the amount of substance. Mass and volume are extensive properties.


A state function is a property of a system that depends only on its present state, which is determined by variables such as temperature and pressure, and is independent of any previous history of the system.


The altitude of a campsite is a state function.

It is independent of the path taken to reach it.


Enthalpy of Reaction

The change in enthalpy for a reaction at a given temperature and pressure

H = H(products) – H(reactants)

Note: means “change in.”

Enthalpy change is equal to the heat of reaction at constant pressure:

H = qP


The diagram illustrates the enthalpy change for the reaction

2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)

The reactants are at the top. The products are at the bottom. The products have less enthalpy than the reactants, so enthalpy is evolved as heat. The signs of both q and H are negative.


Enthalpy and Internal EnergyThe precise definition of enthalpy, H, is

H = U + PV

Many reactions take place at constant pressure, so the change in enthalpy can be given by

H = U + PVRearranging:

U = H – PV

The term (–PV) is the energy needed to change volume against the atmospheric pressure, P. It is called pressure-volume work.


For the reaction

2Na(s) + 2H2O(l) 2NaOH(aq) + H2(g)

–PV

The H2 gas had to do work to raise the piston.For the reaction as written at 1 atm, -PV = -2.5 kJ.

In addition, 368.6 kJ of heat are evolved.


Thermochemical Equation The thermochemical equation is the chemical equation for a reaction (including phase labels) in which the equation is given a molar interpretation, and the enthalpy of reaction for these molar amounts is written directly after the equation

For the reaction of sodium metal with water, the thermochemical equation is

2Na(s) + 2H2O(l)

2NaOH(aq) + H2(g); H = –368.6 kJ

?


Sulfur, S8, burns in air to produce sulfur dioxide. The reaction evolves 9.31 kJ of heat per gram of sulfur at constant pressure. Write the thermochemical equation for this reaction.


S8(s) + 8O2(g) 8SO2(g)

We first write the balanced chemical equation:

Next, we convert the heat per gram to heat per mole.

Now we can write the thermochemical equation:

S8(s) + 8O2(g) 8SO2(g); H = –2.39 × 103 kJ

Note: The negative sign indicates that heat is evolved; the reaction is exothermic.

kJ102.39Δ

Smol1

Sg256.52

Sg1

kJ9.31Δ

3

8

8

8

H

H


Manipulating a Thermochemical Equation

• When the equation is multiplied by a factor, the value of H must be multiplied by the same factor.

• When a chemical equation is reversed, the sign of H is reversed.


Applying Stoichiometry to Heats of Reaction

?


You burn 15.0 g sulfur in air. How much heat evolves from this amount of sulfur? The thermochemical equation is

S8(s) + 8O2(g) 8SO2(g); H = -2.39 x 103 kJ


S8(s) + 8O2(g) 8SO2(g); H = -2.39 x 103 kJ

Molar mass of S8 = 256.52 g

8

3

8

88 Smol1

kJ10x2.39

Sg256.5

Smol1Sg15.0

q

q = –1.40 × 102 kJ


Measuring Heats of Reaction

We will first look at the heat needed to raise the temperature of a substance because this is the basis of our measurements of heats of reaction.


Heat Capacity, C, of a Sample of Substance The quantity of heat needed to raise the temperature of the sample of substance by one degree Celsius (or one kelvin)

Molar Heat Capacity The heat capacity for one mole of substance

Specific Heat Capacity, s (or specific heat)The quantity of heat needed to raise the temperature of one gram of substance by one degree Celsius (or one kelvin) at constant pressure


The heat required can be found by using the following equations.

Using heat capacity:

q = Ct

Using specific heat capacity:

q = s x m x t


A calorimeter is a device used to measure the heat absorbed or evolved during a physical or chemical change. Two examples are shown below.

?


A piece of zinc weighing 35.8 g was heated from 20.00°C to 28.00°C. How much heat was required? The specific heat of zinc is 0.388 J/(g°C).

m = 35.8 gs = 0.388 J/(g°C)t = 28.00°C – 20.00°C = 8.00°C

q = m s t

C8.00Cg

J0.388g35.8

q

q = 111 J(3 significant figures)

?


Nitromethane, CH3NO2, an organic solvent burns in oxygen according to the following reaction:

CH3NO2(g) + 3/4O2(g) CO2(g) + 3/2H2O(l) + 1/2N2(g)

You place 1.724 g of nitromethane in a calorimeter with oxygen and ignite it. The temperature of the calorimeter increases from 22.23°C to 28.81°C. The heat capacity of the calorimeter was determined to be 3.044 kJ/°C. Write the thermochemical equation for the reaction.


We first find the heat evolved for the 1.724 g of nitromethane, CH3NO2.

Now, covert that to the heat evolved per mole by using the molar mass of nitromethane, 61.04 g.

23

23

23rxn NOCHmol1

NOCHg61.04

NOCHg1.724

kJ 20.03- q

H = –709 kJ

kJ 20.03 C22.23C28.81C

kJ3.044

Δ

rxn

calrxn

q

tCq


We can now write the thermochemical equation:

CH3NO2(l) + ¾O2(g) CO2(g) + 3/2H2O(l) + ½N2(g); H = –709 kJ


Hess’s Law of Heat Summation

For a chemical equation that can be written as the sum of two or more steps, the enthalpy change for the overall equation equals the sum of the enthalpy changes for the individual steps.


Suppose we want H for the reaction

2C(graphite) + O2(g) 2CO(g)

It is difficult to measure directly.

However, two other reactions are known:

2CO2(g) 2CO(g) + O2(g); H = – 566.0 kJ

C(graphite) + O2(g) CO2(g); H = -393.5 kJ

In order for these to add to give the reaction we want, we must multiply the first reaction by 2. Note that we also multiply H by 2.


2C(graphite) + O2(g) 2CO(g)

2C(graphite) + 2O2(g) 2CO2(g); H = -787.0 kJ

2CO2(g) 2CO(g) + O2(g); H = – 566.0 kJ

Now we can add the reactions and the H values.

2 C(graphite) + O2(g) 2 CO(g); H = –1353.0 kJ


Hsub = Hfus + Hvap

?


What is the enthalpy of reaction, H, for the reaction of calcium metal with water?

Ca(s) + 2H2O(l) Ca2+(aq) + 2OH-(aq) + H2(g)

This reaction occurs very slowly, so it is impractical to measure H directly. However, the following facts are known:

H+(aq) + OH-(aq) H2O(l); H = –55.9 kJCa(s) + 2H+(aq)

Ca2+(aq) + H2(g); H = –543.0 kJ



First, identify each reactant and product:

H+(aq) + OH-(aq) H2O(l); H = –55.9 kJ

Ca(s) + 2H+(aq) Ca2+(aq) + H2(g); H = –543.0 kJ

Each substance must be on the proper side. Ca(s), Ca2+(aq), and H2(g) are fine.H2O(l) should be a reactant.OH-(aq) should be a product.

Reversing the first reaction and changing the sign of its H accomplishes this.



H2O(l) H+(aq) + OH-(aq); H = +55.9 kJ


The coefficients must match those in the reaction we want.

The coefficient on H2O and OH- should be 2. We multiply the first reaction and its H by 2 to accomplish this.



2H2O(l) 2H+(aq) + 2OH-(aq); H = +111.8 kJ


We can now add the equations and their H’s.Note that 2H+(aq) appears as both a

reactant and a product.



2H2O(l) 2H+(aq) + 2OH-(aq); H = +111.8 kJ


Ca(s) + 2H2O(l) Ca2+(aq) + 2OH-(aq) + H2(g); H = –431.2 kJ


Standard Enthalpies of Formation

The term standard state refers to the standard thermodynamic conditions chosen for substances when listing or comparing thermodynamic data: 1 atm pressure and the specified temperature (usually 25°C). These standard conditions are indicated with a degree sign (°).

When reactants in their standard states yield products in their standard states, the enthalpy of reaction is called the standard enthalpy of reaction, H°. (H° is read “delta H zero.”)


Elements can exist in more than one physical state, and some elements exist in more than one distinct form in the same physical state. For example, carbon can exist as graphite or as diamond; oxygen can exist as O2 or as O3 (ozone).

These different forms of an element in the same physical state are called allotropes.

The reference form is the most stable form of the element (both physical state and allotrope).


The standard enthalpy of formation, Hf°, is the enthalpy change for the formation of one mole of the substance from its elements in their reference forms and in their standard states. Hf° for an element in its reference and standard state is zero.

For example, the standard enthalpy of formation for liquid water is the enthalpy change for the reaction

H2(g) + 1/2O2(g) H2O(l)

Hf° = –285.8 kJ

Other Hf° values are given in Table 6.2 and Appendix C.


C(graphite) + 2Cl2(g) CCl4(l); Hf° = –135.4 kJ

We first identify each reactant and product from the reaction we want.

Standard enthalpies of formation can be used to calculate the standard enthalpy for a reaction, H°.

CH4(g) + 4Cl2(g) CCl4(l) + 4HCl(g); H° = ?

Table 6.2 shows the Hf° values:

1/2 H2(g) + 1/2 Cl2(g) HCl(g); Hf° = -92.3 kJ

CH4(g) C(graphite) + 2H2(g); Hf° = +74.9 kJ



Each needs to be on the correct side of the arrow and is. Next, we’ll check coefficients.




1/2 H2(g) + 1/2 Cl2(g) HCl(g); Hf° = -92.3 kJ




Cl2 and HCl need a coefficient of 4. Multiplying the second equation and its H by 4 does this.




1/2 H2(g) + 1/2 Cl2(g) HCl(g); Hf° = -92.3 kJ




Now, we can add the equations.




2H2(g) + 2Cl2(g) 4HCl(g); Hf° = -369.2 kJ







2H2(g) + 2Cl2(g) 4HCl(g); Hf° = -369.2 kJ

CH4(g) + 4Cl2(g) CCl4(g) + 4HCl(g); H° = –429.7 kJ


?


Methyl alcohol, CH3OH, is toxic because liver enzymes oxidize it to formaldehyde, HCHO, which can coagulate protein. Calculate Ho for the following reaction:

2CH3OH(aq) + O2(g) 2HCHO(aq) + 2H2O(l)

Standard enthalpies of formation, :ofΔH

CH3OH(aq): -245.9 kJ/molHCHO(aq): -150.2 kJ/molH2O(l): -285.8 kJ/mol


We want H° for the reaction:2CH3OH(aq) + O2(aq) 2HCHO(aq) + 2H2O(l)

reactants

fproducts

freaction ΔΔΔ HnHnH

mol

kJ0mol 1

mol

kJ245.9 mol 2

mol

kJ285.8-mol 2

mol

kJ150.2 mol 2Δ o

reactonH

kJ 491.8kJ 571.6kJ 300.4 Δ oreaction H

kJ 491.8kJ 872.0Δ oreaction H

kJ 380.2Δ oreaction H


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Chapter 6 Thermochemistry. Copyright © Houghton Mifflin Company. All rights reserved.6 | 2 Contents and Concepts Understanding Heats of Reaction The first