LaurenHill Chemistry 534

25

5. Energy and Enthalpy A. What is Energy?

Energy is the capacity to do work. Although the definition may not seem to do justice to a poet's concept of energy, think of any of its forms, and it involves work. For instance, visible light does work on any material that absorbs it, causing molecules to vibrate and electrons to jump levels.

Example Give another example of how a form of energy is a capacity to do work.

Energy is neither created nor destroyed; it is merely transformed from one form to another. For example in moving a muscle, chemical energy is transformed into mechanical energy; if you move backwards, you can actually trace every little body action to the sun.

How?

B. Potential Energy and Enthalpy

To understand the difference between two types of reactions (exothermic and endothermic), we need to explore a couple of other concepts. In addition to kinetic energy (vibrational, rotational and translational motion), molecules also have potential energy. Potential energy in a chemical context is energy due to composition. It is associated with the coulombic force within atoms and molecules (intramolecular bonds); with the attractions between different molecules (intermolecular bonds).

In water for example, potential energy is associated with bonds between oxygen and the two hydrogen atoms in each molecule, and also with the oxygen atom of one molecule and one of the hydrogen atoms of another molecule.

Intramolecular bonds Intermolecular bonds

O

H

H

O

H

H

II. Thermochemistry

26

Example1 Show the intermolecular and intramolecular bonds in HCl. Example 2 a)Imagine two pieces of plasticine bonded together. If they represent a diatomic

molecule, what kind of bond is represented? b)What holds the real pieces together?

Example 3 How do bonds between protein molecules play a role in the cooking of egg

white? ( see http://www.sumanasinc.com/webcontent/animations/content/proteinstructure.html )

Enthalpy Definition: H is the total heat content of a substance at constant pressure.1

C. Exothermic reactions

If in a reaction molecule A becomes molecules B and C, and if molecule A has more energy that both B and C combined, then the excess energy will be released into the environment. The environment becomes hotter; we have an exothermic reaction:

A B + C + energy Reactant products

Examples of exothermic reactions:

Condensation of water

C + O2 CO2 + 250kJ

Adding an alkali metal to water (2 Na + 2 H2O 2 NaOH + H2 + energy)

All combustion reactions (fires)

1 Enthalpy is also defined as the internal energy of the system plus the product of pressure difference and volume(work). But if pressure is constant, then it is simply equal to its internal energy.

LaurenHill Chemistry 534

27

Example Think of everyday reactions which release heat (exothermic).

On a graph, exothermic reactions are represented as follows:

Energy

Progress of reaction If we examine the graph more closely, we will notice that exothermic reactions have a negative

change in enthalpy. A change in enthalpy, H, is defined as the enthalpy of products – heat of reactants:

H = Hp - Hr

What is that little hill labeled, Ae? Ae = activation energy. This is the energy that reactants must absorb in order to form products, even if the products will not need the energy

to store within their bonds. So Ae = Hmaximum - Hreactants

Example 1 Find H if the reactants have 120 kJ, and the products have 40 kJ of enthalpy.

Example 2 If a reaction has a negative value for H, what will happen to the temperature of its environment?

reactants

products

II. Thermochemistry

28

Example 3 When chlorine and gently heated sodium are placed in the same beaker there is an incredible amount of light and heat released as they react to form table salt, NaCl. Where does that energy come from?

D. Endothermic Reactions If substance A must take energy away from the environment in order to form product D, then the reaction is said to be endothermic, and the victimized environment will feel colder after the reaction.

H = (+) for endothermic reactions and their profile looks like the following:

Examples of endothermic reactions:

Melting of ice absorbs energy

Dissolving ammonium nitrate in water (the essence of commercial cold packs)

NH4 NO3(s) + energy NH4 NO3(aq) Example 1 Think of everyday reactions that take heat away from their environment.

(endothermic)

LaurenHill Chemistry 534

29

Example 2 If a reaction has a positive value for H, what will happen to the temperature of its environment?

Check out these next two pages for an analogy between physics and chemistry so that you gain a better understanding of potential energy. Potential Energy from Physics Point of View

Ep = (mg)h Work = ( Fg )d

After work is done against the force of gravity(Fg) to raise the object to a greater height, the energy is conserved as potential energy . If the box falls it will release the energy in the form of kinetic energy, which can do ”work “ on the floor, a spring, or whatever your fancy.

Energy

Work is done

against gravity

to increase the

height of the

object.

LaurenHill Chemistry 534

29

Potential Energy from the Chemistry Point of View but with Physics also in Mind I2(g) + energy = 2 I(g)

When iodine atoms are sharing two electrons in the I2 molecule, the shared electrons are much closer to each other’s nucleus. The separation distance, d, is small, just like the height in the physics analogy. To break up the bond requires energy because you are separating the shared electrons as they become two separate atoms. “d” has increased and so has the potential energy. We have an endothermic reaction. In every form of chem. potential energy (intraatomic, intramolecular or intermolecular, a similar idea is at work.)

Exercises (the last few exercises are from old exams) 1. The Romantic poet William Blake defined energy as eternal

delight. But from a more practical perspective, just what is energy?

2. The energy associated with physical or chemical changes may

appear in different forms such as heat or electricity. List at least two other forms of energy. 3. A 5.0 gram mass is moving at 1000 m/s. Calculate its kinetic

energy without forgetting to convert mass to kilograms. 4. What is the difference between potential and kinetic energy? 5. Give 3 examples of potential energy.

Work is done against coulombic attraction to increase the distance between what were shared electrons(-) and the nucleus(+).

Work = ( Fc )d

Which becomes chemical potential energy

I..I

I. .I

Energy

II. Thermochemistry

30

6. The Law of Conservation of Energy states that energy can be transformed from one form to the other, yet it cannot be created or destroyed. What other Law important to balancing equations does this remind you of?

7. a. A solar cell absorbs light. Into what form of energy is light transformed?

b. If the electricity produced is allowed to flow into a toaster, what two forms of energy is electricity turned into? 8. a. Discuss the three types of kinetic energy associated with molecules.

b. Describe what happens to the molecules or atoms of a solid as its temperature rises to its melting point.

c. Repeat (b) for the condensation of water. 9. a. Why do molecules have potential energy?

b. What is the difference between intermolecular and intramolecular bonds? c. List 3 everyday examples where the potential energy of a chemical substance is

released. 10. What happens to the energy of molecules that react chemically? 11. What is enthalpy? 12. What is an endothermic reaction? Exothermic? 13. Record and complete the following:

H of reactants (kJ) H of products (kJ) Enthalpy Change, (kJ)

Exo or Endo?

44 73

288 88

44 41

555 -55

222 444

33 255

222 -88

55 38

14. a. Draw an energy-versus-progress-of-reaction profile, using numbers from the above chart's first row.

b. Repeat for second-row data.

LaurenHill Chemistry 534

31

15. A student dissolved 1 g of each of four substances in water in a laboratory experiment. The following table illustrates the change in temperature after the solids dissolved.

SUBSTANCES

Tinitial

Tfinal

NH4Cl

23°C

19°C

NaOH

23°C

60°C

NaCl

23°C

23°C

Drano

23°C

60°C

Which of the above represent exothermic reactions? 16. Endothermic? Or Exothermic?

OH 1. (g)2 nergye + OH (l)2

O 2

1 + H 2. 22 kJ 241.8- = OH2 H

eath + SOH 3. 4(l)2 O2 + S 8

1 + H 2(g)8(s)2(g)

O + N 4. 2(g)2(g) kJ 180.8+ = NO 2 (g) H

O + C 5. 2(g)(s) kJ394 + CO2(g)

17.

A lighter has a reservoir containing a flammable gas under pressure. When the lighter is ignited, two things happen:

o gaseous butane escapes through an opening; o friction from the flint creates heat and sparks

The combustion of butane is exothermic.

Associate the expressions on the right with the numbers on the energy diagram on the left.

II. Thermochemistry

32

Energy Diagram

Reaction Proceeds

1

2

}}

3

4

5

E (kJ)

Expressions a. The energy contained in butane when the

lighter is opened b. The energy associated with the butane

heated by the sparks c. The energy liberated by the combustion of

butane d. The energy contained in the products

formed by the combustion of butane (CO2(g)

and H2O(g)).

18. Which of the following are endothermic changes? a. Melting ice b. A burning candle c. Dew forming on a lawn d. Moth balls undergoing sublimation e. Iron rusting f. Water decomposing by electrolysis

6. Hess' Law

Hess' Law states that the enthalpy of a reaction is independent of

whether the reaction occurs in one or several steps. This allows us to

algebraically add equations and their accompanying H's to obtain the

H for the desired or target equation.

Keep the following rules in mind:

1. If an equation is multiplied or divided by a number, that factor also applies to H.

2. If an equation is reversed, then the sign of H changes.

3. Remember: there are different enthalpy changes associated with different states of

matter. Do not, for example interchange H2O(l) with H2O(s).

Germain Henri Hess

LaurenHill Chemistry 534

33

Example 1 Carbon disulfide is a very flammable solvent. It burns according to the

following equation:

CS2(l) + 3 O2(g) CO2(g) + 2 SO2(g)

Calculate H for the above reaction using the following data:

(1) C(s) + 2 S(s) CS2(l) H = 88 kJ

(2) C(s) + O2(g) CO2(g) H = -394 kJ

(3) S(s) + O2(g) SO2(g) H = -297 kJ

II. Thermochemistry

34

Example 2 Given:

Al2O3(s) + 1676 kJ 2 Al(s) + 2

3 O2(g)

H2O(g) H2(g) + 2

1 O2(g) H = + 242

mol

kJ

What is the energy change associated with the following reaction in terms of

Al mol

kJ?

Al2O3(s) + 3 H2(g) 2 Al(s) + 3 H2O(g)

Example 3 Use the table from HW question 2 to find the heat of combustion for

ethane (C2H6).

LaurenHill Chemistry 534

35

Exercises

1. Methane (CH4) burns in the presence of oxygen according to this balanced

equation :

CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)

Below are several useful equations:

H2(g) + 1/2 O2(g) H2O(g) + 242 kJ

C(s) + O2(g) CO2(g) H = -394 kJ/mol

C(s) + 2 H2(g) CH4(g) + 75 kJ

What is the heat of combustion for 1 mole of methane?

2. The molar heat of reactions of various elements are listed below:

Elements Formula Name H (kJ)

O 2

1 + H 2(g)2(g) OH (g)2 water vapour -241.8

O + S 8

12(g)8(s) SO2(g) sulfur dioxide -296.9

O2 + S 8

1 + H 2(g)8(s)2(g) SOH 4(l)2 sulfuric acid -811.4

O 2

1 + C 2(g)(s) CO(g) carbon monoxide -110.5

O + C 2(g)(s) CO2(g) carbon dioxide -393.5

H 2 + C 2(g)(s) CH4(g) methane - 74.8

H 3 + C2 2(g)(s) HC 6(g)2 ethane - 84.7

H 4 + C3 2(g)(s) HC 8(g)3 propane -103.8

a. Calculate H for CH4(g) + O 2

12(g) CO(g) + 2 H 2(g)

b. Calculate H for HC 8(g)3 O 5 + 2(g) OH4 + CO (g)22(g)

II. Thermochemistry

36

3. Calculate the heat of combustion for ammonia, NH3(g).

12 NH3(g) + 21 O2(g) 8 HNO3(aq) + 14 H2O(l) + 4 NO(g)

Given:

NH3(g) + 1.25 O2(g) NO(g) + 1.5 H2O(l) H = -293 kJ

NO2(g) + (1/3) H2O(l) (2/3) HNO3(aq) + (1/3) NO(g) H = -45 kJ

NO(g) + 0.5 O2(g) + 59 kJ NO2(g)

4. a. Find the heat of reaction for the combustion of nitric oxide:

2 NO(g) + O2(g) 2 NO2(g)

The following experimental data is available to you:

0.5 N2(g) + 0.5 O2(g) NO (g) + 93 kJ

0.5 N2(g) + O2(g) NO2 (g)

H = - 34 kJ

b. If you had used the data from #3, could you have gotten the same answer

in one step? How?

5. Diborane, B2H6, is highly reactive and was once considered as a possible rocket

fuel for the U.S. space program. Calculate H for the synthesis of diborane from

its constituent elements:

2 B(s) + 3H2(g) B2H6(l)

Reaction H ( kJ )

2 B(s) + 1.5 O2(g) B2O3(s)

-1273

B2H6(l) + 3 O2(g) B2O3(s) + 3 H2O(g)

-2035

H2(g) + 0.5 O2(g) H2O(l)

-286

H2O(g) H2O(l) -44

LaurenHill Chemistry 534

37

7. Estimating H From Bond Energies

It takes energy to break old bonds but energy is released when new bonds are formed. If

we could do the chemical accounting and sum up what’s invested and what the returns

are, we could estimate the net result or the H for a reaction.

Important reminder:

Energy invested to break bonds; Hbb = (+)

Energy released when bonds are formed; Hbf = (-)

Example 1: Estimate the per mole offor the following

reaction: H2 + Br2 2 HBr

H H

hydrogen

Br Br

bromine

H Br

hydrogen bromide

H Br

progress

Hbb Hbf H =HbbHbf

Table of Bond Energies to consult:

Average bond energies, kJ/mol:

H—H

H—F

H—Cl

H—Br

H—I

C—C

C=C

C≡C

C—N

C=N

C≡N

436

570

432

366

298

347

619

812

293

515

891

C—H

C—O

C=O

C≡O

C—F

C—Cl

C—Br

C—I

N—H

N—O

N=O

414

335

745

1075

485

326

285

239

389

175

590

N—N

N=N

N≡N

N—F

N—Cl

O—O

O=O

O—H

F—F

F—Cl

159

418

941

270

201

138

494

464

159

256

P—H

P—Cl

S—H

Cl—Cl

Br—Br

Br—Cl

I—I

318

326

339

243

193

218

151

kJ

Rxn progress

Hbb Hbf

H

II. Thermochemistry

38

Example 2: Estimate the for the following reaction. Again, consult the bond

energies table on the previous page:

CH4 + 2 O2 CO2 + 2 H2O

Lewis Structures

Hbb Hbf H

=HbbHbf

Example 3 Show an energy- reaction profile (energy diagram) for an endothermic

reaction in terms of Hbband Hbf

LaurenHill Chemistry 534

39

Exercises

1. Estimate H for each of the following using the given table of bond energies:

a) 2H2 + O2 2 H2O

b) C2H6 + 3.5 O2 2 CO2 + 3 H2O

c) 2 CH3OH + 3 O2 2 CO2 + 4 H2O

2 CH3OH + 3 O2 2 CO2 +

Average bond energies, kJ/mol:

H—H

H—F

H—Cl

H—Br

H—I

C—C

C=C

C≡C

C—N

C=N

C≡N

436

570

432

366

298

347

619

812

293

515

891

C—H

C—O

C=O

C≡O

C—F

C—Cl

C—Br

C—I

N—H

N—O

N=O

414

335

745

1075

485

326

285

239

389

175

590

N—N

N=N

N≡N

N—F

N—Cl

O—O

O=O

O—H

F—F

F—Cl

159

418

941

270

201

138

494

464

159

256

P—H

P—Cl

S—H

Cl—Cl

Br—Br

Br—Cl

I—I

318

326

339

243

193

218

151

2. a) Without consulting a table of bond energies, find the bond energy of H--Cl on a per

mole basis if H2’s and Cl2’s bond energies are 436 kJ/mole and 243 kJ/mole,

respectively, and the H for the following reaction is

-185 kJ/mole of H2

H2 + Cl22 HCl

b) Convert H2’s bond energy to kJ/g

c) Show an energy- reaction profile (energy

diagram) for

H2 + Cl22HCl in terms of Hbband Hbf

3. Why is H---Cl’s bond energy greater than that of H---

Br?

Hint: periodic trends.

4. a) Explain why the first 3 steps of the salt-producing

reaction are endothermic.

b) Why are the last two steps are exothermic?

c) Use algebra to show that the overall reaction is

indeed Na(s) + ½ Cl2(g) NaCl(s).

II. Thermochemistry

40

8. Calorimetry and Molar Enthalpy

The purpose of calorimetry is to use an instrument

known as a calorimeter to determine the enthalpy

of a substance undergoing chemical change. In a

calorimeter known as a bomb calorimeter, it is the

enthalpy of combustion that is measured. This is

how the caloric content of foods is determined. In

both cases, since the heat absorbed or released is

proportional to the amount of reactant used, molar

enthalpy = H/n is a more meaningful and

characteristic quantity.

In a bomb calorimeter, the actual chamber holding

the sample is known as a “bomb”. After opening

its lid, we place a weighed sample in a cup at the

bottom of the bomb. We seal it, and through a

valve, we deliver O2, saturating the bomb to get it

ready for ignition. We secure the bomb within a

calorimeter bucket that is filled with water (the water is the environment which will

absorb the heat of combustion). A stirrer keeps the temperature of the water evenly

distributed. A thermometer allows us to measure the initial temperature; the ignition wire

connected to a high voltage source initiates the explosion; heat is released, and we

measure the maximum temperature attained.

Q = mc T

m = mass of the water in the calorimeter in grams (because of c ‘s units; see below), or

the mass of whatever substance is acting as the environment. In reality we should assume

that the material part of the calorimeter also absorbs heat. But in this course, we usually

ignore that part.

c = specific heat of water or whatever is acting as the environment.

Note c is usually expressed in , so that Q is in Joules

T = Tf – Ti.

To get H, remember:

H = - Q. ( see "I'm so hot and she's so cold”, on page 43")

We convert J to kJ by 1000, and then

Molar enthalpy = H/n. n = number of moles of reactant. So we convert the

carefully measured mass in to moles by dividing by molar

mass.

LaurenHill Chemistry 534

41

In molar heat of neutralization problems, n = CV, where

C = concentration in “M” = moles/L.

V = volume in liters.

In-Class Examples

1. 9.0 grams of charcoal (C) were completely consumed in a bomb calorimeter. If

we assume that the 2.0 L of water absorbed all of the heat released by the

charcoal, and if the temperature of the water increased from 20.25 to 56.04oC,

what is the molar enthalpy of carbon?

2. CS2, a very flammable liquid, has a molar enthalpy of -1028 kJ/mole. What do

you expect aluminum's final temperature to be if 1.0 kg of Al is initially at 20.0 C,

and it absorbs all the heat from the following sample of CS2:

mass of CS2 before burning: 22.6 g

mass of CS2 after burning: 11.6 g

specific heat capacity of Al: 0.900 J/[g C]

II. Thermochemistry

42

3. 300.0 mL of 0.20 M aqueous KOH neutralizes 150 mL of aqueous 0.20 M H2SO4.

We go from an average initial temperature of 22.3 C to a maximum of 29.2 oC.

Calculate the molar heat(enthalpy) of neutralization of KOH. Assume that the

amount of water created in the neutralization is negligible and that the specific

heat and density of all aqueous solutions is 4.19J/(goC) and 1.0 g/ml, respectively.

4. Find the final temperature of the following mixture:

400.0 g of Cu initially at 99.0 oC

25.0 L of water initially at 10.0 oC

c for Cu = 0.39 J/[g C]

LaurenHill Chemistry 534

43

Example: Relate the "He's so hot and she's so cold" to why Q = -H.

Throughout the explanation, imagine a man with hot hands and a woman with cold

hands.

1st Point of View: Man is reaction; woman is environment. Man

releases or loses heat; he represents

an exothermic reaction. H for man is (-). Woman feels heat; she

absorbs it, and her temperature increases. Her Q = (+) . Recall that Q

= mc , where = Tf - Ti.

Note H = -Q.

2nd Point of View: Woman is reaction; man is environment. Woman

takes heat from man. She is an endothermic reaction. She steals or

absorbs heat from the man. H for woman is (+).

Man feels a cold hand; he loses heat to the woman, so his temperature drops. His Q = (-).

Again H = -Q; the signs are again opposite of one another.

Chemical Case.

Suppose that NaOH(aq) is neutralizing HCl(aq). Both are dilute solutions in water. When

they react they release heat into the surrounding water, just like the man. Their H = (-).

But you record an increase in temperature because you are inserting a thermometer into

the water. The water is like the woman in the previous analogy. So for water, Q = (+).

If NaNO3 dissolves in water, the sodium nitrate absorbs or steals heat from the water. The

H for the reacting nitrate is (+), just like the woman's when she is regarded as the

reactant. The water is now like the man; it experiences a drop in temperature and its Q is

(-).

II. Thermochemistry

44

Exercises (questions slightly modified from old exams)

1. Calculate the heat released by the combustion of one mole of C4H10(g) based on

the data below:

initial mass of butane 7.25 g

remaining mass of butane 1.25 g

water's initial temperature 25.0 oC

water's final temperature 60.7 oC

volume of water in calorimeter 2.0 L

2. Find the molar heat of KOH(s) if dissolving 4.0 g of this base in 200.0 mL of

water caused the water in the calorimeter raised from 25.0 to 31.5 oC.

3. When 200 mL of 0.50 moles/L NaOH solution completely neutralizes 200 ml of

HCl, the temperature of the solution rises by 6.6 oC. Calculate the molar heat of

neutralization of NaOH, assuming that the resulting solution has the same density

and specific heat as those of water.

4. In an experiment, a piece of iron, Fe, with a mass of 4.0 grams was heated from

22°C to 42°C.

During the heating process, the piece of iron absorbed 5.04 101 kJ/mol. What

is the specific heat capacity of this piece of iron?

5. Uncle Ivan prefers his pre-dinner drink to be about 15°C. What mass of tap water

(at 11°C) must he add to his 30.0 mL of alcohol at 22°C so that his drink can be at

his preferred temperature (15°C)?

(Assume the alcohol has the same density and specific heat capacity as ethanol =

Cg

J 2.45

; density of ethanol = 0.79g/ml)

6. A piece of metal with a mass of 32.6 grams at a temperature of 2.00 102°C is

dropped into a calorimeter containing 1.00 102 mL of water at 25.0°C.

The specific heat capacity of the metal is 0.448 Cg

J

.

Assuming complete heat transfer between the water and the metal, what will be

the maximum temperature of the metal-water system in the calorimeter?

LaurenHill Chemistry 534

45

Styrofoam

Thermometer

Water

7. A student was asked to check the purity of the paraffin, C25H52, in a candle.

Knowing that the molar heat of combustion of pure paraffin is 15 300 kJ/mol, she

conducted an experiment to determine the molar heat of combustion of the

paraffin in the candle. She used a calorimeter and made the following

observations:

Mass of the candle at the beginning of the experiment

Mass of the candle at the end of the experiment

Initial temperature of the water in the calorimeter

Final temperature of the water in the calorimeter

Volume of water in the calorimeter

28.52 g

24.29 g

20.0°C

42.0°C

2000. mL

Note: All of the heat released during the combustion of the paraffin was absorbed

by the water in the calorimeter.(question on next page……)

Can the student conclude that the paraffin in the candle is relatively pure, given

that the paraffin in the candle is considered pure if the difference between its

molar heat of combustion and the molar heat of combustion of pure paraffin is

between 0 and 350 kJ/mol).

8. Methyl alcohol, CH3OH(1), is generally used as a fuel to heat the oil in fondue

sets. What mass of methyl alcohol is needed to heat 1.10 kg of fondue oil from

22.0°C to 98.0°C?

Specific heat capacity of fondue oil is: 8.9 kJ/[kg°C]

Equation for the combustion of methyl alcohol is:

CH3OH(1) + 3/2 O2(g) CO2(g) + 2 H2O(g) + 639 kJ

9.

During a calorimetry experiment, 4.01 g of LiCl(s) are dissolved in 100. mL of

water. The temperature of the water increased from 22.0°C to 28.0°C. What

quantity of heat energy would be produced if 2.0 moles of LiCl were dissolved?

II. Thermochemistry

46

10. A blacksmith heats a horseshoe to a very high temperature; the horseshoe has a

mass of 550 g.

Then he plunges it into an insulated bucket containing 8.50 liters of water at

22.0°C.

He notes that the maximum water temperature reached was 26.9°C.

Given that the specific heat of the iron is 0.45 kJ/[kg °C], determine the

temperature to which the horseshoe must have been heated.

Extra Calorimetry (solutions also on web site)

1. Phileas Fogg, the character who went around the world in 80 days, was very

fussy about his bathwater temperature. It had to be exactly 38.0o C. You are

his butler, and one morning while checking his bath temperature, you notice

that it’s 42.0oC. You plan to cool the 100.0 kg of water to the desired

temperature by adding an aluminum-duckie originally at freezer temperature

(-24.0oC). Of what mass should the Al-duckie be? [Specific heat of Al = 0.900

J/(goC); density of water =1 .00 g/ml]. Assume that no heat is lost to the air.

2. A certain material’s (environment) temperature increases by 1.00oC for every

1560 J that it gains. A 0.1964 g sample of quinone (molar mass = 108.1

g/mole) was burnt, and the surrounding material’s temperature increased from

20.3 oC to 23.5 oC. Find the molar heat of combustion for quinone.

3. A 1.55 g of CH4O sample is burnt in a calorimeter. If the molar heat of

combustion of CH4O is -725 kJ/mole, and assuming that the 2.0 L of water

absorbed all of the heat of combustion, what temperature change did the water

experience?

4. 0.20 moles of HX were neutralized by NaOH. The concentrations of the base

and acid were equal. If the temperature of the water in the calorimeter

increased from 19.9 to 24.6 C, what was the original concentration of HX?

Molar heat of neutralization = -80 kJ/mole of HX

5. In real calorimeters, most of the heat released by the bomb is absorbed by

water, but a certain amount is also absorbed by the metal and insulation

surrounding the water tank. A certain calorimeter absorbs 24 J/oC. If 50.0 g of

52.7oC water is mixed with the calorimeter’s original 50.0 g of 22.3oC water,

what will be the final temperature of the mixture?