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D_Heat of Neutralization

The heat of neutralization is the heat change when one mole of water is formed from thereaction between an acid and an alkali.

Example:

H+(aq) + OH

(aq)

H2O(l)

(, = heat of neutralization

All neutralization reaction are exothermic reaction.

Energy level diagram for the neutralization between a strong acid and a strong alkali

Example:

HNO3(aq) + NaOH(aq) NaNO3(aq) + H2O(l)

(, = -57 kJmol-1

This means 57 kJ of heat energy is released when 1 mole of water is produced in theneutralization reaction between HNO3 and NaOH.

The heat of neutralization between HNO3 and NaOH is-57 kJmol

-1

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Experiment 3: Heat of Neutralization between strong acid and strong alkali

Apparatus:

250 cm3

polystyrene cups, thermometer, 50cm3

measuring cylinders

Materials :

1.0 moldm3

sodium hydroxide, NaOH solution, 1.0 moldm3 potassium hydroxide, KOH solution. 1.0

moldm3hydrochloric acid,HCl, 1.0 moldm

3nitric acid.

Procedures:

1. Measure 50cm3of1.0moldm3 sodium hydroxide solution using a measuring cylinder. Pour itinto a polystyrene cup.

2. Measure 50cm3of1.0moldm3 hydrochloric acid using another measuring cylinder. Pour itinto another polystyrene cup.

3. Leave the polystyrene cup containing the solutions on a table for five minutes.4. After 5 minutes, measure and record the initial temperature of both solution.5. Pour the hydrochloric acid, HCl quickly and carefully into the polystyrene cup containing

sodium hydroxide, NaOH solution.

6. Stir the mixture using the thermometer while observing the change in its temperature.7. Record the highest temperature of the mixtures.8. Repeat steps 1 to 7 using 50cm3of1.0moldm3 potassium hydroxide solution and 50cm3of

1.0moldm3 nitric acid, to replace the sodium hydroxide, NaOH solution and hydrochloric

acid, HCl respectively.

Inference:

Neutralization of any strong and any strong alkali is an exothermic reactions.

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Discussion:

1. The value of heat of neutralization for both reactions are the same. This is because, HCl,HNO3, NaOH and KOH are completely ionized in aqueous solution. The neutralization

occurred involves the reaction between one mole of hydrogen ions and one mole of

hydroxide ions to form one mole of water

2. 4. The initial temperatures of both solutions of acid and alkali were measured after 5minutes to allow each solution to achieve a uniform temperature in equilibrium with the

surroundings

3. 5. The experimental value is slightly lower than the theoretical value because a smallamount of heat energy is lost to the surroundings from the surface of the solution and the

polystyrene cup during the reaction.

Determination of the Heat of Neutralization

Total heat released in neutralization depends on three factors:

a. Quantity of acid and alkali usedb. Basicity of acid and alkalic. Strength of acid and alkali

Note : Total Heat Released { Heat of Neutralization

Heat released is directly proportional to the quantity of acid and alkali used.

Example :

1 mole HCl + 1 mole NaOH

------released 57 kJ heat energy

2 mols HCl + 2 mols NaOH

-------released 2 v 57 kJ heat energy

Factor 2: Basicity ofAcid and Alkali

Basicity of an acid is the number of moles of OH-

ions that are required to react with onemole of the acid.

Complete neutralization of diprotic acid (dibasic acid) released the heat energy that 2 timesgreater than the complete neutralization of monoprotic acid (monobasic acid).

Example:

HCl is a monoprotic acid

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1 mole HCl + 1 mole NaOH

------released 57 kJ heat energy

H2SO4 is a diprotic acid

1mole of H2SO4 + 2 mols NaOH

-------released 2 v 57 kJ heat energy

Factor 3: The Strength of acid dan alkali

Heat released in the neutralization between a weak acid or weak alkali is lesser than thatbetween a strong acid and strong alkali.

Example:

1. HCl is a strong acid

1 mol HCl + 1 mol NaOH

------released 57 kJ heat energy

2. Ethanoic acid, CH3COOH is a weak acid

1 mol CH3COOH + 1 mol NaOH

------released less than 57 kJ heat energy

3. Ammonia solution, NH3 is a weak alkali

1 mol HCl + 1 mol NH3

------released less than 57 kJ heat energy

Why are the values of heat of neutralization between Strong acid and weak alkali Weak acid and strong alkali Weak acid and weak alkali

are lower than that the value between strong acid and strong alkali?

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Weak acid and weak alkali ionize partially in water where most of the particles in theirsolution are still unionized molecules.

Hence, some of the heat energy released during the neutralization reaction is used tobreak the bonds in these molecules, so that they are ionized to form H

+and OH

respectively.

Practice D

1. Heat energy is released when 50cm3 of 1.0 moldm3 nitric acid is added to 50cm3 of 1.0moldm

3sodium hydroxide solution. Which of the following reactions will release the same

amount of heat?

I. 50cm3 of 1.0 moldm3 HCl and 50cm3 of 1.0 moldm3 NaOHII. 25cm3 of 1.0 moldm3 CH3COOH and 50cm3 of 1.0 moldm3 NH3 solution

III. 50cm3 of 1.0 moldm3 H2SO4 and 25cm3 of 1.0 moldm3 Ca(OH)2 solutionIV. 25cm3 of 2.0 moldm3 CH3COOH and 25cm3 of 2.0 moldm3 NaOH

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2. When 100cm3 of 1 moldm3 CH3COOH is mixed with 100cm3 of 1 moldm3 NaOH solution,the temperature of the mixture increases by 5.5rC. Calculate the rise in temperature if

100cm3

of 0.8 moldm3

CH3COOH is mixed with 100cm3

of 0.8 moldm3

NaOH solution.

3. When 50cm3

of 1moldm3

NaOH solution is added to an excess of acid HA, 2.65kJ of heat is

released.

a. Calculate the heat of neutralizationb. Is the acid HA a strong acid or a weak acid?c. State the assumptions that you made in answer (b).

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E_Heat of Combustion

The heat of combustion is the heat change when one mole of a substance is completely burnt in

oxygen under standard conditions.

Example:

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

(H = 890 kJ mol1

This means 890 kJ of heat energy is released when 1 mole of CH4 is burnt completely in excess of

oxygen.

Heat of Combustion of CH4 is 890 kJ mol1

Experiment 4: Heat of Combustion of Alcohol

Apparatus:

250 cm3

copper can, tripod stand, thermometer, 100cm3

measuring cylinder, spirit lamps, electronic

balance, wooden block, windshield.

Materials :

Methanol, CH3OH, ethanol, C2H5OH, propan-1-ol, C3H7OH, butan-1-ol, C4H9OH, water

Procedure:

1. Measure 200cm3 of water using a measuring cylinder. Pour it into a copper can.2. Place the copper can on a tripod stand (without a wire gauze). Leave it aside for 5 minutes.3. After 5 minutes, measure and record the initial temperature of the water.4. Pour 50cm3 of methanol into a spirit lamp and weigh it.5. Place the lamp on a wooden block under the copper can. Light up the wick of the lamp

immediately.

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6. Stir the water continuously using a thermometer while observing the increase in thetemperature.

7. Once the temperature increases by about 30rC, put out the flame immediately and recordthe highest temperature of the water.

8. Immediately weigh the spirit lamp with methanol in it again.9. Repeat steps 1 to 8 using ethanol, propan-1-ol and butan-1-ol respectively to replace

methanol.

Calculation: Heat of combustion of methanol

Alcohol methanol ethanol propan-1-

ol

butan-1-ol

Mass of lamp + alcohol before

combustion (g)

139.36 142.78 141.03 147.35

Mass of lamp + alcohol after

combustion (g)

138..23 141.93 140.28 146.65

Mass of alcohol burnt (g) 1.13 0.85 0.75 0.70

1714-methanolofcombustiontheofHeat

714

713881

0.035325200

releasesmethanolmole1ofCombustion

energyheatof25200releasesmethanolofmol0.0353ofCombustion

0.0353mol25200

1163(1)12

1.13304.2200

burntmethanolofmolesofNumberreleasedheatTotal

can.copperin thewaterby theabsorbediscombustionduringoutgivenheattheallthatAssume

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kJmol

kJ

J

J

J

J

mcU

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Alcohol Number of Carbon atoms per

molecule (n)

Heat of Combustion

( kJmol1)

Methanol, CH3OH 1 714

Ethanol, C2H5OH 2 1 364

Propan-1-ol, C3H7OH 3 2 016

Butan-1-ol

C4H9OH

4 2 664

11364-ethanolofcombustiontheofHeat

1364

6363631

0.01848

25200

releasesethanolmole1ofCombustion

energyheatof25200releasesethanolofmol0.01848ofCombustion

mol0.0184825200

1165(1)2(12)

0.85304.2200

burntethanolofmolesofNumberreleasedheatTotal

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}

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kJmol

kJ

J

J

J

J

mcU

GRAPH OF HEAT OF COMBUSTION OF ALCOHOLS AGAINST THE NUMBER OF

CARBON ATOM

714

1364

2016

2664

0

500

1000

1500

2000

2500

3000

1 2 3 4

NUMBER OF CARBON ATOM PER MOLECULE OF ALCOHOL

HEATOFCOMBUS

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Inference From Graph

From the graph, it can be deduced that as the number of carbon atoms per molecules of alcohol

increases, the value of heat of combustion of the alcohol also increases

Discussion

1. The copper can cannot be replace by a glass beaker because glass is a poor conductor ofheat, thus some of the heat given out may not be transferred to the water in the glass

beaker but is lost to the surroundings instead.

2. The water in the copper can must be stirred continuously throughout the experiment to ensure

that the water is uniformly heated so that a uniform temperature is achieved.

3. The values of the heat of combustion obtained in this experiment are less than the expected

values. This may due to:

a small amount of heat is lost to the surroundings through the surface of water, the

flame and absorbed by the copper can.

A small amount of alcohol is incompletely burnt

Precautions to be taken in Experiment

1. The water must be stirred continuously throughout the experiment.2. The flame from the combustion of alcohol must be as close as possible to the bottom of the

copper can.

3. A windshield is used to shield the apparatus from being disturbed by wind4. The spirit lamp must be weighed immediately before and after the burning of alcohol.5. Wire gauze is not used when the water in the copper can is heated

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Fuel Value

The fuel value of a fuel is the amount of heat energy given out when one gram of the fuel iscompletely burnt in excess of oxygen.

The unit for fuel value is kJg1 The selection of a fuel depends on the following factors:

Fuel value Cost of the fuel Density of the fuel Effect of the fuel on the environment

Practice E

1. When 2.7g of glucose, C6H12O6 is burnt completely in excess oxygen, the heat releasedincreases the temperature of 600g of water by 12.5rC. Calculate the heat of combustion of

glucose.

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2. When 0.0125 mol of methane, CH4 is completely burnt, the heat energy released raises the

temperature of 250cm3

of water by 10rC. What is the fuel value of methane?

[specific heat capacity of water =4.2Jg1rC

1 ; density of solution=1.0gcm3

; relative atomic mass:

H,1 ; C,12; O,16 ]

The thermochemical equation for the combustion of heptane is shown below:

C7H16(l) + 11O2 (g) 7CO2(g) + 8H2O (l)

(H= 5520 kJ mol1

The combustion of heptane in excess oxygen releases 1104 kJ of heat energy. What is the mass of

heptane used?