TOPIC 4

WAJA F5 Chemistry 2010 Chapter 4: Thermochemistry

CHAPTER 4 : THERMOCHEMISTRYAENERGY CHANGES IN CHEMICAL REACTION

Activity 1: (a)What is an exothermic reaction?

An exothermic reaction is a chemical reaction that gives out heat to the surroundings.Heat energy given out from the reaction is (1)to the surroundings

The temperature of the surroundings (2).

(b)What is an endothermic reaction?

An endothermic reaction is a chemical reaction that absorbs heat from the surroundings .The reactants (3) . heat energy from the surroundings. The temperature of the surroundings (4) .

Activity 2

(a)

Identify the following reactions as exothermic or endothermic reaction by writing a ( at the appropriate box as shown in Question (a).Reaction

Exothermic

Endothermic

(a) Combustion of ethanol(b) Burning of magnesium

(c) Neutralisation between acid and alkali(d) Adding water to concentrated sulphuric acid

(e) Photosynthesis

(f) Reaction between acid and magnesium(g) Reaction between acid and calcium carbonate(h) Dissolving ammonium salt in water(i) Thermal decomposition of copper(ll) carbonate(j) Thermal decomposition of zinc nitrate

(b)The amount of heat energy released or absorbed during a chemical reaction is called the heat of reaction. It is given a symbol (1). and the unit is (2) ..

(c)(d) The heat of reaction , H = H products - H reactants(a) Exothermic reaction : The reactants lose heat energy to form the products . Thus the products formed have less energy than the reactants, Therefore, H is (3)(b) Endothermic reaction : The reactants absorb heat energy to form the products . Thus the products formed have (4) energy than the reactant. Therefore, H is (5).List two other examples of exothermic and endothermic reactionExothermic reaction

Endothermic reaction

1)

2)Activity 3 :

(a)Energy level diagram

(i)(ii)

(iii)

Energy level diagram for exothermic reaction :

The products have less energy than the reactants, Energy

H = negative

Construct the energy level diagram based on the given chemical equation, Mg(s) + H2SO4 (aq) ( MgSO4 (aq) + H2 (g) H = -467 kJ

(reactants) ( products )

Energy level diagram for endothermic reaction :

The products have more energy than the reactants,

Energy

H = positive

Construct energy level diagram based on the given chemical equation

CaCO3(s) ( CaO(s) + CO2 (g) H = + 178 kJ

Information that can be obtained from the energy level diagram ,

Energy

H = - 190 kJ Figure 1Figure 1 shows the energy level diagram for the reaction between zinc and copper(ll) sulphate,

The reaction between (1) and (2). is an (3) .. reaction.

During the reaction, the temperature of the mixture (4) The total energy of one mole of (5). and one mole of (6) is (7). than the total energy of one mole of copper and one mole of zinc sulphate by (8). kJ

When one mole of (9).. reacts with one mole of (10)to form (11).. .. mole of copper and (12).. mole of zinc sulphate, (13).. kJ of heat is (14) Energy

H = +53 kJ

Figure 2Figure 2 shows the energy level diagram for the reaction between hydrogen gas and iodine The reaction between (15)and (16). is an (17) reaction.

During the reaction, the temperature of the mixture (18) The total energy of one mole of (19). and one mole of (20). is (21) than the total energy of two moles of hydrogen iodide by (22) kJ

When one mole of (23).reacts with one mole of (24) to form (25) .mole of hydrogen iodide (26)..kJ of heat is (27)

(b)(i) A chemical reaction involves (1) .... (2) ............... of the reactants and

(3).. (4) ........................of the product

(ii) Bonds breaking always(1) . energy and bonds formation always

(2) energy

Bond breaking Bond formation (iii) If the energy absorbed during bonds breaking is less than the energy released during

bonds formation ,energy is (1).. to the surrounding. The reaction is an (2) .. reaction(iv) If the energy absorbed during bonds breaking is more than the energy released during bonds formation, energy is(1). from the surrounding.

Hence it is an (2) reaction

(c)Explain the application of exothermic and endothermic reaction in our daily lives.

(i)(ii)

Instant cold packs : Instant cold packs are used to treat (1), have separate compartments of (2) and (3)in a plastic bag. When the barrier between the two is broken by squeezing the outer bag, the (4).. dissolves in the(5) .endothermically to provide instant coldness.A reusable heat pack:

B HEAT OF PRECIPITATION

HEAT OF REACTION

Activity 4: Match the chemical equation with the type of reaction below :

Chemical equation

Type of reaction

HNO3 + KOH ( KNO3 + H2O Precipitation

C2H5OH + 3O2 ( 2CO2 + 3H2ODisplacement

Mg + CuSO4 ( MgSO4 + Cu Neutralization

Pb(NO3)2 + 2KI ( PbI2 + 2KNO3Combustion

Activity 5 :

(a) What is meant by heat of precipitation ?

..

..

(b) To determine the heat of precipitation of silver chloride, AgCl

Procedure :1. Measure 25 cm3 0.5 mol dm-3 silver nitrate solution and pour it into the polystyrene cup,2. Put the thermometer into the silver nitrate solution. Record the initial temperature,

3. Measure 25 cm3 0.5 mol dm-3 sodium chloride solution and record the initial temperature,

4. Pour the sodium chloride solution quickly into the silver nitrate solution in the polystyrene cup.5. Stir the solution mixture with the thermometer and record the highest temperature achieved.

Result : Initial temperature of silver nitrate solution = 28.5 0C

Initial temperature of sodium chloride solution = 29.5 0C

Highest temperature of the mixture = 32.0 0C

Calculation

Step 1 :

Calculate the heat change using the formula H = mc

Temperature change, = 32.0 0C - 29.0 0C = 3.0 0CHeat change , H = mc m = ( 25 + 25 ) g = 50 g

c = 4.2 J g-1 oC-1

= 3.0 0C H =(1) .........J

Step 2:

Write balanced chemical equation or ionic equation for the reaction that occurs

Chemical equation : AgNO3 (aq) + NaCl(aq) ( AgCl (s) + NaNO3 (aq)

PrecipitateIonic equation : (2).Deduce the mole ratio from the ionic equation :(3)mol silver ion, Ag+ react with (4) mol of chloride ion, Cl- to produce (5). mol of silver chloride , AgCl.

Step 3 :

Calculate the number of moles of reactant that reacts,Use the formula :

n = MV ( V in dm3 ) Number of moles of silver ion = the number of moles of silver nitrate

= (6) .. mol

Number of moles of chloride ion = the number of moles of sodium

chloride

= (7).. mol

Number of moles of silver chloride formed = (8) .. mol

Step 4

Calculate the heat of precipitation of silver chloride, H

Step 5

Energy level diagram When (9) .. mol of silver chloride formed, heat released is (10) ...When 1 mol of silver chloride formed, heat released is (11)......................Heat of precipitation of silver chloride, H = (12) - kJ mol 1Draw the energy level diagram for the reaction that occurs in this experiment (13)

(c)(d)Calculate the heat change when 200 cm3 of 0.5 mol dm-3 calcium chloride, CaCl2 solution is added to 200 cm3 of 0.5 mol dm-3 sodium carbonate, Na2CO3 solution if the heat of precipitation of calcium carbonate, CaCO3 is +12.6 kJ mol-1[ Specific heat capacity of solution : 4.2 J g-1 0C-1 . Density of solution : 1 g cm-3 ]The thermochemical equation for the precipitation of silver chloride is as follows : Ag+ (aq) + Cl- (aq) ( AgCl H = 65.5 kJ mol-1

Calculate the temperature change when 100 cm3 of 0.5 mol dm-3 silver nitrate, AgNO3 , solution is added to 100 cm3 of 0.5 mol dm-3 potassium chloride, KCl solution

C HEAT OF DISPLACEMENT

Activity 6 (a) What is meant by the heat of displacement ?

..

..

(b) To determine the heat of displacement of copper from a copper (ll) sulphate solution by zinc.

Procedure :

1. Measure 25 cm3 0.2 mol dm-3 copper(ll) sulphate solution and pour into a polystyrene cup.

2. Put the thermometer into the copper(ll) sulphate solution. Record the initial temperature,

3. Add half a spatula of zinc powder (in excess) quickly into copper(ll) sulphate solution.

5. Stir the mixture with the thermometer and record the highest temperature achieved.

Result : Initial temperature of copper(II) sulphate solution = 30.0 0C

Highest temperature of the mixture = 40.0 0CCalculation

Step 1 :

Calculate the heat change using the formula H = mc

Changes of temperature, = 40.0 0C - 30.0 0C = 10.0 0C

Heat change , H = mc m = 25 g

c = 4.2 J g-1 oC-1

= 10.0 0C

H = (1).. J

Step 2:

Write balanced chemical equation or ionic equation for the reaction that occurs

Chemical equation : Zn (s) + CuSO4(aq) ( Cu (s) + ZnSO4 (aq)

copper displacedDeduce the mole ratio from the equation :(2) mol copper metal, Cu is displaced from (3) .. mol of copper(ll) sulphate solution, CuSO4 by zinc

Step 3 :

Calculate the number of moles of reactant that reactsNumber of moles copper(ll) sulphate = (4).. mol

Number of moles of copper = (5).. mol

Step 4

Calculate the heat of displacement of copper, H

Step 5

Energy level diagram When (6) .. mol of copper is displaced, the heat released is (7) ...

When 1 mol of copper is displaced, the heat released is (8)................. Heat of reaction , H = (9) kJ mol 1Draw the energy level diagram for the reaction that occurs in this experiment (10)

(c) Figure shows an experiment carried out to determine the heat of displacement of silver

from silver nitrate by copper metal.

Based on figure above, calculate the heat of displacement for the reaction.

(d)In an experiment, excess magnesium powder is added to 50 cm3 of 0.25 mol dm-3 iron(ll) sulphate solution at 29.0 0C. The thermochemical equation is shown below, Mg(s) + Fe2+ (aq) ( Mg2+ (aq) + Fe (s) H = -80.6 kJ mol - 1

What is the highest temperature reached in this experiment ?

D HEAT OF NEUTRALIZATION

Activity 7

(a) What is meant by the heat of neutralization

..

(b) To determine the heat of neutralisation between a strong acid ( hydrochloric acid )

and a strong alkali ( sodium hydroxide )

Procedure :

1. Measure 50 cm3 2.0 mol dm-3 sodium hydroxide solution and pour it into the polystyrene cup,2. Put the thermometer into the sodium hydroxide solution. Record the initial temperature,

3. Measure 50 cm3 2.0 mol dm-3 hydrochloric acid solution and record the initial temperature,

4. Pour the hydrochloric acid solution quickly into the sodium hydroxide solution in the

polystyrene cup.

5. Stir the mixture with the thermometer and record the highest temperature achieved.

Result :

Initial temperature of sodium hydroxide solution = 29.0 0C

Initial temperature of hydrochloric acid solution = 29.0 0C

Highest temperature of the mixture = 42.0 0C

Calculation

Step 1 : Calculate the heat change using the formula H = mcChanges of temperature, = 42.0 0C - 29.0 0C = (1) .0CHeat change , H = mc m = ( 50 + 50 ) g = 100 g

c = 4.2 J g-1 oC-1

= (2) .0C

H = (3).. J

Step 2:

Write balanced chemical equation or ionic equation for the reaction that occurs

Chemical equation (4) .Ionic equation (5) .

Deduce the mole ratio from the ionic equation : (6)mol hydrogen ion, H+ react with (7)mol of hydroxide ion, OH- to produce (8). mol of water , H2O

Step 3 :

Calculate the number of moles of reactant that reacts,Use , n = MV

( V in dm3 ) Number of moles of H+ = the number of moles of hydrochloric acid = (9).. mol

Number of moles of OH- = the number of moles of sodium hydroxide = (10).. mol

Number of moles of water formed = (11) .. mol

Step 4

Calculate the heat of neutralisation of hydrochloric acid and sodium hydroxide, H

Step 5 : Energy level diagram

When (12) .. mol of water formed, heat released is (13) ...

When 1 mol of water formed, heat released is (14) .................Heat of neutralisation , H = (15) kJ mol - 1Draw the energy level diagram for the reaction that occurs in this experiment (16)

(c)(d)The thermochemical equation for the reaction between ethanoic acid and sodium hydroxide is given below,

CH3COOH (aq) + NaOH (aq) ( NaCH3COO (aq) + H2O (l) H = -55 kJ mol - 1

Calculate the heat given out when 200 cm3 of ethanoic acid 0.5 mol dm-3 is added to

200 cm3 of sodium hydroxide 0.5 mol dm-3

The energy level diagram of a neutralization reaction is shown in figure below

Energy

H2SO4 + 2NaOH

H = - 114 kJ Na2SO4 + 2H2O

When 100 cm3 of 1.0 mol dm-3 sulphuric acid is added to 100 cm3 of 1.0 mol dm-3

sodium hydroxide solution. What is the change in temperature?

E : HEAT OF COMBUSTION

Activity 8 :

(a) What is meant by the heat of combustion

.

(b) To determine the heat of combustion of ethanol

Experiment to determine the heat of combustion.Procedure :

1. Measure 200 cm3 of water and pour it into a copper can. Record the initial temperature of

the water and place the copper can on a tripod stand.

2. Fill a lamp with ethanol and weigh it. Record the mass of the lamp together with its

content.

3. Light up the wick of the lamp immediately. Stir the water continuously until the temperature of the water increases by about 30 0C.5. Put off the flame and record the highest temperature reached by the water

6. Weigh again the lamp and record the mass of the lamp. Result :

Mass of lamp and ethanol before burning = 190.55 gMass of lamp and ethanol after burning = 189.80 gInitial temperature of water = 29.00C

Highest temperature of water = 59.00C

Calculation

Step 1 :

Calculate the heat change/heat absorbed by the water using the formula,

H = mcChanges of temperature, = 59.00C - 29.00C = 30.0 0C

Mass of water, m = 200 gHeat change/ heat absorbed by water , H = mc c = 4.2 J g-1 oC-1

= 30.0 0C

H = (1).. J

Step 2:

Write a balanced chemical equation for the combustion of ethanol C2H5OH + (2)= (3)+ (4) .

Step 3 :

Calculate the number of moles of ethanol that is used in the experiment,[ Relative atomic mass : H, 1 : C,12; O,16 ]

Mass of ethanol burnt/used = (5) . gNumber of moles of ethanol burnt = (6) __________ molar mass of ethanol = .. mol

Step 4

Calculate the heat of combustion of ethanol, H

Step 5

Energy level diagram When(7) .. mol of ethanol is burnt in oxygen heat released is(8) . When 1 mol of ethanol is burnt in oxygen, heat released is (9) ..........Heat of combustion of ethanol , H = (10) kJ mol - 1Draw an energy level diagram for the combustion of ethanol in this experiment (11)

(c) The following results are obtained by a student in an experiment to determine the heat of

combustion of alcohols. Complete the table and calculate the heat of combustion of methanol, propanol and

butanol based on the data given in the table below .

[ Relative atomic mass : H,1 ; C,12 ; O,16 . Specific heat capacity of water , 4.2 J g-1 0C-1 ]AlcoholMethanolPropanolButanolVolume of water/cm3200200200Initial temperature of water / 0C28.028.028.0Final temperature of water / 0C60.058.059.0Mass of alcohol burnt/g

1.184

0.750

0.719

Changes in temperature / 0C

(1)(2)(3)Molecular formula

(4)(5)(6)Number of carbon atoms

(7)(8)(9)Molar mass

(10)(11)(12)Number of moles of alcohol burnt(13)(14)(15)Heat change/ absorbed by the water / J

(16)(17)(18)Heat of combustion of alcohol /

kJ mol-1(19)(20)(21)(i) State the relationship between the number of carbon atoms in an alcohol and the heat

of combustion,

.

.

(ii) Predict the heat of combustion for pentanol

.

(d)(e)When 1 mole of butanol, C4H9OH is burnt in excess of oxygen, 2600 kJ of heat is produced. Calculate the mass of butanol needed to burn completely in oxygen in order to raise the temperature of 500 cm3 of water by 300C( Relative atomic mass : H , 1 ; C , 12 ; O , 16,

Specific heat capacity of water , 4.2 J g-1 0C-1) The heat of combustion of propanol, C3H7OH is -2016 kJ mol -1 What is its fuel value ?

Solution :

(i) Calculate the molar mass of propanol, C3H7OH. (ii) Calculate the fuel value of propanol, C3H7OH.

reactants

products

reactants

products

Zn + CuSO4

Cu + ZnSO4

H2 (g) + I2(g)

2HI (g)

HEAT OF REACTION

The change in the amount of heat in a chemical reaction

Symbol : H

The

HEAT OF PRECIPITATION

HEAT OF DISPLACEMENT

HEAT OF NEUTRALIZATION

HEAT OF COMBUSTION

The heat change in a reaction can be calculated using the formula , H = mc

m = mass of solution, g

c = specific heat capacity of the solution, J g-1 oC-1

= temperature change , 0C

Assumption : i) The solution is dilute, it has the same density as water , 1 g cm-3

( 1 cm3 = 1 g )

ii) The solution has the same specific heat capacity as water, 4.2 J g-1 oC-1

Example :

Calculate the amount of heat change when the temperature of 200 cm3 of water is raised from 280C to 400C,

Q = mc m = 200 g

= 200 g x 4.2 J g-1 oC-1 x 12 0C c = 4.2 J g-1 oC-1

= 10080 J = 40 - 28 = 12 0C

= 10.08 kJ

Guidelines for the calculation of the heat of reaction

Steps to follow:

Step 1 - Calculate the heat change using the formula, H = mc ( p Joule )

Step 2 - Write chemical equation or ionic equation for the reaction that occurs

Step 3 - Calculate the number of moles of reactant that reacts using either the following

formulae :

Number of moles = mass = q mol or

molar mass

Number of moles = MV ( q mol) M : Molarity of the solution

(Solution) V : volume of the solution in dm3

Step 4 - Link the number of moles of reactants ( step 3 ) with the heat change ( step 1 )

q mol of reactants react ( heat lost/gain is p J

1 mol of reactants react ( heat lost/gain is p J

q

Heat of reaction , H = +/- p kJ mol-1

1000 x q

= +/- r kJ mol-1

Note : + is used for endothermic reaction and - is used for exothermic reaction

Step 5 - Draw the energy level diagram

25 cm3 0.5 mol dm-3

sodium chloride solution

25 cm3 0.5 mol dm-3

silver nitrate solution

Average initial temperature = 29.00C

zinc powder

25 cm3 0.2 mol dm-3

copper(ll) sulphate solution

Average initial temperature = 29.0 0C

50 cm3 2.0 mol dm-3

hydrochloric acid solution

50 cm3 2.0 mol dm-3

sodium hydroxide solution

Learning Outcomes :

You should be able to:

State what exothermic reaction is,

State what endothermic reaction is,

Identify exothermic reactions,

Identify endothermic reactions,

Give examples of exothermic reactions,

Give examples of endothermic reactions,

Construct energy level diagrams for exothermic reactions,

Construct energy level diagrams for endothermic reactions,

Interpret energy level diagram,

Interrelate energy change with formation and breaking of bonds,

Describe the application of knowledge of exothermic and endothermic reactions in everyday life.

Fuel value ( kJ g-1 ) = heat of combustion of alcohol ( kJ mol-1 )

molar mass

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

Learning Outcomes

You should be able to:

State what heat of reaction is,

State what heat of precipitation is,

Determine the heat of precipitation for a reaction,

Construct an energy level diagram for a precipitation reaction,

Solve numerical problems related to heat of precipitation

Learning Outcomes,

You should be able to:

State what heat of displacement is,

Determine the heat of displacement,

Construct an energy level diagram for a displacement reaction,

Solve numerical problems related to heat of displacement,

Learning Outcomes :

You should be able to:

State what heat of neutralization is,

Determine the heat of neutralization,

Construct energy level diagrams for various types of neutralization reactions,

Compare the heat of neutralization for the reactions between a strong acid and a strong

alkali with the heat of neutralization for a reaction between a weak acid and a weak

alkali,

Explain the difference of the heat of neutralization for a strong acid and a strong alkali

with heat of neutralization for a reaction involving a weak acid and/or a weak alkali.

solve numerical problems related to heat of neutralization,

Learning Outcomes

You should be able to :

state what heat of combustion is,

determine heat of combustion for a reaction,

construct an energy level diagram for a combustion reaction,

compare the heats of combustion of various alcohols,

state what fuel value is,

describe the difference between heats of combustion of various alcohols,

describe the applications of fuel value,

compare and contrast fuel values for various fuels,

solve numerical problems related to heat of combustion.

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