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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