WAJA F5 Chemistry 2010 Chapter 4: Thermochemistry

WAJA F5 Chemistry 2010 Chapter 4: Thermochemistry

CHAPTER 4 : THERMOCHEMISTRY

AENERGY CHANGES IN CHEMICAL REACTION

Activity 1:

(a) / What is an exothermic reaction?
An exothermic reaction is a chemical reaction that gives out heatto 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 , Δ 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, Δ H is (3)…………………
(b) Endothermic reaction : The reactants absorbheat energy to form the products . Thus the products formed have (4) ……………energy than the reactant. Therefore, Δ H is (5)…………….
List twoother examples of exothermic and endothermic reaction
Exothermic 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

ΔH = negative

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

Energy level diagram for endothermic reaction :
The products have more energy than the reactants,
Energy

ΔH = positive

Construct energy level diagrambased on the given chemical equation
CaCO3(s)  CaO(s) + CO2(g) ΔH = + 178 kJ

Information that can be obtained from the energy level diagram,
Energy

ΔH = - 190 kJ

Figure 1
Figure 1shows 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

ΔH = +53 kJ

Figure 2
Figure 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 breakingalways(1) …………. energy and bondsformationalways
(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 inthe(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 + 3H2O / Displacement
Mg + CuSO4  MgSO4 + Cu / Neutralization
Pb(NO3)2 + 2KI  PbI2 + 2KNO3 / Combustion

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 = 28.50C
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 = mcӨ / Temperature change, Ө = 32.0 0C - 29.0 0C = 3.0 0C
Heat change , H = mcӨ m = ( 25 + 25 ) g = 50 g
c = 4.2 J g-1 oC-1
Ө = 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)
Precipitate
Ionic 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, ΔH

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, ΔH = (12) - ……………kJ mol – 1
Draw 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 ΔH = –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-3potassium 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 0C
Calculation
Step 1 :
Calculate the heat change using the formula H = mcӨ / Changes of temperature, Ө = 40.0 0C - 30.0 0C = 10.0 0C
Heat change , H = mcӨ m = 25 g
c = 4.2 J g-1 oC-1
Ө = 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 displaced
Deduce 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 reacts / Number of moles copper(ll) sulphate = (4)…………………….. mol
Number of moles of copper = (5)…………………….. mol
Step 4
Calculate the heat of displacement of copper, ΔH
Step 5
Energy level diagram / When (6) ….. mol of copper is displaced, the heat released is(7) .……..
∴When 1 mol of copper isdisplaced, the heat released is(8)......
Heat of reaction , ΔH = (9) ……………… kJ mol – 1
Draw the energy level diagram for the reaction that occurs in this experiment (10)

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) ΔH = -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 = mcӨ / Changes of temperature, Ө = 42.0 0C - 29.0 0C= (1)……………….0C
Heat change , H = mcӨ m = ( 50 + 50 ) g = 100 g
c = 4.2 J g-1 oC-1
Ө= (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, ΔH
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 , ΔH = (15) ……………… kJ mol - 1
Draw 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) ΔH = -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

ΔH = - 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 g
Mass of lamp and ethanol after burning = 189.80 g
Initial 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 = mcӨ / Changes of temperature, Ө =59.00C - 29.00C = 30.0 0C
Mass of water, m = 200 g
Heat change/ heat absorbed by water , H = mcӨ
c = 4.2 J g-1 oC-1
Ө = 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) ………………. g
Number of moles of ethanol burnt = (6) ______
molar mass of ethanol
= …………………….. mol
Step 4
Calculate the heat of combustion of ethanol, ΔH
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 , ΔH = (10) ……………… kJ mol - 1
Draw 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 Jg-1 0C-1 ]
Alcohol / Methanol / Propanol / Butanol
Volume of water/cm3 / 200 / 200 / 200
Initial temperature of water / 0C / 28.0 / 28.0 / 28.0
Final temperature of water / 0C / 60.0 / 58.0 / 59.0
Mass 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.