THERMOCHEMISTRY THERMOCHEMICAL EQUATIONS · 2021. 4. 30. · The School For Excellence 20 Unit 3 & 4 Chemistry 21 Notes & Tests – Page 1 THERMOCHEMISTRY . THERMOCHEMICAL EQUATIONS

The School For Excellence 2021 Unit 3 & 4 Chemistry – Notes & Tests Page 1

THERMOCHEMISTRY

THERMOCHEMICAL EQUATIONS Chemical reactions such explosions and the burning of fuels are exothermic which means energy is released. The amount of energy released depends on the amount of explosive or fuel used in the reaction that is, energy released is directly proportional to the amount of reactant or reactants used. When water evaporates, energy is absorbed from the surroundings (endothermic reaction) as the state of water changes from liquid to a gas. Once again, the amount of energy absorbed depends on the amount of water that evaporates. Chemical reactions involving the release or the absorption of energy, are represented by thermochemical equations which have the following features:

1. Chemical formulae of reactants and products.

2. Coefficients of amounts in moles for reactants and products.

3. States of reactants and products.

4. Amount of energy either released or absorbed when a specified amount of reactants is used.

EXAMPLE 1 Combustion of methane gas:

)(2)()(2)( 2224 lOHgCOgOgCH +→+ 1890 −−=∆ kJmolH The above equation is telling us that when 1mole of methane gas reacts with 2 mole of oxygen, 890 kJ of energy are released. If 2 mole of methane were to react, then 4 mole of oxygen would be required and the amount of energy released would be 1780 kJ. EXAMPLE 2 Photosynthesis:

)(6)()(6)(6 2612622 gOaqOHClOHgCO +→+ 12803 −+=∆ kJmolH This means that when 6 mole of carbon dioxide react with 6 mole of water, 1 mole of glucose is formed and 2803 kJ of energy are absorbed. If 2 mole of glucose were to be produced, 12 mole of carbon dioxide would react with 12 mole of water and 5606 kJ of energy would be absorbed.


EXAMPLE 3 Change of state for water: Evaporation: )()( 22 gOHlOH → 144 −+=∆ kJmolH Condensation: )()( 22 lOHgOH → 144 −−=∆ kJmolH The evaporation of 1 mole of water absorbs 44 kJ of energy while the condensation of the same amount of water releases 44 kJ of energy. This example illustrates the importance of including states in the equation and also highlights that for a reverse reaction involving the same amounts of reactants and products, the amount of energy transferred remains the same but the sign of ∆H is reversed. Note: The units of ∆H, kJ mol-1 do not refer to the amount of kJ per 1 mole of substance but to the amount of kJ of energy either released or absorbed per number of moles of reactants and products, as indicated in the balanced thermochemical equation.

ENERGY PROFILES Energy profiles are a graphical representation of the heat content (H) of reactants compared to that of products. Also, the activation energy and the heat of reaction (∆H) are represented. An energy profile can tell us whether a reaction is exothermic or endothermic and, subsequently whether ∆H is positive or negative. Activation Energy is the energy absorbed to break the bonds of the reactants, so that the reaction can occur. ∆H is the difference in energies between products and reactants and is given by the expression: ( ) ( )tsacHoductsHH tanRePr −=∆


1. Energy profile for an exothermic reaction.

In exothermic reactions, the energy in the bonds of the products is less than the energy in the bonds of the reactants therefore the excess energy is released to the surroundings as heat. 2. Energy profile for an endothermic reaction.

In endothermic reactions, the energy in the bonds of the products is greater than the energy in the bonds of the reactants therefore the extra energy required is absorbed from the surroundings.


CALCULATIONS INVOLVING THERMOCHEMICAL EQUATIONS

Thermochemical equations allow us to calculate: 1. The heat of reaction for any given amount of substance involved in the reaction, given a

standard heat of reaction, ∆H.

2. The standard heat of reaction given the heat of reaction from a known amount of substance.

EXAMPLE 1 The combustion of methane is represented by the following thermochemical equation:

12224 890);(2)()(2)( −−=∆+→+ kJmolHlOHgCOgOgCH

(a) State whether the reaction is exothermic or endothermic. (b) How many kilojoules of energy would be released by the combustion of 1.6g of

methane? Solution (a) The reaction is exothermic because ∆H is negative.

(b) Find number of moles of methane in 1.6g; molCHn 1.016

6.1)( 4 ==

From equation: 1 mol of methane releases 890 kJ 0.1 mol of methane releases x kJ

By proportion: kJxx 891

1.08908900.1

1.0=

×=∴=


EXAMPLE 2 The process of cellular respiration can be summarised by the following thermochemical equation:

12226126 2803);(6)(6)(6)( −−=∆+→+ kJmolHlOHgCOgOaqOHC

Calculate the volume of oxygen gas reacted, at STP in order to release 140 kJ of energy. Solution When 2803 kJ of energy is released, 6 moles of oxygen react. When 140 kJ of energy are released, x moles of oxygen react.

By proportion: molxx 3.02803

14062803140

6=

×=∴=

0.3mol of oxygen react at STP. Since 1 mol of gas occupies 22.4 L at STP, the volume occupied by 0.3 mol of oxygen is given by:

LOV 72.64.223.0)( 2 =×=

EXAMPLE 3 Ammonia is produced according to the following reaction:

)(2)(3)( 322 gNHgHgN →+ When 0.1 mol of ammonia is formed, 4.62 kJ of energy is released. Calculate the heat of reaction for the production of ammonia, according to the above reaction. Solution The equation shows the formation of 2 mole of ammonia, so we can say that: 0.1 mol releases 4.62 kJ 0.1 mol releases x kJ

By proportion: kJxx 4.921.0

262.462.41.0

0.2=

×=∴=

Finally: 14.92 −−=∆ kJmolH


COMPARING ENERGY OF FUELS Thermochemical equations also allow us to compare fuels in terms of energy released per unit mass or unit volume.

Example comparing butane and methane

1. Look at heats of combustion for each fuel: Butane: 1

222104 5772);(10)(8)(13)(2 −−=∆+→+ kJmolHlOHgCOgOgHC

Methane: 12224 890);(2)()(2)( −−=∆+→+ kJmolHlOHgCOgOgCH

2. Convert mole coefficient shown in equation into grams:

Butane: 2 mol release 5772 kJ gHCm 116582)( 104 =×=∴ Methane: 1 mol releases 890 kJ gCHm 16161)( 4 =×=∴

3. Calculate kJ of energy released per gram of fuel:

Butane: 11 76.491165772)( −− == kJgkJgEnergy

Methane: 11 6.5516890)( −− == kJgkJgEnergy

Methane has a slightly higher energy per unit mass.

Note: Given the density of each fuel, the amount of energy per unit volume can be calculated by

using the following formula: Density

massvolumevolumemassDensity =⇒=


UNIT 3 CHEMISTRY

THERMOCHEMICAL EQUATIONS – TOPIC TEST 1

QUESTION 1 Which of the following best represents a thermochemical equation? A OHOH 222 22 →+ B 1

2 2( ) ( ) 44H O l H O g H kJmol−→ ∆ = + C 1

2 2( ) ( ) 44H O l H O g H kJmol−→ ∆ = − D )()( 22 lOHgOH → QUESTION 2 Consider the following reaction:

N2(g) + 3H2(g) ⇒ 2NH3(g) ∆H= –92.4Kj mol-1

The ∆H value above indicates that: A The reaction is endothermic. B The reaction is exothermic. C The reaction can only proceed at high temperatures. D The reaction can only proceed at low temperatures. QUESTION 3 For the reaction:

N2(g) + 3H2(g) ⇒ 2NH3(g) ∆H= –92.4Kj mol-1

A 92.4Kj of energy are absorbed for every mole of ammonia formed. B 92.4Kj of energy are released for every mole of ammonia formed. C 92.4Kj of energy are absorbed for every 2 moles of ammonia formed. D 92.4Kj of energy are released for every 2 mole of ammonia formed.

QUESTION 4 The thermochemical equation: 1

2 161);(2)()( −+=∆→+ kJmolHgCOsCgCO indicates that: A When 2 mole of CO form from carbon dioxide and solid carbon, 161 kJ of energy is

absorbed

B For every 1 mole of CO formed 161 kJ of energy is absorbed

C When 2 mole of CO form from carbon dioxide and solid carbon, 161 kJ of energy is released

D For every 1 mole of CO formed 161 kJ of energy is released


QUESTION 5 The heat of formation for the reaction )(3)(2)(8 812 ggs SOOS →+ is –395.7 1kJ.mol− of 3SO . Which of the equations below represents this equation? A )(3)(2)(8 812 ggs SOOS →+ +3166 kJ B )(3)(2)(8 812 ggs SOOS →+ - 397.7 kJ C )(3)(2)(8 812 ggs SOOS →+ + 397.7 kj D )(3)(2)(8 8kJ 316612 ggs SOOS →++ QUESTION 6 What does it mean if the ΔH value for a reaction is positive? A the kinetic energy of the system is increasing B the potential energy of the system is decreasing C the products have less potential energy than the reactants D the reactants have less potential energy than the products QUESTION 7 When ClNH4 dissolves, the resulting solution is cool to the touch. Which of the following equations best represents the dissolution of ClNH4 ? A −+ +→ )()(4)(4 aqaqs ClNHClNH ΔH = –33.6 kJ/mol

B −+ +→+ )()(4)(4 kJ 33.6 aqaqs ClNHClNH

C −+ +→ )()(4)(4 aqaqs ClNHClNH + 33.6 kJ D Heat is released when ammonium chloride dissolves QUESTION 8 Which is an exothermic process? A Ice melting B Water evaporating C Water boiling D Water vapour condensing QUESTION 9 Which statement is true for the combustion of ethanol?

)(2)(2)(2)(52 323 gggl OHCOOOHHC +→+ ∆H = –1370 kJ/mol A The enthalpy change would be the same is gaseous water were produced. B The potential energy of the products is less than the potential energy of the reactants. C The products of the reaction occupy a larger volume than the reactants. D The reaction is endothermic.


ANSWERS QUESTION 1 Answer is B QUESTION 2 Answer is B QUESTION 3 Answer is D QUESTION 4 Answer is A QUESTION 5 Answer is A QUESTION 6 Answer is D QUESTION 7 Answer is B QUESTION 8 Answer is D QUESTION 9 Answer is B


UNIT 3 CHEMISTRY

THERMOCHEMICAL EQUATIONS – TOPIC TEST 2

QUESTION 1 Consider the following flowchart: Reaction A Reaction B Sun Energy Stored in Plants 2 2,CO H O and heat

If reaction A is photosynthesis and reaction B is respiration, then:

A Both reactions are endothermic B Both reactions are exothermic C Reaction A is endothermic and reaction B is exothermic D None of the above QUESTION 2 Which of the following best represents a thermochemical equation? D OHOH 222 22 →+ E 1

2 2( ) ( ) 44H O l H O g H kJmol−→ ∆ = + F 1

2 2( ) ( ) 44H O l H O g H kJmol−→ ∆ = G )()( 22 lOHgOH →

QUESTION 3 The Thermochemical equation: 1

2 ( ) ( ) 2 ( ) 161CO g C s CO g H kJmol−+ → ∆ = + indicates that: E When 2 mole of CO form from carbon dioxide and solid carbon, 161 kJ of energy is

absorbed

F For every 1 mole of CO formed 161 kJ of energy is absorbed

G When 2 mole of CO form from carbon dioxide and solid carbon, 161 kJ of energy is released

H For every 1 mole of CO formed 161 kJ of energy is released


QUESTION 4 Ethane burns in air according to the following thermochemical equation:

12 6 2 2 22 ( ) 7 ( ) 4 ( ) 6 ( ) 3120C H g O g CO g H O l H kJmol−+ → + ∆ = −

(a) Calculate the amount of heat released when 1 mol of ethane burns in air. (b) Calculate the amount of energy released when 10.5 g of ethane is burnt. (c) Calculate the volume of CO2 produced at SLC when 350 kJ of energy is released.


QUESTION 5 The chemical reaction ++ →+ )(4)()(3 aqaqaq NHHNH is exothermic, with molkJH /0.53−=∆ .

Calculate the amount of heat evolved when ml40 of a M20.0 )(3 aqNH solution is added to

ml40 of a M040.0 solution of )(aqHCl . A 28.5 10 kJ−− ×

B 28.5 10 kJ−×

C 14.2 10 kJ−− ×

D 14.2 10 kJ−× QUESTION 6 Sketch the following energy profiles: (a) An exothermic reaction with a large ∆H. (b) An endothermic reaction with a small ∆H. Solution


QUESTION 7 Two commonly used fuels, methane and ethane burn according to the reactions below: Methane: 1

4 2 2 2( ) 2 ( ) ( ) 2 ( ) 890CH g O g CO g H O l H kJmol−+ → + ∆ = −

Ethane: 12 6 2 2 22 ( ) 7 ( ) 4 ( ) 6 ( ) 3120C H g O g CO g H O l H kJmol−+ → + ∆ = −

Calculate the energy output for each fuel expressed in kilojoules per kilogram of fuel (kJ Kg-1). Solution


SOLUTIONS

QUESTION 1 Answer is C QUESTION 2 Answer is B QUESTION 3 Answer is A QUESTION 4 (a) Since the equation shows the combustion of 2 mole of ethane releasing 3120 kJ:

kJmolHeat 15602

3120)1( ==

(b) molHCn 35.030

5.10)( 62 ==

2 mol release 3120 kJ 0.35 mol release x kJ

kJxx 5462

312035.031202

35.0=

×=∴=

(c) 2 mole release 3120 kJ

x mole release 350 kJ

molxx 224.03120

23503120350

2=

×=∴=

2( @ ) 0.224 24.8 5.56V CO STP L= × =

QUESTION 5 Answer is B QUESTION 6 (a)

ENERGY

∆H


(b) QUESTION 7 For methane: 890 kJ given by 1 mol or 16g.

Convert to Kg: KgKg 016.0100016

==

11 625,55016.0

890)( −− == kJKgkJKgEnergy

For ethane: 3120 kJ given by 2 mole or 60g

KgKg 06.01000

60==

11 000,5206.0

3120)( −− == kJKgkJKgEnergy

This comparison shows that the two fuels have similar energy outputs per kilogram, with methane having a slight advantage.

ENERGY ∆H


ENERGY PROFILES – TOPIC TEST 1

QUESTION 1 The best definition of enthalpy is A The amount of energy needed for one mole of a substance to react. B The energy change that occurs during a chemical reaction C The total kinetic and potential energy contained in a chemical substance D The energy released during a chemical reaction Refer to the following diagram for Questions 2 and 3

QUESTION 2 The above profile represents A An endothermic reaction because the energy of the products is greater than the energy

of the reactants.

B Exothermic because the energy of the reactants is greater than the energy of the products.

C Endothermic because the products have a low energy content.

D Exothermic because the reactants have a high energy content. QUESTION 3 The energy profile indicates that A The reactants are more stable and have stronger bonds on average than the products B The reactants are less stable and have stronger bonds on average than the products C The reactants are more stable and have weaker bonds on average than the products D The reactants are less stable and have weaker bonds on average than the products


Questions 4 8 refer to the following diagram

QUESTION 4 The value of (a) in the diagram represents A The amount of energy needed to break the bonds in the reactants B The activation energy C The amount of energy released when products are formed in the reverse reaction D All of the above QUESTION 5 The activation energy for the forward reaction is A 300 kJ/mol B -300 kJ/mol C 200 kJ/mol D -200 kJ/mol QUESTION 6 The change in enthalpy for the reverse reaction is A 300 kJ/mol B -300 kJ/mol C 200 kJ/mol D -200 kJ/mol QUESTION 7 The activation energy for the reaction 2C 2A + 2B is A 800 kJ/mol B -800 kJ/mol C 200 kJ/mol D -200 kJ/mol

kJ/mol A + B C


QUESTION 8 The value of (b) represents A the energy released when bonds are formed in the products B the energy absorbed when bonds are formed in the products C the change in enthalpy D the activation energy QUESTION 9 The enthalpy change and activation energy in kJ/mol for the reverse reaction of the system described above are A molkJEmolkJH A /35,/25 =−=∆ B molkJEmolkJH A /60,/25 =−=∆ C molkJEmolkJH A /35,/25 ==∆ D molkJEmolkJH A /60,/25 ==∆


The following information refers to Questions 10 12. The diagram below represents an energy profile for the formation of carbon monoxide from carbon dioxide and carbon: )()(2)( 2 ggs COCOC →+ .

QUESTION 10 The H∆ for the forward reaction in 1−molkJ is A 170− B 220− C 170 D 220 QUESTION 11 The activation energy in 1−molkJ for the reverse reaction is A 220− B 50− C 50 D 220 QUESTION 12 If a catalyst was used in the reaction, which of the following values would remain unchanged? A the activation energy B the energy released when bonds are formed in the products C the change in enthalpy D the activation energy for the reverse reaction

-220

-390

-170

Energy (kJ/mol)

CCO +2

CO2


QUESTION 13 In a coal fired power station, coal is burnt to generate electricity. The transfer of energy can be represented in a flow chart. A B C Which of the following statements is true? A Process A is exothermic, and Process B is exothermic B Process A is exothermic, and Process C is exothermic C Process A is endothermic, and Process B is exothermic D Process A is endothermic, and Process C is exothermic QUESTION 14 The following diagram represents the energy changes as a chemical reaction proceeds The diagram represents an A exothermic reaction and the chemical energy of the reactants is greater than the

products

B exothermic reaction and the chemical energy of the products is greater than the reactants

C endothermic reaction and the chemical energy of the reactants is greater than the products

D endothermic reaction and the chemical energy of the products is greater than the reactants

QUESTION 15 Which of the following molecules has the greater activation energy barrier? A ethane, C2H6 B butane, C4H10 C hexane, C6H14 D octane, C8H18

Coal

is Burnt

Water Boils

Steam is

Condensed

Steam is used to spin turbines

Reactants

Product


QUESTION 16 Consider the following two reaction profiles:

Reaction A

Energy

Carbon dioxide water

Glucose + oxygen

Reaction B

Energy

Glucose + Oxygen

Carbon Dioxide + Water

(a) Name each of the two reactions. (b) Outline the biological significance of each reaction.


QUESTION 17 Iron can react with carbon dioxide to form iron oxide and carbon monoxide

(a) On the set of axes below, sketch an energy level diagram to show how the energy of the reactants is related to that of the products. If the activation energy is 40 kJ mol -1, label and show values for H and the activation energy on your diagram. (4 marks) (b) Is the activation energy greater for the forward reaction or the back reaction? (1 mark) (c) Are the reactants or the products of this reaction more stable? Explain this in terms of activation energy. (2 marks)


ANSWERS

QUESTION 1 Answer is C QUESTION 2 Answer is B QUESTION 3 Answer is D QUESTION 4 Answer is D QUESTION 5 Answer is A QUESTION 6 Answer is D QUESTION 7 Answer is C QUESTION 8 Answer is A QUESTION 9 Answer is D QUESTION 10 Answer is C QUESTION 11 Answer is C QUESTION 12 Answer is C QUESTION 13 Answer is B QUESTION 14 Answer is D QUESTION 15 Answer is D QUESTION 16 (a) Reaction A is photosynthesis, reaction B is respiration. (b) Photosynthesis creates high-energy molecules such as glucose, which maintains the food chain through plants.

Respiration releases energy from high-energy molecules, the process is essential for functioning and growth of most living organisms.


QUESTION 17 (a) (b) Forward reaction (c) Reactant molecules are more stable because it takes more energy to break the bonds

kJ/mol

40 kJ/mol

26 kJ/mol

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THERMOCHEMISTRY THERMOCHEMICAL EQUATIONS · 2021. 4. 30. · The School For Excellence 20 Unit 3 & 4 Chemistry 21 Notes & Tests – Page 1 THERMOCHEMISTRY . THERMOCHEMICAL EQUATIONS