Energy Changes, Exothermic/Endothermic Reactions and Bond Energy Calculations

1. A reaction mixture becomes warmer during a chemical reaction.

Which statement is correct?

A The reaction is endothermic because energy is taken in from the surroundings.
B The reaction is exothermic because energy is transferred to the surroundings.
C The reaction is endothermic because the products have less energy than the reactants.
D The reaction is exothermic because bonds are broken without any bonds forming.

Answer: B

A wrong: Endothermic reactions usually make surroundings colder.
B right: Temperature rises because energy is transferred to surroundings.
C wrong: Products lower than reactants means exothermic.
D wrong: Bonds are broken and bonds are formed in chemical reactions.

2. A student dissolves ammonium nitrate in water. The temperature of the water decreases.

Which statement explains this?

A More energy is released when bonds form than is absorbed when bonds break.
B More energy is absorbed than released overall.
C No energy change occurs because dissolving is physical.
D The surroundings gain thermal energy from the solution.

Answer: B

A wrong: That would make the process exothermic.
B right: The process absorbs more energy than it releases.
C wrong: Physical changes can still involve energy changes.
D wrong: The surroundings lose thermal energy, so temperature falls.

3. Which process is endothermic?

A combustion of methane
B neutralisation of hydrochloric acid with sodium hydroxide
C thermal decomposition of calcium carbonate
D respiration of glucose

Answer: C

A wrong: Combustion is exothermic.
B wrong: Neutralisation is exothermic.
C right: Thermal decomposition needs heat energy.
D wrong: Respiration releases energy.

4. Which process is exothermic?

A photosynthesis
B cracking of hydrocarbons
C reaction of magnesium with hydrochloric acid
D electrolysis of molten lead(II) bromide

Answer: C

A wrong: Photosynthesis absorbs energy.
B wrong: Cracking requires heat.
C right: Magnesium reacting with acid releases heat.
D wrong: Electrolysis requires electrical energy.

5. Which statement about bond breaking and bond forming is correct?

A Bond breaking releases energy and bond forming absorbs energy.
B Bond breaking absorbs energy and bond forming releases energy.
C Both bond breaking and bond forming release energy.
D Both bond breaking and bond forming absorb energy.

Answer: B

A wrong: Reversed.
B right: Energy is absorbed to break bonds and released when bonds form.
C wrong: Bond breaking absorbs energy.
D wrong: Bond forming releases energy.

6. A reaction has these energy changes.

Energy absorbed to break bonds = 820 kJ/mol
Energy released when bonds form = 950 kJ/mol

What is the overall energy change?

A –130 kJ/mol, exothermic
B +130 kJ/mol, endothermic
C –1770 kJ/mol, exothermic
D +1770 kJ/mol, endothermic

Answer: A

Energy change = energy absorbed – energy released
= 820 – 950
= –130 kJ/mol

A right: Negative value means exothermic.
B wrong: Sign is wrong.
C wrong: Adds both values.
D wrong: Adds both values and uses wrong sign.

7. In an energy profile diagram for an exothermic reaction, which statement is correct?

A Products are at a higher energy level than reactants.
B Products are at a lower energy level than reactants.
C Activation energy is zero.
D The reaction pathway must go downward first.

Answer: B

A wrong: That describes an endothermic reaction.
B right: Exothermic products have less energy than reactants.
C wrong: Activation energy is not zero.
D wrong: Energy profile rises first to the activation energy peak.

8. In an energy profile diagram for an endothermic reaction, which statement is correct?

A Products have less energy than reactants.
B Products have more energy than reactants.
C The reaction releases energy to surroundings overall.
D Bond forming releases more energy than bond breaking absorbs.

Answer: B

A wrong: That is exothermic.
B right: Endothermic products have more energy than reactants.
C wrong: Endothermic reactions absorb energy.
D wrong: In endothermic reactions, bond breaking absorbs more energy than bond forming releases.

9. Which statement about activation energy is correct?

A It is the total energy released by a reaction.
B It is the minimum energy particles need for successful reaction.
C It is always equal to the overall enthalpy change.
D It is the energy difference between products and reactants only.

Answer: B

A wrong: That is not activation energy.
B right: Activation energy is the minimum energy needed for particles to react successfully.
C wrong: Activation energy is not the same as energy change.
D wrong: That describes overall energy change.

10. A catalyst increases the rate of a reaction.

Which statement about the energy profile is correct?

A The catalyst lowers the activation energy.
B The catalyst increases the overall energy change.
C The catalyst makes an endothermic reaction exothermic.
D The catalyst raises the energy of the products only.

Answer: A

A right: A catalyst provides an alternative route with lower activation energy.
B wrong: Catalysts do not change overall energy change.
C wrong: Catalysts do not change whether a reaction is exothermic or endothermic.
D wrong: The products’ energy is unchanged.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

11. The combustion of hydrogen is shown.

2H2 + O2 → 2H2O

Bond energies are shown.

H–H = 436 kJ/mol
O=O = 498 kJ/mol
O–H = 464 kJ/mol

What is the energy change for the reaction?

A –486 kJ
B –242 kJ
C +486 kJ
D +1856 kJ

Answer: A

Bonds broken = 2(H–H) + 1(O=O)
= 2(436) + 498
= 1370 kJ

Bonds formed = 4(O–H)
= 4(464)
= 1856 kJ

Energy change = 1370 – 1856
= –486 kJ

A right: Exothermic.
B wrong: Half the value.
C wrong: Sign is wrong.
D wrong: This is energy released forming bonds only.

12. Methane burns completely in oxygen.

CH4 + 2O2 → CO2 + 2H2O

Bond energies are shown.

C–H = 413 kJ/mol
O=O = 498 kJ/mol
C=O = 805 kJ/mol
O–H = 464 kJ/mol

What is the energy change for the reaction?

A –818 kJ/mol
B –346 kJ/mol
C +818 kJ/mol
D +2648 kJ/mol

Answer: A

Bonds broken = 4(C–H) + 2(O=O)
= 4(413) + 2(498)
= 1652 + 996
= 2648 kJ

Bonds formed = 2(C=O) + 4(O–H)
= 2(805) + 4(464)
= 1610 + 1856
= 3466 kJ

Energy change = 2648 – 3466
= –818 kJ/mol

A right.
B wrong: Incorrect value.
C wrong: Sign is wrong.
D wrong: Energy absorbed breaking bonds only.

13. Ethene reacts with hydrogen.

C2H4 + H2 → C2H6

Bond energies are shown.

C=C = 612 kJ/mol
C–C = 347 kJ/mol
C–H = 413 kJ/mol
H–H = 436 kJ/mol

Which energy change is correct?

A –125 kJ/mol
B +125 kJ/mol
C –265 kJ/mol
D +265 kJ/mol

Answer: A

Bonds broken = C=C + H–H
= 612 + 436
= 1048 kJ

Bonds formed = C–C + 2(C–H)
= 347 + 2(413)
= 1173 kJ

Energy change = 1048 – 1173
= –125 kJ/mol

A right.
B wrong: Sign is wrong.
C wrong: Wrong value.
D wrong: Wrong sign and value.

14. Chlorine reacts with hydrogen.

H2 + Cl2 → 2HCl

Bond energies are shown.

H–H = 436 kJ/mol
Cl–Cl = 242 kJ/mol
H–Cl = 431 kJ/mol

What is the energy change?

A –184 kJ/mol
B +184 kJ/mol
C –247 kJ/mol
D +247 kJ/mol

Answer: A

Bonds broken = H–H + Cl–Cl
= 436 + 242
= 678 kJ

Bonds formed = 2(H–Cl)
= 2(431)
= 862 kJ

Energy change = 678 – 862
= –184 kJ/mol

A right.
B wrong: Sign is wrong.
C wrong: Wrong value.
D wrong: Wrong sign and value.

15. Nitrogen reacts with hydrogen.

N2 + 3H2 → 2NH3

Bond energies are shown.

N≡N = 944 kJ/mol
H–H = 436 kJ/mol
N–H = 391 kJ/mol

What is the energy change?

A –94 kJ/mol
B +94 kJ/mol
C –1173 kJ/mol
D +1173 kJ/mol

Answer: A

Bonds broken = N≡N + 3(H–H)
= 944 + 3(436)
= 944 + 1308
= 2252 kJ

Bonds formed = 6(N–H)
= 6(391)
= 2346 kJ

Energy change = 2252 – 2346
= –94 kJ/mol

A right.
B wrong: Sign is wrong.
C wrong: Uses only formed bonds badly.
D wrong: Sign and value wrong.

16. Which calculation correctly gives the energy change using bond energies?

A energy change = energy released forming bonds – energy absorbed breaking bonds
B energy change = energy absorbed breaking bonds – energy released forming bonds
C energy change = energy of products + energy of reactants
D energy change = activation energy – catalyst energy

Answer: B

A wrong: Reversed.
B right: ΔH = bonds broken – bonds formed.
C wrong: That is not bond energy calculation.
D wrong: Catalyst energy is not used.

17. A reaction has energy change –75 kJ/mol.

Which statement is correct?

A Reactants have 75 kJ/mol less energy than products.
B Products have 75 kJ/mol less energy than reactants.
C The reaction is endothermic.
D The surroundings become colder.

Answer: B

A wrong: Reversed.
B right: Negative energy change means products are lower in energy.
C wrong: Negative means exothermic.
D wrong: Surroundings usually become warmer.

18. A reaction has energy change +210 kJ/mol.

Which statement is correct?

A It is exothermic and products have lower energy.
B It is endothermic and products have higher energy.
C It is exothermic and products have higher energy.
D It is endothermic and products have lower energy.

Answer: B

A wrong: Positive energy change is not exothermic.
B right: Endothermic products are higher in energy.
C wrong: Exothermic reactions have negative energy changes.
D wrong: Products are higher, not lower.

19. Which pair contains only exothermic changes?

A combustion and neutralisation
B thermal decomposition and photosynthesis
C electrolysis and cracking
D evaporation and melting

Answer: A

A right: Both release energy.
B wrong: Both are endothermic.
C wrong: Both require energy.
D wrong: Both are endothermic physical changes.

20. Which pair contains only endothermic changes?

A freezing and condensation
B combustion and respiration
C thermal decomposition and photosynthesis
D neutralisation and precipitation

Answer: C

A wrong: Both are exothermic.
B wrong: Both are exothermic.
C right: Both absorb energy.
D wrong: Neutralisation is exothermic.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

21. In a calorimetry experiment, 0.500 g of ethanol is burned and heats 100 g of water from 21.0°C to 47.0°C.

[Specific heat capacity of water = 4.2 J/g°C]

How much energy is transferred to the water?

A 10.9 kJ
B 5460 kJ
C 10 920 kJ
D 5.46 kJ

Answer: A

Energy = mcΔT
= 100 x 4.2 x 26
= 10 920 J
= 10.9 kJ

A right.
B wrong: Incorrect unit/value.
C wrong: 10 920 kJ is far too high.
D wrong: Half the correct value.

22. 200 g of water is heated from 18°C to 33°C.

[Specific heat capacity of water = 4.2 J/g°C]

What is the energy transferred?

A 12.6 kJ
B 25.2 kJ
C 2520 kJ
D 12 600 kJ

Answer: A

Energy = mcΔT
= 200 x 4.2 x 15
= 12 600 J
= 12.6 kJ

A right.
B wrong: Doubles the answer.
C wrong: Unit/value wrong.
D wrong: 12 600 kJ is far too high.

23. 50.0 cm3 of water is heated from 20.0°C to 28.4°C.

[Density of water = 1.00 g/cm3; specific heat capacity = 4.2 J/g°C]

What is the energy transferred to the water?

A 1764 J
B 420 J
C 352.8 J
D 17 640 J

Answer: A

Mass of water = 50.0 g
ΔT = 8.4°C

Energy = 50.0 x 4.2 x 8.4
= 1764 J

A right.
B wrong: Uses wrong temperature change.
C wrong: Too low.
D wrong: Ten times too high.

24. In a simple spirit burner experiment, the calculated energy released by the fuel is much lower than the data book value.

Which reason best explains this?

A Some heat is lost to the surroundings.
B The water absorbs more heat than released by the fuel.
C The fuel does not combust.
D The thermometer produces heat.

Answer: A

A right: Heat loss to air, container and surroundings lowers the experimental value.
B wrong: Water cannot absorb more heat than the fuel releases.
C wrong: Fuel combusts, but heat transfer is inefficient.
D wrong: The thermometer does not produce heat.

25. Which improvement would make a simple combustion calorimetry experiment more accurate?

A use a larger flame and remove the lid
B stir the water and use a draught shield
C hold the flame further from the beaker
D use a plastic beaker directly above the flame

Answer: B

A wrong: Increases heat loss.
B right: Stirring spreads heat evenly and a draught shield reduces heat loss.
C wrong: More heat is lost before reaching water.
D wrong: Plastic beaker may melt and is unsuitable.

26. A student burns 0.230 g of ethanol, C2H5OH, and 11.5 kJ of energy is transferred to water.

[Mr of ethanol = 46]

What is the energy released per mole of ethanol burned?

A 230 kJ/mol
B 1150 kJ/mol
C 2300 kJ/mol
D 46 000 kJ/mol

Answer: C

Moles ethanol = 0.230 / 46
= 0.00500 mol

Energy per mole = 11.5 / 0.00500
= 2300 kJ/mol

A wrong: Ten times too low.
B wrong: Half the correct value.
C right.
D wrong: Far too high.

27. 0.120 g of methanol, CH3OH, releases 2.73 kJ of energy.

[Mr of methanol = 32]

What is the energy released per mole of methanol?

A 91.0 kJ/mol
B 364 kJ/mol
C 728 kJ/mol
D 22 750 kJ/mol

Answer: C

Moles methanol = 0.120 / 32
= 0.00375 mol

Energy per mole = 2.73 / 0.00375
= 728 kJ/mol

A wrong: Too low.
B wrong: Half the correct value.
C right.
D wrong: Uses mass incorrectly.

28. Which statement about fuels is correct?

A A fuel is useful only if its combustion is endothermic.
B A fuel releases energy when it burns.
C All fuels burn without oxygen.
D Fuels must be ionic compounds.

Answer: B

A wrong: Useful combustion is exothermic.
B right: Fuels release energy during burning.
C wrong: Burning usually requires oxygen.
D wrong: Many fuels are covalent compounds.

29. Which fuel releases only water when burned completely?

A methane
B hydrogen
C ethanol
D carbon monoxide

Answer: B

A wrong: Methane forms carbon dioxide and water.
B right: Hydrogen burns to form water only.
C wrong: Ethanol forms carbon dioxide and water.
D wrong: Carbon monoxide forms carbon dioxide.

30. Which product forms during incomplete combustion of a hydrocarbon and is poisonous?

A carbon monoxide
B carbon dioxide
C water
D oxygen

Answer: A

A right: Carbon monoxide is poisonous and forms during incomplete combustion.
B wrong: Carbon dioxide forms in complete combustion.
C wrong: Water is not the poisonous product.
D wrong: Oxygen is a reactant, not a product.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

31. A reaction pathway diagram has these values.

Energy of reactants = 80 kJ/mol
Energy of highest point = 220 kJ/mol
Energy of products = 30 kJ/mol

What are the activation energy and overall energy change?

	activation energy	overall energy change
A	140 kJ/mol	–50 kJ/mol
B	190 kJ/mol	–50 kJ/mol
C	140 kJ/mol	+50 kJ/mol
D	220 kJ/mol	–190 kJ/mol

Answer: A

Activation energy = 220 – 80 = 140 kJ/mol
Overall energy change = 30 – 80 = –50 kJ/mol

A right.
B wrong: Activation energy is wrong.
C wrong: Sign of energy change is wrong.
D wrong: Both values wrong.

32. A reaction pathway diagram has these values.

Energy of reactants = 50 kJ/mol
Energy of highest point = 180 kJ/mol
Energy of products = 120 kJ/mol

Which statement is correct?

A activation energy = 130 kJ/mol and reaction is exothermic
B activation energy = 130 kJ/mol and reaction is endothermic
C activation energy = 60 kJ/mol and reaction is endothermic
D activation energy = 70 kJ/mol and reaction is exothermic

Answer: B

Activation energy = 180 – 50 = 130 kJ/mol
Energy change = 120 – 50 = +70 kJ/mol, so endothermic.

A wrong: Correct activation energy but wrong reaction type.
B right.
C wrong: Activation energy is not 60.
D wrong: Both wrong.

33. A catalyst changes an energy profile.

Uncatalysed activation energy = 90 kJ/mol
Catalysed activation energy = 35 kJ/mol
Overall energy change = –25 kJ/mol

What is the overall energy change for the catalysed reaction?

A –25 kJ/mol
B +25 kJ/mol
C –55 kJ/mol
D +35 kJ/mol

Answer: A

A right: A catalyst does not change the overall energy change.
B wrong: Sign does not change.
C wrong: Confuses activation energy reduction with ΔH.
D wrong: Uses catalysed activation energy.

34. Which statement about a catalyst is correct?

A It is used up in the reaction.
B It provides an alternative route with lower activation energy.
C It increases the energy released by exothermic reactions.
D It increases the energy absorbed by endothermic reactions.

Answer: B

A wrong: Catalysts are chemically unchanged overall.
B right: Catalysts lower activation energy.
C wrong: They do not change energy released.
D wrong: They do not change overall energy absorbed.

35. A reaction is exothermic but does not start until a spark is supplied.

Why is a spark needed?

A The reaction is actually endothermic.
B The spark provides activation energy.
C The spark changes products into reactants.
D Exothermic reactions cannot occur at room temperature.

Answer: B

A wrong: It can still be exothermic.
B right: The spark provides the activation energy needed to start the reaction.
C wrong: The spark does not reverse the reaction.
D wrong: Some exothermic reactions occur at room temperature.

36. Which reaction has the largest activation energy?

reaction	energy of reactants / kJ/mol	highest point / kJ/mol
A	40	90
B	80	160
C	120	170
D	200	240

A A
B B
C C
D D

Answer: B

Activation energy:

A: 90 – 40 = 50
B: 160 – 80 = 80
C: 170 – 120 = 50
D: 240 – 200 = 40

B right: Largest activation energy is 80 kJ/mol.

37. Which reaction is most exothermic?

reaction	energy of reactants / kJ/mol	energy of products / kJ/mol
A	100	20
B	100	70
C	50	10
D	80	100

A A
B B
C C
D D

Answer: A

Energy change = products – reactants.

A: 20 – 100 = –80
B: 70 – 100 = –30
C: 10 – 50 = –40
D: 100 – 80 = +20

Most exothermic = most negative = A.

38. Which reaction is most endothermic?

reaction	energy of reactants / kJ/mol	energy of products / kJ/mol
A	20	90
B	60	100
C	120	130
D	150	110

A A
B B
C C
D D

Answer: A

Energy change = products – reactants.

A: 90 – 20 = +70
B: 100 – 60 = +40
C: 130 – 120 = +10
D: 110 – 150 = –40

Most endothermic = largest positive = A.

39. A reaction has an activation energy of 120 kJ/mol and an energy change of –40 kJ/mol.

What is the activation energy for the reverse reaction?

A 40 kJ/mol
B 80 kJ/mol
C 120 kJ/mol
D 160 kJ/mol

Answer: D

Forward reaction is exothermic by 40 kJ/mol.
Reverse activation energy = 120 + 40
= 160 kJ/mol

A wrong: Uses ΔH only.
B wrong: Subtracts instead of adding.
C wrong: Same as forward activation energy.
D right.

40. A reaction has an activation energy of 75 kJ/mol and an energy change of +25 kJ/mol.

What is the activation energy for the reverse reaction?

A 25 kJ/mol
B 50 kJ/mol
C 75 kJ/mol
D 100 kJ/mol

Answer: B

Forward reaction is endothermic by 25 kJ/mol.
Reverse activation energy = 75 – 25
= 50 kJ/mol

A wrong: Uses ΔH only.
B right.
C wrong: Same as forward activation energy.
D wrong: Adds instead of subtracting.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.

41. The reaction between nitrogen and oxygen is shown.

N2 + O2 → 2NO

Bond energies are shown.

N≡N = 944 kJ/mol
O=O = 498 kJ/mol
N=O = 607 kJ/mol

What is the energy change?

A –228 kJ/mol
B +228 kJ/mol
C –1214 kJ/mol
D +1214 kJ/mol

Answer: B

Bonds broken = N≡N + O=O
= 944 + 498
= 1442 kJ

Bonds formed = 2(N=O)
= 2(607)
= 1214 kJ

Energy change = 1442 – 1214
= +228 kJ/mol

A wrong: Sign is wrong.
B right.
C wrong: Uses bonds formed only.
D wrong: Uses wrong calculation.

42. Hydrogen peroxide decomposes.

2H2O2 → 2H2O + O2

Bond energies are shown.

O–H = 464 kJ/mol
O–O = 146 kJ/mol
O=O = 498 kJ/mol

Each H2O2 molecule contains two O–H bonds and one O–O bond.

What is the energy change for the equation as written?

A –206 kJ
B +206 kJ
C –498 kJ
D +498 kJ

Answer: A

Bonds broken in 2H2O2:
4(O–H) + 2(O–O)
= 4(464) + 2(146)
= 1856 + 292
= 2148 kJ

Bonds formed:
4(O–H) + 1(O=O)
= 4(464) + 498
= 1856 + 498
= 2354 kJ

Energy change = 2148 – 2354
= –206 kJ

A right.
B wrong: Sign is wrong.
C wrong: Uses O=O value incorrectly.
D wrong: Wrong sign and value.

43. Ethanol combusts completely.

C2H5OH + 3O2 → 2CO2 + 3H2O

Bond energies are shown.

C–C = 347 kJ/mol
C–H = 413 kJ/mol
C–O = 358 kJ/mol
O–H = 464 kJ/mol
O=O = 498 kJ/mol
C=O = 805 kJ/mol

In ethanol, CH3CH2OH, there are five C–H bonds, one C–C bond, one C–O bond and one O–H bond.

What is the energy change?

A –1276 kJ/mol
B –837 kJ/mol
C +1276 kJ/mol
D +837 kJ/mol

Answer: A

Bonds broken:
5(C–H) + C–C + C–O + O–H + 3(O=O)
= 5(413) + 347 + 358 + 464 + 3(498)
= 2065 + 347 + 358 + 464 + 1494
= 4728 kJ

Bonds formed:
4(C=O) + 6(O–H)
= 4(805) + 6(464)
= 3220 + 2784
= 6004 kJ

Energy change = 4728 – 6004
= –1276 kJ/mol

A right.
B wrong: Too small.
C wrong: Sign wrong.
D wrong: Sign and value wrong.

44. The reaction between carbon monoxide and hydrogen is shown.

CO + 2H2 → CH3OH

Bond energies are shown.

C≡O = 1077 kJ/mol
H–H = 436 kJ/mol
C–H = 413 kJ/mol
C–O = 358 kJ/mol
O–H = 464 kJ/mol

In methanol, CH3OH, there are three C–H bonds, one C–O bond and one O–H bond.

What is the energy change?

A –112 kJ/mol
B +112 kJ/mol
C –721 kJ/mol
D +721 kJ/mol

Answer: A

Bonds broken:
C≡O + 2(H–H)
= 1077 + 2(436)
= 1949 kJ

Bonds formed:
3(C–H) + C–O + O–H
= 3(413) + 358 + 464
= 1239 + 358 + 464
= 2061 kJ

Energy change = 1949 – 2061
= –112 kJ/mol

A right.
B wrong: Sign wrong.
C wrong: Value wrong.
D wrong: Sign and value wrong.

45. Which reaction is endothermic based on the bond energies shown?

reaction	energy absorbed breaking bonds / kJ/mol	energy released forming bonds / kJ/mol
A	500	900
B	1200	800
C	750	900
D	300	600

A A
B B
C C
D D

Answer: B

Endothermic means energy absorbed > energy released.

A wrong: 500 – 900 = –400, exothermic.
B right: 1200 – 800 = +400, endothermic.
C wrong: 750 – 900 = –150, exothermic.
D wrong: 300 – 600 = –300, exothermic.

46. Which reaction releases the greatest amount of energy?

reaction	energy absorbed breaking bonds / kJ/mol	energy released forming bonds / kJ/mol
A	300	450
B	600	950
C	900	1100
D	400	700

A A
B B
C C
D D

Answer: B

Energy released overall = released – absorbed.

A: 450 – 300 = 150
B: 950 – 600 = 350
C: 1100 – 900 = 200
D: 700 – 400 = 300

B right: Greatest energy released.

47. Which statement explains why most combustion reactions are exothermic?

A More energy is absorbed breaking fuel and oxygen bonds than is released forming product bonds.
B More energy is released forming product bonds than is absorbed breaking reactant bonds.
C No bonds are broken during combustion.
D Oxygen bonds are never broken during combustion.

Answer: B

A wrong: That would make combustion endothermic.
B right: Product bond formation releases more energy than reactant bond breaking absorbs.
C wrong: Bonds are broken during combustion.
D wrong: Oxygen bonds are broken.

48. Which change is always endothermic?

A forming a chemical bond
B breaking a chemical bond
C neutralising an acid
D burning a hydrocarbon

Answer: B

A wrong: Bond forming releases energy.
B right: Breaking a chemical bond always requires energy.
C wrong: Neutralisation is usually exothermic.
D wrong: Burning hydrocarbons is exothermic.

49. Which statement about an endothermic reaction is correct?

A The temperature of surroundings usually increases.
B The products have lower energy than the reactants.
C The reaction absorbs energy from the surroundings.
D The energy released forming bonds is greater than energy absorbed breaking bonds.

Answer: C

A wrong: Surroundings usually cool.
B wrong: Products have higher energy.
C right: Endothermic reactions absorb energy.
D wrong: That describes exothermic reactions.

50. Which row correctly compares exothermic and endothermic reactions?

	exothermic	endothermic
A	takes in energy	gives out energy
B	products higher energy	products lower energy
C	temperature of surroundings usually rises	temperature of surroundings usually falls
D	bond breaking releases energy	bond forming absorbs energy

Answer: C

A wrong: Reversed.
B wrong: Exothermic products are lower; endothermic products are higher.
C right: Exothermic reactions usually warm surroundings; endothermic reactions usually cool surroundings.
D wrong: Bond breaking absorbs energy; bond forming releases energy.

Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total A Grades, 11 World Records and 7 Distinctions, Educate A Change.