Calculate energy changes from bond energies

Question 1

H₂ + Cl₂ → 2HCl. Bond energies (kJ/mol): H–H 436, Cl–Cl 243, H–Cl 432. Calculate the energy change, in kJ/mol.

Answer kJ/mol

Question 2

In a reaction, 600 kJ is needed to break bonds and 450 kJ is released when new bonds form. Is the reaction exothermic or endothermic?

• A. exothermic
• B. endothermic

Question 3

Why is breaking bonds endothermic?

• A. Atoms repel each other
• B. Bonds contain no energy
• C. Breaking bonds releases heat
• D. Energy must be supplied to overcome the attraction holding atoms together

Question 4

N₂ + 3H₂ → 2NH₃. Bond energies (kJ/mol): N≡N 945, H–H 436, N–H 391. Calculate the energy change, in kJ/mol.

Answer kJ/mol

Question 5

In a reaction, 550 kJ is needed to break bonds and 670 kJ is released when new bonds form. Is the reaction exothermic or endothermic?

• A. exothermic
• B. endothermic

Question 6

Hydrogen fuel cells combine hydrogen and oxygen. Why do they release energy overall?

• A. More energy is released forming O–H bonds than is needed to break H–H and O=O
• B. No bonds are broken
• C. Water absorbs energy
• D. Hydrogen is radioactive

Question 7

H₂ + Br₂ → 2HBr. Bond energies (kJ/mol): H–H 436, Br–Br 193, H–Br 366. Calculate the energy change, in kJ/mol.

Answer kJ/mol

Question 8

In a reaction, 1050 kJ is needed to break bonds and 1170 kJ is released when new bonds form. Is the reaction exothermic or endothermic?

• A. exothermic
• B. endothermic

Question 9

Why is breaking bonds endothermic?

• A. Bonds contain no energy
• B. Breaking bonds releases heat
• C. Atoms repel each other
• D. Energy must be supplied to overcome the attraction holding atoms together

Question 10

N₂ + 3H₂ → 2NH₃. Bond energies (kJ/mol): N≡N 945, H–H 436, N–H 391. Calculate the energy change, in kJ/mol.

Answer kJ/mol

Question 11

In a reaction, 1750 kJ is needed to break bonds and 1670 kJ is released when new bonds form. Is the reaction exothermic or endothermic?

• A. exothermic
• B. endothermic

Question 12

In an exothermic reaction, how do the energies of bonds broken and bonds made compare?

• A. more energy is released making bonds than is taken in breaking bonds
• B. no bonds are made
• C. they are always equal
• D. breaking bonds releases more energy

Question 13

2H₂ + O₂ → 2H₂O. Bond energies (kJ/mol): H–H 436, O=O 498, O–H 464. Calculate the energy change, in kJ/mol.

Answer kJ/mol

Question 14

In a reaction, 600 kJ is needed to break bonds and 450 kJ is released when new bonds form. Is the reaction exothermic or endothermic?

• A. exothermic
• B. endothermic

Question 15

Why is breaking bonds endothermic?

• A. Bonds contain no energy
• B. Energy must be supplied to overcome the attraction holding atoms together
• C. Breaking bonds releases heat
• D. Atoms repel each other

Question 16

2H₂ + O₂ → 2H₂O. Bond energies (kJ/mol): H–H 436, O=O 498, O–H 464. Calculate the energy change, in kJ/mol.

Answer kJ/mol