Pupil sheet · Bond energies

Calculating ΔH from bond energies — questions

Name ______________________ Date ____________

Try every question before you use a nudge. Write in full sentences where you are asked to explain.

1. identify

Supplement. Which expression gives the enthalpy change, ΔH, of a reaction?

Circle one answer.

Energy given out making bonds − energy taken in breaking bonds
Energy taken in breaking bonds + energy given out making bonds
Energy taken in breaking bonds − energy given out, made positive
Energy taken in breaking bonds − energy given out making bonds

Nudge (only if you are stuck): An exothermic reaction gives out more than it takes in, and its ΔH is negative.

2. calculate

Supplement. Hydrogen reacts with bromine: H₂ + Br₂ → 2HBr. Use the bond energies in the table to calculate (a) the energy taken in breaking bonds; (b) the energy given out making bonds; (c) ΔH.

Bond	Bond energy / kJ/mol
H–H	436
Br–Br	193
H–Br	366

(a) Energy in: ______________ unit ________

(b) Energy out: ______________ unit ________

Nudge (only if you are stuck): 2HBr is two molecules, each with one H–Br bond.

3. deduce

Supplement. Hydrogen burns in oxygen: 2H₂ + O₂ → 2H₂O. A water molecule is H–O–H. Deduce the number of (a) H–H bonds broken; (b) O=O bonds broken; (c) O–H bonds made.

(a) H–H bonds broken: ______________ unit ________

(b) O=O bonds broken: ______________ unit ________

Nudge (only if you are stuck): Count the bonds in one molecule, then multiply by the number in front of the formula.

4. calculate

Supplement. Use the bond energies in the table to calculate ΔH for burning hydrogen: 2H₂ + O₂ → 2H₂O.

Bond	Bond energy / kJ/mol
H–H	436
O=O	498
O–H	463

ΔH: ______________ unit ________

Nudge (only if you are stuck): A water molecule is H–O–H, and the 2 in 2H₂O counts molecules.

5. calculate

Supplement. Ammonia is made from nitrogen and hydrogen: N₂ + 3H₂ → 2NH₃. Nitrogen has a triple bond, N≡N, and each ammonia molecule has three N–H bonds. (a) Use the bond energies in the table to calculate ΔH for the equation as written. (b) The equation makes two moles of ammonia. Calculate ΔH for making one mole of ammonia. Give your answer to 3 significant figures.

Bond	Bond energy / kJ/mol
N≡N	945
H–H	436
N–H	391

(a) ΔH for N₂ + 3H₂ → 2NH₃: ______________ unit ________

(b) ΔH for making 1 mol of NH₃: ______________ unit ________

Nudge (only if you are stuck): ΔH belongs to the amounts written in the equation.

6. calculate

Supplement. Methane reacts with chlorine in sunlight: CH₄ + Cl₂ → CH₃Cl + HCl. Count every bond and use the bond energies in the table to calculate (a) the energy taken in breaking all the bonds in the reactants; (b) the energy given out making all the bonds in the products; (c) ΔH.

Displayed formulae: methane, a carbon atom with four C–H bonds, plus chlorine, Cl–Cl, gives chloromethane, a carbon atom with three C–H bonds and one C–Cl bond, plus hydrogen chloride, H–Cl.

Bond	Bond energy / kJ/mol
C–H	412
Cl–Cl	242
C–Cl	338
H–Cl	431

(a) Energy in, every bond in CH₄ and Cl₂: ______________ unit ________

(b) Energy out, every bond in CH₃Cl and HCl: ______________ unit ________

Nudge (only if you are stuck): Chloromethane keeps three of methane's C–H bonds.

7. calculate / explain

Supplement. Hydrogen is made from methane and steam: CH₄ + H₂O → CO + 3H₂. Carbon monoxide has a triple bond, C≡O. (a) Use the bond energies in the table to calculate ΔH. (b) The reaction is carried out at about 900 °C. Use your answer to (a) to explain why the reaction must be heated all the time.

Bond	Bond energy / kJ/mol
C–H	412
O–H	463
C≡O	1077
H–H	436

(a) ΔH: ______________ unit ________

(b) Why the reaction must be heated all the time.

Nudge (only if you are stuck): Count the bonds in one molecule, then multiply by the number in front of the formula.

8. calculate

Supplement. Hydrogen reacts with iodine: H₂ + I₂ → 2HI. ΔH = −9 kJ/mol. Use the bond energies in the table to calculate the bond energy of the H–I bond.

Bond	Bond energy / kJ/mol
H–H	436
I–I	151

H–I bond energy: ______________ unit ________

Nudge (only if you are stuck): Which side of the equation holds the unknown bond, and how many of it are there?

9. identify / calculate

Supplement. A pupil calculates ΔH for burning ethene, C₂H₄ + 3O₂ → 2CO₂ + 2H₂O. Ethene has a C=C double bond and four C–H bonds. Their working: energy in = 348 + 4 × 412 + 498 = 2494 kJ/mol; energy out = 4 × 805 + 4 × 463 = 5072 kJ/mol; ΔH = 5072 − 2494 = +2578 kJ/mol. (a) Identify three mistakes. (b) Calculate the correct ΔH.

Bond	Bond energy / kJ/mol
C–C	348
C=C	612
C–H	412
O=O	498
C=O	805
O–H	463

(b) The correct ΔH: ______________ unit ________

(a) The three mistakes.

Nudge (only if you are stuck): Check the bond energy used for each bond, the number of each bond, and the subtraction.

10. calculate / suggest

Supplement. Ethanol is used as a fuel: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O. (a) Use the displayed formula of ethanol and the bond energies in the table to calculate ΔH. (b) A data book gives ΔH = −1367 kJ/mol for burning ethanol. Suggest one reason why your answer is different.

Displayed formulae: ethanol has a C–C bond, five C–H bonds, a C–O bond and an O–H bond. Oxygen is O=O, carbon dioxide is O=C=O and water is H–O–H. The equation is C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O; one molecule of each is drawn.

Bond	Bond energy / kJ/mol
C–C	348
C–H	412
C–O	358
O–H	463
O=O	498
C=O	805

(a) ΔH: ______________ unit ________

(b) One reason why your answer differs from the data-book value.

Nudge (only if you are stuck): Tick off each bond in the displayed formula of ethanol as you count it.