1.4 Energetics

Question 1

State what is meant by the term mean bond enthalpy

Answer 1

• energy required to break a given covalent bond
• averaged over a range of compounds

Question 2

Answer 2

bi) 4 -xCH Bonds + C-C bond + C=O bond + 2.5 x O=O bond = +3980
ii) -4 x H-O + -4 x C-O = -4800
iii) = bond broken - bonds made
3980 - 4800 =-820

Question 3

Define the term standard enthalpy of combustion

Answer 3

enthalpy change when 1 mole of a substance is completely burned in oxygen with all products and reactant in their standard state

Question 4

State why the standard enthalpy of formation of oxygen is zero.

Answer 4

by definition enthalpy of formation is from an element

Question 5

Write an equation for the decomposition of hydrazine (N2H4) into ammonia and nitrogen only.

Answer 5

3N2H4 -> 4NH3 + N2

Question 6

Give one reason why the value calculated from mean bond enthalpies is different from the value given in a data book.

Answer 6

Mean bond enthalpies are average values from different compounds

Question 7

Suggest one reason, other than the use of mean bond enthalpies, why a value for the enthalpy of combustion of a liquid alkane is different from the value obtained using the expression

Answer 7

• alkanes is not gaseous as equation relates to gaseous alkanes
• it takes energy to convert it to a gas

Question 8

Suggest one environmental advantage of manufacturing methanol fuel by this reaction.
CO2(g) + 3H2(g) -> CH3OH(g) + H2O(g)

Answer 8

The carbon dioxide (produced on burning methanol) is used up in this reaction

Question 9

Answer 9

Bonds broken - bonds formed
Leads to F-F = 158 kJmol^-1
Student is correct because F-F bond energy is much less

Question 10

Answer 10

Question 11

Answer 11

Product - reactant = -104
iii) Sb is not in its standard state, as is it a solid in standard state, not liquid
iii) Reduction / redox

Question 12

How to remember how you should construct your Hess cycles

Answer 12

Green arrow for combustion
Red for formation

Question 13

Write an equation, including state symbols, for the reaction with an enthalpy change equal to the enthalpy of formation for iron(III) oxide.

Answer 13

2Fe(s) + O2(g) ⟶ Fe2O3(s)

Question 14

Answer 14

Question 15

Answer 15

For enthalpy change it is broken bonds - bonds formed
Make carbon monoxide and steam the products
Final answer is B

Question 16

Answer 16

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