Chapter 9 - Enthalpy

Question 1

What is the law of conservation of energy ?

Answer 1

Energy cannot be created or destroyed, just converted from one form to another.

Question 2

Define enthalpy

Answer 2

The heat content that is stored in a chemical system.

Question 3

Define enthalpy change

Answer 3

• The heat exchanged with the surroundings during a chemical reaction.
• The difference between the enthalpy of the products and the reactants.

Question 4

Define exothermic

Answer 4

A reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings.

Question 5

Define endothermic

Answer 5

A reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken out of the surroundings.

Question 6

Define activation energy

Answer 6

The minimum energy required for a reaction to take place.

Question 7

What are the standard conditions ?

Answer 7

• Standard pressure - 100kPa
• Standard temperature - 298K
• Standard concentration - 1mol/dm^3
• Standard state - The physical state of a substance under standard conditions

Question 8

Define standard enthalpy change of reaction

Answer 8

The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states.

Question 9

Define standard enthalpy change of formation

Answer 9

The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states.

Question 10

Define standard enthalpy change of combustion

Answer 10

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.

Question 11

Define standard enthalpy change of neutralisation

Answer 11

The enthalpy change that accompanies the reaction of an acid by a base to from one mole of H(2)O, under standard conditions, with all reactants and products in their standard states.

Question 12

What is the equation to measure an energy change ?

Answer 12

• Q=mcΔT
• Q - energy change with surroundings (J)
• m - mass (g)
• c - specific heat capacity (J/g/K)
• ΔT - change in temperature (K)

Question 13

How is enthalpy change worked out from the energy change?

Answer 13

• ΔH = -Q/n
• ΔH - enthalpy change (J/mol)
• Q - energy change with surroundings (J)
• n - number of moles (mol)

Question 14

What are the causes for less energy being transferred than expected when working out ΔH(c) ?

Answer 14

• Heat loss to the surroundings
• Incomplete combustion
• Evaporation
• Non-standard conditions

Question 15

How can heat loss be accounted for using a graph of temperature against time?

Answer 15

Extrapolate the cooling curve back to when it was added.

Question 16

Enthalpy change of neutralisation always has the same value, what is it?

Answer 16

-57.5 kJ/mol

Question 17

Define average bond enthalpy

Answer 17

The energy required to break one mole of a specified type of bond in a gaseous molecule

Question 18

How is average bond enthalpy calculated ?

Answer 18

It is calculated from the actual bond enthalpies in different chemical environments

Question 19

What are some properties of bond enthalpies ?

Answer 19

• Energy is always required to break bonds
• Bond enthalpies are always endothermic
• Bond enthalpies always have a positive enthalpy value

Question 20

What determines whether an overall reaction is exothermic or endothermic ?

Answer 20

The difference between the energy required for bond breaking and the energy released by bond making

Question 21

What kind of energy and enthalpy changes are bond formation ?

Answer 21

They are exothermic and they release energy

Question 22

What is Hess’ law ?

Answer 22

If a reaction can take place by more than one route and the initial and final concentrations are the same, the total energy change is the same for each route.

Question 23

Which idea does Hess’ law come from ?

Answer 23

It comes from the idea of the conservation of energy

Question 24

What can be said about the routes using Hess’ law ?

Answer 24

The total enthalpy change is the same for each route

Question 25

Which direction do the arrows go in the enthalpy change of combustion ?

Answer 25

They go down from the reactants and products to the elements

Question 26

Which direction do the arrows go in the enthalpy change of formation ?

Answer 26

They go up from the elements to the reactants and products

Question 27

Enthalpy change of formation routes

Answer 27

R1 : B + A
R2 : C

A + B = C
A = C - B

Question 28

Enthalpy change of combustion routes

Answer 28

R1 : C + A
R2 : B

A + C = B
A = B - C