Enthalpy

Question 1

What is enthalpy change, ΔH?

Answer 1

The heat energy transferred in a reaction at constant pressure.

Question 2

What are the units of ΔH?

Answer 2

Kj mol -1

Question 3

What is standard enthalpy change of reaction, ΔH^-Θr?

Answer 3

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

Question 4

What is standard enthalpy change of formation, ΔH^-Θf?

Answer 4

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.

Question 5

What is standard enthalpy change of combustion, ΔH^-Θc?

Answer 5

The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in their standard states.

Question 6

What are exothermic reactions?

Answer 6

They give out energy to their surroundings.
The products of the reaction end up with less energy than the reactants.
This means the enthalpy change, ΔH, will be negative.

Question 7

What are endothermic reactions?

Answer 7

They take in energy from their surroundings.
This means that the products of the reaction have more energy that the reactants, so the enthalpy change for the reaction, ΔH, is positive.

Question 8

What is the activation energy?

Answer 8

The minimum amount of energy required to begin breaking the reactant bonds and start a chemical reaction.

Question 9

What is the bond dissociation enthalpy?

Answer 9

The amount of energy you need per mole to break the bond in gaseous compounds.

Question 10

Why type of reaction is bond breaking?

Answer 10

You need energy to break bonds- so endothermic.

Question 11

Why type of reaction is bond formation?

Answer 11

Energy is released when bonds are formed- so exothermic.

Question 12

How do you calculate enthalpy change?

Answer 12

Enthalpy change of a reaction = total energy absorbed - total energy released

Question 13

Define bond enthalpy

Answer 13

The amount of energy needed to break one mole of a particular bond in one mole of gaseous molecules.

Question 14

State Hess’s Law.

Answer 14

The total enthalpy change for a chemical reaction is independent of the rate by which the reaction takes place, provided initial and final conditions are the same.

Question 15

Suggest reasons why standard enthalpy changes of combustion determined experimentally are less exothermic than the calculated theoretical values.

Answer 15

Heat released to the surroundings
Incomplete combustion
Non standard conditions

Question 16

Endothermic- bond breaking or making?

Answer 16

You need energy to break bonds, so bond breaking is endothermic (delta H is positive).

Question 17

Exothermic- bond breaking or making?

Answer 17

Energy is released when bonds are formed, so this is exothermic (delta H is negative).

Question 18

What is bond dissociation enthalpy?

Answer 18

You need energy to break the attraction between bonds- stronger bonds take more energy to break. The amount of energy you need per mole is called the bond dissociation enthalpy.
Bond dissociation enthalpies always involve bond breaking in gaseous compounds.

Question 19

Average bond enthalpy definition.

Answer 19

The energy needs to break one mole of bonds in the gas phase, averaged over many different compounds.