CH5 Enthalpy Definitions

Question 1

Define Enthalpy

Answer 1

Enthalpy (H) is a measure of the amount of heat energy contained in a substance. It is stored in the chemical bonds and intermolecular forces as potential energy. (Additionally : When substances react, the difference in the enthalpy between the reactants and products (at constant pressure) results in a heat change which can be observed.)

Question 2

Define exothermic reactions and endothermic reactions

Answer 2

For exothermic reactions heat is given out by
the system and ∆H is negative.

For endothermic reactions heat is absorbed by the system and ∆H is positive.

Question 3

What are the standard conditions for enthalpy changes

Answer 3

The standard conditions for enthalpy changes are:
* a pressure of 100 kPa
* concentrations of 1 mol dm–3 for all solutions
* all the substances in their standard states.

Question 4

Define Standard State of a substance

Answer 4

The standard state of a substance is the pure form of the substance under standard conditions of 298 K (25 °C) and 1.00 × 10^5 Pa.

Question 5

Define Specific heat capacity

Answer 5

The specific heat capacity (c) is defined as the heat needed to increase the temperature of unit mass of material by 1 K

Question 6

Define heat capacity

Answer 6

The heat capacity (C) is defined as the heat needed to increase the temperature of an object by 1 K

Question 7

Define standard enthalpy of combustion

Answer 7

The standard enthalpy change of combustion (∆Hc⊖) is the enthalpy change for the complete combustion of one mole of a substance in its standard state in excess oxygen under standard conditions.

Question 8

Define Hess’s Law

Answer 8

Hess’s law states that the enthalpy change for any chemical reaction is independent of the route, provided the starting conditions and final conditions, and reactants and products, are the same.

Question 9

Define standard enthalpy of formation and the corresponding equation to find enthalpy change.

Answer 9

The standard enthalpy of formation of a substance is the enthalpy change that occurs when one mole of the substance is formed from its elements in their standard states under standard conditions of 298 K (25 °C) and 1.00 × 10^5 pa

∆H reaction = ∑∆Hf⊖ (products) – ∑∆Hf⊖ (reactants)

Question 10

Define average bond enthalpy

Answer 10

The average bond enthalpy is the energy needed to break one mole of bonds in gaseous molecules under standard conditions averaged over similar compound

Question 11

Define free radical

Answer 11

A free radical is a species with an unpaired electron

Question 12

Define first ionization energy

Answer 12

The first ionization energy is the minimum energy required to remove one mole of electrons from one mole of gaseous atoms.

Question 13

Define first electron affinity

Answer 13

The first electron affinity is the enthalpy change when one mole of gaseous electrons is added to one mole of gaseous atoms.

Question 14

Define lattice enthalpy

Answer 14

The lattice enthalpy is the enthalpy change that occurs when one mole of a solid ionic compound is separated into gaseous ions under standard conditions.

Question 15

Define enthalpy change of atomization

Answer 15

The enthalpy change of atomization ∆H⊖atom
is the enthalpy change that occurs when one mole of gaseous atoms is formed from the element in its standard state.

Question 16

Define enthalpy change of solution

Answer 16

The enthalpy change of solution is the enthalpy change when one mole of a solute is dissolved in a solvent to infinite dilution under standard conditions of temperature (298 K) and pressure (1.0 × 105 Pa).

Question 17

Define enthalpy change of hydration

Answer 17

The enthalpy change of hydration of an ion is the enthalpy change that occurs when one mole of gaseous ions is dissolved to form an infinitely dilute solution of one mole of aqueous ions under standard conditions of temperature and pressure