A Level Chemistry - Energetics

Front 1

Standard conditions

Back 1

■ 100kPa

■ 298K

■ 1 mol/dm3

Front 2

Enthalpy change

Back 2

heat energy change measured at constant pressure in kJ/mol

Front 3

Standard enthalpy change of combustion

Back 3

Enthalpy change for

- the complete combustion

- of 1 mole of substance

- in oxygen

- under standard conditions

Hint 3

Always negative

Front 4

Standard enthalpy change of reaction

Back 4

Enthalpy change for

- a reaction

- that occurs in the molar quantities given in the chemical equation

- under standard conditions

Front 5

Standard enthalpy change of neutralization

Back 5

Enthalpy change when

- an acid and alkali react

- to form 1 mole of water

- under standard conditions

Hint 5

Always negative

Front 6

Standard enthalpy change of formation

Back 6

Enthalpy change for

- the formation

- of 1 mole of substance

- from the elements in their standard states

- under standard conditions

Hint 6

A more negative enthalpy change of formation produces a more energetically stable compound

Front 7

Mean bond enthalpy

Back 7

Energy required to break 1 mole of bonds in the gas phase, averaged over many different compounds

Hint 7

The specific bond enthalpies of the molecules in the reaction will be slightly different to the mean bond enthalpies used in the calculations

Front 8

Bond enthalpy

Back 8

Amount of energy required to break 1 mole of a type of bond in a molecule in the gas phase

Front 9

Enthalpy change of vaporisation

Back 9

Enthalpy change when

- 1 mole of the liquid

- is converted to gas

- at its boiling point

- with a pressure of 1 bar (100 kPa).

Hint 9

Always positive

Front 10

Enthalpy change of atomisation

Back 10

Enthalpy change for

- the formation

- of 1 mole of gaseous atoms

- from the element in its standard state

Hint 10

Always positive

Given as 'per mole of atoms formed'

Front 11

Enthalpy change of solution

Back 11

Enthalpy change when

- 1 mole of an ionic substance

- dissolves in sufficient water

- to give a solution of infinite dilution

Front 12

First electron affinity

Back 12

Energy released when

- 1 mole of electrons

- are added

- to 1 mole of gaseous atoms

- to form 1 mole of gaseous 1- ions

Hint 12

Always negative

Front 13

Standard lattice formation energy

Back 13

Energy change for

- the formation

- of 1 mole of an ionic solid

- from its gaseous ions

- under standard conditions

Hint 13

Always negative

Lattice enthalpy is a measure of the strength of the ionic bonds in an ionic solid. The greater the lattice enthalpy, the stronger the bonds.

Front 14

Factors affecting lattice energy

Back 14

■ smaller ionic radius means the ions are closer together in the lattice, increasing the strength of the attraction between the oppositely charged ions, so ionic bonds are stronger and lattice energy is larger

■ greater ionic charge means a stronger attraction between the oppositely charged ions, so ionic bonds are stronger and lattice energy is larger

Front 15

Comparing lattice energies

Back 15

Comparison of the experimental lattice energy value (from a Born-Haber cycle) with the theoretical value (obtained from electrostatic theory) in a particular compound indicates the degree of covalent bonding

Hint 15

● If there is reasonable agreement between the experimental value and the theoretical value, that means that the assumptions about the solid being ionic are correct.

● If the experimental and theoretical values don't agree, that means that the solid is not 100% ionic and it actually has a significant amount of covalent bonding because there isn't enough electronegativity difference between the 2 ions to allow for complete transfer of an electron.

Front 16

Enthalpy change of hydration

Back 16

Enthalpy change when

- 1 mole of gaseous ions

- dissolve in sufficient water

- to give a solution of infinite dilution

Hint 16

Always negative

The size of the hydration enthalpy is governed by the amount of attraction between the ions and the water molecules.

Front 17

Factors affecting hydration enthalpy

Back 17

■ smaller ionic radius means the ions are closer to the water molecules, increasing the strength of the attraction between the ions and oppositely charged dipoles on the water, so hydration enthalpy is larger

■ greater ionic charge means a stronger attraction between the ions and oppositely charged dipoles on the water, so hydration enthalpy is larger

Front 18

Polarisation

Back 18

● the greater the charge density of the cation, the greater it's polarizing power, so it causes more distortion to the electron cloud of the anion, so ionic bond is polarised more and has greater covalent character

● the greater the charge and radius of the anion, the greater it's polarizability, as the electrons are further from the nucleus and there's greater repulsion between them. So the electron cloud of the anion is more easily distorted by the cation, so ionic bond is polarised more and has greater covalent character

Hint 18

Polarisation is the distortion of the electron cloud around the anion by the cation.

Front 19

Exothermic reactions

Back 19

● reactants release heat energy to the surroundings

● temperature rises

● products have less enthalpy than reactants, so are more stable

● enthalpy change is negative

● bond forming is exothermic

● reactions are exothermic if more energy is released from forming bonds in products than is absorbed to break bonds in reactants

Front 20

Endothermic reactions

Back 20

● reactants absorb heat energy from the surroundings

● temperature falls

● products have more enthalpy than reactants, so are less stable

● enthalpy change is positive

● bond breaking is endothermic

● reactions are endothermic if more energy is absorbed to break bonds in reactants than is released from forming bonds in products

Front 21

Entropy

Back 21

Entropy is a measure of the disorder of a system (the number of ways of arranging the energy) and the natural direction of change is increasing total entropy (positive entropy change) as this makes substances more energetically stable.

Front 22

Entropy and changing state

Back 22

Solids have the lowest entropy, as the particles are the least disordered.

Gases have the greatest entropy, as the particles are the most disordered.

Entropy changes as the system becomes more or less disorder as state changes.

Front 23

Entropy and dissolving

Back 23

Dissolving a substance increases entropy as the particles move around more in the solution, so are more disordered

Front 24

Entropy and number of particles

Back 24

Entropy increases as the number of moles of particles increases, since there are more ways to arrange the energy of the system making it more disordered

Front 25

Activation energy

Back 25

Minimum amount of kinetic energy the particles need to have to break bonds in reactants and start a chemical reaction

Front 26

Reversible reactions

Back 26

Reactions with negative ΔG, and so are theoretically feasible, have equilibrium constants > 1.

Reactions with positive ΔG, and so are not theoretically feasible, have equilibrium constants < 1.

Front 27

Feasibility

Back 27

An endothermic reaction can still be feasible due to an increase in entropy which overcomes this change in enthalpy.

Even if the ΔG value is negative and the reaction is thermodynamically feasible, it may not occur in practice due to kinetic factors like a very high activation energy or a very slow rate.

Front 28

Hess's law

Back 28

Enthalpy change is independent of route taken