5. Thermo Chemistry

Question 1

Enthalpy (definition)

Answer 1

The amount of energy or heat content of a substance. The energy is stored in chemical bonds (including kinetic and potential energy)

Question 2

Standard Enthalpy Change of Reaction (dH)

Answer 2

The difference between the enthalpy of the products and the enthalpy of the reactants at 298k and 1.00x10⁵Pa

dH = p - r

Question 3

Exothermic Reactions

Answer 3

The temperature of the surroundings increases and dH is negative, energy is given out

• combustion and neutralization reactions
• products from an exothermic reaction have less stored energy, are more stable and have stronger bonds

Question 4

Endothermic Reactions

Answer 4

The temperature of the surroundings decreases as dH is positive, energy is absorbed

• the higher the nethalpy the less stable the substance

Question 5

Standard Enthalpy of Combustion (dHc)

Answer 5

The enthalpy change that results when 1 mole of a compound reacts with oxygen at 298K and 1.00 x 10⁵Pa. All reactants and products are in their standard states

dH = r - p

Question 6

Heat Energy Change (2 equations)

Answer 6

Q = mcdT

m = mass in g (volume in cm³)

c = specific heat capacity of the substance

dT = change in temperature in K

dH = Q / n

Q = heat energy change

dH = heat energy change per mole

Question 7

Calorimetry

Answer 7

The technique used to measure enthalpy change

Assumptions and sources of error

• that no heat is lost to or gained from the surroundings
• significant errors are associated with reactions
• heat rises and dT under these conditions are reported to be less than the literature stipulates
• Thermometers often have precision uncertainty of +-0.1C or greater

Question 8

Hess’s Law

Answer 8

In a chemical reaction the total change in chemical potential energy must be equal to the energy lost or gained by the reaction system

dH₂ = 2dH₁ - dH₃

Question 9

Standard Enthalpy Change of Formation (dHf)

Answer 9

The enthalpy change that results when 1 mole of a compound is formed from its elements at 298K and 1.00 x 10⁵ Pa, in the standard states

dHf = p - r

Question 10

Bond Enthalpy

Answer 10

The amount of energy required to break 1 mole of bonds in the gaseous state averaged across a range of compounds containing that bond

• breaking bonds = endothermic
• making bonds = exothermic

dH = r - p

Question 11

Bond enthalpies of Ozone

Answer 11

Ozone is decomposed more easily than oxygen

• O₂ requires 498kJ/mol to break
• O₃ requires 364kJ/mol to break

Question 12

Enthalpy of Atomization (dH_at)

Answer 12

The energy change required to change 1 mole of atoms from their standard state to a gaseous state

M_(s) –> M_(g)

0.5X_2(g) –> X_(g)

_{dH > 0}

Question 13

Ionization Energy (dH_IE)

Answer 13

The standard enthalpy change that occurs by the removal of 1 mole of electrons from 1 mole of atoms or positive ions in the gaseous phase

IE₁: M_(g) –> M⁺_(g) + e^-

IE₂: M⁺_(g) –> M²⁺_(g) + e^-

^{dH_IE > 0}

Question 14

Electron Affinity (dH_EA)

Answer 14

The energy change that occurs when 1 mole of electrons joins to one mole of atoms in the gaseous phase

X_(g) + e^- –> X^-_(g)

^{dH_EA < 0}

Question 15

Lattice Enthalpy (dH_lat)

Answer 15

The energy required to convert 1 mole of the solid compound into gaseous ions (association/ dissociation)

MX_(s) –> M⁺_(g) + X^-_(g)

^{dH > 0}

Magnitude of Lattice Enthalpy

• charge on the ions
  
  • greater charge, greater the attraction, greater lattice energy
• size of the ions
  
  • larger the ions the further apart they are and the more separate the charges, the smaller the lattic energy

Question 16

Born-Haber Cycle

Answer 16

Diagrammatic representation of the formation of ionic compounds, starting from atoms, moving to gaseous atoms to gaseous ions to a solid compound

dH_f = dH_atm + dH_IE + dH_EA + dH_lat

Question 17

Enthalpy of Solution (dH_sol)

Answer 17

The energy change that occurs when 1 mole of substance dissolves in excess solvent

MX_(s) –> M⁺_(aq) + X^-_(aq)

Question 18

Enthalpy of Hydration (dH_hyd)

Answer 18

The energy change that occurs when 1 mole of gaseous ions are added to water to form a dilute solution

• value is always negative
• if another solvent other than water is used it is enthalpy of solvation

M⁺_(g) –> M⁺_(aq)

X^-_(g) –> X^-_(aq)

Question 19

Entropy

Answer 19

The measure of the distribution of total available energy between the particles. The greater the disorder the particles, the great the entropy

G > L > S

• The entropy of a substance increases with temperature

dS = entropy of p - entropy of r

An increase in disorder produces a positive dS

• decomposition reactions
• changes in state
• dissolution reactions
• increases in temperature

Question 20

Gibbs Free Energy

Answer 20

The energy available to do work

dG = dH - TdS

• negative dG = spontaneous reaction
• positive dG = not spontaneous reaction
• For any element in its standard state dG = 0

Question 21

Spontaneity

Answer 21