3.1.8 Thermodynamics

What is the definition of the enthalpy change of formation?

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions (298K, 100kPa)

What is the definition of the enthalpy change of combustion?

The enthalpy change when one mole of a substance undergoes complete combustion in oxygen under standard conditions

What is the definition of the enthalpy change of dissociation?

The enthalpy change when all of the bonds of the same type in one mole of a gaseous molecule is broken

What is the definition of the first electron affinity?

The enthalpy change when one mole of a gaseous atom gains one electron per atom to become one mole of a gaseous 1- ion

What is the definition of the second electron affinity?

The enthalpy change when one mole of a gaseous 1- ion gains an electron to become one mole of a gaseous 2- ion

What is the definition of the enthalpy change of the first ionisation? 

The enthalpy change when one mole of a gaseous atom loses one electron per atom to become one mole of a gaseous 1+ ion 

What is the definition of the enthalpy change of the second ionisation?

The enthalpy change when one mole of a gaseous 1+ ion loses one electron to become one mole of a gaseous 2+ ion

What is the definition of the enthalpy change of atomisation?

The enthalpy change when one mole of gaseous atoms is formed from an element or compound in its standard state under standard conditions

What is the definition of the lattice enthalpy of dissociation?

The enthalpy change when one mole of a solid ionic compound is dissociated into its gaseous ions under standard conditions 

What is the definition of the lattice enthalpy of formation?

The enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions under standard conditions 

What is the definition of the enthalpy change of hydration?

The enthalpy change when one mole of gaseous ions are fully dissolved in water to form one mole of aqueous ions

What is the definition of the enthalpy change of solution?

The enthalpy change when one mole of an ionic solid completely dissolves in water to form an infinitely dilute solution under standard conditions

What are the two equations to calculate enthalpy change of solution?

What should the Hess’ cycle look like when calculating the enthalpy change of solution?

(or could have lattice enthalpy of formation replacing dissociation arrow in the opposite direction)

What are lattice enthalpies?

A measure of how strong an ionic bond is:

• the more negative the lattice enthalpy value, the stronger the ionic bond

What affects the lattice enthalpy of an ionic compound?

The charge & size of the ions:

• ions with a smaller ionic radii will have a more negative lattice enthalpy → attract more strongly due to higher charge density

What is the perfect ionic model?

• Assumes that ions are perfectly spherical & there is an even charge distribution (point charges)

• 100% polar bonds & no covalent character

Why is the perfect ionic model often not accurate?

• In an ionic lattice, a cation can distort the electron cloud of a nearby anion (i.e. ion polarisation) → this distortion reduces the anion's spherical shape

• A cation's ability to do this is its polarising power → greater polarisation results in more covalent character

What happens when a solid is dissolved in terms of interactions of the ions with water molceules?

• The lattice breaks forming gaseous ions, which are dissolved in water

• The aqueous ions are surrounded by water molecules (permanent dipole due to polar O-H bond)

What three things can the enthalpy change of solution be calculated from knowing?

• Lattice enthalpy of dissociation

• Lattice enthalpy of formation

• Enthalpy of hydration

What is entropy (ΔS) ?

The measure of disorder in a system:

• the number of ways energy can be shared out between particles

• higher entropy value = more disorder

What units is entropy measured in?

Jules per kelvin per mole

Compare the entropy for solids, liquids & gases

Solids → low entropy

• atoms held firmly in place by strong bonds

• particles only vibrate about a fixed point; hardly any randomness

• (dissolving it increases its entropy)

Liquids → increased entropy

• particles are able to move about more freely

• many more ways particles & energy can be distributed

Gases → highest entropy

• particles whizz around wherever

• highest number of random arrangements

What is the second law of thermodynamics?

Entropy always increases (it’s more likely for molecules to be disordered than ordered)

Is a reaction with a positive or negative entropy change more likely to be spontaneous?

Positive → reactions always try to increase the amount of disorder

What is the equation to calculate the total entropy change?

What is the equation to calculate the entropy change of a system?

(per mole)

What is the equation to calculate the entropy of the surroundings?

kJ → J (x1000)

What does it mean if a reaction is spontaneous & feasible?

If a reaction is spontaneous & feasible, it will take place of its own accord/reacts naturally

Is a reaction with a positive or negative enthalpy change more likely to be spontaneous?

Negative (exothermic)

What is the equation for Gibbs free energy?

°C → K (+273)

What are the units for Gibbs free energy?

kJ mol^-1

What does the value for Gibbs free energy for a reaction show?

• If G < 0, reaction is feasible

• If G = 0, reaction is JUST feasible

• If G > 0, reaction is not feasible

What is the significance of the temperature at which G = 0?

This is the temperature (in Kelvin) at which the reaction becomes feasible

What is the equation to calculate the temperature at which a reaction becomes feasible?

(convert ΔH from kJmol^-1 to Jmol^-1 by x1000)

If ΔH is negative & ΔS is positive, what is the value of ΔG & is the reaction feasible?

• ΔG is always negative

• Reaction is feasible at any temperature

If ΔH is positive & ΔS is negative, what is the value of ΔG & is the reaction feasible?

• ΔG is always positive

• Reaction is not feasible at any temperature

If ΔH is negative & ΔS is negative, what is the value of ΔG & is the reaction feasible?

• ΔG is negative at lower temperatures

• Reaction is only feasible at lower temperatures

If ΔH is positive & ΔS is positive, what is the value of ΔG & is the reaction feasible?

• ΔG is negative at higher temperatures

• Reaction is only feasible at higher temperatures

What are the two key things to look for to decide if entropy increases, decreases or stays relatively constant?

• Number of moles → more moles made = entropy increases

• Going from solid to liquid/gas or liquid to gas 

What is the equation for Gibbs free energy rearranged in the same form as the equation of a straight line (y = mx + c) ?

ΔG = (−ΔS x T) + ΔH

What is Hess’ Law?

The enthalpy change for a reaction is independent of the route taken

What is the definition of enthalpy change?

The heat energy transferred in a reaction at constant pressure