17.0 Thermodynamics Flashcards
What is the standard enthalpy of formation, ∆Hөf?
Is this enthalpy change exothermic or endothermic?
Give an example.
• The enthalpy change when one mole of a compound is
formed from its elements under standard conditions
with all reactants and products in their standard states
• Exothermic
• 2Na (s) + ½O2 (g) → Na2O (s)
What is the standard enthalpy of combustion, ∆Hө
c? Is this enthalpy change exothermic or endothermic? Give an example.
• The enthalpy change when one mole of a compound is
completely burned in oxygen under standard
conditions with all reactants and products in their
standard states
• Exothermic
• H2 (g) + ½O2 (g) → H2O (l)
What is the mean bond enthalpy (bond dissociation
enthalpy), ∆Hөdiss?
Is this enthalpy change exothermic or endothermic?
Give an example.
• The standard enthalpy change when one mole of
covalent bonds is broken in the gaseous state averaged
over a range of compounds
• Endothermic
• I2 (g) → 2I (g)
What is the standard enthalpy of atomisation, ∆Hө
at?
Is this enthalpy change exothermic or endothermic?
Give an example.
• The enthalpy change which accompanies the
formation of one mole of gaseous atoms from the
element in its standard state under standard
conditions
• Endothermic
• ½I2 (s) → I (g)
What is the first ionisation, ∆Hөi1 or IE1?
Is this enthalpy change exothermic or endothermic?
Give an example.
• The standard enthalpy change when one mole of
gaseous atoms is converted into one mole of gaseous
ions each with a single positive charge
• Endothermic
• Mg (g) → Mg+ (g) + e-
What is the second ionisation, ∆Hөi2 or IE2?
Is this enthalpy change exothermic or endothermic?
Give an example.
• The standard enthalpy change when one mole of
gaseous 1+ ions are converted into one mole of 2+
gaseous ions
• Endothermic
• Mg+ (g) → Mg2+ (g) + e-
What is the lattice enthalpy of formation, ∆HөL?
Is this enthalpy change exothermic or endothermic?
Give an example.
• The standard enthalpy change when one mole of a
solid ionic compound is formed from its gaseous ions
• Exothermic
• Mg2+ (g) + 2Cl- (g) → MgCl2 (s)
What is the lattice enthalpy of dissociation, ∆HөL?
Is this enthalpy change exothermic or endothermic?
Give an example.
• The standard enthalpy change when one mole of a
solid ionic compound separates into its gaseous ions
• Endothermic
• MgCl2 (s) → Mg2+ (g) + 2Cl- (g)
What is the enthalpy of hydration, ∆Hөhyd?
Is this enthalpy change exothermic or endothermic?
Give an example.
The standard enthalpy change when water molecules
surround one mole of gaseous ions
• Exothermic
• Mg2+(g) + aq → Mg2+(aq)
What is the enthalpy of solution, ∆Hөsol?
Is this enthalpy change exothermic or endothermic?
Give an example.
• The standard enthalpy change when one mole of
solute dissolves completely in sufficient solvent to
form a solution in which the molecules are far enough
apart not to interact with each other
• Endothermic
• MgCl2(s) + aq → Mg2+(aq) + 2Cl-(aq)
What is the perfect ionic model of a lattice?
All the ions in the lattice are spherical, and have their charge evenly distributed around them
Why do most ionic compounds have some covalent
character?
The positive and negative ions in a lattice are not usually spherical causing the positive ions to polarise their neighbouring negative ions resulting in covalent character
Why are theoretical lattice enthalpies (based on the
perfect ionic model of a lattice) often different from
experimental values?
This is because most ionic compounds have a degree of covalent character
What 2 things happen when an ionic lattice dissolves in water?
- The bonds between the ions break to form free ions
- Bonds between the ions and the water are made
What word describes reactions which take place of
their own accord?
Feasible or spontaneous
What is the randomness or disorder of a system called?
What is its symbol and unit?
• Entropy
• S
• JK-1 mol -1
What 2 factors affect the entropy of a reaction?
- The physical states of the reactants; solids are
ordered and have low entropy whereas gases are disordered and have high entropy - The number of gas molecules; if the number of moles
of gas increases from reactants to products, the
entropy change is positive and vice versa
What equation is used to calculate the entropy change, ∆S, for a reaction?
∆S = sum of Sproducts - sum of Sreactants
What is the standard entropy change, ∆Sө, of a
substance?
The entropy change of 1 mole of a substance under
standard conditions (a pressure of 100 kPa and
temperature of 298 K)
What equation is used to calculate the Gibbs free
energy change, ∆G, for a reaction?
What are the units of each term in this equation?
∆G = ∆H - T∆S
• ∆G = kJmol-1
• ∆H = kJmol-1
• T = K
• ∆S = JK-1 mol -1
What value must the Gibbs free energy change, ∆G,
take for a reaction to be feasible?
Negative or equal to zero
What equation is used to calculate temperature, T, for a feasible reaction i.e. when ∆G = 0?
T = ∆H/∆S
What equation is used to calculate entropy change, ∆S, for a feasible reaction i.e. when ∆G = 0?
∆S = ∆H/T
