Thermodynamics Flashcards
What is the standard enthalpy of formation
The enthalpy change when one mole of a compound is formed from its elements under standard conditions, all reactants and products in their standard states
e.g. Na(s) + ½Cl2(g) -> NaCl(s)
What is the standard enthalpy of combustion
The enthalpy change when one mole of a compound is completely burned in oxygen under standard conditions, all reactant and products in their standard states
e.g. C2H6(g) + 3½O2(g) -> 2CO2(g) + 3H2O(l)
What is the standard enthalpy of atomisation
The enthalpy change when one of gaseous atoms is formed from an element in its standard state
e.g. Mg(s) -> Mg(g)
e.g. ½ Br2(l) -> Br(g)
What is the mean bond enthalpy
The enthaply change when one mole of gaseous molecules each break a covalent bond to form two free radicals, averaged over a range of compounds
e.g. CH4(g) -> H(g) + CH3(g)
What is the first ionisation enthalpy
The standard enthalpy change when one mole of electrons is removed from onr mole of gaseous atoms to give one mole of gaseous ions each with a single positive charge
e.g. Na(g) -> Na+(g) + e-
e.g. Ca(g) -> Ca+(g) + e-
What is the second ionisation enthalpy
The standard enthalpy change when one mole of electrons is removes from one mole of gaseous 1+ ions to give one mole of gaseous ions, each with a 2+ charge
e.g. Ca+(g) -> Ca2+(g) + e-
e.g. Mg+(g) -> Mg2+(g) + e-
What is the First Electron affinity
The standard enthalpy change when one mole of gaseous atoms is converted into a mole of gaseous ions, each with a single negative charge under standard conditions
e.g. Cl(g) + e- -> Cl-(g)
e.g. S(g) + e- -> S-(g)
What is the second electron affinity
The standard enthalpy change when one mole of electrons is added to a mole of gasous ions each with a single negative charge, to form a mole of ions each with a 2- charge
e.g. S-(g) + e- -> S2-(g)
e.g. O- (g) + e- -> O2-(g)
What is the Lattice formation enthalpy
The standard enthalpy change when one mole of solid ionic lattice is formed from its gaseous ions
e.g. Mg2+(g) + 2Br-(g) -> MgBr2(s)
What is Lattic dissociation enthalpy
The standard enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions
e.g. NaCl(s) -> Na+(g) + Cl-(g)
e.g. MgBr2(s) -> Mg2+(g) + 2Br-(g)
What is the standard enthalpy of hydration
The standard enthalpy change when one mole of gaseous ions is converted into one mole of aqueou ions
e.g. Na+(g) + aq -> Na+(aq)
e.g. Cl-(g) + aq -> Cl-(aq)
What is the standard enthalpy of solution
The standard enthalpy change when one mole of solute dissolves in enough solvent to form a solution in which the ions are far enough apart not to react with each other
e.g. NaCl(s) + aq -> Na+(aq) + Cl-(aq)
Explain why the bond enthalpy of a Cl-Cl bond is greeater than that of a Br-Br bond
(2 marks)
- Cl is a smaller atom
- So attraction between nucleus and bonding pair is stronger
Suggest why the electron affinity of chlorine is an exothermic change
(1 mark)
Net attraction between the chlorine nucleus and the extra electron
Explain the meaning of the term perfect ionic model
(1 mark)
- No covalent character
- Ion is point charge
Suggest why the calculated value of lattice dissociationof AgCl is greater than the theoretical value
(2 marks)
- AgCl has covalent character
- Forces in the lattice are stronger than pure ionic attractions
In terms of the forces acting on the particles, suggest one reason why the first electron affinity of oxygen is an exothermic process
(1 mark)
There is an attractive force between the nucelus of an O and an external electron
The freezing of water is an exothermic process. Give one reason why the temperature of a sample of water can stay at a constant value of 0 degrees when it freezes
(1 mark)
The heat given out escapes
Pure ice can look pale blue when illuminated by white light. Suggest why
(2 marks)
- Red light absorbed by light
- Blue light transmitted
Explain why the electronegativity of fluorine is greater than the electronegativity of chlorine
(2 marks)
- Fluorine atom is smaller than chlorine
- Electrons attracted more strongly to the nucleus
Explain why the hydration enthalpy of a fluoride ion is more negative than the hydration enthalpy of a chlorine ion
(2 marks)
- Fluoride smaller that chloride
- So attracts water more strongly
Suggest why a value for the enthalpy of solution of MgO is not found in any data books
(1 mark)
MgO reacts with water/ forms Mg(OH)2
