(1) Thermodynamics Flashcards
Define Hess’s Law.
The enthalpy change of a chemical reaction is independent of route taken.
Define Standard Enthalpy of Formation.
Give an example in the form of an equation.
- Enthalpy change when one mole of a compound is formed from its elements under standard conditions, all reactants and products in their standard states
- Na(s) + ½Cl2(g) –> NaCl(s)
Define Standard Enthalpy of Combustion.
Give an example in the form of an equation.
- Enthalpy change when one mole of a compound is completely burned in oxygen under standard conditions, all reactants and products in their standard states.
- C2H6(g) + 3½O2(g) —> 2CO2(g) + 3H2O(l)
Define Standard Enthalpy of Atomisation.
Give an example in the form of an equation.
- Enthalpy change when one mole of gaseous atoms is formed from an element in its standard state.
- ½Br2(l) —> Br(g)
Define Enthalpy of 1st Ionisation.
Give an example in the form of an eqaution.
- Enthalpy change when one mole of electrons is removed from one mole of gaseous atoms to give one mole of gaseous ions each with a single positive charge.
- Ca(g) —> Ca+(g) + e
Define Enthalpy of 2nd Ionisation.
- Enthalpy change when one mole of electrons is removed from one mole of gaseous 1+ ions to give one mole of gaseous ions each with a 2+ charge.
- Ca+(g) —> Ca2+(g) + e-
Define Enthalpy of 1st Electron Affinity.
Give an example in the form of an equation.
- Enthalpy change when one mole of electrons is added to one mole of gaseous atoms to give 1 mole of gaseous ions, each with a single negative charge under standard conditions.
- Cl(g) + e- —> Cl-(g)
Define Enthalpy of 2nd Electron Affinity.
Give an example in the form of an equation.
- Enthalpy change when one mole of electrons is added to a mole of gaseous 1- ions to give one mole of ions each with a 2- charge
- O- (g) + e- —> O2-(g)
Define Enthalpy of Lattice Formation.
Give an example in the form of an equation.
- Enthalpy change when one mole of solid ionic compound is formed from its gaseous ions.
- Mg2+(g) + 2Br-(g) —> MgBr2(s)
Define Enthalpy of Lattice Dissociation.
Give an example in the form of an equation.
- Enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions
- MgBr2(s) —> Mg2+(g) + 2Br-(g)
Define Enthalpy of Hydration.
Give an example in the form of an equation.
- Enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions
- Na+(g) + aq —> Na+(aq)
Define Standard Enthalpy of Solution.
Give an example in the form of an equation.
- 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 interact with each other
- NaCl(s) + aq —> Na+(aq) + Cl-(aq)
Define Mean Bond Enthalpy.
Give an example in the form of an equation.
- Enthalpy change when 1 mole of gaseous molecules each break a covalent bond to form 2 mol of gaseous atoms
- bond enthalpy = enthalpy of atomisation x 2
- F2 (g) –> 2F(g)
What are the rules which should be followed when constructing a Born-Haber Cycle?
- enthalpy change is POSITIVE the arrow should point UP.
- enthalpy change is NEGATIVE the arrow should point DOWN.
- must have state symbols in every step of the cycle
Steps when constructing a Born- Haber Cycle?
Pg. 10 for reference
(1) Elements in their standard states
Formation = negative
(2) Atomise the metal element
Atomisation = positive
(3) Atomise the non-metal element
Atomisation = positive
(4) Ionise the metal element (more than once if needed)
Ionisation = positive
(5) Electron Affinity for the non-metal element.
Electron affinity = negative
(6) Lattice formation of the whole ionic compound.
Lattice formation = negative
What do you have to ensure you do when finding out Enthalpy of Atomisation using a Born-Haber Cycle for molecules that have two atoms such as 2K(g)?
Multiply by 2.
What are the two processes involved in dissolving an ionic solid in water? (use NaCl as example)
- lattice dissociation enthalpy which is endothermic (separating the ions)
- NaCl (s) → Na+ (g) + Cl- (g)
AND
- enthalpy of hydration which is exothermic (bonds are made with the water molecules)
- Na+ (g) + aq → Na+ (aq)
- Cl- (g) + aq → Cl- (aq)
Two key factors that determine how exothermic a lattice enthalpy are …
- Charge on the ions
- Size of the ions (ionic radius)
Explain the link between Size and Attraction. (of ions)
Small ions= pack together closely in a lattice = stronger attraction
Larger ions =further apart in their lattice = weaker attraction
As the ionic radius increases….
- The attraction between the ions decreases
- The lattice enthalpy becomes less exothermic
As charge increases….
- The attraction between the ions increases
- The lattice enthalpy becomes more exothermic
What structure should you use when answering which compound has ‘more exothermic lattice enthalpy’?
CRAM
C- charge on ions
R- Radius/size of ions
A- attraction between ions
M- more exothermic (state which)
What do we say a compound has when the theoretical value for the enthalpy of lattice formation is close to experimental value?
Shows compound has almost purely ionic bonding.
What is meant by ‘Covalent Character’?
- shared electron density between the two ions is described as covalent character.
What characteristics do compound have to have Covalent Character?
- A positive ion which is small and highly charged (very polarising)
- A negative ion which is large and highly charged (very polarisable)
Describe the Theoretical Model.
Model name : Perfect ionic model
Type of ions: Ions are point charges, perfect spheres which cannot be distorted
Nature of Bonding: Perfectly ionic with NO covalent character
Describe the Experimental Model.
Model name: Born Haber Cycle
Type of Ions: Ions are polarisable
Nature of Bonding: Ionic bonding WITH covalent character
Does a compound with perfect ionic model value equal to experimental model value display Covalent Character?
no covalent character
- purely ionic bonding
Does a compound with a perfect ionic model value is less exothermic than experimental model value display Covalent Character?
Displays covalent character.
What are the factors affecting Hydration Enthalpies?
Charge= larger charge (more +ve) the stronger the attraction between ion and water
Size= the smaller the ion the greater the attraction between ion and water
In terms of electrostatic forces, suggest why the electron affinity of iodine has a negative value..
There is an attraction formed between the nucleus and the electron
Energy is released since the process is exothermic
Explain why there is a difference between the hydration enthalpies of the calcium 2+ and potassium 1+ ions. Answer with respect to the Calcuim ion.
- Calcium ions are smaller and more charged
- will attract water more strongly
Explain why the enthalpy of lattice dissociation of rubidium oxide is less endothermic than that of Lithium oxide.
Rb is bigger than Li
- Attractions between ions is weaker
What is the link between Bond Enthalpy and Enthalpy of Atomisation?
Bond enthalpy = 2 molecules
Atomisation= 1 molecule
Atm= Bond/2
Bond= Atmx2
Suggest why the hydration of Cl- is exothermic.
Chloride ion attacks positively charged H in water.
Calculation of lattice formation of AgI based on perfect ionic model gives smaller value than experimental value.
Explain why.
AgI has covalent character
Bonds are stronger than predicted.
In terms of forces acting on particles, suggest one reason why the first electron affinity of O2 is exothermic.
Attractive force between nucleus of oxygen atom and outer electron.
Suggest why a value for enthalpy of solution of MgO is not found in any data book.
MgO reacts with water.
Explain why the Enthalpy of Atomisation of Chlorine is exactly half the bond dissociation enthalpy of Chlorine.
Breaking one bond gives two atoms.
