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Chemistry > (1) Thermodynamics > Flashcards

(1) Thermodynamics Flashcards

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1
Q

Define Hess’s Law.

A

The enthalpy change of a chemical reaction is independent of route taken.

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2
Q

Define Standard Enthalpy of Formation.

Give an example in the form of an equation.

A
  • 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)
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3
Q

Define Standard Enthalpy of Combustion.

Give an example in the form of an equation.

A
  • 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)
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4
Q

Define Standard Enthalpy of Atomisation.

Give an example in the form of an equation.

A
  • Enthalpy change when one mole of gaseous atoms is formed from an element in its standard state.
  • ½Br2(l) —> Br(g)
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5
Q

Define Enthalpy of 1st Ionisation.

Give an example in the form of an eqaution.

A
  • 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
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6
Q

Define Enthalpy of 2nd Ionisation.

A
  • 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-
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7
Q

Define Enthalpy of 1st Electron Affinity.

Give an example in the form of an equation.

A
  • 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)
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8
Q

Define Enthalpy of 2nd Electron Affinity.

Give an example in the form of an equation.

A
  • 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)
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9
Q

Define Enthalpy of Lattice Formation.

Give an example in the form of an equation.

A
  • Enthalpy change when one mole of solid ionic compound is formed from its gaseous ions.
  • Mg2+(g) + 2Br-(g) —> MgBr2(s)
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10
Q

Define Enthalpy of Lattice Dissociation.

Give an example in the form of an equation.

A
  • Enthalpy change when one mole of solid ionic compound dissociates into its gaseous ions
  • MgBr2(s) —> Mg2+(g) + 2Br-(g)
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11
Q

Define Enthalpy of Hydration.

Give an example in the form of an equation.

A
  • Enthalpy change when one mole of gaseous ions is converted into one mole of aqueous ions
  • Na+(g) + aq —> Na+(aq)
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12
Q

Define Standard Enthalpy of Solution.

Give an example in the form of an equation.

A
  • 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)
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13
Q

Define Mean Bond Enthalpy.

Give an example in the form of an equation.

A
  • 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)
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14
Q

What are the rules which should be followed when constructing a Born-Haber Cycle?

A
  • 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
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15
Q

Steps when constructing a Born- Haber Cycle?

A

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

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16
Q

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)?

A

Multiply by 2.

17
Q

What are the two processes involved in dissolving an ionic solid in water? (use NaCl as example)

A
  • 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)
18
Q

Two key factors that determine how exothermic a lattice enthalpy are …

A
  • Charge on the ions

- Size of the ions (ionic radius)

19
Q

Explain the link between Size and Attraction. (of ions)

A

Small ions= pack together closely in a lattice = stronger attraction

Larger ions =further apart in their lattice = weaker attraction

20
Q

As the ionic radius increases….

A
  • The attraction between the ions decreases

- The lattice enthalpy becomes less exothermic

21
Q

As charge increases….

A
  • The attraction between the ions increases

- The lattice enthalpy becomes more exothermic

22
Q

What structure should you use when answering which compound has ‘more exothermic lattice enthalpy’?

A

CRAM

C- charge on ions

R- Radius/size of ions

A- attraction between ions

M- more exothermic (state which)

23
Q

What do we say a compound has when the theoretical value for the enthalpy of lattice formation is close to experimental value?

A

Shows compound has almost purely ionic bonding.

24
Q

What is meant by ‘Covalent Character’?

A
  • shared electron density between the two ions is described as covalent character.
25
Q

What characteristics do compound have to have Covalent Character?

A
  • A positive ion which is small and highly charged (very polarising)
  • A negative ion which is large and highly charged (very polarisable)
26
Q

Describe the Theoretical Model.

A

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

27
Q

Describe the Experimental Model.

A

Model name: Born Haber Cycle

Type of Ions: Ions are polarisable

Nature of Bonding: Ionic bonding WITH covalent character

28
Q

Does a compound with perfect ionic model value equal to experimental model value display Covalent Character?

A

no covalent character

  • purely ionic bonding
29
Q

Does a compound with a perfect ionic model value is less exothermic than experimental model value display Covalent Character?

A

Displays covalent character.

30
Q

What are the factors affecting Hydration Enthalpies?

A

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

31
Q

In terms of electrostatic forces, suggest why the electron affinity of iodine has a negative value..

A

There is an attraction formed between the nucleus and the electron

Energy is released since the process is exothermic

32
Q

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.

A
  • Calcium ions are smaller and more charged

- will attract water more strongly

33
Q

Explain why the enthalpy of lattice dissociation of rubidium oxide is less endothermic than that of Lithium oxide.

A

Rb is bigger than Li

  • Attractions between ions is weaker
34
Q

What is the link between Bond Enthalpy and Enthalpy of Atomisation?

A

Bond enthalpy = 2 molecules

Atomisation= 1 molecule

Atm= Bond/2

Bond= Atmx2

35
Q

Suggest why the hydration of Cl- is exothermic.

A

Chloride ion attacks positively charged H in water.

36
Q

Calculation of lattice formation of AgI based on perfect ionic model gives smaller value than experimental value.

Explain why.

A

AgI has covalent character

Bonds are stronger than predicted.

37
Q

In terms of forces acting on particles, suggest one reason why the first electron affinity of O2 is exothermic.

A

Attractive force between nucleus of oxygen atom and outer electron.

38
Q

Suggest why a value for enthalpy of solution of MgO is not found in any data book.

A

MgO reacts with water.

39
Q

Explain why the Enthalpy of Atomisation of Chlorine is exactly half the bond dissociation enthalpy of Chlorine.

A

Breaking one bond gives two atoms.

Chemistry (29 decks)

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