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Chemistry A2 > Chapter 22: Enthalpy and Entropy > Flashcards

Chapter 22: Enthalpy and Entropy Flashcards

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1
Q

Define Lattice Enthalpy.

A

Formation of one mole of an ionic compound from its gaseous ions under standard conditions.

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2
Q

Why are all lattice enthalpy values exothermic?

A

Because bonds are being made.

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3
Q

What is the Born Haber cycle?

A

An energy cycle from which lattice enthalpy can be determined indirectly.

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4
Q

Give the 2 routes of the Born Haber cycle.

A

Route 1 = 3 processes:

  • Forming gaseous ATOMS (atomisation)
  • Forming gaseous IONS (ionisation/affinity)
  • Lattice Enthalpy

Route 2:
-Enthalpy change of formation from elements in standard states.

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5
Q

Define enthalpy change of ATOMISATION.

A

enthalpy change that takes place for the formation of one mole of gaseous atoms from elements in their standard state under standard conditions.

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6
Q

What sign do all values of enthalpy change of atomisation have and why?

A

POSITIVE

as the reaction is endothermic. (bonds being broken)

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7
Q

Give an equation for the enthalpy change of atomisation for SODIUM and for CHLORINE.

A

Na(s) —> Na(g)

1/2Cl (g) —> Cl (g)

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8
Q

Define first ionisation energy.

A

Enthalpy change required to remove one electron from each atom in one mole of gaseous atoms to form one mole of 1+ gaseous ions.

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9
Q

Give an equation for the first ionisation energy for sodium.

A

Na(g) —-> Na+(g) +e-

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10
Q

What is Electron affinity?

A

The OPPOSITE to ionisation energy.

Enthalpy change required to add one electron to each atom in one mole of gaseous atoms.

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11
Q

What sign do the values for FIRST electron affinity and ionisation energy have and why?

A

Electron affinity = negative
-exothermic

Ionisation Energy = positive
-endothermic, energy required to overcome attraction between electron and nucleus.

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12
Q

Why is the second electron affinity ENDOTHERMIC?

A

second electron is being accepted by a negative ion -> repulsion so energy is required to overcome this.

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13
Q

When would you need a second electron affinity?

A

For ions that have a higher negative charge than -1. eg. oxygen -2

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14
Q

Define enthalpy change of solution.

A

Enthalpy change for one mole of a solute to dissolve in a solvent.

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15
Q

Give the eqaution for enthaply change of solution for sodium chloride with water.

A

Na+Cl- (s) —-> Na+(aq) + Cl-(aq)

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16
Q

Describe the arrangement of the ions when sodium chloride is a solid and when it is dissolved in water.

A

Solid - electrostatic attraction between the oppositely charged ions acting all around.

AQ - each ion is surrounded by water molecules. Positive sodium with partially negative oxygen; negatice chloride with partially positive hydrogen of water molecule.

17
Q

What are the 2 main processes for when a solid ionic compound dissolves in water?

A

1) Ionic lattice is broken up forming seperate gaseous ions.

2) seperate gaseous ions interact with polar water molecules to form hydrated ions. (ENTHALPY CHANGE OF HYDRATION)

18
Q

Define enthalpy change of hydration.

A

Enthalpy change for the dissolving of gaseous ions in water to form one mole of aqueous ions.

19
Q

What factors affect lattice enthalpy and how?

A

IONIC SIZE
-increase in radius = decrease in attraction between ions = less negative lattice enthalpy value.

IONIC CHARGE
-increase in charge = increase in attraction = more negative lattice enthalpy value.

20
Q

Give the trend of lattice enthalpy across a period.

A

FOR METALS (going right) = supporting effects:
-increase in positive charge
-decrease in radius
BOTH increase attraction.

FOR NON METALS (going left) = opposing effects:

  • increase in negative charge INCREASING attraction BUT
  • increase in radius DECREASING attraction.
21
Q

Why do compounds with more negative lattice enthalpy have high melting points?

A

Because there is stronger ionic bonding so more energy is required to break the bonds.

22
Q

What factors affect enthalpy of hydration and how?

A

IONIC SIZE:
-increase in ionic radius = ionic attraction decreases = hydration energy LESS negative

IONIC CHARGE:
-charge increases = attraction with water increases = hydration energy becomes MORE negative

23
Q

What must be required regarding hydration enthalpies and lattice enthalpy in order for a compound to dissolve?

A

Sum of hydration enthalpies should be larger than the lattice enthalpy so that overall enthalpy change is EXOthermic.

Energy equal to lattice enthalpy is required to break the ionic lattice

24
Q

What is entropy?

A

measure of dispersal of energy.

the greater the dispersal of energy and greater the disorder of particles, the greater the entropy.

Gas> liquid > solid

25
Q

What are the units of entropy and what should you do when using entropy to calculate free energy?

A

J K-1 mol-1

divide by 1000 to make kJ

26
Q

When do substances have an entropy value of 0?

A

at 0K when there is no energy

above 0K all substances have positive entropy

27
Q

When is Entropy CHANGE positive and negative?

A

POSITIVE
- when there are more gas moles produced than reactants

negative = higher entropy reactants than products so more gas moles on left of equation

28
Q

Define standard entropy of a substance.

A

the entropy of one mole of a substance under standard conditions (100kPa and 298K)

29
Q

Give the equation to calculate the entropy change of a reaction.

A

entropy = sum of entropy of products - sum of entropy of reactants

30
Q

What does feasibility mean?

A

a term used to describe whether a reaction is able to happen and is energetically feasible

31
Q

Give the Gibbs’ equation.

A

free energy change = enthalpy change (delta H) - T(entropychange)

32
Q

What is required for a reaction to be feasible?

A

there must be a decrease in free energy

delta G < 0

33
Q

What is free energe used for?

A

To predict the feasibility of a reaction.

34
Q

What reasons may explain why some reactions with a negative free energy value but not take place?

A

-Too high activation energy resulting in a very slow rate. (catalyst would be required)

also if some reactants are left too long they may start to decompose.

delta G does not take into account of kinetics or rate of reaction.

Chemistry A2 (4 decks)

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