Thermodynamics Flashcards by Tania Ahmed

What is the Born-Haber cycle? When is it used?

‘Special type of Hess’s law cycle’

- Used to calculate the lattice enthalpy of ionic compounds

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Define atomisation

Formation of 1 mole of gaseous atoms

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Define Ionisation

Formation of gaseous ions

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Define standard enthalpy change of atomisation

The enthalpy change when 1 mole of gaseous atoms is formed under standard conditions

The enthalpy change for this reaction:
Ca(s) ——> Ca(g)

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Is atomisation ENDO or EXO. Why?

ENDO

because, bonds are being broken

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Define standard enthalpy change of the 1st I.E

(1st I.E)
the enthalpy change of this reaction:
Ca(g) ——> Ca+ (g) + e-

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Is the 1st I.E ENDO or EXO

Why?

ENDO

because, energy is needed to overcome the electrostatic attraction between the nucleus and the electron being removed

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Define standard enthalpy change of the 2nd I.E

The enthalpy change for this reaction:

Ca+(g) ——> Ca2+(g) + e-

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Is the 2nd I.E ENDO or EXO

Why?

MORE ENDO
because, more energy is needed to overcome the electrostatic attraction between the nucleus and the electron being removed

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Define the standard enthalpy of the first electron affinity

The enthalpy change for this reaction:

F(g) + e- ——> F- (g)

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Is the first electron affinity ENDO or EXO

Why?

EXO
because, there is an electrostatic force of attraction between the nucleus of the gaseous atom and the electron being added

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Define the standard enthalpy of the second electron affinity

The enthalpy change for this reaction:

O-(g) + e- ——> O2- (g)

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Is the second electron affinity ENDO or EXO

Why?

ENDO
because energy is needed to overcome the repulsion between the negatively charged gaseous ion and the negatively charged electron being added

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Define Enthalpy of Lattice Formation

The enthalpy change when 1 mole of an ionic solid is formed from the gaseous ions under standard conditions

the enthalpy change for this reaction:
K+(g) + F-(g) ——> KF(s)

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Is the lattice of formation ENDO or EXO

Why?

EXO

because, there is a strong electrostatic force of attraction between the oppositely charged ions

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Define Enthalpy of Lattice Dissociation

The enthalpy change when 1 mole of an ionic solid lattice is broken to form the separate gaseous ions under standard conditions.

The enthalpy change for this reaction:

KF(s) ——> K+(g) + F-(g)

Is the lattice of dissociation ENDO or EXO

Why?

ENDO

because, energy is needed to break the electrostatic force of attraction between the oppositely charged ions

What 2 factors affect Lattice enthalpy

charge of ions

- Size/ radius of ion

What do you call lattice enthalpy values from Born- Harber cycles

Experimental lattice energies

What are theoretical lattice energy values

Values calculated from the ‘perfect ionic model’

Name the 2 main assumptions of the ‘perfect ionic model’

The ions are perfectly spherical /(they are point charges)
The compound is 100% ionic. There is no distortion or polarisation of the ions (no covalent character present)

Define the enthalpy change of solution

The enthalpy change when 1 mole of an ionic solid lattice is dissolved completely in water under standard conditions

The enthalpy change for this reaction:
KCl(s) ——> K+(aq) + Cl-(aq)

When an ionic solid is dissolved in water, which 2 thermodynamic processes are involved

-Lattice Dissociation
The ionic solid lattice is broken into the gaseous ions

-Hydration
The gaseous ions are converted into aqueous ions

What is the equation for the 🔺H(sol) ?

🔺H(sol)

= lattice dissociation + 🔺H(hyd)

Define the enthalpy change of Hydration

The enthalpy change when 1 mole of aqueous ions is formed from 1 mole of gaseous ions under standard conditions OR The enthalpy change for this reaction: Na+ (g) ——> Na+(aq)

Is the enthalpy of hydration ENDO or EXO | Why?

EXO | because of the strong electrostatic attraction between the ion and the diapoles in water

What 2 factors affect the enthalpy of hydration?

- Size of ion | - Charge of ion

What does it mean if an ion has a higher ‘charge density’

SMALLER ion | HIGHER charge

Define what it means for a reaction to be spontaneous/ ‘feasible’

Can occur freely/ of it’s own accord

What is Entropy and give it’s units

‘The measure of the degree of disorder in a reaction’ UNITS: Jk-1 mol-1 J= joules K= kelvin

Why is the entropy of elements not zero?

Entropy is measured at 298K | - the particles still have some energy to distribute (still moving)

Define the state of ‘perfect/ maximum order’

The state of a substance when the particles are stationary (only occurs at 0K)

What does a graph showing entropy against temperature show?

Entropy increases as temperature increases A change in state leads to a BIG INCREASE in entropy Boiling results in a bigger increase in entropy than melting

What is the equation for the entropy change of surroundings

🔺S = (-)🔺H / T

What is the 2nd law of thermodynamics

For a spontaneous reaction | 🔺S total must be positive

What is the equation for ‘Gibbs free energy change’

🔺G = 🔺H - T🔺S

Why might a reaction with negative GFEC not occur spontaneously and give solutions

Rate is too slow - Add a catalyst - Increase the temperature

What is the equation for the entropy change of a system

entropy change of (products) - entropy change of (reactants)