5.2 Energy Flashcards by Kamile Garuolyte

Define lattice enthalpy

enthalpy change associated with the formation of one mole of an ionic lattice from its gaseous ions under standard conditions K+(g) +Cl- –> KCl(s)

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Define enthalpy change of formation

The enthalpy change associated with the formation of one mole of a compound from its constituent elements in their standard conditions

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How do lattice enthalpy values indicated the relative strength of ionic bonds

More exothermic lattice enthalpy values mean stronger ionic bonds and higher melting and boiling point
It also indicates smaller ions and larger charges

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What can we use to measure lattice enthalpy

Born - Haber cycles because we can’t measure lattice enthalpy directly

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What are the key features of a born haber cycle

continuous cycle
includes on step that shows the formation of one mole of the solid ionic lattice from the gaseous atoms
the remaining steps show intermediate changes
the lattice enthalpy can be calculated by applying hess’ law

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Define the enthalpy change of atomisation

The enthalpy change associated when one mole of gaseous atoms is formed from its element in its standard state
K(s) –> K(g)

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Define first ionisation energy

When one mole of gaseous 1+ ions is formed from its gaseous atoms
K (g)- e –> K+(g)

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Define second ionisation energy

When one mole of gaseous 2+ ions are formed from one mole of gaseous 1+ ions
Mg+(g) - e- –> Mg2+(g)

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Define the first electron affinity

when one mole of gaseous 1- ions is formed from gaseous atoms
Cl (g)+ e- –> Cl- (g)

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Define second electron affinity

When one mole of gaseous 2- ions is formed from one mole of gaseous 1- ions

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Describe the steps in a born haber cycle to determine lattice enthalpy of sodium chloride

atomisation of sodium
atomisation of chlorine
ionisation of sodium
electron affinity of chlorine

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What happens when a solid dissolves

The ionic lattice breaks down
the free ions become a part of the solution (hydration )

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Define the standard enthalpy of solution

The enthalpy change that take place when one mole of a solute is completely dissolved in water under standard conditions
KCl (s) = + aq –> K+ (aq) + Cl- (aq)

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What is the enthalpy change associated with the breakdown of an ionic lattice

Lattice enthalpy of dissociation

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How do lattice enthalpies differ energetically

Association is exothermic
Dissociation is endothermic

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What dictates the size of the lattice enthalpy

the size of the ions involved
the charge on the ions

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How does ionic radius affect lattice enthalpy

smaller ions can get closer together and will attract one another more strongly and give rise to a more exothermic lattice enthalpy values

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How does ionic charge affect lattice enthalpy

Ions with higher charges cause greater electrostatic attraction and in turn more exothermic lattice enthalpy values

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hat ions have the most negative lattice enthalpy values

The smallest most highly charged ions will give rise to the most exothermic lattice enthalpies as they can pack closer to the oppositely charged ions with a higher attraction

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Define the standard enthalpy of hydration

The enthalpy change that takes place when dissolving one mole of gaseous ions in water under standard conditions
K+ (g) –> K+(aq)

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What occurs during hydration

The positive ions will be attracted to the slightly negative oxygen
The negative ions will be attracted to the slightly positive hydrogen
the water molecules will completely surround the ions

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What kind of process is hydration

Exothermic

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What dictates enthalpy of hydration

Ionic radius
ionic charge

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How does ionic radius affect enthalpy of hydration

ions with a smaller radii are able to get closer to the water molecule and are able to attract them more strongly so on hydration more energy is released and becomes more exothermic

How does ionic charge affect enthalpy of hydration

The higher the charge on an ion the greater the attraction it will have on the water molecule giving a more exothermic value

define entropy

the quantitative measure of the degree of disorder in a system

Define standard entropy

The entropy content of one mole of a substance under standard conditions

define standard entropy change of a reaction

the entropy change that accompanies a reaction in the molar quantities expressed in the chemical reaction under standard conditions

what are the units for entropy

J/K/mol

what figure is entropy

Always a positive figure

When is entropy 0

At 0K in perfect crystals

What state has the highest entropy

gas

Why do gases have a higher entropy

more spreading out of energy and more random arrangement of molecules

How is entropy affected by temperature

entropy increases as temperature increases

How does entropy change when dissolving ionic solids

the reaction results in the products having more disorder Ions can spread out and the positions of the ions are more disordered within the lattice

How does the number of gas molecules affect entropy

An increase in the number of gas molecules causes an increase in entroy

How do you calculate entropy change

entropy of products - entropy of reactants

How do you calculate total change in entropy

Entropy change of the system + entropy change of the surroundings

What do positive and negative entropy changes indicate

+ve - the change made the system more random -ve - made the system more ordered

What is the Gibbs equation

ΔG = ΔH-TΔS

What must the value of ΔG must be for a reaction to occur spontaneously

negative

What do we assume Δ G is when calculating free energy

What are the limitations of using ΔG

whether or not a reaction happens also depends on factors such as activation energy and the reaction might be very slow

What is a half cell

comprises of an element in its two oxidation states The simplest has a metal placed in its aqueous sloution of ions

What is a hydrogen half cell

Contains hydrogen gas in contact with hydrogen ions

Why is a platinum electrode used in a hydrogen half cell

No solid piece of metal can act as the electrode except platinum as it is inert and does not react at all Its sole purpose is to be in contact with both the H2 and H+ ions and allow transfer of electrons in and out of the half cell via a connecting wire

What are the standard conditions in a half cell

298k 100kPa 1mol/dm

What is the standard electrode potential of a half cell

the e.m.f. of a half cell compared to a standard hydrogen half cell measured at 298k with solution concentrations of 1mol/dm at 100kPa

what are metal ion/ metal ion half cells

This type of cells contain ions of the same element in different oxidation states for example Fe 3+ and Fe2+ this type of half cell requires a platinum electrode

Define equimolar

solution containing ions at same concentrations

How do we measure standard electrode potentials using a hydrogen half cell

the tendency for different half cells to accept or release electrons is measured as an electromotive force which is measured in volts The hydrogen half cell can be used as a reference as it has an emf of 0V

What is a salt bridge

This connects the two solutions and allows ions carrying charge to be transferred between the half cells Filter paper soaked in KNO3 or NH4NO3

What is the relationship between the E value and the metal

The more negative the value the greater the tendency towards the half cell undergoing oxidation (loss of electrons) so the reaction shifts to the left Fe2+ + 2e- ⇌ Fe whereas Cl2 +2e- ⇌ 2Cl- is positive so it is more likely to be reduced and shifts to the right

How do you make a cell from two half cells

Two half cells with different electrode potentials are joined using a wire and salt bridge One half cell releases electrons and the other gains electrons

How would you determine which half cell is being reduced and oxidised

the half cell with the more negative value releases electrons more readily

How is a reading aquired in a cell

The negative terminal releases electrons into the wire and they flow to the other electrode The reading of the voltmeter measures the difference between electrode potentials of the half cells, The bigger the value the further away from the equilibrium position the reaction moves

How would you calculate E of the cell

E of positive terminal - E of negative terminal

What can we find out from reactions by using standard electrode potentials

a species will react with the species below it