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Chemistry > Enthalpy and Entropy A2 > Flashcards

Enthalpy and Entropy A2 Flashcards

1
Q

What is the definition of Enthalpy of Formation? (ΔfH) Exo or endothermic?

A

Enthalpy change when one mole of a substance is formed from its constituent elements with all substances in their standard states.

Exothermic for most substances.

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2
Q

What is the definition of Enthalpy of Combustion? (ΔcH) Exo or endothermic?

A

Enthalpy change when one mole of substance undergoes complete combustion in oxygen with all substances in standard states.

Exothermic.

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3
Q

What is the definition of Enthalpy of Neutralisation?(ΔneutH) Exo or endothermic?

A

Enthalpy change when one mole of water is formed in a reaction between an acid and alkali under standard conditions.

Exothermic.

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4
Q

What are the definitions of Ionisation enthalpy? (ΔIEH) Exo or endothermic?

A

First ionisation energy = enthalpy change when each atom from one mole of atoms loses an electron to form one mole of gaseous 1+ ions.

Second ionisation energy = enthalpy change when each ion from one mole of 1+ ions loses an electron to form one mole of gaseous 2+ ions.

Endothermic

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5
Q

What are the definitions of Electron Affinity (ΔEAH) Exo or endothermic?

A

First electron affinity = enthalpy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous 1- ions.

Exothermic for many non-metals.

Second electron affinity = enthalpy change when each ion in one mole of gaseous 1- ions gains one electron to form one mole of gaseous 2- ions.

Endothermic as adding -ve electron to -ve ion = +ve

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6
Q

What is the definition of Enthalpy of Atomisation? (ΔatH) Exo or endothermic?

A

Enthalpy change when one mole of gaseous atoms is produced from an element in its standard state.

e.g. I2 (s) –> I (g)

Endothermic

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7
Q

What is the definition of Hydration enthalpy? (ΔhydH) Exo or endothermic?

A

Enthalpy change when one more of gaseous ions becomes hydrated.

e.g. Mg2+(g) + aq –> Mg2+(aq)

Exothermic

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8
Q

What is the definition of Enthalpy of Solution? (ΔsolH) Exo or endothermic?

A

Enthalpy change when one mole of ionic solid dissolves in a solvent.

e.g. MgCl2(s) + aq –> Mg2+(aq) + 2 Cl-(aq)

Varies

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9
Q

What is the definition of Bond Dissociation enthalpy? (ΔdisH) Exo or endothermic?

A

Enthalpy change when one mole of covalent bonds are broken in gaseous state.

Endothermic.

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10
Q

What is the definition of lattice enthalpy? (ΔLEH) Exo or endothermic?

A

Enthalpy change when one moles of a solid ionic compound is formed from its constituent gaseous ions.

Exothermic

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11
Q

What is the definition of Enthalpy of vaporisation? (ΔvapH) Exo or endothermic?

A

Enthalpy change when one mole of a liquid is turned into a gas.

Endothermic

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12
Q

What is the definition of Enthalpy of Fusion? (ΔfusH) Exo or endothermic?

A

Enthalpy when one mole of a solid is turned into a liquid.

Endothermic

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13
Q

What type of enthalpy cycle relates lattice enthalpy and enthalpy of formation?

A

Born-Haber cycle

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14
Q

What two processes make up dissolving a substance in water?

A

Breaking ionic lattice (reverse lattice enthalpy) and water molecules surrounding (hydrating) the ions.

Enthalpy of solution = reverse lattice enthalpy* + hydration enthalpy

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15
Q

How does ionic size/radius and ionic charge affect lattice enthalpy?

A

IONIC RADIUS:
- The larger the ionic radius, the further apart and the ions are and so the forces of attraction are weaker. (So less exothermic)
- The smaller the ionic radius, the more closely packed together the ions are so greater forces of attraction. (So lattice enthalpy more exothermic).

IONIC CHARGE:
As ionic charge increases, attraction between ions increases and latice enthalpy becomes more negative/exothermic.

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16
Q

How does ionic radius and ionic charge affect hydration enthalpy?

A

IONIC RADIUS:
As ionic radius increases, attraction between water molecules and ions decreases and hydration enthalpy becomes less negative/exothermic.
(smaller molecules have a greater attraction between water molecules).

IONIC CHARGE:
As ionic charge increases, attraction between water molecules and ions increases and hydration enthalpy becomes more negative/exothermic.

17
Q

What is the definition of Entropy? What is the symbol to represent entropy? Units?

A

A measure of THE DISPERSAL OF ENERGY IN A SYSTEM, which increases as the system gets more disordered.

ΔS - J / K / mol

(higher the entropy, greater the chaoticness!)

18
Q

Does lowering the entropy make the value more or less negative?

A

Decreasing entropy = more negative

19
Q

Why are the values for enthalpy negative?

A

Energy is lost (to the surroundings).

20
Q

Put the 3 states of matter in order of increasing entropy.

A

Solid –> liquid –> gas

21
Q

When…
- a liquid boils to become a gas
- a lquid freezes to become a solid

is ΔS postive or negative?

A

Evaporation = Positive

Freezing = negative

22
Q

If there are more moles of products than reactants, does entropy increase or decrease?

A

Increase

23
Q

What is the equation to work out standard entropy? What is standard entropy?

A

ΔS = ΣS (products) - ΣS (reactants)

The entropy of one mole of a substance under standard conditions (always positive values).

24
Q

If there are the same number of moles of products and reactants and they are in the same state, what happens to entropy?

A

Entropy change is very small so you can’t tell a difference (increase/decrease) without calculations.

25
Q

In an equation, when gas is a product but not a reactant, is ΔS positive or negative?

A

Positive

e.g. CaCO3 + HCl –> CaCl2 + H2O + CO2(g)

26
Q

What is free energy change? What is the symbol?

A

ΔG
The overall energy change caused by the:
- heat transfer between the system and surroundings (ΔH)
- dispersal of energy within the system at the temperature of the reaction (TΔS)

Internal and external energy change

27
Q

What is the Gibbs equation?

A

ΔG = ΔH - (T x ΔS)

Free energy change = enthalpy change - (Temperature (K) x entropy change)

28
Q

When is a reaction feasible, in relation to free energy?

A

When ΔG < 0 (negative value)

29
Q

If ΔH is positive and ΔS is negative, can the reaction ever be feasible?

A

No - two negatives make a positive

30
Q

How do you arrange the Gibbs equation to calculate the minimum temperature that the reaction is feasible?

A

ΔG = ΔH - (T x ΔS)

Set ΔG to 0.

Then rearrange to T = ΔH / ΔS

31
Q

How can endothermic reactions be made feasible?

A

If ΔS is positive enough and T really high, then then -TΔS may be negative enough to outweigh ΔH.

32
Q

Why might a feasible reaction not take place?

A

May have a very high activation energy, so is very slow at normal conditions.

33
Q

What is the thing I keep forgetting to do?

A

Use a negative sign for exothermic reaction!!!! ΔH

Ask yourself for everything ΔH reaction - is this endo or exothermic?

Chemistry (22 decks)

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