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Chemistry > 17 - Thermodynamics > Flashcards

17 - Thermodynamics Flashcards

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1
Q

What is an enthalpy change?

A

Thermal energy change of a reaction under constant pressure.

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2
Q

What are standard conditions?

A

100kPa
298K (25℃)

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3
Q

What is the standard molar enthalpy of formation?

A

The enthalpy change when one mole of a compound is formed from its constituent elements under standard conditions, with products and reactants in their standard states.
Endo or exo

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4
Q

What is the standard molar enthalpy combustion?

A

The enthalpy change when one mole of a substance is completely burnt in excess oxygen.
Exo

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5
Q

What is the standard molar enthalpy of atomisation?

A

The enthalpy change which accompanies the formation of one mole of gaseous atoms from the element in its standard state under standard conditions.
Endo

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6
Q

What is first ionisation energy?

A

The standard enthalpy change when one mole of gaseous atoms are converted into one mole of gaseous ions, each with a single positive charge.
Endo

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7
Q

What is second ionisation energy?

A

The standard enthalpy change of the loss of a mole of electrons from a mole of singly charged positive ions.
Endo

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8
Q

What is first electron affinity?

A

The standard enthalpy change when one mole of gaseous atoms are converted to a mole of gaseous ion each with a single negative charge.
Endo or exo

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9
Q

What is second electron affinity?

A

The standard enthalpy change when one mole of electrons is added to a mole of gaseous ions, each with a single negative charge to form ions each with two negative charges.
Endo

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10
Q

What is the lattice enthalpy of formation?

A

The standard enthalpy change when one mole of solid ionic compound is formed from its gaseous ions.
Exo

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11
Q

What is the lattice enthalpy of dissociation?

A

The standard enthalpy change when one mole of solid ionic compound dissociates into its ions.
Endo

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12
Q

What is the standard enthalpy of hydration?

A

The standard enthalpy change when one mole of gaseous ions are dissolved in water to form an aqueous solution under standard conditions.
Exo

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13
Q

What is the standard enthalpy of solution?

A

The standard enthalpy change when one mole of solute dissolves completely in sufficient solvent to form a solution in which the molecules or ions are far enough apart to not interact with each other.
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14
Q

What is the mean bond dissociation enthalpy?

A

The enthalpy change when one mole of gaseous molecules break a covalent bond to form two free radicals, arranged over a range of compounds.
Endo

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15
Q

What is mean bond enthalpy?

A

The energy needed to break one mole of a certain type of bond in a gaseous molecule, under standard conditions.
Endo

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16
Q

What are the enthalpy changes when forming ionic compounds?

A

1) Atomisation of both elements.
2) Ionisation to form cation
3) Electron affinity to form anion
4) Lattice formation

17
Q

What is Hess’s law?

A

The total enthalpy change for a chemical reaction is the same regardless of the route taken, provided conditions are constant.

18
Q

What is a Born-Haber cycle?

A

A thermochemical cycle that includes all the enthalpy changes involved in the formation of an ionic compound.

19
Q

What are Born-Haber cycles used for?

A

To see if compounds might be expected to exist (-ΔHf).
To find the unknown quantity of lattice formation enthalpy.

20
Q

What is the correlation of the size of the ion and the lattice enthalpies?

A

The larger the ion the smaller the lattice enthalpy as the opposite charges do no approach each other as closely.

21
Q

What is the correlation of the size of the nuclear charge and the lattice enthalpies?

A

As nuclear charge increases the lattice enthalpy increases as ions with double the charge give out roughly twice as much energy when they come together.

22
Q

What is the solubility of ionic solids like?

What are they soluble in

A

Only dissolve well in polar solvents.

23
Q

What happens during hydration of ionic solids?

A

The serparate ions are solvated by the solvent molecules which cluster around the ions so the cation are surrounded by the negative ends of the dipole of water molecules and vice versa.

24
Q

How do you calculate the enthalpy of solution of an ionic compound?

A

1) Find the lattice dissociation enthalpy
2) Find the enthalpy of solution of the ions. (Hydrating the ion)

25
Q

Why is there a large discrepancy between the experimental and theoretical values for the lattice formation enthalpy?

A

There may be some covalent character.
If the cation can approach the anion closely and attract the electron clouds of the anion the anion is polarised and there is a degree of electron sharing covalency.

26
Q

What factors increase polarisation?

A

Cation - small size, high charge.
Anion - large size, high charge.

27
Q

What is a feasible/ spontaneous reaction?

A

A reaction that could take place on its own.

28
Q

What is entropy?

A

The randomness of a system.

29
Q

What affects whether a reaction is feasible or not?

A

The enthalpy change - negative
The entropy - high

30
Q

What increases entropy?

A

If the reaction involves disordering, the arrangement of the particles in the products is more reactive than in the reactants.

31
Q

What effect does temperature have on the entropy?

A

Entropy increases with temperature as at a higher temperature the particles spread out and randomness increases.

32
Q

What is the formula for Gibbs free energy?

A

△G = △H - T△S

33
Q

What is the formula for entropy?

A

△S = △S(products) - △S(reactants)

34
Q

What does it mean if △G is negative?

A

The reaction is feasible.

35
Q

What does it mean if △G = 0?

A

The point where the reaction is just feasible.
At what temperature equilibrium exists in a closed system.

36
Q

What is △G used for?

A

To investigate under what temperature the reaction is feasible.

37
Q

What is the issue with △G?

A

It tells us nothing about how quickly a reaction is likely to go, so the reaction might be feasible but it goes so slowly that for practical reasons it doesn’t occur at all. (High activation energy)
Thermodynamically unstable but kinetically stable.

Chemistry (17 decks)

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