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A level Chemistry > Thermochemistry & thermodynamics > Flashcards

Thermochemistry & thermodynamics Flashcards

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1
Q

Standard enthalpy change of formation

A

The enthalpy change when 1 mole of a substance is formed from its constituent elements in their standard states at 298K and 1 bar

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2
Q

Standard enthalpy change of combustion

A

The heat evolved when 1 mole of a substance is completely burnt in excess oxygen at 298 K and 1 bar

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3
Q

Standard enthalpy change of reaction

A

The enthaply change when molar quantities of reactants as specified by their chemical equation react to form products at 298K and 1 bar

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4
Q

Hess’s Law

A

The enthalpy change of a reaction is determined only by the initial and final states and is independent of the reaction pathway taken

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5
Q

General formula for enthalpy change of reaction

A

enthalpy change of reaction = enthalpy change of combustion/formation of (reactants)- (products)

following stoichiometric coeff of eqn

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6
Q

Standard enthalpy change of neutralisation

A

The heat evolved when 1 mole of water is formed in the neutralisation reaction between an acid and a base at 298K and 1 bar

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7
Q

Nature of neutralisation reaction (endo or exothermic)

A

Neutralisation is an exothermic reaction since it involves the attraction of H+ and OH- ions to form an O-H bond

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8
Q

Why neutralisation of stronger acids and bases is more exothermic

A

Strong acids and strong bases ionise completely in dilute aqueous solution and reaction between them is effectively the reaction b/w H+ and OH- ions.

Weak acids/bases do not ionise completely in aqueous solution. During neutralisation, more energy is absorbed to ionise the un-ionised weak acid/base so less energy is released and the resulting enthalpy change of neutralisation is less exothermic.

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9
Q

eqn for heat change of solution (q)

A

q=mc delta T

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10
Q

eqn for enthalpy change of reaction

A

delta H = q/n, -q/n if reaction is exothermic

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11
Q

assumptions in calculation of delta H

A
  • no heat loss or gain from surroundings

- heat capacity of calorimeter is omitted

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12
Q

Bond dissociation energy

A

Bond dissociation energy is the energy required to break 1 mole of a particular covalent bond in a specific molecule in the gaseous state

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13
Q

What affects the magnitude of bond dissociation energy?

A

Bond dissociation energy is a measure of the strength of covalent bonds. The more endothermic the bond dissociation energy, the stronger the covalent bond

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14
Q

Bond energy

A

Bond energy is the average energy required to break 1 mole of a covalent bond in the gaseous state

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15
Q

Standard enthalpy change of atomisation (element)

A

The energy required when 1 mole of gaseous atoms is formed from the element at 298K and 1 bar

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16
Q

Standard enthapy change of atomisation (compound)

A

The energy required to convert 1 mole of the compound into gaseous atoms at 298K and 1 bar

17
Q

Lattice energy

A

The heat evolved when 1 mole of solid ionic compound is formed from its constituent gaseous ions

18
Q

Factors affecting magnitude of lattice energy

A
  • the charges on the ions

- the sizes of ions or inter-ionic distance

19
Q

Ionisation energy

A

The first ionisation energy is the energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of singly charged gaseous cations

20
Q

Electron affinity

A

The first electron affinity is the enthalpy change when 1 mole of electrons is added to 1 mole of singly charged gaseous anions

21
Q

Why 1st EA is -ve

A

1st EA is usually negative as the effective nuclear charge of the atom leads to an attraction of the incoming electron

22
Q

Why 2nd EA is +ve

A

2nd and subsequent EA are always positive because energy is required to overcome the electrostatic repulsion between the incoming electron and anion

23
Q

Experimental versus theoretical lattice energy

A

Theoretical lattice energy refers to the value calculated based on the model that assumes that the compound is completely ionic

A large differences shows that there is covalent character in the ionic compound. This is most apparent when a cation with a high charge density distorts an anion with a large electron cloud

24
Q

Standard enthalpy change of hydration

A

The heat evolved when 1 mole of free gaseous ions is dissolved in an infinite volume of water at 298K and 1 bar

25
Q

Sign of enthalpy change of hydration

A

enthalpy change of hydration is always negtaive as heat evolved in forming ion-dipole interactions between ion and water molecules

26
Q

What affects the magnitude of enthalpy change of hydration?

A

The magnitude of enthalpy change of hydration depends on charge density.

The higher the charge density of the ion, the stronger the ion-dipole interaction and enthalpy change of hydration will be more exothermic

27
Q

Standard enthalpy change of solution

A

The enthalpy change when 1 mole of solute is completely dissolved in an infinite volume of solvent at 298K and 1 bar

\+ve = likely insoluble 
-ve = likely soluble
28
Q

spontaneous changes

A

spontaneous changes are changes that have a natural tendency to occur

29
Q

entropy (S)

A

entropy is a measure of the randomness/disorder in a system, reflected in the number of ways that the energy can be distributed through the motion of its particles

30
Q

Effect of change in temperature on S

A

As temp increases, the average kinetic energy of the particles and the range of energies increase. There are more ways to disperse the energy among the particles. Hence S increases

31
Q

Effect of change in phase on S

A

During (stated change in phase), the particles move more freely in the liquid/gaseous state and become more disordered. Hence there is an increase in entropy as there are more ways to distribute the particles and their energy in the liquid/gaseous state

32
Q

Effect of change in number of particles on S (for gaseous systems)

A

Increase in number of gas particles causes a large increase in entropy. This is because the particles are the most disordered so the number of ways that the particles and the energy can be distributed increase greatly.

33
Q

Effect of mixing of particles on S

A

As the volume available for each gas is increased, there are more ways to distribute the particles and hence their energy. Thus, entropy increases

34
Q

Effect of dissolution of an ionic solid on S

A

Entropy both increases and decreases.

  • Entropy increases because the ions in the solid are free to move in solution
  • Entropy decreases because the water molecules that were originally free to move become restricted in motion as they arrange themselves around the ions

A level Chemistry (5 decks)

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