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Chemistry > 5. Energetics & Thermochemistry > Flashcards

5. Energetics & Thermochemistry Flashcards

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1
Q

Define ‘bond enthalpy’.

A

The energy required to break one mole of bonds by homolytic fission, in one mole of gaseous covalent molecules under standard conditions.

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2
Q

ΔH= ?

A

-Q/ n

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3
Q

Q= ?

A

mcΔT

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4
Q

Define ‘specific heat capacity’.

A

The energy required to heat 1g of a substance by 1°C

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5
Q

State Hess’ Law.

A

The enthalpy change for a chemical reaction will always be equal, regardless of which route is taken.

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6
Q

What is enthalpy change?

A

The heat energy transferred at constant pressure.

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7
Q

Enthalpy change= ?

A

Enthalpy of products - enthalpy of reactants

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8
Q

Define ‘heat’.

A
  • Thermal potential energy.
  • Heat will transfer from a warmer species to a cooler species down a potential energy gradient.
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9
Q

Define ‘temperature’.

A

A measure of the kinetic energy of a particle.

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10
Q

What is an endothermic reaction?

A
  • Involve a transfer of heat from the surroundings to the system.
  • The enthalpy of the system increases, so is positive.
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11
Q

What is an exothermic reaction?

A
  • Involve a transfer of heat from the system to the surroundings.
  • The enthalpy of the system decreases, so is negative.
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12
Q

What are the x and y axis of an energy profile graph of a reaction?

A
  • X= reaction coordinate
  • Y= potential energy
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13
Q

What four factors affect enthalpy change?

A
  • Mass of the surroundings
  • Amount of heat the surroundings will store
  • The temperature change during the reaction
  • The moles of the system
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14
Q

Describe metal oxides.

A
  • Basic as an OH- ion is released.
  • Dissociates when dissolved in water (if soluble).
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15
Q

Describe non-metal oxides.

A
  • Acidic as H+ ion released
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16
Q

Describe how global warming is caused.

A
  • The earth’s surface absorbs the IR and reradiates it a lower frequency.
  • Greenhouse gases absorb lower frequency IR, which is used to stretch covalent bonds like a spring.
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17
Q

Where do fossil fuels come from?

A
  • Oil from fossilised marine organisms
  • Coal from fossilised plants in swamps.
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18
Q

Name the 5 steps of creating biofuels.

A
  1. Photosynthesis
  2. Harvest crops
  3. Transportation
  4. Fermentation
  5. Purification
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19
Q

Define ‘standard enthalpy change of combustion’.

A

The enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions.

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20
Q

Define ‘average bond enthalpy’.

A
  • The average amount of energy required to break one mole of covalent bonds in a gaseous molecule under standard conditions.
  • In diatomic molecule only.
  • Average refers to the fact that the bond enthalpy is different in different molecules.
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21
Q

Define ‘standard enthalpy change of formation’.

A

The enthalpy change when one mole of a substance is formed from its elements in their standard states under standard conditions.

22
Q

Define ‘standard enthalpy change of atomisation’.

A

The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state under standard conditions.

23
Q

Define ‘first electron affinity’.

A

The enthalpy change when one electron is added to each atom in one mole of gaseous atoms under standard conditions.

24
Q

Give the generic equation representing first electron affinity.

A

X(g) + e- –> X-(g)

25
Q

Why will the data book value for the standard enthalpy of combustion differ from the calculated value?

A
  • Bond enthalpy values are average values deduced from across a range of molecules and are not entirely accurate.
  • Standard enthalpy of combustion refers to substances in their standard states, so there may be differences in the states of products formed.
26
Q

Define ‘lattice enthalpy’

A

The enthalpy change when one mole of ionic solid forms from its constituent gaseous ions.

27
Q

What is the enthalpy of formation of an element?

A

Zero

28
Q

What is entropy?

A
  • A measure of disorder.
  • The number of ways that particles can be arranged and the number of ways that energy can be distributed between those particles.
29
Q

How can entropy be increased?

A
  • Increase the number of moles.
  • Solid –> liquid –> gas.
30
Q

What is a spontaneous reaction?

A

A reaction that takes place continually without intervention.

31
Q

What must the entropy be for a spontaneous reaction?

A

Greater than 0.

32
Q

What are the units of entropy?

A

J/mol/K

33
Q

What happens to entropy between phase changes? Why?

A
  • There is a sharp increase.
  • Bonds are weakened or broken so there are suddenly more ways to arrange molecules.
34
Q

Why is there a gradual increase of entropy when heating occurs?

A

There is a larger distribution of the molecular energies.

35
Q

Total entropy change = ?

A

Entropy change of the system + entropy change of the surroundings.

36
Q

Entropy change of the system= ?

A

Entropy of products- entropy of reactants

37
Q

∆G= ?

A

∆H - T∆S

38
Q

What are the units of Gibbs energy?

A

kJ/mol

39
Q

What is Gibbs energy dependent on?

A

Temperature

40
Q

Is the reaction feasible if ∆H is positive, ∆Ssys is negative, and ∆G is positive?

A

Never

41
Q

Is the reaction feasible if ∆H is positive, ∆Ssys is positive, and ∆G is negative?

A

Only if the temperature is high.

42
Q

Is the reaction feasible if ∆H is negative, ∆Ssys is positive, and ∆G is negative?

A

Always

43
Q

Is the reaction feasible if ∆H is negative, ∆Ssys is negative, and ∆G is negative?

A

Yes, depending on the temperature being low.

44
Q

What is free energy?

A

A suitable measure of a reaction’s feasibility as it indicates how far a reaction will proceed towards completion.

45
Q

What is free energy proportional to for gas phase systems?

A

The equilibrium constant.

46
Q

∆G= ?

A

-RTlnK

47
Q

What does kinetic inertness refer to?

A

A compound or mixture that does not undergo a reaction, despite the reaction appearing to be feasible.

48
Q

What is the principal reason for feasible reactions being inert?

A

High activation energy

49
Q

What do ∆G and the activation energy have to be for a reaction to tale place?

A

Negative ∆G and low activation energy

50
Q
A

Chemistry (10 decks)

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