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GAMSAT March 2016 > Gen 8 - Enthalpy & Thermochemistry > Flashcards

Gen 8 - Enthalpy & Thermochemistry Flashcards

1
Q

What is thermochemistry ?

A

The study of energy absorbed or released in chemical reactions or any physical transformations (ie. phase change)

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2
Q

What equation represents the enthalpy change of a reaction ?

A
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3
Q

What does a positive enthalpy change (delta H+) represent ?

A

The flow of heat into a system as a reaction occurs - E.g. An Endothermic reaction

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4
Q

What does a negative enthalpy change represent ?

A

Heat energy is given off to its surroundings - an exothermic reaction.

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5
Q

What is the law of conservation of energy ?

A

The total energy of an isolated system remains constant over time (if there is no associated external force)

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6
Q

What is the second law of thermodynamics ?

A

The entropy or randomness of the universe increases in any real system.

For a reaction to proceed, the entropy of the system must increase.

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7
Q

What is the difference between enthalpy and entropy ?

A

Enthalpy is the heat transfer that takes place at constant pressure.

Entropy gives an idea of the randomness of a system.

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8
Q

What is the first law of thermodynamics ?

A

Energy can neither be created nor destroyed.

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9
Q

What is delta Hf ?

A

The standard enthalpy (or heat) of formation is the change of enthalpy that would occur when 1 mole of a substance is formed from its elemental constituents, in a standard state reaction.

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10
Q

What is the standard enthalpy of formation of an element in its standard state ?

A

Zero, as no enthalpy change is involved in their formation.

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11
Q

What are the SI units for enthalpy ?

A

KJ/mole

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12
Q

What is delta Hrxn ?

A

The standard enthalpy change for a reaction - the change of enthalpy that would occur if one mole of matter is transformed by a chemical reaction with all reactants and products under standard state.

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13
Q

What is Hess’s law ?

A
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14
Q

Why is it important to define standard conditions in terms of Thermochemistry ?

A

Because enthalpy changes depend on the conditions under which the reactions are carried out.

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15
Q

What is standard pressure ?

A

1 atmosphere, approx 1 barr

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16
Q

What is standard temperature with regards to thermochemistry ?

A

Generally 25 ºC if unspecified

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17
Q

What is the standard physical state of an element ?

A

The “natural” physical state of an element under standard temperature and pressure

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18
Q

What is the standard temperature for an ideal gas ?

A

0ºC

19
Q

What is the bond dissociation energy ?

A

A measure of the bond strength within a particular molecule - the standard enthalpy change in the homolytic cleavage of a chemical bond.

20
Q

Why are bond energies always positive values ?

A

Because it always takes energy to break bonds apart.

21
Q

Describe bond dissociation energy and bond energy in relation to CH4 - why are they not the same ?

A

The bond dissociation energies for each of the homolytic C-H bonds cleavage are all slightly different. The average of the four bond dissociation energies is known as the bond energy.

22
Q

When is bond dissociation energy and bond energy the same ?

A

In all diatomic molecules because there is cleavage of only one bond.

23
Q

Define bond energy

A

A measure of bond strength. The energy required to sever 1 mole of a chemical bond in a gas.

24
Q

How would you calculate Energy transferred (Q) in a thermochemical reaction ?

A
25
Q

When is heat absorbed or released but without a change in temperature of the surroundings ?

A

During a phase change

26
Q

What is a calorie ?

A

The amount of thermal energy required to raise the temperature of 1g of water by 1ºC

27
Q

What is a calorie in joules ?

A

1 cal = 4.184 Joules

28
Q

Describe the 3 enthalpy phase changes ?

A
  1. Heat of fusion Solid –> Liquid
  2. Heat of vaporization Liquid –> Gas
  3. Heat of sublimation Solid –> Gas
29
Q

How do we calculate the heat required for a phase change ?

A

Q = ML

Where:

Q = heat transferred to system

M = mass of sample

L = Heat of transformation

30
Q

What is Calorimetry ?

A

The science of measuring the heat evolved or exchanged due to a chemical reaction.

31
Q

What are the most spontaneous transformation with regards to the 2nd law of thermodynamics ?

A

Transformations which require the smallest amount of energy and lead to the largest disorder of the system are the most spontaneous.

32
Q

What is entropy (S) ?

A

A state function which measures the degree of disorder in a system.

Generally speaking, it increases from a solid to a liquid to a gas.

33
Q

Describe entropy in terms of a rubber band

A

Entropy decreases when the elastic band is stretched. In the relaxed elastic band, the molecules of the rubber polymer are coiled up and form a disorganised structure.

As the rubber is stretched, the molecules tend to line up with each other and adopt a more organised structure.

34
Q

What are the units of entropy ?

A

J/K or Cal/K

35
Q

What is the law of disorder ?

A

The natural tendancy is for systems to move to the direction of maximum disorder, not vice versa.

E.g. Your room never cleans itself !

36
Q

What is the equation for entropy change ?

A
37
Q

What is Gibbs free energy ?

A

A state function of entropy that can be used as a criterion for spontaneity.

38
Q

How do you calculate Gibbs free energy ?

A
39
Q

What are the unit of Gibb’s free energy ?

A

KJMol-1

40
Q

How does the value of delta G carried out at constant pressure, determine spontaneity of a reaction ?

A

A reaction carried out at constant pressure is spontaneous is delta G <0. It is not spontaneous if delta G >0.

41
Q

How is a reaction proceeding if delta G = 0 ?

A

The reaction is at equilibrium.

42
Q

Describe the best time to add milk to a hot drink in order to maximally cool it ?

A

Heat moves to an environment fastest when there is the greatest temperature difference, therefore for maximal heat loss, milk should be added just before drinking.

43
Q

What is the difference between a burn from steam at 100ºC, and a burn from water at 100ºC ?

A

The steam burn would be more severe as it has more heat energy to be released onto the skin. Energy is lost to the skin by condensation.

GAMSAT March 2016 (26 decks)

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