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Chemistry AS Rafa > 3.2 Physical Chemistry > Flashcards

3.2 Physical Chemistry Flashcards

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1
Q

Define enthalpy change of reaction

A

Energy change associated with a given reaction

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2
Q

Define enthalpy change of formation.

A

The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions.

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3
Q

Define enthalpy change of combustion.

A

The energy change that takes place when 1 mole of substance is completely combusted.

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4
Q

Define enthalpy change of neutralisation.

A

The energy change associated with the formation of 1 mole of water from a neutralisation reaction

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5
Q

What are standard conditions?

A

100kPa
298K
All substances in standard states

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6
Q

What is the standard enthalpy change of the formation of an element?

A

0kJ/mol

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7
Q

What is calorimetry?

A

The quantitative study of energy in a chemical reaction.

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8
Q

What is specific heat capacity?

A

The energy required to raise the temperature of 1g of a substance by 1K.

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9
Q

What is q in the equation for calorimetry?

A

Heat exchanged with the surroundings (J)

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10
Q

What is m in the equation for calorimetry?

A

Mass of the substance heated or cooled (g)

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11
Q

What is c in the equation for calorimetry?

A

The specific heat capacity of the substance that is heated or cooled (J/g/K)

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12
Q

How do you reduce heat lost to surroundings in calorimetry?

A

Insulating equipment with draught shields.

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13
Q

What is average bond enthalpy?

A

The mean energy needed for 1 mole of a given type of gaseous Bonds to undergo homolytic fission

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14
Q

Examples of exothermic reactions

A

Combustion

respiration

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15
Q

Examples of endothermic reactions

A

Photosynthesis

Thermal decomposition

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16
Q

Use ideas about the enthalpy changes that take place during bond breaking and bond
making to explain why some reactions are exothermic

A

Bond breaking absorbs energy
AND bond making releases energy
More energy released than absorbed

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17
Q

Suggest three reasons why standard enthalpy changes of combustion determined
experimentally are less exothermic than the calculated theoretical values.

A

Heat released to the surroundings
Incomplete combustion OR incomplete reaction
OR not everything burns
Non-standard conditions

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18
Q

Name the catalyst used in C2H4(g) + H2O(g) C2H5OH

A

acid

19
Q

State le Chatelier’s principle.

A

When conditions on a system in equilibrium are changed

The equilibrium moves to minimise the effects of the change

20
Q

What is important when calculating kJmol-1?

A

Remember + or - sign

21
Q

It is very difficult to determine the standard enthalpy change of formation of
hexane directly. Suggest a reason why

A

many different hydrocarbons would form
OR activation energy too high
OR reaction too slow
OR they don’t react together

22
Q

Choose a reaction that you have studied that is catalysed by chlorine radicals

A 
Cl + O3 → ClO + O2
ClO + O → Cl + O2
overall: O3 + O → 2O2
OR
Cl + CH4 → CH3 + HCl
CH3 + Cl2 → CH3Cl + Cl
overall: CH4 + Cl2 → CH3Cl + HCl
23
Q

What is a catalyst?

A

A substance that increases rate of reaction without being used up during the process.

24
Q

What is a homogenous catalyst?

A

A catalyst that is in the same phase as the reactants (eg liquid catalyst that is mixed with liquid reactants)

25
Q

Give an example of a homogenous catalyst

A

Enzymes in blood and saliva

26
Q

What is a heterogeneous catalyst?

A

A catalyst that is in a different phase from the reactants.

27
Q

What is an example of a heterogeneous catalyst?

A

A catalytic converter in a car.

28
Q

What catalyst breaks down the triple bond in nitrogen in the Haber process?

A

Iron

29
Q

What is the Boltzmann distribution?

A

The distribution of energies of molecules at a particular temperature, often shown as a graph.

30
Q

What does the area under the curve of a Boltzmann distribution graph represent?

A

The number of molecules

31
Q

Why does the Boltzmann curve start at the origin?

A

There are no molecules with 0 energy.

32
Q

Why does the Boltzman curve not touch the energy axis?

A

There is no maximum energy of a molecule

33
Q

When is a chemical system in dynamic equilibrium?

A

the concentrations of the reactants and the products remain constant
the rate of forward reaction is the same as the rate of the reverse reaction

34
Q

How can the position of equilibrium be altered?

A

By changing:
concentrations of reactants and products
pressure in reactions involving gas
temperature

35
Q

What does a catalyst do to equilibrium?

A

Speeds up the rate of the forward and reverse reactions equally
Establish it faster

36
Q

Examples of important equilibrium systems?

A

Haber process

the conversion of sulphur dioxide into sulphur trioxide in the Contact process.

37
Q

Where do nitrogen and hydrogen come from in the Haber process?

A

Nitrogen is obtained from air by fractional distillation

Hydrogen is prepared by reacting methane and water

38
Q

What are the optimum conditions for the Haber process?

A

High pressure and low temperature
Forward reaction produces less molecules
The forward reaction is exothermic

39
Q

What are the drawbacks of the optimum conditions of the Haber process?

A

Low temperature means low rate

High pressure very expensive high concentration bad safety

40
Q

What are the actual conditions used on a modern ammonia plant?

A

450 degrees
200atm
Iron catalyst

41
Q

Why do bond enthalpies have positive values?

A

Breaking bonds is endothermic and energy must be put in

42
Q

Define the term bond enthalpy

A

The energy needed to break 1 mile of bonds in the substance in the gaseous state

43
Q

How do catalysts improve sustainability?

A 
Not used up in reaction
Lower temperatures
Lower Ea
Less fuel
Less CO2
Different reactions can be used
Better atom economy
Less hazardous chemicals 
Generate specific products

Chemistry AS Rafa (9 decks)

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