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A Level Chemistry Year 1 > Physical Chemistry > Flashcards

Physical Chemistry Flashcards

1
Q

What is enthalpy change, ΔH?

A

The heat energy transferred in a reaction at constant pressure

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2
Q

What are the units of ΔH (enthalpy change)?

A

kj mol^-1

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3
Q

What are standard conditions?

A

100kPa (1 atm) pressure and a temperature of 298K (25°C)

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4
Q

What is standard state?

A

A substance’s physical state in standard conditions

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5
Q

What are the conditions needed to measure standard enthalpy changes?

A

Standard state and standard conditions (as enthalpy change is affected by temperature and pressure)

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6
Q

What is standard enthalpy change of reaction?

A

The enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation, under standard conditions with all reactants and products in their standard states

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7
Q

What is standard enthalpy change of formation?

A

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

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8
Q

What is standard enthalpy of combustion?

A

The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in standard states

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9
Q

What is standard enthalpy of neutralisation?

A

The enthalpy change when solutions of an acid and an alkali react together to form 1 mole of water under standard conditions

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10
Q

What are exothermic reactions?

A

Reactions that give out energy to the environment

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11
Q

Describe exothermic reactions in terms of energy of products and reactants

A

The products have less energy than the reactants (as energy is released to the environment)

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12
Q

Will ΔH be +ve or -ve for exothermic reactions?

A

-ve

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13
Q

Will ΔH be +ve or -ve for endothermic reactions?

A

+ve

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14
Q

Give an example of an exothermic reaction?

A

Combustion

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15
Q

What are endothermic reactions?

A

Reactions that take in energy from their surroundings

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16
Q

Describe endothermic reactions in terms of energy of products and reactants

A

Products have more energy than reactants

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17
Q

Give an example of an endothermic reaction?

A

Thermal Decomposition

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18
Q

What is an enthalpy profile diagram?

A

A graph showing how the enthalpy changes during a reaction

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19
Q

What is the activation energy?

A

The minimum amount of energy needed to begin breaking reactant bonds and start a chemical reaction

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20
Q

What is the activation energy shown by in enthalpy profile diagrams?

A

The difference between the highest point on the graph and the reactants

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21
Q

When is a substance most stable, in terms of energy?

A

When it’s lost all its internal energy

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22
Q

Lower positions on an enthalpy profile diagram will be…

A

… more stable than higher positions

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23
Q

What is bond enthalpy?

A

The energy required to break a bond between 2 atoms

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24
Q

What is average bond enthalpy?

A

The energy needed to break one mole of a bond in the gas phase, averaged over the different compounds that the bond is found in

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25
Q

Is bond breaking endothermic or exothermic?

A

Endothermic, because it needs energy to break the bonds

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26
Q

Is bond forming endothermic or exothermic?

A

Exothermic, as it releases energy

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27
Q

Stronger bonds release….. energy when they are formed

A

More

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28
Q

Stronger bonds take….. energy to breaks

A

More

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29
Q

What equation links enthalpy change of reaction, total energy absorbed and total energy released?

A

Enthalpy change of reaction = Total energy absorbed - Total energy released

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30
Q

When measuring enthalpy changes in the lab, why do you use a polystyrene cup?

A

It’s an insulator, so that you don’t lose or gain much heat through the sides

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31
Q

How do you find the enthalpy changes in the lab for a reaction such as neutralisation?

A

Undergo the reaction in a polystyrene cup and place a thermometer to measure the temperature change

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32
Q

How do you find the enthalpy changes in the lab for a reaction such as combustion?

A

Use a bomb calorimeter containing a known mass of water. You burn a known mass of the reactant and record the temperature change of the water

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33
Q

What is the equation for enthalpy change?

A

q = mcΔT

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34
Q

In the equation q = mcΔT, what does q stand for?

A

Heat lost or gained (measured in J). This is the same as the enthalpy change if the pressure is constant

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35
Q

In the equation q = mcΔT, what does m stand for?

A

Mass (in g) of solution in the insulated container

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36
Q

In the equation q = mcΔT, what does c stand for?

A

Specific heat capacity of the solution/water (4.18 J g^-1 K^-1)

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37
Q

In the equation q = mcΔT, what does ΔT stand for?

A

The change in temperature of the solution

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38
Q

What is the specific heat capacity of a substance?

A

The amount of heat energy it takes to raise the temperature of 1 g of a substance by 1°C

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39
Q

What are the 3 steps for calculating the standard enthalpy change of combustion using data from an experiment in a lab?

A
  • Calculate the amount of heat lost of gained during the combustion using q = mcΔT then convert units from J to kJ
  • Calculate the number of moles of fuel that caused this enthalpy change, from the mass that reacted
  • Calculate the standard enthalpy change of combustion by dividing the actual heat change of the reaction, q (in kJ) and the number of moles of fuel that burned
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40
Q

What are the 3 steps for calculating the standard enthalpy change of reaction using data from an experiment in a lab?

A
  • Calculate the amount of heat lost of gained during the combustion using q = mcΔT then convert units from J to kJ
  • Calculate the number of moles of one of the reactants that caused this enthalpy change
  • Calculate the standard enthalpy change of reaction using the equation ΔH°r = (Heat lost or gained (measured in kJ) / number of moles that reacted) x number of moles reacting in balanced chemical equation
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41
Q

Why might a measured enthalpy change value be different than the one in the data book?

A

Possibly to do with heat loss to the surroundings, or incomplete combustion

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42
Q

What does Hess’s Law state?

A

The total enthalpy change of a reaction is always the same, no matter which route is taken

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43
Q

If ΔH values are for formation ΔfH, what is the energy change equal to?

A

Sum of products - sum of reactants

ΔfH = p - r

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44
Q

If ΔH values are for combustion ΔcH, what is the energy change equal to?

A

Sum of reactants - sum of products

ΔcH = r - p (think CRaP)

45
Q

What is collision theory?

A

States that a reaction won’t take place between 2 particles unless they collide in the right direction and they collide with at least a certain minimum amount of kinetic energy

46
Q

What is the activation energy?

A

The minimum amount of kinetic energy that particles need to have in order to react when they collide (in order to start breaking the bonds)

47
Q

What is Boltzmann Distribution?

A

A theoretical model that describes the distribution of kinetic energies of molecules in a gas

48
Q

What is the effect of temperature on rate of reaction?

A

When you increase the temperature, the molecules will , on average, have more kinetic energy and move faster.. This means a higher proportion of molecules will have at least the activation energy needed to react

49
Q

Describe what Boltzmann Distribution means?

A

With molecules in a gas/liquid, they don’t all have the same kinetic energy. Some have more and some have less

50
Q

When drawing a graph of Boltzmann Distribution, how do you transform the graph if the temperature is increasing?

A

Move the curve to the right (but keep the same area under the curve)

51
Q

When drawing a graph of Boltzmann Distribution, how do you transform the graph if the temperature is decreasing?

A

Move the curve to the left (but keep the same area under the curve)

52
Q

What is the effect of concentration on rate of reaction?

A

A higher concentration means a faster rate of reaction

53
Q

Why does a higher concentration mean a faster rate of reaction?

A

The particles in the solution will be closer together so they’ll collide more often

54
Q

What is the effect of increasing pressure on the rate of reaction for gases?

A

The rate of reaction will increase

55
Q

Why does increasing the pressure on gases cause the rate of reaction to increase?

A

Because the particles are closer together, increasing the chance of collisions

56
Q

If one of the reactants is a solid, what is the effect of increasing surface area on the rate of reaction?

A

A larger surface area makes the reaction faster

57
Q

What is a catalyst?

A

A substance that increases rate of reaction by providing an alternative pathway with a lower activation energy

58
Q

At the end of a reaction, the catalyst is…?

A

Chemically unchanged

59
Q

Why are catalysts used in industrial processes?

A

Because you can save a lot of money and energy, as they don’t get used up in the reaction so you only need tiny amounts to catalyse huge amounts

60
Q

What catalyst does the Haber process use?

A

An iron catalyst to increase the rate of forming ammonia from nitrogen and hydrogen

61
Q

How does a catalyst lower the activation energy of reaction?

A

When a suitable catalyst is present, the reactant molecules bind to the catalyst. This makes it easier to break the bonds, so the activation energy decreases. The broken reactant molecules then form product molecules, and break away from the catalyst

62
Q

What is a heterogeneous catalyst?

A

A catalyst that is in a different phase from the reactants (different physical state)

63
Q

Name a process that uses a heterogeneous catalyst

A

Haber process (gases are passed over a solid iron catalyst)

64
Q

What is a homogeneous catalyst?

A

A catalyst that is in the same physical state as the reactants

65
Q

What state are homogeneous catalysts usually found as?

A

An aqueous catalyst for a reaction between 2 aqueous solutions

66
Q

How does a homogeneous catalyst work?

A

One or more reactants combine with the catalyst to from an immediate species, which then reacts to form the product and reform the catalyst

67
Q

How does using catalysts effect the environment?

A

Using catalysts means a lower temperature and pressure can be used. This means less CO2 is produced and less fossil fuels are used

68
Q

Howe do catalysts reduce waste?

A

They allow a different reaction to be used to make the same product, but with a greater atom economy

69
Q

What are catalytic converters made from?

A

Alloys of plutonium, palladium and rhodium

70
Q

How do catalytic converters work?

A

They reduce pollution into the atmosphere by speeding up the reaction:

2CO + 2NO —> 2CO2 + N2

71
Q

What is an issue with catalysts?

A

They don’t last forever so eventually they need to be disposed of

72
Q

What is the problem of disposing catalysts?

A

Most contain nasty toxic compounds, which may leach into the soil if they’re sent directly to a landfill

73
Q

In what causes would a catalyst be recycled rather than sent to landfill?

A

If it contains precious metals such as plutonium

74
Q

What does uncatalysed mean?

A

No catalyst was used

75
Q

What is the reaction rate defined as?

A

The change in the amount of reactants or products per unit time

76
Q

What is reaction rate?

A

How fast a reaction takes place

77
Q

What is the units of reaction rate?

A

‘whatever you’re measuring per unit time’ (e.g. g s^-1)

78
Q

What is the equation for rate of reaction?

A

Rate of reaction = Amount of product formed or reactant used / Time

79
Q

What are the 5 steps to measure the progress of a reaction when the product is a gas?

A
  • Set up a mass balance and place on top a conical flask with the reactants in it
  • Measure the start mass
  • When the reaction starts, start the stopwatch
  • Take regular mass readings at regular intervals
  • Make a table of column time and column mass
80
Q

The rate of reaction… as time progresses

A

Slows down

81
Q

Why does the rate of reaction slow down as time progresses?

A

The reactants get used up, and so collisions between molecules become less likely

82
Q

How do you work out the rate of reaction from a mass-time graph?

A

It’s the gradient (draw a tangent at a given point and find gradient)

83
Q

What is dynamic equilibrium?

A

In a reversible reaction, dynamic equilibrium is reached when the concentration of reactants and products are constant, and the forward and backwards reactions are going at the same rate

84
Q

What is the only condition needed for a dynamic equilibrium?

A

For the reaction to be in a closed system, so nothing can get in or out

85
Q

What is Le Chatelier’s Principle?

A

If there’s a change in concentration, pressure or temperature, the equilibrium will move to help counteract the change

86
Q

In a reversible reaction, what does it mean if the position of equilibrium moves left?

A

The backwards reaction is faster, so you’ll get more reactants

87
Q

What is the effect of a catalyst on the position of equilibrium?

A

No effect, as they speed up the forward and reverse reaction at the same rate

88
Q

For Le Chatelier’s Principle, what happens if you increase the concentration of reactant?

A

The equilibrium tries to get rid of the extra reactant, so it shifts to the right so that more product is made

89
Q

For Le Chatelier’s Principle, what happens if you increase the concentration of product?

A

The equilibrium tries to get rid of the extra product, so it shifts to the left, making the reverse reaction faster, so that more reactant is made

90
Q

For Le Chatelier’s Principle, what happens if you increase the pressure?

A

The equilibrium shifts to the side with the fewest gas molecules to reduce the pressure

91
Q

For Le Chatelier’s Principle, what happens if you decrease the pressure?

A

The equilibrium shifts to the side with the most gas molecules to increase the pressure

92
Q

What state does the reactants and products have to be in for a change in pressure to make any difference to the position of the equilibrium?

A

Changing the pressure only affects equilibria involving gases

93
Q

If there are the same number of moles of gas on each side of the reaction, what happens to the equilibrium when you change the pressure?

A

It won’t affect the position of the equilibrium

94
Q

For Le Chatelier’s Principle, what happens if you increase the temperature?

A

Increasing the temperature means adding heat energy, so the equilibrium shifts in the endothermic (positive ΔH) direction to absorb this energy

95
Q

For Le Chatelier’s Principle, what happens if you decrease the temperature?

A

Decreasing the temperature means removing heat energy, so the equilibrium shifts in the exothermic (negative ΔH) direction to try and replace this energy

96
Q

If the forward reaction is endothermic, what will the reverse reaction be?

A

Exothermic

97
Q

What is yield?

A

The amount of product you get from a reaction

98
Q

What are the industrial conditions for the production of ethanol?

A

Pressure: 60-70 atm
Temperature: 300 °C
Phosphoric acid catalyst

99
Q

Why did they chose the industrial conditions for the production of ethanol to be what they are?

A

To balance cost and yield (due to Le Chatelier’s Principle)

100
Q

How do you investigate the position of equilibrium due to a change in temperature?

A

The brown gas NO2 exists in equilibrium with the colourless gas N2O4

  • Place 2 sealed tubes of containing the equilibrium mixture in a water bath (one in a warm and one in a cool)
  • The tube in the warm water will change to a dark brown colour
  • The tube in the cold water will go a paler colour
101
Q

Why aren’t ideal conditions for maximum yield used in reality?

A

Because of the cost of generating the conditions or their safety

102
Q

What is a homogeneous reaction?

A

A reaction where all the reactants and products are in the same physical state

103
Q

What is the equation to work out the equilibrium constant (Kc) for the general reaction aA + bB ⇌ dD + eE (where lower case = number of moles)?

A

Kc = ( [D]^d x [E]^e ) / ( [A]^a)x [B]^b )

the square brackets mean the concentration in mol dm^-3

104
Q

What is the equilibrium constant?

A

A ratio worked out from the concentration of the products and reactants once a reversible reaction has reached equilibrium

105
Q

When calculating equilibrium constant, what goes on the top line and what goes on the bottom line of the fraction?

A

Products on top, reactants on bottom

106
Q

How do you work out the units of the equilibrium constant?

A

Put the units in the equation on their own and simplify the fraction

107
Q

What does it mean if the equilibrium constant is a large number?

A

At equilibrium there will be lots of product and not much reactant

108
Q

What does it mean if the equilibrium constant is a small number?

A

At equilibrium there will be lots of reactant and not much product

109
Q

How do you estimate the position of equilibrium?

A

Kc will increase when the quantities of the products increase, so an increase in Kc means the equilibrium has shifted to the right

A Level Chemistry Year 1 (9 decks)

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