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Alevel Chemistry OCR A > Module 3: Section 2 - Physical Chemistry > Flashcards

Module 3: Section 2 - Physical Chemistry Flashcards

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1
Q

Enthalpy changes
Definition of enthalpy change AH (delta H)

A

Energy transferred between a system and its surroundings at a constant pressure and final temperature is the same as the start temperature

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2
Q

Enthalpy changes
What are the units for enthalpy change

A

kJ mol ^-1

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3
Q

Enthalpy changes
What is a system

A

Chemical or chemicals being studied and if system is closed there is no exchange between the system and the surroundings

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4
Q

Enthalpy changes
What does standard enthalpy changes refer to

A

Reactions done under standard conditions and with everything in their standard stats

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5
Q

Enthalpy changes
What are standard conditions

A

298 K (25*C)
Pressure of 1atm (100kPa)
Where solutions involved have concentration of 1 mol dm-3

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6
Q

Enthalpy changes
What is exothermic change

A

Give out heat to surroundings

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7
Q

Enthalpy changes
Examples of exothermic changes

A

Combustion
Respiration
Hot packs

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8
Q

Enthalpy changes
Is delta H positive or negative for exothermic changes

A

Negative as the products are at a lower energy level than reactants

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9
Q

Enthalpy changes
On an enthalpy profile diagram for exothermic changes where are the products

A

Products are below the reactants

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10
Q

Enthalpy changes
What are endothermic changes

A

Take in energy from the surroundings

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11
Q

Enthalpy changes
Examples of endothermic changes

A

Melting and vaporisation
Photosynthesis

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12
Q

Enthalpy changes
Is delta H popsoitve or negative for endothermic changes

A

Positive as products have more energy than the reactants

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13
Q

Enthalpy changes
In and energy profile diagram are the products above or below

A

Products above reactants

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14
Q

Enthalpy changes
What must happen for a reaction to occur

A

Bonds must be broken and new ones formed so energy must be supplied to start both endothermic and exothermic reactions

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15
Q

Enthalpy changes
What is activation energy Ea

A

Minimum amount of energy needed to begin breaking reactants bonds and start a chemical reaction

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16
Q

Enthalpy changes
Units for activation energy Ea

A

kJ mol-1

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17
Q

Enthalpy changes
How is the symbol of delta H modified fr standard enthalpy change

A

A H °298(small)
Delta symbol triangle then capital H then degree sign with lien through horizontally then small 298 at bottom

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18
Q

Enthalpy changes
What is the enthalpy change of a reaction

A

Enthalpy change associated with a stated reaction

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19
Q

Enthalpy changes
Sign for enthalpy change of reaction

A

AH r
Delta symbol then capital H them small r

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20
Q

Enthalpy changes
What is enthalpy change of combustion

A

Enthalpy change when one mole of a substance undergoes complete combustion

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21
Q

Enthalpy changes
Sign for enthalpy change of combustion

A

A H c
Delta sign then capital H the small c

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22
Q

Enthalpy changes
What are enthalpy change of combustion always

A

Always exothermic so negative delta H

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23
Q

Enthalpy changes
Example of enthalpy change of combustion for ethanol

A

C2H5OH (l) 3 O2 (g) —> 2 CO2 (g) + 3 H2O (l)

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24
Q

Enthalpy changes
Important for equation of enthalpy change of combustion

A

Include state symbols, always balanced, COMPLETE combustion

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25
Q

Enthalpy changes
What is the enthalpy change of formation

A

Enthalpy change when one mole of a compound is formed from its elements

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26
Q

Enthalpy changes
Sign for enthalpy change of formation

A

A H f
Delta sign then capital H then small f

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27
Q

Enthalpy changes
What is the reaction for enthalpy cage of formation usually

A

Usually but not exclusively exothermic

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28
Q

Enthalpy changes
Equation for enthalpy of formation

A

Elements have to be in standard states which have 0 enthalpy
Include state symbols
Carbon usually taken as graphite

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29
Q

Enthalpy changes what is enthalpy change of neutralisation

A

Enthalpy change when one mole of water is formed from its ions in dilute solution

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30
Q

Enthalpy changes
What type of reaction is enthalpy of neutralisation

A

Exothermic so negative delta H

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31
Q

Enthalpy changes
Equation for enthalpy of neutralisation

A

H+ (aq) + OH- (aq) —> H2O (l)

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32
Q

Enthalpy changes
What value is obtained when strong acids react with strong alkalis

A

-57 kJ mol-1

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33
Q

Calorimetry
What is calorimetry

A

Practical determination of enthalpy changes usually involving heating or cooling known amounts of water

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34
Q

Calorimetry
What reaction is it when water is heated up in calorimetry

A

Exothermic

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35
Q

Calorimetry
What reaction is it when water cooled as down in calorimetry

A

Endothermic

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36
Q

Calorimetry
What equation can be used to calculate energy required to change the temperature of a substance

A

q = m x c x AT

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37
Q

Calorimetry
What is q in q = m x c x AT

A

Heat energy/ energy transferred in kJ

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38
Q

Calorimetry
What is m in q = m x c x AT

A

Mass of solution in kg

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39
Q

Calorimetry
What is c in q = m x c x AT

A

Specific heat capacity in kJ K-1 kg-1

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40
Q

Calorimetry
What is AT in q = m x c x AT

A

Change in temperature in K

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41
Q

Calorimetry
What is specific heat capacity of water

A

4.18 kJ K-1 kg-1

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42
Q

Calorimetry
Equation to calculate enthalpy change

A

AH =q / moles in reaction

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43
Q

Calorimetry
What is AH in AH = q/moles in reaction

A

Enthalpy change in kJ mol-1

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44
Q

Calorimetry
What is q in AH = q / moles in reaction

A

Energy transfer in J

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45
Q

Calorimetry
What is the graphical method to calculate a value of AT (change in temp)

A

Temperature taken every half minute before mixing the reactants
Reactants mixed after 3 mins
Further reading taken every half minute as reaction mixture cools
Extrapolate the lines and calculate value

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46
Q

Bond enthalpy
What is average bond enthalpy

A

Energy required to break one mole of a specified type of bond
In a gaseous molecule

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47
Q

Bond enthalpy
What is always required to break bond and what are bond enthalpies always

A

Energy and always endothermic with a positive enthalpy value

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48
Q

Bond enthalpy
Example. Of bond dissociation ethalpy Cl

A

Cl2 (g) —> 2 Cl (g)

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49
Q

Bond enthalpy
What does the strength of a bond depend on

A

It’s environment so mean values are quoted

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50
Q

Bond enthalpy
What type of reaction is making a bond

A

Exothermic

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51
Q

Bond enthalpy
What does a smaller bond enthalpy mean

A

Weaker bond so easier to break

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52
Q

Bond enthalpy
What type of reaction is breaking a bond

A

Endothermic

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53
Q

Bond enthalpy
What determines what type of reaction the overall reaction is

A

The difference between the energy required for bond breaking and the energy released by a bond making

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54
Q

Hess law
What is Hess Law used for

A

Determining enthalpy change of difficult reactions

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55
Q

Hess law
What does Hess law state

A

Enthalpy change is independent of route taken

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56
Q

Hess law
What are diagrams that illustrate Hess law called

A

Enthalpy cycle

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57
Q

Bond enthalpy
How do you do a calculation for overall change

A

Using given data workout out energy required to break bonds of reactants and energy required to make bonds in products
Calculate break-make and see if positive or negative value

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58
Q

Hess law
What is it used for calculating

A

Enthalpy change of reaction
Enthalpy change of combustion
Enthalpy change of formation

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59
Q

Hess law
If given enthalpy of formation what put at bottom and where do the arrows go

A

Put elements at bottom and arrow upwards away from elements

60
Q

Hess law
If given enthalpy of combustion, what put at bottom and where do arrows go

A

Oxides
Down towards oxides

61
Q

Hess law
If given bond enthalpy what put at bottom and where do arrows go

A

Gaseous atom
Down towards gaseous atoms

62
Q

Hess law
If going against arrow what do u do

A

Minus it

63
Q

Rates of reaction
What is the basis of rates

A

The collision theory

64
Q

Rates of reaction
How must particles collide for a reaction to happen

A

Must collide in the right direction
Collide with at least a certain amount of kinetic energy to over c0me the activation energy

65
Q

Rates of reaction
What is the activation energy

A

Minimum amount of kinetic energy particles need to react as is needed to brews the bonds to start the reaction

66
Q

Rates of reaction
Distribution of kinetic energy between molecules

A

Dont all have the same amount
Some have lots and move quickly
Some not about and move slower
Most are somewhere in between

67
Q

Rates of reaction
What factors affect rate of reaction

A

Temperature
Concentration pressure
Surface area
Catalysts

68
Q

Rates of reaction
What happens to rate of reaction as increase temperatures

A

Rate of reaction increases

69
Q

Rates of reaction
Why does increasing temperature increase rate of reaction

A

Particles will have more kinetic energy and move faster, be able to get over activation energy and react
Also moving faster so more collisions will occur

70
Q

Rates of reaction
Affect of concentration on rate of reaction

A

Increasing concentration increases rate of reaction

71
Q

Rates of reaction
Why does increasing concentration increase the rate of reaction

A

More particles in a set volume so will collide more frequently therefore more chance to react

72
Q

Rates of reaction
Impact of pressure on rate of reaction

A

Increasing pressure increases rate of reaction

73
Q

Rates of reaction
Why does increasing pressure increase rate of reaction

A

More gas particles in set volume so more frequent and successful collisions

74
Q

Rates of reaction
Impact of catalysts on rate of reaction

A

Catalysts increase rate of reaction

75
Q

Rates of reaction
Why can catalysts spoked up a reactions

A

Lowers the activation energy by providing a alternate pathways therefore more particles will have enough energy to react

76
Q

Rates of reaction
Impact of surface areas on rate of reaction

A

Larger surface area increases rate of reaction

77
Q

Rates of reaction
Why does increasing surface area increase rate of reaction

A

More area from them particles to collide with

78
Q

Rates of reaction
Definition for catalyst

A

Increases the rate of reaction by providing an alternative reaction pathways with a lower activation energy and is chemically unchanged at the end of the reaction

79
Q

Rates of reaction
On an energy profile what is the lowered activation energy after a catalysts labelled as

A

Ec

80
Q

Rates of reaction
What two things can catalysts be

A

Homogeneous
Heterogeneous

81
Q

Rates of reaction
What is a heterogenous catalyst

A

One that is in a different physical state from the reactants

82
Q

Rates of reaction
Where does the reaction happen for heterogeneous catalysts

A

On the surface of the catalyst

83
Q

Rates of reaction
What does increasing the surface area of a heterogeneous catalyst do

A

Increase the rate of reaction because the number of molecules that can react at the same time increases

84
Q

Rates of reaction
What are homogeneous catalysts

A

In the same physical state as the reactant

85
Q

Rates of reaction
What are homogeneous catalysts usually used for

A

An aqueous catalyst for the reaction between two aqueous solutions

86
Q

Rates of reaction
How does a homogeneous catalyst work

A

Reactants combine with the catalyst to make an intermediate species, which then react to form the products and reform the catalyst

87
Q

Rates of reaction
Why are catalysts good for industries

A

Dramatically lower production costs, give more product in a shorter time and help make better products

88
Q

Rates of reaction
Example of iron used as a catalyst

A

In ammonia production

89
Q

Rates of reaction
What is the Hager process that iron is used in

A

N2 (g) + 3 H2 (g) <=> 2 NH3 (g)

90
Q

Rates of reaction
Why is using catalysts good for industry

A

Lowers production cost
Gives more product in shorter time
Helps make better products

91
Q

Rates of reaction
Why is iron used in the Haber process

A

If not for catalyst, temp would have to be raised loads which would be more expensive and reduce the amount of ammonias produced

92
Q

Rates of reaction
How catalysts help environmental sustainability (2)

A

Lowers temp and pressure needed so less energy requires so less CO2 released and fossil fuels saves
Greater atom economy so less waste

93
Q

Rates of reaction
What does the maxwell Boltzmann distribution curve show

A

Distribution of particles in a reaction

94
Q

Rates of reaction
Why does the MB distribution curve always start at the origin

A

No particles have no energy

95
Q

Rates of reaction
Why does the MB curve never cross the x axis

A

Particles cannot gave negative energy and energy is infinite

96
Q

Rates of reaction
What is on the x axis of the MB curve

A

Molecular energy

97
Q

Rates of reaction
What’s on the y axis of the MB curve

A

Number of particles with a particular energy

98
Q

Rates of reaction
What is the shape of the MB curve

A

Peak then slopes of but never touches x axis

99
Q

Rates of reaction
What part of the graph tells us the number of particles that can collide successfully

A

Area under graph with more energy that the EA

100
Q

Rates of reaction
Where is the EA on a MB curve

A

To the right of the x axis

101
Q

Rates of reaction
what happens to the shape of a MB curve when the temperature is increased

A

Pearl at most probable energy shifts to right and lowers and tail not as close to x axis

102
Q

Rates of reaction
Why does increasing the temperature increase rate of reaction including MB distribution

A

Increasing temperature means area under curve with energy higher than the EA will increase as the particles have more kinetic energy. This means that more particles will collide and be able to overcome the Ea
Also collisions more frequent

103
Q

Rates of reaction
What happens to the MB curve when catalyst added

A

EA moves to left but res of curve stays the same

104
Q

Rates of reaction
Why does adding a catalyst increase the rate of reaction using the MB curve

A

Catalysts lowers the activation energy by providing g an alternate pathways which Samsung that the area under the curve with more energy than the EA increases. Therefore more particles collide with enough energy to overcome the EA, increasing the rate of reaction

105
Q

Rates of reaction
Equation for rate of reaction

A

Rate= change in y axis/ time

106
Q

Rates of reaction
How do you calculate the rate of reaction from a graph

A

Draw tangent at time and find change in y/change in x

107
Q

Dynamic equilibrium

A
108
Q

Dynamic equilibrium
Do all reactions proceed to completion

A

No some end up with a mixture of reactants and products

109
Q

Dynamic equilibrium
What is the definition for dynamic equilibrium

A

In a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction and the concentrations of the reactants and products is constant

110
Q

Dynamic equilibrium
Process of reaching equilibrium

A

Forward reaction starts fast but slows as reactants gets less concentrated
Initially no backwards reaction by as products form will get faster
Once backwards and forward reactions are equal and opposite the reaction has reached equilibrium

111
Q

Dynamic equilibrium
Why is a reversible chemical reaction a dynamic process

A

Everything may appear stationary but reactions are moving both ways

112
Q

Dynamic equilibrium
How can the position of equilibrium be changed

A

By changing certain conditions

113
Q

Dynamic equilibrium
What in equations shows the reactions are reversible

A

<=> symbol

114
Q

Dynamic equilibrium
What five things are true when a chemical equilibrium is established

A

-in a closed system
-equilibrium can be approached from either side
-reaction is dynamic as is moving forwards and backwards
-concentrations remain constant
-forward and backward reaction are equal and opposite

115
Q

Dynamic equilibrium
What is a closed system

A

One in which none of the reactants or products escape from the reaction mixture

116
Q

Dynamic equilibrium
What is an open system

A

Matter and energy can be lost to the surroundings

117
Q

Dynamic equilibrium
What’s le chateliers principle

A

When a change is applied to a system in dynamic equilibrium the system reacts in such a way to oppose the effect of the change

118
Q

Dynamic equilibrium
What factors can affect the equilibrium position

A

Concentration
Pressure
Temperature

119
Q

Dynamic equilibrium
What happens to position if increase concentration on left side

A

Position of equilibrium moves to right

120
Q

Dynamic equilibrium
What happens to position is decrease concentration on the left

A

Position of equilibrium moves to left

121
Q

Dynamic equilibrium what happens to position if increase concentration on the right

A

Position of equilibrium moves to the left

122
Q

Dynamic equilibrium
What happens to position os equilibrium if decrease concentration on the right

A

Moves to right

123
Q

Dynamic equilibrium
What happens to position of increase pressure

A

Moves to side with fewer gaseous molecules

124
Q

Dynamic equilibrium
What happen to position ifdecrease pressure

A

Moves to side with more gaseous molecules

125
Q

Dynamic equilibrium
What happens to position if increase temperature

A

Moves to endothermic products

126
Q

Dynamic equilibrium
What happens if decreas the temperature to the positions

A

Moves toexothemic products

127
Q

Dynamic equilibrium
What happens to position of equilibrium if add a catalyst

A

Doesn’t affect position but equilibrium is reached quicker

128
Q

Dynamic equilibrium ]
What does Kc stand for

A

Equilibrium constant

129
Q

Dynamic equilibrium
What is Kc defined as for the reaction aA + bB <=> cC + dD

A

[C]^c [D]^d
—————-
[A]^a [B]^b

130
Q

Dynamic equilibrium
What does the notation [X]

A

The concentration of X

131
Q

Dynamic equilibrium
What does Kc give you an idea of

A

How far to the left or right the position of equilibrium is

132
Q

Dynamic equilibrium \
What is an equilibrium expression

A

When the products in square brackets raised to number mole is written over the reactants in square brackets raised to the number of moles

133
Q

Dynamic equilibrium
How do you calculate the value of Kc

A

Substitute concentrations into the equilibrium expression

134
Q

Dynamic equilibrium
Why des the units of Kc change

A

In equilibrium expression each figure is represents concentration in mol dm^-3

135
Q

Dynamic equilibrium
How do you work out the units of Kc

A

Since varies for each equilibrium expression, substitute the units mol dm^-3 into the expression and simplify

136
Q

Dynamic equilibrium
How come some Kc have no units

A

Concentration units cancel out

137
Q

Dynamic equilibrium
What can you estimate using Kc

A

Position of equilibrium

138
Q

Dynamic equilibrium
Where is the position of equilibrium of Kc is larger than 1

A

To the right as more products then reactants

139
Q

Dynamic equilibrium
Where is the position of equilibrium is Kc=1

A

Lies in centre as equal concentrations of products and reactants

140
Q

Dynamic equilibrium
Where is the position of equilibrium if Kc is less than 1

A

To the left because larger concentrations of reactants than products

141
Q

Dynamic equilibrium
What do you use for moles at equilibrium calculation

A

ICE

142
Q

Dynamic equilibrium
What does ICE stand for

A

Initial
Change
Equilibrium

143
Q

Dynamic equilibrium
How do you set out a moles at equilibrium question

A

With equationat top and ICE down side as a table
Put known values into table

144
Q

Dynamic equilibrium
What units do the values have to be in for ICE

A

Concentration (mol dm^-3)
If 1:1:1:1 mol ratio can use in mol

145
Q

Dynamic equilibrium
How do you do an ICE calculation

A

Work out change for each compound and work out conc for each compound at equilibrium

Alevel Chemistry OCR A (13 decks)

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