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A-level chemistry > AS - Unit 2 - Energy > Flashcards

AS - Unit 2 - Energy Flashcards

1
Q

What is enthalpy?

A

The heat content that is stored in a chemical system

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2
Q

What does exothermic mean?

A

It refers to a reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings

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3
Q

What determines enthalpy change?

A

When the products of a reaction have more or less chemical energy than the reactants, resulting in a change of enthalpy

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4
Q

What is enthalpy change?

A

The heat exchange with the surroundings during a chemical reaction, at constant pressure
The difference between the enthalpy of the products and the enthalpy of the reactants

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5
Q

Give three things with occur in an exothermic reaction

A

The enthalpy of the products is smaller than the enthalpy of the reactants
There is heat loss from the chemical system to the surroundings
delta H has a negative sign because heat has been lost by the chemical system

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6
Q

What does endothermic mean?

A

It refers to a reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken in from the surroundings

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7
Q

Give three things that happen in an endothermic reaction

A

The enthalpy of the products is greater than the enthalpy of the reactants
There is a heat gain to the chemical system from the surroundings
delta H has a positive sign because heat has been gained by the chemical system

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8
Q

Is combustion endo or exothermic?

A

It is exothermic, and the energy given out is often used for everyday purposes such as powering a car by combusting fuels

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9
Q

Give two lifestyle changes that people can make to cut down on the production of CO2

A

Walk or cycle distances that don’t necessarily need a car

Avoid long haul flights to holidays abroad

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10
Q

What reaction is possibly the most important exothermic reaction for life?

A

Respiration

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11
Q

Give an example of an endothermic reaction

A

Thermal decomposition of limestone

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12
Q

What is an enthalpy profile diagram?

A

A diagram for a reaction to compare the enthalpy of the reactants with the enthalpy of the products

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13
Q

In exothermic reactions, which way does the enthalpy profile diagram begin and end?

A

The reactants begin at the top, the arrow points down and the products end at the bottom

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14
Q

In endothermic reactions, which way does the enthalpy profile diagram begin and end?

A

The reactants begin at the bottom, the arrow points up and the products end at the top

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15
Q

What is activation energy?

A

The minimum energy required to start a reaction by the breaking of bonds

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16
Q

Why is activation energy important in exothermic reactions?

A

Natural gas needs a spark to overcome its activation energy, once the energy barrier has been overcome the net output of energy provides more energy that can be used to overcome the activation energy for the reaction to continue.
Without this barrier exothermic reactions would take place spontaneously and fuels would not exist as they would spontaneously combust

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17
Q

What are standard conditions?

A

A pressure of 100kPa
Temperature usually of 298K (25 degrees C)
Concentration of 1.0 mol dm-3

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18
Q

What is a standard state?

A

The physical state of a substance under the standard conditions of 100kPa and 298K

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19
Q

What is the standard enthalpy change of a reaction?

A

The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states

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20
Q

What is the standard enthalpy change of combustion?

A

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states

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21
Q

What is the standard enthalpy change of formation?

A

The enthalpy change that takes place when one mole of a compound is formed from its constituent elements in their standard states and under standard conditions

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22
Q

What is the value for the enthalpy change of formation of an ELEMENT?

A

0kJ mol-1

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23
Q

What is specific heat capacity?

A

The energy required to raise the temperature of 1g of a substance by 1 degree C

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24
Q

Heat loss in a chemical system = ???

A

Heat gain by surroundings

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25
Heat gain in a chemical system = ???
Heat loss by surroundings
26
To determine the heat exchange during a reaction what do you need to know?
m - the mass of the surroundings involved in the heat exchange c - the specific heat capacity of the surroundings delta T - the temperature change of the surroundings delta T = T(final) - T(initial)
27
How do you calculate the value of heat exchange to the surroundings (Q)?
Q = mc delta T
28
What is combustion?
A reaction involving oxygen to form oxides
29
How do you calculate the enthalpy change of combustion experimentally?
Burn a known mass of a substance in air Heat a known mass of water Measure the temperature change in water e.g. Measure a volume of water into the beaker 1cm3 of water weighs 1g so the mass of water is easy to deduce The burner containing the fuel is weighed the initial temperature of the water is taken the burner is lit and the water heated until the temperature has risen by a reasonable amount max temperature is taken and the temp change (delta T) can be determined Flame is extinguished and the burner re-wighed to find the mass of the fuel that has been burnt
30
Give two reasons for experimental and theoretical enthalpy values being different when combusting
May have been incomplete combustion | May have been heat loss to the surroundings
31
What is bond enthalpy?
The enthalpy change that takes place when breaking by homolytic fission 1 mol of a given bond in the molecules of a gaseous species
32
What is average bond enthalpy?
The average enthalpy change that takes place when breaking by homolytic fission 1 mol of a given type of bond in the molecules of a gaseous species
33
What do bond enthalpies tell you?
The amount of energy needed to break a certain bond
34
Is bond breaking endo or exothermic?
It is endothermic because it uses energy to break the bonds
35
Is bond making endo or exothermic?
It is exothermic because when a bond is formed energy is released
36
How do you calculate enthalpy change with bond enthalpy data?
sum (bond enthalpies of bonds broken) - sum (bond enthalpies of bonds made)
37
What does Hess' law state?
If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route
38
Give three reasons why it is not always possible to measure enthalpy change
High activation energy Slow reaction rate More than one reaction taking place
39
What is an enthalpy cycle?
A diagram showing alternative routes between reactants and products which allows the indirect determination of an enthalpy change from other known enthalpy changes using Hess' law
40
When constructing a Hess' cycle for combustion which way do the arrows point?
Both bottom arrows point down
41
When constructing a Hess' cycle for formation which way do the arrows point?
Both bottom arrows point up
42
Define the rate of reaction
The change in concentration of a reactant or a product in a given time
43
How do you calculate the rate of reaction?
Change in concentration -------------------------------------- time
44
What are the units for rate of reaction?
mol dm-3 s-1
45
Give three main points about the rate of reaction for most reactions
At the start of the reaction, each reactant has its greatest concentration - the rate of the reaction is at its fastest As the concentration proceeds the concentrations of the reactants decrease - the rate of the reaction slows down When one of the reactants has been used up the rate becomes zero - the reaction stops
46
Give 5 things which can alter the rate of a chemical reaction
``` Temperature Pressure (with gases) Concentration Surface area Adding a catalyst ```
47
What are the two conditions that have to be met in order for a reaction to be successful?
The molecules must have sufficient energy to overcome the activation energy of the reaction The molecules must collide in the correct orientation
48
How does increasing the concentration of a reactant help to increase the rate of reaction?
There are more molecules occupying the same volume So the molecules will be closer together so there is a greater chance of the molecules colliding Meaning collisions are more frequent and more collisions will occur in a certain length of time
49
What effect does pressure have on the rate of a chemical reaction?
The same number of molecules occupy a smaller volume (for gas, increasing the pressure is like increasing the concentration) so more collisions will take place, there will be more collisions with energy greater than the activation energy. There will be more frequent collisions and the rate of reaction will increase
50
What does a catalyst do?
It increases the rate of a chemical reaction without being used up in the process It also lowers the activation energy of a reaction by providing an alternative route for the reaction to follow
51
What catalyst is used for the Haber process?
Iron
52
What catalyst is used in the production of H2SO4?
V2O5
53
What catalyst is used in the hydrogenation of margarine?
Nickel
54
What is heterogeneous catalysis?
The catalysis of a reaction in which the catalyst has a different physical state from the reactants; frequently, reactants are gases whilst the catalyst is a solid
55
What is homogeneous catalysis?
The catalysis of a reaction in which the catalyst and reactants are in the same physical state, which is most frequently the aqueous or gaseous state
56
How are catalytic converters effective for the environment?
They improve our air quality by reducing toxic emissions from vehicles and preventing photochemical smog
57
How does iron help the Haber process?
The N=-N (triple bond) has to be broken in the Haber process, which requires a large input of energy. Iron help to weaken the N=-N bond and therefore lowers the activation energy saving a lot of money for the companies producing ammonia
58
Give three ways in which the industrial use of enzymes has many benefits
Lower temperatures and pressures can be used than with conventional inorganic catalysts, saving energy and costs Enzymes allow a reaction to take place which forms pure products, with no side reactions (the often remove the need for complex separation techniques, thus reducing costs) Conventional catalysts are often poisonous and can pose disposal problems at the end of their industrial life. Enzymes are biodegradable
59
Name three food products which are produced using enzymes
Dairy products Alcoholic drinks Fruit juices
60
How have biological washing powders help to reduce household costs?
Before biological washing powders, clothes were often washed at very high temperatures, meaning the water had to be heated, costing a lot in energy Clothes often faded and turned out misshapen Enzymes reduce the temperature needed to wash clothes which is less damaging towards the clothes and your energy bill
61
What type of enzyme is used to remove stains from clothes?
Proteases break down proteins and remove stains caused by blood, egg, gravy and other proteins
62
What enzyme breaks down any starches found on clothes when using biological washing powders?
Amylases
63
Which enzymes break down grease and fats?
Lipases
64
What is the Boltzmann distribution?
The distribution of energies of molecules at a particular temperature, often shown as a graph
65
Give 4 important features of a Boltzmann distribution graph
The area underneath the curve is equal to the total number of molecules in the sample. The area does not change with conditions There are no molecules in the system with zero energy There is no maximum energy for a molecule Only the molecules with an energy grater than the activation energy are able to react
66
Explain the effect of temperature on a Boltzmann distribution graph
At higher temperatures the kinetic energy of all the molecules increases. The graph flattens and shifts to the right. The number of molecules in the system does not change so the area underneath the curve does not change
67
When you increase the temperature of a reaction why does the rate increase?
Because more collisions take place in a certain length of time - molecules are moving faster and have more kinetic energy Higher proportion of molecules have an energy that is greater than the activation energy - more collisions will lead to a chemical reaction
68
What effect does a catalyst have on the rate of a reaction in a Boltzmann distribution graph
A catalyst lowers the activation energy meaning a greater proportion of molecules exceed the lower activation energy and the reaction rate increases
69
What drawing a Boltzmann distribution graph what is it important to consider?
Make sure the curve does not touch the X axis!!
70
What is dynamic equilibrium?
The equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction
71
What is le Chatelier's principle?
The principle states that when a system is dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise the change
72
An equilibrium only applies when.....
The system remains isolated. An isolated system is when no materials are being added or taken away and no external conditions, such as temperature or pressure are being altered
73
What three factors can alter the position of the equilibrium?
Concentrations of the reactants or products Pressure in reaction involving gases Temperature
74
What is the effect of concentration on equilibrium?
Increasing the concentration of a reactant causes the position of the equilibrium to move in the direction that decreases the concentration, the equilibrium moves to the right forming more products Increasing the concentration of a product causes the position of the equilibrium to move in the direction that decreases this increased product. The equilibrium will move to the left forming more reactants
75
What is the effect of pressure on equilibrium?
Changing the pressure of an equilibrium will only have an effect if there are gases present Increasing the total pressure causes the equilibrium to shift to the side with fewer gas moles, this will decrease the pressure. Decreasing the total pressure of the system causes the position of the equilibrium to move to the side with the most number of gas moles, this will increase the pressure
76
Explain the effect of temperature on equilibrium?
Increasing the temperature of the system causes the position of the equilibrium to move in the direction that decreases the temperature. Decreasing the temperature causes the equilibrium to move in the direction that will increase the temperature
77
What is the effect of a catalyst on equilibrium?
A catalyst does not alter the position of the equilibrium, it just speeds up the forward and backward reaction at the same speed. It just increases the rate at which equilibrium is established
78
Give two examples of equilibrium in industry
Preparation of ammonia in the Haber process | Conversion of sulfur dioxide into sulfur trioxide in the Contact process
79
What conditions favour production of ammonia in the Haber process?
High pressure - the forward reaction produces less gas moles, when you increase the pressure the equilibrium goes to the side with the least gas moles, favouring the forward reaction which is what we want! The forward reaction is exothermic meaning if you lower the temperature, the equilibrium will favour the forward reaction forming more products!!!
80
What is the drawback of the Haber process favouring a lower temperature?
Although low temperatures should produce a high equilibrium yield, the reaction would take place at a very low rate. Meaning at this low temperature the molecules wouldn't have enough energy to exceed the activation energy :(
81
Why is the high pressure needed for the Haber process a drawback?
Creating a high pressure uses a lot of energy making the process costly. It also comes with safety risks too!
82
What compromise has been reached in order for the industry to produce sufficient ammonia?
Temperature - 400-500 degrees C Pressure - High pressure must still be used, 200 atm Catalyst - Iron catalyst is used to speed up the rate of reaction allowing the equilibrium to be established faster and for lower temperatures to be used
83
With the compromised conditions used for ammonia production, how much ammonia is actually made and how do the industry ensure the process is effective?
15% nitrogen and hydrogen is actually converted. Ammonia product is liquefied and removed. However unreacted nitrogen and hydrogen gases are passed through the reactor again so nothing is wasted. This happens until almost all of the nitrogen and hydrogen gas has been converted into ammonia

A-level chemistry (15 decks)

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