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Higher chemistry based off course support notes > Unit 3 > Flashcards

Unit 3 Flashcards

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1
Q

How are industrial processes designed?

A

Industrial processes should be designed to maximise profit and minimize impact on the environment.

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2
Q

Name some of the factors that influence the design of an industrial process to maximise profit?

A
  • Availability, sustainability, and cost of feedstocks.
  • Opportunities for recycling products of the reaction
  • The energy requirements of the reaction taking place/
  • The marketability of the byproducts of the reaction must also be taken into account
  • The yield of the product should also be taken into account.
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3
Q

What are some of the features of a chemical process that can help to reduce the environmental impact of the process?

A
  • Minimising waste
  • Avoiding the use or production of toxic or corrosive substances as far as possible.
  • Designing biodegradable products
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4
Q

Explain what is meant by molar volume?

A

Molar volume is the volume occupied by one mole of any gas under the same conditions of temperature and pressure. The molar volume is the same for all gasses and are linked together by the equation n = v/vmol

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5
Q

Explain why Availability, sustainability and cost of feedstocks must be taken into account when we are designing a chemical process?

A

profit can be gained by making use of local feedstocks
However profit can also be lost due to transportation and the safety concerns from transporting large quantities of feedstock. The cost of the feedstock must be taken into account as well as the more rare a feedstock the more expensive it will be. feedstocks such as crude oil will be subject to fluctuating costs.

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6
Q

What is meant by the term feedstock?

A

A feedstock is a reactant from which other chemicals extracted or synthesised. feedstocks are usually derived from raw materials such as crude oil.

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7
Q

Why must opportunities for recycling be taken into account when designing a chemical process?

A

profit can be gained when unreacted feedstocks are fed back into the reaction reducing the amount of the feedstock that you will need to purchase and reducing the waste produced by the process.
However if you do not take this into account separating unreacted starting molecules from the end reaction can be costly and is inefficient and wasteful.

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8
Q

Why must energy requirements be taken into account when we are designing a chemical process?

A

Profit can be gained by using exothermic reactions that release heat which can be used to provide energy for the continued reaction or to provide energy for other chemical processes in the plant. catalysts can also be used to increase the rate of the reaction meaning that higher temperatures will not be needed and the energy requirements will be reduced.
However profit can be lost when reactions have to be heated, as this is costly and requires energy. Reactions that require cooling can also be very costly as coolants must be used to remove energy.

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9
Q

Why must the marketability of the byproducts be taken into account when we are designing a chemical process?

A

Profit can be gained when products of the reaction can be used in other processes in the chemical plant. Alternatively the byproducts could be sold to other chemical industries for a profit.
however profit can also be lost as it is expensive to deal with toxic, corrosive or environmentally damaging chemicals.

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10
Q

Why must the product yield be taken into account when designing a chemical process?

A

Profit can be gained when the product yield is high as large quantities of product can be produced in little time.
However profit can be lost if the product yield is too low as low yielding reactions require large amounts of energy to generate sufficient amounts of product.

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11
Q

What is meant by atom economy?

A

The atom economy is the mass of the desired product as a percentage of the mass of the total reactants being used in the reaction.

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12
Q

What can be said about reactions with high atom economies?

A

They are efficient as little to no waste product will be produced and most of the reactants will be converted into products

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13
Q

What can be said about reactions that have low atom economies?

A

Reactions with low atom economy will be inefficient and high amounts of waste product will be produced which can be costly to remove.

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14
Q

What is meant by the percentage yield?

A

Percentage yield is the mass of the product that is actually made as a percentage of the product that is theoretically possible to be made.

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15
Q

Give some examples of scenarios that could cause the theoretical yield to not be equal to the actual yield of the product?

A

reactions can have a low percentage yield if the feedstocks used are not pure, if side reactions are occurring, or if the reaction is in equilibrium.

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16
Q

What equation do we use to calculate the atom economy of a reaction?

A

%AE = mass of desired product/mass of reactants * 100

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17
Q

How can we calculate the percentage yield of a reaction?

A

%yield = mass of of product produced/theoretical mass of product * 100

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18
Q

What do ideas of excess state about a reaction?

A

In most reactions there will be a reactant that is in excess and another reaction that limits the amount of products produced by the reaction this is called the limiting reactant.

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19
Q

Explain a common method used do decide which reactant should be the limiting reactant of a process?

A

The limiting reactant should be the most expensive reactant as this ensures that all of the most expensive reactant is being used up in the reaction. Whereas the reactant that is in excess should be less expensive.

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20
Q

What are some of the things that we have to look out for when using molar volumes to compare volumes of gases in a reaction?

A
  • We must ensure that we only use this to compare gases and must be aware of the state symbols
  • If the reaction is cooled at temperatures below 100 degrees then water will no longer be a cass and therefore will not be taken into account when totaling the volume of gas
  • If one of the reactants is in excess it should be ignored and you should use the limiting reactant to compare other gases.
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21
Q

Explain the principles of collision theory?

A

Collision theory states that for a successful collision to take place particles must collide with sufficient activation energy to break down existing chemical bonds and form new ones and particles must collide with the correct collision geometry.

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22
Q

Explain why all chemical reactions have the highest relative rate at the beginning of the reaction?

A

As at the beginning of the reaction this is where the number of particles is the greatest meaning that the most collisions will take place here, and as the reaction progresses the frequency of collisions will reduce meaning that less successful collisions will take place

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23
Q

What effect does increasing the surface area of solids have on the rate of the reaction?

A

Increasing the surface area by decreasing the particle size will mean that more collisions will occur hence the rate of the reaction will increase.

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24
Q

What effect does increasing the concentration of a solution have on the rate of a reaction?

A

Increasing the concentration of a solution will increase the number of particles present in the solution which increase the number of collisions taking place increasing the rate of the reaction.

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25
Q

What effect does increasing the temperature have on the rate of the reaction?

A

Increasing the temperature will increase both the frequency of the collisions and the energy with which particles collide. This increases the number of successful collisions increasing the rate of the reaction

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26
Q

How can we calculate the rate of a reaction?

A

Change in quantity/Change in time = rate

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27
Q

What factors can we measure to find the rate of the reaction?

A
  • volume of gas produced
  • mass of reactants lost
  • concentration of the reactants
  • pH of the reaction
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28
Q

What does the steepness of the slope of a graph about a reactions progress over time indicate?

A

The slopes gradient is the rate of the reaction hence when the slope is steep the rate of the reaction is high.

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29
Q

What formula do we use to calculate the relative rate of a reaction?

A

Relative rate = 1/t

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30
Q

What is meant by the term enthalpy?

A

Enthalpy is the chemical energy that is stored in all chemicals.

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31
Q

How is enthalpy influenced by a reaction?

A

During a collision bonds are broken and new bonds are formed this causes a change in enthalpy during a reaction.

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32
Q

What is meant by the term activated complex?

A

Activated complex refers to an unstable arrangement of atoms (that forms at the peak potential energy on an enthalpy diagram)

(Activated complexes are shown by dotted lines indicative of bonds being broken and new bonds forming) H —– H
Br —– Br

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33
Q

What can be said about the potential energy diagrams of endothermic reactions?

A

In an endothermic reaction the products will have a higher potential energy than the reactants. As endothermic reactions absorb heat from their surroundings.

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34
Q

What can be said about the potential energy diagrams of exothermic reactions?

A

In exothermic reactions the products will have less energy than the reactants as during an exothermic reaction heat energy will be released to the environment.

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35
Q

What is enthalpy change?

A

Enthalpy change is the change in the potential energy of the reactants and the products. Hence when we look at a potential energy diagram we can calculate enthalpy change by looking at the difference in the potential energy of the reactants and the products.

36
Q

What can be said about the activation energy of the forwards and backwards reaction?

A

The activation energy of the forwards and backwards reaction will not be the same.

37
Q

Explain the effect of adding a catalyst to a reaction?

A

Adding a catalyst to a reaction will increase the rate of the reaction by lowering the activation energy, and this happens because the catalyst provides and alternative reaction pathway. Use of catalysts means that reactions can be carried out at lower temperatures this will save energy.

(Note that when drawing out the effect of a catalyst on a reaction on a potential energy diagram the catalysed reaction should still start at the reactants and end at the products but the activation energy should be decreased)

38
Q

Define the term temperature?

A

Temperature is defined as a measure of the average kinetic energy of the particles in a chemical. (Note that an increase in temperature has no effect on the activation energy however means that more particles possess the activation energy required for a successful collision to take place.

39
Q

Explain the importance of knowing if a reaction is exothermic or endothermic in a chemical process?

A

It is important to control an exothermic reaction as an exothermic reaction can lead to thermal explosions, in addition to this reactions that get too hot can produce undesirable products as the additional energy causes side reactions. The energy released by an exothermic reaction can also save money and heat energy can be used in other parts of the chemical plant.

Endothermic reactions are very costly as they require heat energy to be supplied to sustain the reaction. Ideally the energy would come from another exothermic reaction taking place elsewhere in the plant.

40
Q

What equation do we use to measure the amount of energy released by a fuel?

A

Eh = cm(change in)T

41
Q

Give reasons for the standard apparatus used to measure energy released by fuel to give results lower than in the databook?

A
  • The alcohol can evaporate
  • non uniform distribution of heat
  • Incomplete combustion taking place
  • Heat loss from the system
  • Not all energy being transferred to the water
42
Q

What apparatus should we use to obtain values of the energy released by fuels that would match the databook?

A

We should use a bomb calorimeter?

43
Q

Comment on the effectiveness of the design of the bomb calorimeter?

A
  • The apparatus is well insulated to prevent heat loss to the surroundings
  • A pure supply of oxygen is used to ensure that complete combustion takes place
  • Water completely surrounds the container where combustion is taking place meaning a majority of the temperature produced by combustion will be transferred to the water
  • The water is stirred evenly to distribute heat equally
44
Q

Define the term enthalpy of combustion?

A

Enthalpy of combustion is the energy released when one mole of a substance is burned completely in oxygen at a standard temperature and pressure.

(Note that enthalpies of combustion equations must be balanced and we cannot change the number of moles of the substance being burned it will always be one mole of a substance being burned hence we must instead change the number of moles of other chemicals and we can use 1/2 moles or even 1/4 moles.)

45
Q

Explain the process of calculating the enthalpy of combustion of a substance?

A
  • use Eh = cm(delta)T to obtain the energy released by the substance when burned completely in oxygen
  • use n = m/gfm with the mass of the fuel burned to figure out how many moles of the fuel were burned
  • use enthalpy of combustion = Eh/n
  • Convert your answer to a negative value as we are talking about combustion and so the enthalpy change is negative as it describes a release of heat energy
46
Q

Define the term bond enthalpy?

A

Bond enthalpy is the energy required to break one mole of bonds in a mole of gaseous molecules

47
Q

Explain why to calculate the enthalpy change of certain chemicals we must use average bond enthalpies?

A

As enthalpy of bonds in different chemical compounds changes slightly as the bonds will be affected by the atoms and bonds that surround it hence we must use a quoted average bond enthalpy to estimate the enthalpy change.

48
Q

What is the enthalpy of a reaction?

A

The difference in the energy required to break existing chemical bonds in the compound and the energy released by the chemical when new bonds are formed.

49
Q

What can be said about breaking bonds of a chemical compound?

A

Breaking bonds of a chemical compound requires energy hence the process is endothermic

50
Q

What can be said about the formation of new bonds?

A

Forming new bonds releases energy hence the process will be exothermic.

51
Q

State the principles of Hess’ law?

A

Hess’ law states that the enthalpy change of a chemical reaction is independent of the route taken provided that the conditions at the start of the reaction are the same as the conditions at the end of a reaction.

52
Q

Explain how we can use hess’ law to find out an unknown enthalpy changes if given all the enthalpy changes of reactions required to produce the unknown enthalpy change?

A

We add these equations together to figure out the enthalpy change of the desired product and all of the additional chemicals should in theory cancel out.

(Note that when using hess’ law in this way
- If any chemical appears in more than one equation you are free to ignore it
- You should only ever use an equation once)

53
Q

Why is equilibra not desirable for chemical industry?

A

As in equilibrium reactants react to form products however the products will break back down to form reactants this causes the percentage yield of the process to decrease as less product will be produced.

54
Q

What do we call a the reaction in chemical equilibrium that proceeds from reactants to products?

A

The forwards reaction

55
Q

What do we call the reaction in an equilibrium that proceeds from products to reactants?

A

The backwards reaction

56
Q

Explain what is meant by equilibrium?

A

Equilibrium is a point in a reversible system where the rate of the forwards reaction is equal to the rate of the backwards reaction
(Note that in any question involving explaining equilibrium this definition will always be worth a mark)

57
Q

Explain what happens to concentration in a chemical equilibrium?

A

When the rate of the forwards reaction equals the rate of the backwards reaction in a reversible system the concentration of the reactants decreases while the concentration of products increase until a point of equilibrium is reached where concentrations of reactants and products stop changing and will remain constant.

58
Q

explain why equilibriums are described as dynamic?

A

As in a chemical equilibrium both the forwards reaction and the backwards reaction do not stop hence equilibrium is described as dynamic

59
Q

Does the position of the equilibrium change if we start with the products insead of the reactants?

A

No the position of the equilibrium is independent of if we start with products or reactants

60
Q

Are the concentrations of reactants and products in equilibrium equal?

A

No, the concentration of reactants and products is constant however is not equal.

61
Q

Explain the conditions under which a chemical equilibrium can form?

A

Chemical equilibria can only occur in a closed system - which is a system where the reactants and products cannot escape their container. If a reactant or product escaped from the system then chemical equilibrium could not be reached as either the forwards or backwards reaction would stop.

62
Q

Explain Le Chatelier’s principle?

A

Le Chatelier’s principle states that the position of an equilibrium will change to minimise the effect of any imposed change in the conditions of the reaction.

63
Q

What happens to the position of an equilibrium when the temperature increases?

A

The equilibrium position will shift to the endothermic side to counteract the imposed change on the reaction conditions by reducing the temperature

64
Q

What happens to the position of an equilibrium when the temperature is decreased?

A

When the temperature is decreased the position of the equilibrium shifts to the exothermic side as this will counteract the imposed change by increasing the temperature.

65
Q

What happens to the position of an equilibrium where the concentration of reactants is increased?

A

The equilibrium position will shift to the products side to counteract the imposed change on the system

66
Q

What happens to the position of an equilibrium when the concentration of products is increased?

A

When the concentration of products increases the position of the equilibrium will shift to the reactants side to counteract the imposed change.

67
Q

What happens to the position of an equilibrium where the concentration of reactants is decreased?

A

When concentration of reactants are decreased the equilibrium position will shift towards the backwards reaction to counteract the imposed change.

68
Q

What happens to the position of an equilibrium where the concentration of products is decreased?

A

When the concentration of products is decreased the equilibrium position will shift to the products side to counteract the imposed change.

69
Q

What does adding an acid to an equilibrium provide?

A

Adding an acid increases the concentration of hydrogen ions

70
Q

In what conditions does a change in pressure result in a change in the position of an equilibrium position?

A

Where at least one of the components of the reaction is gaseous. Where there is a different number of moles of gas on each side of the reaction.

71
Q

What happens to the position of an equilibrium when we increase the pressure?

A

When we increase the pressure the position of the equilibrium will shift towards the side of the reaction with the least moles of gas to counteract the imposed change on the system.

72
Q

What happens to the position of an equilibrium where we decrease the pressure?

A

When we decrease pressure the equilibrium position will shift to the side of the reaction with the largest number of moles of gas this will counteract the imposed change.

73
Q

Explain the effects of a catalyst on equilibrium?

A

Use of a catalyst will have no impact on equilibrium position as catalysts reduce the activation energy of the forwards and backwards reactions by the same amount. Catalysts also have no effect on the enthalpy change of the reaction. And catalysts help equilibrium to be reached more quickly.

74
Q

Why does the chemical industry in the production of ammonia choose to use temperatures and pressures that only result in a yield of 40%?

A

As higher temperatures lower the yield of ammonia as it favours the exothermic reaction. However temperatures that are too low will mean that the rate of the reaction is far too slow.

The forwards reaction reduces the number of moles of gas hence it is favoured by high pressures, however very high pressures are too difficult to achieve without the use of extremely expensive equipment.

75
Q

What are the feedstocks required for the haber bosch process and from where are they obtained?

A
  • Nitrogen is a feedstock required for the process and is obtained from the raw material air
  • Hydrogen is another feedstock required for the process and is obtained from the raw material natural gas.
76
Q

What kind of catalyst is used in the haber bosch process?

A

An iron catalyst is used in the haber bosch process to increase the rate of the reaction and reducing the temperatures required to maintain the rate of the reaction.

77
Q

What does chromatography allow chemists to do?

A

chromatography is very important in chemical industry as it allows chemists to identify and separate chemicals that are present in a reaction mixture.

78
Q

How are molecules in chromatography separated?

A

Molecules in chromatography are separated based on molecular size and polarity of molecules.

79
Q

What are the three types of chromatography?

A
  • Paper chromatography
  • Thin layer chromatography
  • Gas liquid chromatography
80
Q

How do we calculate the retention factor?

A

Rf = distance traveled by molecule/distance traveled by solvent

81
Q

What are the mobile and stationary phases in paper chromatography?

A

The mobile phase is the solvent used to dissolve the mixture and the stationary phase is the paper which both the solvent and the molecules travel up.

82
Q

What are the stationary and mobile phases in gas liquid chromatography?

A

The mobile phase is usually a non-reactive gas such as nitrogen or helium, and the stationary phase is the powder placed along the long tube that is around 5 - 10 meters long

83
Q

What are the mobile and stationary phases for thin layer chromatography (TLC)?

A

The mobile phase is a solvent used to dissolve the compound and the stationary phase is powdered aluminium oxide (alumina)

84
Q

How does chromatography separate molecules based on polarity?

A

As the stationary phase in chromatography is usually polar meaning it will attract other polar molecules and will attract nonpolar molecules but to a lesser extent. If a polar stationary phase is chosen then the mobile phase will attract polar molecules more than nonpolar molecules and they will be carried along faster.

85
Q

Explain how molecules are separated by size in chromatography?

A

Molecules are separated by size as larger molecules will be more difficult to move along the stationary phase as a result of their size hence smaller molecules will have higher retention factor.

86
Q

What does the y axis of a gas liquid chromatography graph reperesent?

A

The y axis represents the quantities of each substance in the reaction mixture

87
Q

What does the x axis of the gas liquid chromatography graph indicate?

A

How long it takes each substance to travel through the columb

Higher chemistry based off course support notes (4 decks)

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