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CHEMISTRY > Topic 7 > Flashcards

Topic 7 Flashcards

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1
Q

Define yeild

A

The mass of a product made in a chemical process

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2
Q

Theoretical yields. In any chemical reaction :

A

No atoms are gained or lost

The total mass stays the same

This means that for a given mass of a limiting reactent you can make only a maximum mass of a product. This is the theoretical yield

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3
Q

What’s the actual yield

A

Is the mass of product that you really get at the end of a chemical process. This is always less than the theoretical yields

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4
Q

How do you calculate percentage yield

A

Actual yield X 100
—————
Theoretical yield

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5
Q

Reasons for obtaining the theoretical yield

A

incomplete reactions:
Reaction has not finished
Reaction reaches equilibrium

Side reactions: competing, unwanted reactions, so by-products are also made

Practical losses during the experiment:
Losses during purification eg filtration
Losses during transfers eg liquid left begin in containers

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6
Q

What is atom economy

A

A way of measuring the number of atoms wasted when making a substance

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7
Q

How can you calculate atom economy

A

Total Mr of desired products
_______________________________ x100

Total Mr of all products

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8
Q

What do you need to know to calculate atom economy

A

The balanced equation for the reaction

The relative formula masses, Mr of the products

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9
Q

100% atom economy

A

The atom economy of a process is 100% if all atoms in the reactants can be converted to the desired products. This happens when

There is only one product such as in the manufacture of ammonia :
N2 (g) + 3H2 (g) reversible 2NH3 (g)

All of the by-products are used eg as feedstock (reactants) for other processes

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10
Q

Industrial processes

A

In general, the higher the atom economy the lower its impact on the environment. Processes with high atom economies are usually preferred because they -
Produce less waste
Conserve limited resources

Such processes are more sustainable
This means that they allow us a good standard of living today without reducing the chances is this in the future

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11
Q

What does Avogadro law state

A

Equal volumes of all gases, at the same temperature and pressure have the same number of molecules

This means one mole of any gas occupies the same volume at a given temperature and pressure

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12
Q

The volume occupied by a gas depends on

A

The number of particles present

The temperature of the gas

The pressure of the gas

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13
Q

What’s the molar volume

A

The volume occupied by one mole of any gas at room temperature and atmospheric pressure (rtp)

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14
Q

Value for the molar volume, Vm, is

A

24 dm3/mol

24000 cm3/mol

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15
Q

Ammonium chloride decomposed when it’s heated. What’s the chemical formula

A

NH4CL(s) -> NH3 (g) + HCL (g)

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16
Q

Ammonia and hydrogen chloride combine when cool. What’s the equation

A

NH3 (g) + HCL (g) -> NH4CL (s)

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17
Q

At equilibrium

A

Rate of forward reaction = rate of backward reaction

18
Q

When can a reversible reaction reach equilibrium?

A

When it’s in a closed system

19
Q

A chemical equilibrium is a

A

Dynamic equilibrium

The forward and backward reactions keep going

20
Q

What is the haber process

A

Is a reversible reaction between nitrogen and hydrogen to form ammonia

21
Q

Atmospheric pressure and temperature for have process

A

200 atmospheres pressure

450C

22
Q

Change in the equilibrium
TemperTure increased
What’s the effects?

A

Moves in the direction of an endothermic reaction

Rate of reaching equilibrium is increased

23
Q

Pressure increased in equilibrium

What are the effects

A 
Moves in the direction of the fewest molecules of gas 
Rate increased (if reacting gases are present) to equilibrium
24
Q

Concentration of a reacting substance is increased in an equilibrium.
How does this effect?

A

Moves in the direction away from the reacting substance

Rate of reaching equilibrium increases

25
Q

Catalyst added to equilibrium

How does this effect

A

No change on equilibrium position.

Rate of reaching equilibrium increased

26
Q

Conditions chosen for industrial processes are related to the following:

A

Availability of raw materials and energy supplies. In the haber process air and water are easily obtained ; natural gas is more difficult and expensive to obtain

The control of temperature and pressure. Many industrial processes are not allowed to reach equilibrium because this would take too long

The use of a suitable catalyst eg iron in the haber process. Processes work at lower temperatures with catalysts, reducing cost and increasing yield

27
Q

What’s NPK

A

Nitrogen
Phosphorus
Potassium

28
Q

What three substances do plants need to grow well?

A

NPK

29
Q

NPK must be supplied in soluble compounds for plant roots to absorb. What are their typical soluble compound

A

Nitrate and ammonium salts
Phosphate salts
Potassium salts

30
Q

Ammonium salts are useful for

A

Feritilises

31
Q

Ammonium salts are a source of soluble

A

Nitrogen

32
Q

Formula for ammonium nitrate and how much of it is nitrogen

A

NH4NO3

35%

33
Q

Formula for ammonium sulfate

And how much is nitrogen

A

(NH4)2SO4

21%

34
Q

Lavatory-scale producayion of ammonium sulfate needs:

A

Ammonia solution
Dilute sulfuric acid

These are bought from chemical manufacturers. Lavatory production involves:

Only a few stages (titration then crystallisation)
Small-scale production (very little made)

35
Q

Fertiliser in a factory industrial-scale production of ammonium sulfate needs:

A

Natural gas, air and water
Sulfur, air and water

Industrial production involves:
Many stages (making ammonia and sulfuric acid reacting accurate volumes followed by evaporation)
Large-scale production (a lot is made)

36
Q

A voltage can be produced using reactions in

A

Chemical cells and fuel cells

37
Q

You can make a simple chemical cell using

A

Piece of zinc dipped in zinc sulfate sokution

A piece of copper dipped in copper sulfate solurion

A piece of filter paper soaked in potassium chloride solution

Crocodile clips, wires and a
voltmeter

Electrons from through from zinc to copper (zinc is more reactive) ions pass through the filter paper from one beaker to the other completing the circuit. The ordinary batteries you use in torches and clocks are chemical cells but the solutions are mixed with powders to make a paste

38
Q

In a hydrogen-oxygen fuel cell:

A

Hydrogen and oxygen are used to produce a voltage

Water vapour is the only product

Overall : 2H2 (g) + O2 (g) -> 2H2O(g)

This is not combustion (no flames are made)

39
Q

Advantages of chemical cells

A

Suitable for portable appliances

Cheap to manufacture

40
Q

Disadvantages of chemical cells

A

May contain harmful substances

Do not produce a voltage when one of the reactens is used up - they go ‘flat’

41
Q

Advantages of hydrogen - oxygen fuel cells :

A

Produce water as the only waste

Produce a voltage as long as the air are supplied

42
Q

Disadvantages of hydrogen - oxygen fuel cells

A

Not suitable for portable appliances

Are expensice to manufacture

CHEMISTRY (15 decks)

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