Topic 6: Calcultions and Equilibria Triple

Question 1

Define what is meant by yield?

Answer 1

The amount of useful product you can get from something.

Question 2

Define what is meant by theoretical/expected yield?

Answer 2

The maximum calculated amount of a product that can be formed from a given amount of reactants.

Question 3

Define what is meant actual yield?

Answer 3

The actual amount of a product obtained from a chemical reaction.

Question 4

What is the formula to work out %yield?

Answer 4

%yield = actual yield/theoretical yield x 100%

Question 5

What are the three main reasons why reactions don’t produce 100% yield?

Answer 5

1) some of the product is lost or not collected (e.g when transferring liquid from a container)
2) there may be an unwanted side reaction taking place so an unexpected product is also made.
3) the reaction may be reversible or reach an equilibrium. If the reaction is not left for long enough the reactants may have not all been used up.

Question 6

What is the assumption when calculating the theoretical yield?

Answer 6

The theoretical yield is calculated assuming that all the reactants are turned in to products and that the products are successfully separated from the reaction mixture.

Question 7

What does a higher yield mean in terms of benefits?

Answer 7

The higher the percentage yield to more useful the reaction is. A higher yield means that fewer raw materials are used to make the same amount of product, so there is less waste and more profit can be made.

Question 8

What is atoms economy?

Answer 8

This is a method to show how efficiently a particular reaction makes use of atoms in the reactants.

It is also the mass of products as a % of the mass of all products you make.

Question 9

What is the formula for atom economy?

Answer 9

Atom economy=mass of useful product/mass of all productsx100

Question 10

Why is atom economy used?

Answer 10

Many reactions form more than one product and not all the products are useful. The atom economy shows the percentage, by mass of useful products.

Question 11

What method of making ethanol has a higher atom economy and is therefore more useful?

Answer 11

The reaction of ethane with steam is one way of making ethanol and has a 100% atom economy because all the atoms in the reactant are present in the product.

Fermentation is less useful because it does not have a 100% atoms economy as carbon dioxide is given off as a by product.

Question 12

How is ethanol made by reacting it with steam?

Answer 12

Raw materials: ethene obtained from crude oil (non renewable).
Temperature: 300°c
Pressure: high pressure (60-70 atm)
Catalyst: concentrated phosphoric acid
Rate of reaction: fast
Purity: pure

Question 13

How is ethanol made by fermentation?

Answer 13

Raw materials: carbohydrates (e.g sugar from sugar cane, maize) the are renewable.
Temperature: 30°c-40°c
Pressure: atmospheric pressure
Catalyst: enzymes in yeast
Rate of reaction: slow
Purity: impure must be fractionally distilled.

Question 14

What are the 2 ways of making ethomal?

Answer 14

Fermentation of sugars and reacting ethene with steam.

Question 15

What are the disadvantage of using a percentage yield calculation when choosing a reaction path way?

Answer 15

% yield calculations give no indication of how waste products. A reaction can have a high percentage yield but a low atoms economy, meaning waste by products are formed.

Question 16

Give one way to improve the atom economy for a reaction:

Answer 16

Find uses for the by-products of the reaction.

Question 17

What factors must chemists consider when choosing a reaction pathway:

Answer 17

• energy consumption
• rate of reaction
• raw materials
• conditions needed to produce a high yield if reaction reaches equilibrium.

Question 18

What is a solution with an accurate concentration made up in and why?

Answer 18

A solution with an accurate concentration is made up in a volumetric flask. These flasks are calibrated to measure one volume of solution accurately.

To make a solution the solute is dissolved in distilled water and and then made up to the graduation mark before stoppering and shaking the flask.

Question 19

What is a by-product of a reaction?

Answer 19

Substances produced in a chemical reaction that are in addition the desired product.

Question 20

What is volumetric flask?

Answer 20

A flask which is accurately calibrated to hold a given volume of solution.

Question 21

What does calibrated mean?

Answer 21

Marked with a scale for accurate reading.

Question 22

When measuring the volume of a solution in a volumetric flask what is it important to remember and why?

Answer 22

Measure the volume from the bottom of the meniscus and look at it with your eyes on the same level as the graduation mark to get an accurate measurement and avoid random error.

Question 23

What is meant by a concentration of a solution?

Answer 23

The amount of solute dissolved in a stated volume of solution.

Question 24

What are the units for concentration?

Answer 24

gdm -3

1dm = 1000cm = 1litre

Or mol/dm-3

Question 25

What is the calculation for concentration on gdm-3?

Answer 25

Concentration(gdm-3)=mass of solute(g)/volume of solution (dm3)

Question 26

What is the calculation for working out concentration in mol dm-3?

Answer 26

Concentration mol dm-3 = moles of solute (mol)/volume of solute (dm3)

Question 27

How do you work out the concentration in mol dm-3 when you only have the concentration in gdm-3?

Answer 27

Concentration (mol dm-3) = concentration (gdm3)/ relative formula mass

Question 28

What are acid, alkali titration used for?

Answer 28

They are used to find the exact volume of an acid that neutralises a specified volume of of an alkali or vice verse.

Question 29

Why do we use indicators during titration and what indicators do we use?

Answer 29

We use indicators because acids and alkalis are colourless. When the indicator turns colourless then that is the volume of acid needed to neutralise the alkali or vice verse.

We use phenolphthalein because it turns pink in the presence of an acid or alkali and colourless when they neutralise each other.

Question 30

Why can you not use universal indicator in a titration?

Answer 30

Because it changes depending on the pH of a solution rather than a distinct colour change. E.g pink to clear.

Question 31

Why is a burette used in titrations?

Answer 31

It has a fine scale, which gives it a good resolution.

Question 32

What are the steps involved in a titration?

Answer 32

1) rinse a burette with hydrochloric acid, then fill the burette with the acid making sure the jet below the tap is full.
2) record the initial volume of acid in the burette.
3) rinse a pipette with sodium hydroxide solution the fill the pipette to the 25.0cm3 mark and empty the solution in to a conical flask.
4) add a few drops of methyl orange of phenolphthalein to the conical flask and place it under a white tile to make colour change more distinct.
5) add the acid from the burette to the sodium hydroxide solution drop by drop while swirling the flask.
6) when the indicator starts to change colour, continue adding acid until end point is reached. When solution goes clear record volume of acid that was needed to neutralise the alkali.
7) repeat experiment without using indicator to ensure that when the acid neutralises the alkali only water and salt are produced so the indicator does not affect end result.

Question 33

What does the volume occupied by a sample of gas depend on?

Answer 33

The temperature, pressure and number of particles of gas.

THE VOLUME OF GAS DOES NOT DEPEND ON ITS RELATIVE FORMULA MASS.

Question 34

When using phenolphthalein in a titration what is the colour change?

Answer 34

Pink to clear.

Question 35

When using methyl orange in a titration what is the colour change?

Answer 35

Yellow to peach/orange.

Question 36

What does Avogadro law state?

Answer 36

It states that if the temperature and pressure are the same equal volumes of different gases contain an equal number of molecules. This lets you calculate volumes of gases involved in a reaction.

Question 37

What is the molar gas volume?

Answer 37

The volume occupied by one mole of molecules of any gas. This is 24dm3 or 24000cm3 at room temperature and pressure -RTP.

Question 38

How can you calculate moles of gas?

Answer 38

Amount of gas (moles)= volume/molar volume or 24.

The molar volume for gas is 24.

Question 39

When a plant grows in soil why is the concentration of mineral ions reduced in the soil and what impacts may this have in the plant?

Answer 39

Plants absorb minerals and water through their root hair cells. This gradually reduces the concentration of mineral ions in the soil.

Plants may then not grow properly and suffer from deficiency diseases, such as stunted shoots and discoloured leaves.

Question 40

What do fertilisers do and how does this help plants?

Answer 40

Fertilisers replace mineral ions needed by plants and so promote plant growth. Nitrogen, phosphorus and potassium are important elements but fertilisers must provide them to the plant as soluble compounds because root hair cells can only absorb mineral ions that are dissolved in water.

Question 41

Give two everyday examples of devices that use chemical cells?

Answer 41

Batteries used in mobile phones and torches.

Question 42

What components does a simple chemical cell have?

Answer 42

• two different metals, each dipped in a solution of one of their salts.
• a ‘salt bridge’ to allow dissolved ions to pass from one solution to the other.

Question 43

How does a simple chemical cell work?

Answer 43

A voltage (potential difference) is produced between two metals. In general the further apart the metals are in the activity series the greater the voltage. A current flows if the cell is connected to an external circuit.

Question 44

Is the reaction is a chemical cell exothermic or endothermic and why?

Answer 44

Exothermic, but the energy transferred is mainly by electricity rather than heating. When one of the reactants are used up the reaction stops and voltage is no longer produced.

This why electric cars need recharging after a journey or why batteries go ‘flat’.

Question 45

What is a fuel cell?

Answer 45

A device that produces voltage due to reactions involving fuel and oxygen, for as long as these reactions are supplied.

Question 46

How do fuel cells work?

Answer 46

Fuel cells are supplied with fuel and oxygen from outside. No burning takes place inside fuel cells, like chemical cells. Fuel cells do not go ‘flat’ they produce voltage for al long as the reactants are supplied.

Question 47

State in difference between a chemical cell and a fuel cell:

Answer 47

In a chemical cell when one of reactants are used up the reaction stops and voltage is no longer produced.
A fuel cell produces voltage for as long as the reactants are supplied.

Question 48

What is the product of an oxygen-hydrogen fuel cell?

Answer 48

Water is the only product.

Question 49

What is the advantage of using a hydrogen-oxygen fuel cell and an electric motor in a car rather than petrol or diesel?

Answer 49

A hydrogen-oxygen fuel cell is much quieter and uses less maintenance than a petrol or diesel engine but it still needs to be stored in a tank.

They don’t produce carbon dioxide, which is a green house gas meaning they are better for the environment. However, most hydrogen is manufactured with by the reaction of steam with natural gas or oil which does produce CO2 as a by-product.

Question 50

What do nitrogenous fertilisers supply a plant with and how are they manufactured? Give an example.

Answer 50

Nitrogenous fertilisers are a source of soluble nitrogen compounds. E.g ammonium nitrate.
Ammonium nitrate is a salt manufactured by reacting ammonia with dilute nitric acid:
Ammonia + nitric acid ➡️ ammonium nitrate

(Ammonia is manufactured in the hater process which is a reversible reaction.
Nitric acid is manufactured from ammonia in several stages over all the reaction it can be obtained from is:
Ammonia + oxygen ➡️ nitric acid + water. )

Question 51

State another nitrogenous fertilisers apart from ammonium nitrate. How is it manufactured?

Answer 51

Ammonium sulphate. It is made by reacting ammonia solution with dilute sulphuric acid:
Ammonia + sulphuric acid ➡️ ammonium sulphate

Preparing ammonium sulphate in the laboratory is is different from its industrial production.

Question 52

Compare the preparation of ammonium sulphate in a laboratory compared to on an industrial scale:

Answer 52

Laboratory-
Scale: small
Starting material: ammonium solution, dilute sulphuric acid
Stages: titration, then crystallisation 
Type of process: batch

Industrial:
Scale: large
staring materials: raw materials for making ammonia and sulphuric acid 
Stages: several stages
Type of process: continuous

Question 53

What are the differences between a batch and continuous process?

Answer 53

A batch process is when small amount of the product desired is made in a lab, the apparatus is cleaned and another small amount is made. In a continuous process, the reactants are constantly fed into the reactors and the product is removed. The process is only stopped rarely to maintain and clean equipment.

Batch processes are difficult to automate. However, continuous processes are usually automated so they need fewer people to make a given amount of product.

Question 54

Under what type of system may a reversible reaction reach an equilibrium?

Answer 54

A reversible reaction may reach equilibrium in a closed system, such as a stoppered flask in which no substances can enter or leave.

Question 55

What happens in a dynamic equilibrium?

Answer 55

• the forward and backward reaction still happen at the same rate.
• the concentrations of all reacting substances do not change.

Question 56

When conditions, such as temperature, pressure in a reaction involving gases and the concentration of a reacting substance are increased as well as a catalyst being added how does this change the position of equilibria and affect the rate it reaches equilibrium?

Answer 56

Temperature increased- moves in direction of endothermic reaction- time to reacher equilibria decreases.

Pressure increased in a reaction involving gases- moves toward the side in the balanced equation with the fewer molecules of the reacting gas-time reaching equilibria decreases.

Concentration of a reacting substance increases- moves away from a reacting substance in a balanced equation- time decreases.

Catalyst added- no change in position- time decreases.

Question 57

How do chemical manufactures choose reaction conditions?

Answer 57

The must make profit so choose conditions to produce an acceptable amount of yield in an acceptable amount certain time. This may mean the reaction is not allowed to reach equilibrium- (it is not profitable to achieve equilibrium if it takes years). Also choosing conditions that give a high rate of reaction is not worth it if they are too expensive.

Question 58

What is a reaction pathway?

Answer 58

A series of reactions needed to make a certain product.

Question 59

What factors are taken into account when choosing reaction pathways?

Answer 59

• availability and cost of raw materials and supplies.
• the rate of reaction and equilibrium position.
• atom economy, yield and usefulness of by-products.

Question 60

What are the 2 processes which produce nitric acid?

Answer 60

Birkeland-Eyde process:
A high voltage electric arc was created between the 2 electrodes. Nitrogen and oxygen in the air reacted to form Nitrogen monoxide (NO) and then further reacted to produce Nitrogen oxide (NO2), which is dissolved in water to form nitric acid. However, this method uses huge amounts of electricity and nitrogen monoxide only has a yield of 4%. We use the Ostwald process.

Ostwald process:
This is a series of reactions to get nitric acid.
Ammonia + oxygen ➡️ nitrogen monoxide +water
Nitrogen monoxide + oxygen ➡️ nitrogen dioxide
(These are both reversible reactions)
Nitrogen dioxide + oxygen + water ➡️ nitric acid

Question 61

What is the equation describing the haver process?

Answer 61

N2 (g) + 3H2 (g) ➡️ 2NH3(g)

It’s a reversible reaction.

The forward reaction is exothermic and the backwards reaction is endothermic.