Yeilds, Atom Economy, Lots Of Calculations Etc

Question 1

What is the theoretical yield?

Answer 1

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

Question 2

What is the actual yield?

Answer 2

The actual amount of product obtained from a chemical reaction.

Question 3

What is the percentage yield?

Answer 3

The percentage yield compares the actual yield and the theoretical yield.

Question 4

What is the equation for percentage yield?

Answer 4

Actual yield ➗ theoretical yield ✖️ 100

Question 5

When manufacturing substances, the theoretical yield is calculated assuming that all the reactants are turned into products, and that the products are successfully separated from the reaction mixture. What are the 3 main reasons reactions do not give 100%?

Answer 5

The reaction may be incomplete so not all of the reactants are used up.
Some of the product may be lost.
There might be unwanted side reactions taking place.

Question 6

In a yields experiment, why may the reaction be incomplete?

Answer 6

Possibly because the reaction has not been left for long enough, or the reaction may reach equilibrium.

Question 7

In a yields experiment, why may some of the product be lost?

Answer 7

Eg, when a liquid is transferred from one container to another, some of it will be left behind on the walls of the first container.

Question 8

In a yields reaction what unwanted side reactions may be taking place?

Answer 8

Some of the reactants may react to make a different product. These side reactions compete with the main reaction.

Question 9

The higher the percentage yield of a reaction, the more useful the reaction is. What does a high yield mean?

Answer 9

A high 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 10

What is atom economy?

Answer 10

Atom economy is a method of showing how efficiently a particular reaction makes use of the atoms in the reactants. Many reactions form more than one product and not all of the products are useful. The atom economy shows the percentage, by mass of useful products.

Question 11

What is the equation for atom economy?

Answer 11

Atom economy =

Mr of the useful product ➗ sum of the Mr of all reactants ✖️ 100

Question 12

There are two ways of making ethanol. C2H5OH. In one method ethane, C2H4 is reacted with steam. All of the atoms in the reactants are present, so what does this make the atom economy?

Answer 12

100%

Question 13

Ethanol is also produced by the fermentation of sugars, such as glucose. The atom economy of this method is much lower than making ethanol from ethane, why?

Answer 13

Because the carbon dioxide is formed as a by-product.

Question 14

What is a by-product?

Answer 14

Substance produced in a chemical reaction in addition to the desired product.

Question 15

A percentage yield calculation gives no indication of the amount of waste products. A reaction can have a high percentage yield but a low atom economy, meaning that waste by-products are formed. It is better to reduce the amount of waste produced than to have to treat it later. What is one way to improve the atom economy of a reaction.

Answer 15

To find used for the by-products of the reaction.

Question 16

What is a reaction pathway?

Answer 16

A series of reactions needed to make a particular product.

Question 17

What factors must chemists consider when deciding on a reaction pathway?

Answer 17

Energy consumption,
Rate of reaction,
Raw materials,
Conditions needed for a high percentage yield if the reaction reaches equilibrium.

Question 18

Go to page 111

Answer 18

Look at the table that compares the two ways of making ethanol.

Question 19

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

Answer 19

A volumetric flask. These flasks are calibrated to measure one volume of solution accurately. To make a solution, the solution is distilled water and then made up to the graduation mark before stoppering and then shaking the flask. It is important to measure the volume from the bottom of the meniscus and to look at it with your eyes on the same level as the graduation mark to get an accurate measurement and avoid random error.

Question 20

What is the concentration of a solution?

Answer 20

The concentration of a solution is the amount of solution the dissolved in a stated volume of solution. The units of concentration are usually written as g dm -3. 1 dm3 is the same volume as 1 litre or 1000cm3

Question 21

What is the equation for concentration?

Answer 21

Concentration in g dm3 =
mass of solute in g ➗ volume of solute in dm3
If the volume of the solution is given in cm3, convert it to dm3 by dividing by 1000. You can use an equation triangle to help re-arrange this equation.

Question 22

The concentration of a solution if often given as the number of moles of solute dissolved in 1 dm3 of solution. What is this unit written as?

Answer 22

Mol dm-3

Question 23

What is the equation for concentration in mol dm-3?

Answer 23

Number of moles of solute ➗ volume of solution in dm3

Or this equation could be useful

Concentration in g dm-3 ➗ Mr of solute

Question 24

Go to page 113

Answer 24

Memorise the triangle

Question 25

What are acid-alkali titration used to find?

Answer 25

They are used to find the exact volume of an acid that neutralises a specified volume of an alkali or vice versa. Acids and alkalis are colourless so we use indicators.

Question 26

What is the mole ratio?

Answer 26

The ratio in moles of the substances in a balanced equation.

Question 27

What is the equation for number of moles in a substance for solids?

Answer 27

Number of moles of substance = | Mass of substance (g) ➗ Mr

Question 28

What is the equation for moles in liquids?

Answer 28

Concentration ✖️ volume (dm3)

Question 29

What is the equation for moles in gases?

Answer 29

Volume ➗ 24dm3

Question 30

Go to the back of chemistry book

Answer 30

Learn the mole calculations

Question 31

What is the method for titration calculations?

Answer 31

Fill the burette with hydrochloric acid, making sure the jet below tap is also full. Record initial volume of acid in the burette. Then fill the pipettes to the 25.0 cm3 mark with sodium hydroxide solution, and then empty the solution into a conical flask. Add a few drops of methyl orange indicator to the flask and place the flask on a white tile under the burette. Add the acid to the sodium hydroxide solution while swirling the flask. When the indicator starts to change colour, rinse the tip of the burette and the sides of the flask with a small amount of distilled water from a wash bottle to ensure that all the acid is in the mixture, then add the acid drop by drop until the end-point is reached. Record the final volume of acid in the burette. Repeat the experiment, apart from the initial rinsing of the burette and pipette, until concordat results are obtained.

Question 32

The volume occupied by a sample of a gas depends on the temperature, pressure and number of particles of gas. The volume of a gas does not depend on its relative formula mass (Mr). Remember that the mean distance between gas particles is very large compared to what?

Answer 32

The size of the particles.

Question 33

What does Avogadro's law state?

Answer 33

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

Question 34

What is molar gas volume?

Answer 34

The molar gas volume is the volume occupied by one mole of molecules of any gas. It is 24 dm3 or 24 000 cm3 at room temperature and pressure.

Question 35

What is the equation for volume of gas at rtp?

Answer 35

Volume of gas = | Amount of gas (mol) ✖️ molar volume

Question 36

If you know the volume of a gas at rtp, you can calculate its amount, what is the equation?

Answer 36

Amount of gas (mol) ➗ molar volume

Question 37

What is the equation for molar volume?

Answer 37

Molar volume = | Volume of gas ➗ amount of gas (mol)

Question 38

Go to page 118

Answer 38

And look at the triangle

Question 39

Plants absorb mineral ions through their root hair cells. This gradually reduces the concentration of mineral ions in the soil. Plants may then not grow properly and may suffer from deficiency diseases, such as stunted shoots and discoloured leaves. What replaces the mineral ions?

Answer 39

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

Question 40

What are nitrogenous fertilisers a source of?

Answer 40

Soluble nitrogen compounds. Ammonium nitrate is one nitrogen-rich fertilisers. Ammonium nitrate is a salt manufactured by reacting ammonia solution with dilute nitric acid.

Question 41

What is the word equation for ammonium nitrate?

Answer 41

Ammonia + nitric acid ➡️ ammonium nitrate

Question 42

What is the chemical equation for ammonium nitrate?

Answer 42

NH3 (aq) + HNO3 (aq) ➡️ NH4NO3 (aq)

Question 43

What is ammonia manufactured by?

Answer 43

Ammonia is manufactured by the haber process, in which nitrogen and hydrogen react together in a reversible reactions

Question 44

Nitric acid is manufactured from ammonia in stages. What is the word and chemical equation for this reaction?

Answer 44

Ammonia + oxygen ➡️ nitric acid + water NH3 (g) + 2O2 (g) ➡️ HNO3 (aq) + H2O (l)

Question 45

Ammonium sulfate is also a nitrogenous fertiliser. It is made in the lab by reacting ammonia solution with dilute sulfric acid. What is the word and chemical equation for this?

Answer 45

Ammonia + sulfric acid ➡️ ammonium sulfate 2NH3 (aq) + H2SO4 (aq) ➡️ (NH4)2 SO4 (aq)

Question 46

Describe the process of making ammonium sulfate in a lab

Answer 46

Scale of production - small scale Starting materials - ammonia solution and dilute sulfur if acid Stages - titration, then crystallisation Type of process - batch

Question 47

Describe the processes of making ammonium sulfate in industrial production

Answer 47

Scale of production - large scale Starting materials - raw materials for making ammonia and sulfuric acid Stages - several stages Type of process - continuous

Question 48

For the laboratory preparation of ammonium sulfate, ammonia solution and dilute sulfuric acid are brought from chemical manufacturers. For industrial production, these substances are often made on-site from their raw materials.

Answer 48

Go to page 121 and look at the flow chart

Question 49

The laboratory preparation of ammonia sulfate is a batch process. A small amount is made, the apparatus is cleaned, and them another small amount is made. What is the difference with industrial preparation?

Answer 49

Industrial preparation is a continuos process, in which reactants are constantly fed into the reactors and products are removed. The process is stopped only rarely to maintain and clean the equipment. Batch processes are difficult to automate. However, continuos processes are usually automated, so they need fewer people to make a given amount of product.

Question 50

Reversible reactions may reach equilibrium, but when does this only happen?

Answer 50

In a closed system, such as a stoppered flask, in which no substances can leave or enter.

Question 51

What is a closed system?

Answer 51

When substances cannot enter or leave an observed environment.

Question 52

Describe what happens in dynamic equilibrium

Answer 52

The forward and backward reactions still happen, and at the same rate. The concentrations of all reacting substances do not change. The position of a dynamic equilibrium, and how quickly equilibrium is reached, are affected by changes in conditions.

Question 53

What is dynamic equilibrium?

Answer 53

When the forwards and backwards reactions in a reversible chemical reaction are occurring at the same rate.

Question 54

In dynamic equilibrium, what if the temperature is increased?

Answer 54

Equilibrium position shifts in the endothermic direction (transferring energy from the surroundings)

Question 55

What happens to the position of equilibrium if the temperature is decreased?

Answer 55

It moves in the exothermic direction (transferring energy to the surroundings)

Question 56

In dynamic equilibrium what if the gas pressure is increased?

Answer 56

The equilibrium position shifts in the direction that forms fewer gas molecules (as this reduces pressure)

Question 57

In a dynamic equilibrium reaction, what happens if the gas pressure is decreased?

Answer 57

The equilibrium position shifts in the direction that forms more gas molecules (as this is increasing pressure) w

Question 58

What happens if the concentration is increased in a dynamic equilibrium reaction?

Answer 58

The equilibrium position will shift in the direction that uses up substances that has been added.

Question 59

What happens if the concentration is decreased in a dynamic equilibrium reaction?

Answer 59

The equilibrium position shifts in the direction that forms more of the substance that has been removed.

Question 60

What happens if a catalyst has been added to a dynamic equilibrium reaction?

Answer 60

There is no change in position

Question 61

Which way do the dynamic equilibrium position shift in endothermic and exothermic reactions.

Answer 61

Exothermic ➡️ | Endothermic ⬅️

Question 62

Chemical manufacturers must make a profit. They choose reaction conditions to produce an acceptable product yield in an acceptable time. What might this mean?

Answer 62

This may mean that reactions are not allowed to reach equilibrium - it is not profitable to achieve equilibrium if it takes years. In addition, choosing conditions that give a high rate of reaction is not worthwhile if they are too expensive.

Question 63

In the haber process, nitrogen and hydrogen react together to produce ammonia: N2 (g) + 3H2 (g) reversible reaction sign 2NH3 (g) Describe what will happen to the position of equilibrium

Answer 63

There are fewer molecules of reacting gas on the right-hand side of the equation. As the pressure is increased, the position of equilibrium moves to the right.

Question 64

High pressures increase the equilibrium yield of ammonia and increase the rate of reaction. Why is the pressure chosen (200 atmospheres) a compromise?

Answer 64

Because very high pressures are too expensive to maintain.

Question 65

In equilibrium reactions, the forward reaction is exothermic, so the backward reaction is endothermic. As the temperature is increased, the position of equilibrium moves to the left (in the direction of the endothermic reaction. What do low temperatures do?

Answer 65

Low temperatures increase the equilibrium yield of ammonia but reduce the rate of reaction. The temperature chosen, 450°C , is a compromise that gives an acceptable yield of ammonia in an acceptable time. An iron catalyst is used because, although it does not change the position of equilibrium, it does increase the rate of reaction.

Question 66

There is usually more than one way to make a product. How is a particular reaction pathway is chosen?

Answer 66

By taking into account factors such as: Availability and cost of raw materials and energy supplies. The rate of reaction and the equilibrium position. Atom economy, yield and usefulness of by-products.

Question 67

Go to page 123

Answer 67

And see how nitric acid is made

Question 68

The everyday batteries used in mobile phones and torches and chemical cells. What components does a simple chemical cell have?

Answer 68

Two different metals, each dipped into a solution of one of their salts. A 'salt bridge' to allow dissolved ions to pass from one solution to the other.

Question 69

What is produced by the two metals in a chemical cell?

Answer 69

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

Question 70

Explain what happens in a chemical cell

Answer 70

It's an exothermic reaction, but the energy is transferred mainly by electricity rather than by heating. When one of the reactants is used up, the reaction stops and a voltage is no longer produced. This is why household batteries go 'flat', and why electric cars need recharging after a journey. Most modern batteries are 'dry cells' containing various reactants. Their solutions are mixed with a powder to make a paste.

Question 71

How can a simple chemical cell be made?

Answer 71

By connecting two half cells (see exercise book) with different electrode potentials. One half cell releases electrons (oxidised) and one half cell gains electrons (reduced). There will be a voltage difference.

Question 72

In chemical cells, the more reactive a metal is, the greater the what?

Answer 72

The greater the tendency to release electrons and be oxidised. Therefore in the zinc and copper cell, the zinc is oxidised and the copper is reduced.

Question 73

Explain what are fuel cells are

Answer 73

Chemical cells store all their reactants, but fuel cells are supplied with fuel and oxygen from outside. No burning takes place inside fuel cells, just like in chemical cells. Fuel cells do not go 'flat' - they produce a voltage for as long as reactants are supplied. Hydrogen - oxygen fuel cells use hydrogen and oxygen and water is the only product.

Question 74

What's a chemical cell?

Answer 74

A device that produces a voltage due to reactions between reactants stored inside it, until one of the reactants is used up.

Question 75

What is a fuel cell?

Answer 75

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

Question 76

Describe a hydrogen - oxygen fuel cell

Answer 76

At the left hand electrode, hydrogen atoms lose electrons and form hydrogen ions. The electrons flow through the external circuit to the positive electrode. Hydrogen ions pass through a membrane to the right - hand electrode, where they gain electrons and react with oxygen.

Question 77

Go to page 124 and 125

Answer 77

And look at the diagrams

Question 78

What are the advantages of a hydrogen - oxygen fuel cell?

Answer 78

A hydrogen - oxygen fuel cell and electric motor are much quieter and need less maintenance than a petrol or Diesel engine, but the hydrogen still needs to be stored in a tank. They do not produce carbon dioxide (a greenhouse gas). More efficient than power stations or batteries.

Question 79

What are the disadvantages of hydrogen - oxygen fuel cells?

Answer 79

Most hydrogen is manufactured by the reaction of steam with coal or natural gas. These processes release carbon dioxide as a by-product. Hydrogen is very expensive. Fossil fuels are burnt.