Separate Chemistry 1

Question 1

Properties of transition metals

Answer 1

Most of the known metals are transition metals and they have typical properties of metals.
They are very lustrous, they are hard, strong and are good conductors of heat and electricity.
They are highly dense metals and have very high melting points.
Transition metals can have more than one oxidation state as they can lose a different number of electrons, depending on the chemical environment they are in.
Compounds containing transition elements in different oxidation states will have different properties and colours in aqueous solutions.

Question 2

Uses of transition metals

Answer 2

The transition elements are used extensively as catalysts which are substances that speed up the rate of a reaction without being used up in the process.
They do not take part in the reaction.
Their catalytic characteristics stem from their ability to interchange between a range of oxidation states.
This allows them to form complexes with reagents which can easily donate and accept electrons from other chemical species within a reaction system.

The transition metals are also used in medicine and surgical applications such as limb and joint replacement.
Titanium in particular is useful as it is the only element that can bond with bones due to its high biocompatibility.

They are also used to form coloured compounds in dyes and paints for both household and industrial applications.
They are used in creating stained glass, jewellery and in anti-corrosive materials.

Question 3

Corrosion

Answer 3

Corrosion is the destruction of materials by chemical substances in their environment which act on them over a period of time.
Most metals can corrode in the presence of oxygen to form the corresponding metal oxide.
Corrosion is caused by redox reactions:
The metal loses electrons and is oxidised while the oxygen gains electrons and is reduced.
Rusting is the name given specifically to the corrosion of iron in the presence of water and oxygen from the air:
iron + water + oxygen ⟶ hydrated iron(III)oxide

Question 4

Barrier method preventing iron rusting

Answer 4

Rust can be prevented by coating iron with barriers that prevent the iron from coming into contact with water and oxygen.
However, if the coatings are washed away or scratched, the iron is once again exposed to water and oxygen and will rust.
Unlike some other metals, once iron begins to rust it will continue to corrode internally as rust is porous and allows both air and water to come into contact with fresh metal underneath any barrier surfaces that have been broken or scratched.
Common barrier methods include: paint, oil, grease and plastic.

Question 5

Galvanising protection for iron

Answer 5

Iron can be prevented from rusting making use of metals higher in reactivity than iron.
Galvanising is a process where the iron to be protected is coated with a layer of zinc.
ZnCO3 is formed when zinc reacts with oxygen and carbon dioxide in the air and protects the iron by the barrier method.
If the coating is damaged or scratched, the iron is still protected from rusting because zinc preferentially corrodes as it is higher up the reactivity series than iron.
Compared to iron it loses its electrons more readily:
Zn → Zn2+ + 2e-
The iron stays protected as it accepts the electrons released by zinc, remaining in the reduced state and thus it does not undergo oxidation.
The electrons donated by the zinc react with hydrogen ions in the water producing hydrogen gas:
2H+ + 2e– → H2
Zinc therefore reacts with oxygen and water and corrodes instead of the iron.

Question 6

Sacrificial corrosion

Answer 6

Sacrificial corrosion occurs when a more reactive metal is intentionally allowed to corrode.
e.g. - Galvanising is an example of sacrificial protection, where a coat of zinc is put onto an iron object (such as a bucket) to prevent rusting. The zinc acts as sacrificial protection as it’s more reactive than iron, so it’ll corrode in preference to iron. The zinc also acts as a barrier.

Question 7

Electroplating

Answer 7

Electroplating is a process where the surface of one metal is coated with a layer of a different metal.
The metal being used to coat is a less reactive metal than the one it is covering.
The anode is made from the pure metal used to coat
The cathode is the object to be electroplated.
The electrolyte is an aqueous solution of a soluble salt of the pure metal at the anode.

Question 8

Uses of electroplating

Answer 8

Electroplating is done to make metals more resistant to corrosion or damage, e.g. chromium and nickel plating.
When people talk about a ‘tin can’, the amount of tin is very small (only about 1%). The can is made from steel and has a very thin coat of tin on the interior surface that resists corrosion from the liquids inside.
It is also done to improve the appearance of metals, e.g. silver plating cutlery and jewellery.

Question 9

Alloys

Answer 9

Alloys are mixtures of metals, where the metals are mixed together physically but are not chemically combined.
They can also be made from metals mixed with nonmetals such as carbon.
Alloys often have properties that can be very different to the metals they contain, for example they can have greater strength, hardness or resistance to corrosion or extreme temperatures.
Alloys contain atoms of different sizes, which distorts the regular arrangements of atoms.
This makes it more difficult for the layers to slide over each other, so they are usually much harder than the pure metal.
Brass is a common example of an alloy which contains 70% copper and 30% zinc.

Question 10

Steel alloys

Answer 10

Iron from a blast furnace is an alloy of 96% iron, with carbon, phosphorus, silicon and sulfur impurities.
It is called cast iron and is too brittle for many uses, so most of it is converted into steel by removing some of the impurities.
Not all of the carbon is removed as steel contains some carbon, the percentage of which depends on the use of the steel.
Alloys of steel are made from adding other metals to steel such as chromium, manganese or nickel.
By carefully controlling the amounts added, the particular type of alloy required can be produced.
Steel alloys are used in construction, transport, manufacturing and other industries.

Question 11

Types of stainless steel

Answer 11

Mild steel - 0.25% carbon alloyed with iron - Car body panels, wires - Soft and malleable.
High carbon steel - 0.5-1.4% carbon - Tools and chisels - Hard.
Low alloy steel - 1-5% of other metals (Cr, Ni, Ti) - Construction, bridges, high
speed tools - Hard and strong, low ductility and malleability.
Stainless steel - 20% chromium and 10% nickel - Cutlery and sinks, chemical plants - Strong and resistant to corrosion.

Question 12

Alloys of aluminium

Answer 12

Aluminium is mixed with copper, manganese and silicon for aircraft body production as aluminium alloys tend to be stronger and lighter than pure aluminium.
Aluminium and magnesium (5%) make an interesting alloy called magnalium which is also used extensively in automobile and aircraft construction
As well as being lighter and stronger, it is also more corrosion resistant than aluminium.
Magnalium with 50% magnesium is used in the production of fireworks as it is more stable than pure magnesium but still burns brightly.

Question 13

Alloys of copper

Answer 13

Bronze is an alloy made of copper and tin. It is harder than copper and is used to make ornaments and medals.
Brass is a common example of an alloy which contains 70% copper and 30% zinc. It is decorative and corrosion resistant and is used for low friction ornamental purposes such as plumbing and carpentry fittings.

Question 14

Alloys of gold

Answer 14

Gold alloys are used to make jewellery.
Gold metal is relatively soft and malleable so other metals such as copper, zinc and silver are added to provide strength and toughness.
Carats are used to express the purity of gold jewellery.
Pure gold with nothing else added is said to be 24 carat.
Rose, yellow and white gold are alloys of gold with varying proportions of copper, silver and other metals. White gold is a good imitation of platinum which is a very expensive precious metal.

Question 15

Reaching equilibrium

Answer 15

Equilibrium occurs when during the course of a reversible reaction, the rate of the forward reaction equals the rate of the reverse reaction.
This means that products are being formed in the forward reaction as fast as reactants are being formed in the reverse reaction.
It is reached at a faster rate when:
A higher pressure is used as there are more successful collisions.
A higher temperature is used as the particles have greater kinetic energy.
A higher concentration is used as there are more particles per given volume, hence there are more collisions.
A catalyst is used as it speeds up the rate of reaction, allowing it to reach equilibrium faster.

Question 16

Economic consideration

Answer 16

Part of the industrial process is the economic decision on how and where to design and implement a manufacturing site.
The availability and cost of raw materials is a major consideration which must be studied well before any decisions are taken.
In the Haber Process the raw materials are readily available and inexpensive to purify:
Nitrogen - from the air
Hydrogen- from natural gas
If the cost of extraction of raw materials is too high or they are unavailable then the process is no longer economically viable.
Many industrial processes require huge amounts of heat and pressure which is very expensive to maintain.

Question 17

Fertilisers

Answer 17

Compounds containing nitrogen, potassium and phosphorus are used as fertilisers to increase crop yields.
NPK fertilisers are formulations containing appropriate ratios of all three elements.
From these three essential elements:
Nitrogen promotes healthy leaves.
Potassium promotes growth, healthy fruit and flowers.
Phosphorus promotes healthy roots.

Question 18

Artificial fertilisers

Answer 18

A distinct advantage of artificial fertilisers is that they can be designed for specific needs whereas in natural fertilisers, such seaweed or manure, the proportions of elements cannot be controlled.
Fertiliser compounds contain the following water soluble ions:
Ammonium ions, NH4+ and nitrate ions, NO3–, which are sources of soluble nitrogen.
Phosphate ions, PO43-, which are a source of soluble phosphorus.

Question 19

Ammonium nitrate

Answer 19

Ammonia is an alkaline substance and neutralises acids producing a salt and water.
The salt it produces contains the ammonium ion, NH4+, which is a component of several fertilisers.
Ammonia also undergoes oxidation to produce nitric acid, HNO3.
Nitric acid is used as the source of the nitrate ion, NO3–, which is another important ion found in fertilisers.
Ammonium nitrate, a fertiliser and one of the most important ammonium salts, is made by reacting ammonia with nitric acid:
NH3 (aq) + HNO3 (aq) → NH4NO3 (aq)

Question 20

Preparation of ammonium sulphate in a lab

Answer 20

Aim: To prepare ammonium sulphate by titration.
Procedure - Add an exact volume of ammonia to the conical flask and place on the white tile.
Add a few drops of indicator and swirl, it should turn yellow.
Add the acid to the flask solution drop by drop, swirling the flask in between
Continue until the colour turns red sharply and record the titre.
Repeat by adding exactly the same amount of acid but this time without the indicator which is an impurity.
Pour the reaction mixture in an evaporating dish and gently heat in a water bath to remove some of the water.
Stop heating when the volume has been reduced to roughly one third of its volume.
Leave in a dry place so the remaining water evaporates, allowing crystallisation to occur.
This may take a few days depending on ambient conditions.
After a few days ammonium sulphate crystals should appear.
Filter to remove any remaining water.

Question 21

Industrial preparation of ammonium sulphate

Answer 21

The industrial preparation of ammonium sulphate is a large scale operation consisting of several stages.
Ammonia is prepared by the Haber process and sulphuric acid by the Contact process.
Both processes require their own supplies of raw materials, energy and equipment.
The most common industrial process of manufacturing ammonium sulphate involves filling a large reactor chamber with ammonia gas.
Sulphuric acid is sprayed into the chamber from above and ammonium sulphate powder is produced.

Question 22

Comparing preparation of ammonium sulphate

Answer 22

Lab -
Equipment: Simple equipment needed, prepared using a titration apparatus.
Reactant concentration: low concentration, less heat given off.
Separation of product: Crystallisation is used which is a slow process.

Industry -
Equipment: Hugely expensive and complex .
Reactant concentration: High concentrations, very exothermic reaction.
Separation of product: The heat produced is used to evaporate water from the reaction mixture to make very concentrated ammonium nitrate product.

Question 23

Chemical cells

Answer 23

Chemical cells are electrochemical cells that produce a voltage across the cell until one of the reactants has been used up. A fuel cell is a chemical cell that’s supplied with a fuel and oxygen (or air) and uses the reaction between them to efficiently release energy.

Question 24

Fuel cells

Answer 24

A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode.
These cells are becoming more common in the automotive industry to replace petrol or diesel engines.
As the fuel enters the cell it becomes oxidised which sets up a potential difference or voltage within the cell.
Different electrolytes and fuels can be used to set up different types of fuel cells.
An important cell is the hydrogen-oxygen fuel cell which combines both elements to release energy and water.

Question 25

Advantages of fuel cells

Answer 25

They do not produce any pollution: the only product is water.
They produce more energy per kilogram than either petrol or diesel.
No power is lost in transmission as there are no moving parts, unlike an internal combustion engine.
No batteries to dispose of which is better for the environment.
Continuous process and will keep producing energy as long as fuel is supplied.
Quieter so less noise pollution.

Question 26

Disadvantages of fuel cells

Answer 26

Materials used in producing fuel cells are expensive.
High pressure tanks are needed to store the oxygen and hydrogen in sufficient amounts which are dangerous and difficult to handle.
Fuel cells are affected by low temperatures, becoming less efficient.
Hydrogen is expensive to produce and store.
Quieter so potential danger to pedestrians if used in cars and lorries.

Question 27

Hydrogen - oxygen fuel cells

Answer 27

There are a few different types of fuel cells, using different fuels and different electrolytes. One important example is the hydrogen-oxygen fuel cell. The reaction between hydrogen (the fuel) and oxygen releases energy, which is used in hydrogen-oxygen fuel cells to produce a voltage. This voltage can be used to power an electrical device (e.g. an electric car) that the fuel cell is connected to. The reaction in the fuel cell doesn’t produce any harmful pollutants the only product is water. The overall reaction in a hydrogen-oxygen fuel cell is:
2H2 + O2 —– 2H2O

Question 28

Concentration

Answer 28

Concentration (moles / dm^3) = number of moles of solute (mol) / volume of solution (dm^3)

Question 29

Titration calculations

Answer 29

Moles (mol) = volume (dm^3) x concentration (moles / dm^3)

Question 30

Yield

Answer 30

Yield is the term used to describe the amount of product you get from a reaction.
In practice, you never get 100% yield in a chemical process for several reasons.
These include:
Some reactants may be left behind in the equipment.
The reaction may be reversible and in these reactions a high yield is never possible as the products are continually turning back into the reactants.
Some products may also be lost during separation and purification stages such as filtration or distillation.
There may be side reactions occurring where a substance reacts with a gas in the air or an impurity in one of the reactants.
Products can also be lost during transfer from one container to another.

Question 31

Actual and theoretical yield

Answer 31

The actual yield is the recorded amount of product obtained
The theoretical yield is the amount of product that would be obtained under perfect practical and chemical conditions.
It is calculated from the balanced equation and the reacting masses
The percentage yield compares the actual yield to the theoretical yield
For economic reasons, the objective of every chemical producing company is to have as high a percentage yield as possible to increase profits and reduce costs and waste.

Question 32

Percentage yield

Answer 32

The percentage yield is a good way of measuring how successful a chemical process is.
There are often several methods of creating a compound and each method is called a reaction pathway.
Reaction pathways consist of a sequence of reactions which must occur to produce the required product.
Companies often investigate and try out different reaction pathways and these are then compared and evaluated so that a manufacturing process can be chosen.
The percentage yield of each pathway is a significant factor in this decision making process.
Percentage yield = (actual yield / theoretical yield) x 100

Question 33

Atom economy

Answer 33

Along with the percentage yield, atom economy is used to analyse the efficiency of reactions.
Most reactions produce more than one product and very often some of them are not useful.
Atom economy studies the amount of reactants that get turned into useful products.
It illustrates what percentage of the mass of reactants become useful products.
It is used extensively in the analysis of systems and procedures in industries, in an effort to obtain sustainable development.
It is also a very important analysis for economic reasons as companies prefer to use processes with higher atom economies.
The higher the atom economy of a process then the more sustainable that process is.
Atom economy = (total Mr of desired product / total Mr of all product) x 100

Question 34

Choosing a reaction pathway

Answer 34

Reactions that have low atom economies use up a lot of resources and produce a lot of waste material which then needs to be disposed of, a very expensive procedure.
These reactions are thus unsustainable as they use up too much raw material to manufacture only a small amount of product.
They are not economically attractive as raw materials tend to be expensive, as does waste disposal which requires chemicals, equipment, space and transport.
Companies continually analyse reactions and processes and evaluate several factors in an effort to improve efficiency.
Atom economy, percentage yield, rates of reaction and equilibrium position are important factors which need to be considered when choosing a reaction pathway.
High percentage yields and fast reaction rates are desirable attributes in industrial chemical processes.
In reversible reactions, the position of the equilibrium may need to be changed in favour of the products by altering reaction conditions.
If the waste products can be sold or reused in some way that would improve the atom economy.
Alternative methods of production could also be considered that may produce a more useful by-product.

Question 35

Molar volume

Answer 35

At room temperature and pressure, the volume occupied by one mole of any gas was found to be 24 dm3 or 24,000 cm3.
This is known as the molar gas volume at RTP.
RTP conditions are 20 ºC and 1 atmosphere (atm).
Volume (dm^3) = amount of gas (mol) x 24 (dm^3 / mol)

Question 36

Avogadro’s law

Answer 36

In 1811 the Italian scientist Amedeo Avogadro developed a theory about the volume of gases.
Avogadro’s law (also called Avogadro’s hypothesis) enables the mole ratio of reacting gases to be determined from volumes of the gases.
Avogadro deduced that equal volumes of gases must contain the same number of molecules.
At room temperature and pressure(RTP) one mole of any gas has a volume of 24 dm3.