Transitional Metals, Alloys and Corrosion
Most metals are transition metals, found in the central block of the Periodic Table between Groups 2 and 3. Transition metals have high melting points, and high densities. They are malleable, ductile, good electrical conductors and shiny when polished. Transition metal form coloured compounds and the compounds and the metals themselves often show catalytic activity. Iron is a malleable, strong metal that is used as a catalyst in the Haber Process.
The oxidation of metals results in corrosion. Metals form a thin layer of tarnish as they oxidise, which stops oxygen from reaching the metal, preventing further oxidation. Rusting is the corrosion of iron, which occurs when iron reacts with oxygen and water. Hydrated iron oxide is the substance known as rust.
Rusting can be prevented if air, and therefore oxygen, is excluded from its environment. This can be achieved by storing it in an unreactive atmosphere of inert gases like nitrogen. Water vapour can be excluded by using a powder that absorbs water vapour. Paint, plastic coating and oiling are methods that exclude both air and water.
Sacrificial protection is a method that involved galvanising iron. The metal is coated in has attached to it magnesium or zinc, which oxidises more easily than iron, so oxygen reacts with them instead of the iron object. The metal is more reactive and therefore loses electrons more easily, so are more easily oxidised, meaning even if a cut appears in the zinc protective layer, electrons are donated to the iron and the zinc is oxidised instead.
Electroplating coats the surface of one metal with a thin layer of another. Expensive transition metals like silver and gold are electroplated onto cheaper base metals to produce attractive jewellery for a lower cost. Electroplating also reduces a metal’s corrosion as it forms a layer between the metal and oxygen in the air. Electrolysis is used to electroplate a metal object. The object is used as the cathode, while the anode is the plating metal. The electrolyte should contain ions of the plating metal. As a DC current flows through the solution, silver ions in the electrolyte move to the cathode where they are reduced and are deposited as silver atoms. At the anode, silver atoms are oxidised and enter the electrolyte. The longer the current flows, the thicker the layer of protective metal becomes.
An alloy is a mixture of a metal element with one or more other elements. Alloy steels are made by adding other elements to iron. Stainless steels resist rusting due to the chromium present, which reacts with oxygen to form chromium oxide, which stops oxygen from reacting with iron but is also thin enough to be transparent. The higher the carbon content, the stronger and harder the iron is. Mild steel has a low carbon content, but manganese increases its strength while maintaining its malleability, so it is used for car body parts and as a building material.
Alloys are stronger than the pure metals they contain. In a solid pure metal, the atoms are all the same size and are arranged in regular layers, which can move past each other is a strong enough force is applied, making it ductile and malleable. In alloys, the other elements present may be of different sizes, which disrupt the regular structure and make it difficult for layers to slide past each other, making the alloy strong, while still usually being malleable.
The properties of metals are always related to their uses. Gold and copper are unreactive and are malleable, ductile and good electrical conductors, so could be used in wiring; however, the cost of gold makes copper a more suitable choice. Aluminium is also resistant to corrosion, but is a less effective conductor. However, it is stronger and cheaper so is used in overhead cables. Alloys may have more useful properties than individual elements. Brass, made of copper and zinc, is unreactive like copper, but is much stronger so is used in plug pins. Magnalium is 95% aluminium and 5% magnesium. It is less dense and very strong, allowing for it to be used in aircraft and also in scientific instruments.
Quantitative Analysis
Concentrations
CORE PRACTICAL: Acid-Alkali Titration
A – Measure out set amount of sodium hydroxide using a pipette to provide accurate measurements B – Add to conical flask with single-colour indicator e.g. methyl orange or phenolphthalein
C – Place the flask on a white tile to make colour change more obvious D – Pour acid into a clean burette
E – Slowly add acid to the alkali, dropwise near the end
F – Once the colour of the indicator changes, stop the acid flow G – Record the volume of acid added
H – Repeat until concordant results are achieved
The concentration of one of the solutions in a titration can be calculated if the concentration of the other solution is known. First, work out the number of moles using the equation:
Remember that 1000cm3 is equal to 1dm3. Then, work out the number of moles of the other substance by looking at the molar ratio in the balanced equation. Then, rearrange the formula above and substitute the volume for the mean of the concordant results from the titration.
Percentage Yield, Atom Economy, Reaction Pathways and Gases
The percentage yield is always less than the theoretical yield because:
- The reaction is incomplete, meaning not all the reactants have been used and not all of the products have been formed
- Some of the product is lost when transferring substances between containers
- Unwanted side reactions occur to make a different product
Atom Economy is a method of showing how efficiently a reaction makes use of atoms in the reactants to form a desired product. A percentage yield gives no indication of the amount of waste products. Chemists must consider the usefulness of by-products, yield, rate of reaction and the equilibrium position of a reaction before using it if alternatives are available, such as in the production of ethanol.
Avogadro’s Law states that, if the temperature and pressure are the same, equal volumes of different gases contain an equal number of molecules. The molar gas volume is the volume occupied by one mole of any gas. At rtp this value is 24dm3
Dynamic Equilibria
The Haber Process is a reversible reaction between nitrogen and hydrogen to form ammonia. The conditions used are a compromise between yield and rate of reaction. Although a higher temperature favours the backward reaction, the temperature must be high enough to allow for the reaction to reach equilibrium at a fast-enough rate. A pressure over 200atm is too expensive to maintain but the higher pressure increases the rate of reaching equilibrium as well as favouring the forwards reaction. Increasing concentration of the reactants would favour the forward reaction and increase the rate of attaining equilibrium but the availability of raw materials is not high enough. The iron catalyst allows the rate of reactions to increase.
Fertilisers replace mineral ions needed by plants to promote plant growth. Nitrogen, potassium and phosphorous are needed as soluble compounds as only ions in water can be absorbed by plants.
Ammonia reacts with nitric acid to produce ammonium nitrate, a soluble salt which allows for ammonium ions to be absorbed by the plant. Ammonia comes from the Haber Process, whilst nitric acid is produced by oxidising ammonia to produce nitric acid and water. The production of ammonium sulfate, another fertiliser, is different in a lab compared to in industry.
| Laboratory Preparation | Industrial Production | |
| Scale of Production | Small Scale | Large Scale |
| Starting Materials | Ammonia solution and sulfuric acid | Raw materials for ammonia and sulfuric acid |
| Stages | Titration then crystallisation | Several Stages |
| Process | Batch | Continuous |
Chemical Cells
A chemical cell, two metals dipped into a solution of one of their own salts with a salt bridge allowing dissolved ions to pass from one solution to the other, produces a voltage until one of the reactants is used up. In a hydrogen-oxygen fuel cell, hydrogen and oxygen are used to produce a voltage and water is the only product.
| Advantages of Fuel Cells | Disadvantages of Fuel Cells |
| No greenhouse gases produced – only waste product is water | Hydrogen is manufactured by the reaction of steam with coal or gas |
| Very quiet and require less maintenance than fuels like petrol | Hydrogen must be stored in tanks which takes up space |
| Very efficient as no energy lost through friction | Hydrogen is explosive |