5a Flashcards
transition metals
Most metals can be classified as transition metals, including a lot of everyday metals (e.g. copper, iron, zinc, gold, silver, platinum). Transition metals have the typical properties of metals (see p.65-66) - they’re relatively hard, strong, shiny and malleable materials that conduct heat and electricity well.
transition metal properties - Melting point and density
Transition metals have high melting points. They also have high densities.
transition metal properties - Uses of transition metals
The properties of some transition metals give them a wide variety of uses, for example:
- Gold is used in jewellery because it’s shiny and malleable. It’s also a great electrical conductor and highly corrosion resistant, so it’s used in some electronic components.
- Copper is used for water pipes because it’s malleable and corrosion resistant. It’s a good electrical conductor, so it’s used in electrical wires.
transition metal properties - Transition metals as catalysts
Transition metals and their compounds all make good catalysts. e.g., Iron is the catalyst used in the Haber process for making ammonia.
transition metal properties - Transition metal compounds
The compounds of transition metals are colourful. What colour they are depends on what transition metal ion they contain.
What are alloys?
Pure metals are malleable (easily shaped by hammering) because they have a regular arrangement of identical ions. The layers can slide over each other. This means some metals aren’t strong enough for certain uses, so alloys are used instead. Alloys are made by adding another element to a metal. The added element could be a non-metal or another metal.
Different elements have different sized atoms. So, for example, when an element such as carbon is added to pure iron, the smaller carbon atoms will disrupt the layers of pure iron atoms, making it more difficult for them to slide over each other. So, alloys are stronger and harder than pure metals.
Alloys of iron
Pure iron is soft and easily shaped, making it too bendy for most uses. Most pure iron that is produced is used to make alloys called steels, which are stronger and harder than iron. Steels are formed by adding small amounts of carbon, and sometimes other metals, to the iron. There are different types of steel. They have different uses because of their different properties.
Copper alloys
Brass is an alloy of copper and zinc and has a gold-like appearance. It’s more malleable than bronze and is ideal in applications which require moving parts with low friction. For example, it is often used in water taps and door fittings.
Bronze is an alloy of copper and tin. It’s harder than copper and it’s good for making statues, decorative objects and medals
Gold alloys
Gold alloys are used to make jewellery, as pure gold is too soft. Metals such as zinc, copper and silver are used to harden the gold. The amount of gold in a gold alloy is described in terms of carats. Pure gold is described as 24 carat, so 18 carat means that 18 out of 24 parts of the alloy are gold. In other words, 18 carat gold is 75% gold.
Eg - An 18-carat gold ring with a mass of 20 g contains: 20 x (18÷24) = 15 g of gold.
Aluminium alloys
Aluminium has a low density which is an important property in aircraft manufacture. But pure aluminium is not strong enough for making aeroplanes, so it’s alloyed with small amounts of other metals to increase its strength.
Magnalium is an alloy of aluminium and magnesium. When it’s made with small amounts of magnesium (about 5%), magnalium is stronger, lighter and corrodes less easily than pure aluminium. This type of magnalium is strong enough to be used to make parts for aeroplanes. Magnalium with a higher magnesium content (about 50%) is used in fireworks, as it’s reactive and burns brightly, like magnesium, but is more stable than pure magnesium.
What is corrosion?
Metals can corrode in the presence of oxygen and water to form their metal oxides. The metal gains oxygen, so it’s oxidised.
Rusting is the name for the corrosion of iron. Rusting only happens when the iron is in contact with both oxygen (from the air) and water. Here’s the equation for the corrosion of iron:
iron + oxygen + water→ hydrated iron(III) oxide
Rust experiment
Experiments can show that both oxygen and water are required for rust to form:
* If you put an iron nail in a boiling tube with just water, it won’t rust. (The water is boiled to remove oxygen and oil is used to stop air getting in.)
* If you put an iron nail in a boiling tube with just air, it won’t rust. (Calcium chloride can be used to absorb any water from the air.)
* However, if you put an iron nail in a boiling tube with air and water, it will rust. The mass of a rusty nail will increase as the iron atoms in the nail have now bonded to oxygen and water molecules, resulting in a compound that is heavier than iron alone.
Preventing corrosion
Stopping corrosion is really important, as structures such as bridges can become damaged over time and become dangerous to use. There are two main ways to prevent rusting from happening - creating a barrier and the sacrificial method.
Creating a barrier
Creating a barrier that keeps oxygen, water or both away from the metal will prevent corrosion. There are different ways of creating a barrier. For example:
* Painting a metal or coating it with a plastic. This is ideal for big and small structures alike. It has the added advantage that it can be decorative.
* Electroplating uses electrolysis to reduce metal ions onto an iron electrode. For example, it can be used to coat iron with a layer of a different metal that won’t be corroded away (see below for more).
* Oiling or greasing is used when moving parts are involved. For example, this is used on bike chains to prevent them from corroding.
Sacrificial Protection
You can also prevent rusting using sacrificial protection - this involves placing a more reactive metal with the iron. The water and oxygen react with this ‘sacrificial’ metal instead of the iron in the object you’re protecting.
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.
Electroplating example
Silver can be electroplated onto a brass cup to make it look more attractive. To do this, you make the brass cup the cathode (to attract the positive silver ions), and use a bar of pure silver for the anode. Then you dip them both in a solution of silver ions (for example, silver nitrate). The silver ions in solution are attracted towards the cathode and silver gets deposited on the cup The anode releases more silver ions into the solution for more electroplating:
Cathode: Ag+ + e- → Ag
Anode: Ag → Ag+ + e-
