SC9-13-Calculations involving masses, Electrolytic processes, Obtaining and using metals, Reversible reactions and equilibria, Transition metals, alloys and corrosion

SC9b-Conservation of mass

During a chemical reaction, no atoms are created or destroyed. Therefore there are the same amount of atoms on either side. You can see this if you are using a closed system (no gas can get in or out).

Sometimes, the mass will change. This is because the reaction is in a non enclosed system and therefore gases can get in and affect the mass. If the mass increases, it is because other gases have entered and reacted with the other reactants. If the mass decreases, it is because at least one of the products is a gas. The gas can escape from the non enclosed practical and will make it weigh less.

SC10a-Electrolysis

Electrolysis is the breaking down of a substance using electricity. Electrolysis can be carried out on any ionic compound but only if it is melted or dissolved. This allows the ions to move freely-this is called an electrolyte. An electric current is passed through the electrolyte, causing it to decompose.

The Positive ions (cations) move towards the negative electrode (cathode). The negative ions (anions) move towards the positive electrode (anode). Anions travel to the anode and lose electrons forming atoms. Cations travel to the cathode and gain electrons forming atoms.

Oxidation is the loss of electrons and reduction is the gain of electrons (OIL RIG). Therefore when a positive ion gains electrons, it is reduced;a negative ion losing electrons is oxidised.

At the anode negative ions lose electrons to become neutral.This can be written as a half equation: e.g. Cl- -> Cl + e- Oxide: O2- -> O + 2e-

Positive ions gain electrons to form neutral atoms. e.g. copper: Cu2+ + 2e- -> Cu Aluminium: Al3+ 3e- -> Al

If an ionic salt is dissolved in water, there are additional ions present because: H+ ions travel to the cathode and can form hydrogen gas; OH- ions travel to the anode and make O2 gas.

Less reactive metals like silver or copper will be formed instead of hydrogen whereas metals like sodium or aluminium will not be formed.

Core practical: If we electrolyse CuSO4 (aq) with graphite electrodes, copper is formed at the cathode and oxygen is formed at the anode. If copper electrodes are used, the copper disappears from the anode and is plated on the cathode. The mass of the cathode should increase and the mass of the anode should increase.

Corrosion is an oxidation reaction. Rusting is only the corrosion of iron.

SC11b-Ores

A metal ore is a rock which contains enough metal to make it economically worthwhile. Ores are mined and the metals can be extracted. Unreactive metals such as gold or platinum, are uncombined and can be mined as they are.

The position of the metal in the reactivity series determines whether it can be extracted using carbon or electrolysis. Metals higher than carbon have to be extracted using electrolysis which is expensive. Metals below carbon are extracted by reduction using carbon. Carbon can only take the oxygen away from the metals which are less reactive than the carbon. Metals more reactive are extracted using electrolysis. They must be in a molten state so the ions can move. Large amounts of electricity is needed and it is more expensive than carbon.

There are biological methods of extracting metals:

Bioleaching uses bacteria grown on a low grade ore. Bacteria produce leachate -a solution containing copper ions. Copper is extracted and purified by electrolysis. This method does not require high temperatures but toxic substances and sulphuric acid can be produced by the process, and damage to the environment.
Phytoextraction involves growing plants that absorb metal compounds. The plants are burnt and metal is extracted. This can extract metals from contaminated soils but it is more expensive than mining and growing plants depend on weather.
There are no harmful gases and both methods cause less damage to landscape than mining. These conserve supplies of higher grade ores. But, these methods are very slow.

SC11d-Life cycle assessment and recycling

Advantages of recycling:

Natural reserves of metal ores will last longer.
The need to mine ores is reduced. Mining can damage landscapes as well as create noise and dust pollution.
Less pollution is produced.
Many metals need less energy to recycle than to extract new metal from the ore.
Less waste metal ends up in landfill sites.
Recycling creates lots of jobs.

Disadvantages of recycling:

Cost and energy used in collecting, transporting and sorting metals to be recycled.

Life cycle assessment (LCA) looks at each stage of the life of the product-It works out the potential environmental impact of each stage.

Process:

Choice of material: Metals must first be extracted from mines which requires energy and causes pollution.
Manufacture: This uses a lot of energy and can cause pollution. Most chemical manufacture needs water. Factories must be careful not to pollute nearby water sources.
product use: This can damage the environment-Paint gives off toxic fumes etc.
Disposal: Products are disposed at landfill sites at the end of their life. This reduces space and pollutes air and water and also can be burnt to pollute the air.

SC12a-Dynamic equilibrium

Moat reactions are reversible. The arrow to show this in an equation looks like ⇌

Dynamic-Reaction proceeds in both directions even after equilibrium is reached. Equilibrium-The point at which the concentration of reactants and products is constant.

As the reactants react, their concentrations fall-so the forward reaction will slow down. But as more of the products are being made, the backwards reaction speeds up. When they are both going at the same rate, this is when equilibrium is reached. Both reactions are still happening, but there is no overall effect. Equilibrium can only be reached if it is in a closed system so nothing can escape. If equilibrium lies to the left we have more reactant than product but if it lies to the right we have more product than reactant.

Endothermic: A reaction that, overall, takes in heat (temp decreases). Energy change is positive.

Exothermic: A reaction that gives out heat (Temp increases). Energy change is negative.

Equilibria rules:

If we alter the conditions, we can move the position of equilibrium right or left. The further right the more product we get (high yield).

Temperature: A higher temperature will favour the endothermic direction.
Pressure: A higher pressure will favour the side with fewer gas moles.

SC13a-Transition metals

Transition metals:

Have high melting points.
Are not very reactive.
Are useful in catalysis.
Form colour compounds
Are strong and hard.
Have a high density.
Form different ions.

Group 1 metals:

Have relatively low melting points.
Are very reactive
Form white compounds
Are soft-can be cut with a knife.
Have low densities-some float on water.
React with water-forms alkaline solution and H2 gas.
Forms 1+ ions (some transition can too).

Both:

For ionic compounds reacting with non metals.
Are thermal and electrical conductors.

Metals can be protected from corrosion:

Oxide layers-some metals react with O2 forming a thin protective layer e.g. Al.
Oil paint the metal-a protective layer.
Sacrificial protection-Connect a more reactive metal to the one being protected. The more reactive one will react first.
Galvanising-Cover the metal in zinc, which forms a barrier and sacrificially protects the object.

SC13c & d-Electroplating and Alloys
Electroplating coats the surface of one metal with a thin layer of another metal. It may be used to improve a metal object's ability to resist corrosion. To electroplate something, you can put that object at the cathode.
An alloy is a mixture of Metals (and possible carbon) formed by melting constituents together.

A pure metal can have atoms that push past each other but in an alloy, there is a distorted structure that makes it difficult for layers to move past each other as the atoms are different sizes.

	Creado por Jessica Diamond hace más de 7 años

Apunte sobre SC9-13-Calculations involving masses, Electrolytic processes, Obtaining and using metals, Reversible reactions and equilibria, Transition metals, alloys and corrosion, creado por Jessica Diamond el 11/07/2017.