Topic 4 - extracting metals and equilibria

deduce​​ the​ ​relative​ reactivity​ ​of​ ​some​ ​metals,​ by​ ​their​​ reactions​​ with water,​ ​ acids​​ and​ ​salt​ solutions

• the most reactive metals will react with cold water:

  • products are a metal hydroxide (forming an alkaline solution) and hydrogen gas

  • eg. with potassium: 2K + 2H₂O → 2KOH + H₂

• fairly reactive metals react with acids: acid + metal → salt + hydrogen

• almost all metals react with oxygen: metal + oxygen -→ metal oxide, through more reactive metals will react with oxygen more quickly

• only metal that does not react with any of the above is gold, because it is extremely unreactive

• you can therefore deduce the relative reactivity of some metals by seeing if they react with water (ie. VERY reactive), acid (reactive), and oxygen (not that reactive). for these reactions, you can see if they have taken place by looking for bubbles (if hydrogen is produced)

• you can see if one metal is more reactive than another by using displacement reactions…

  • easily seen when a salt of the less reactive metal is in the solution

    • more reactive metal gradually disappears as it forms a solution

    • less reactive metal coats the surface of the more reactive metal

explain​ ​the displacement reactions as redox reactions, in terms of gain or loss of electrons

• more reactive metals form a cation (+) as they displace the less reactive metal, losing electrons and therefore being OXIDISED (oxidation is the loss of electron), it forms an ion as it replaces the less reactive metal in the salt solution

• less reactive metals form atoms from negative ions as they are displaced, gaining electrons and therefore being REDUCES (reduction is gain of electrons), it forms an atom as it is replaced in the salt solution

explain the reactivity series of metals in terms of the reactivity of the metals with water and dilute acids and that these reactions show the relative tendency of metal atoms to form cations

• when metals react with other substances, metal atoms form positive ions

• reactivity of a metal is related to its tendency to form positive ions - more reactive metals can form positive ions more easily

• metals can be arranged in order of their reactivity in a reactivity series

  • metals potassium, sodium, lithium, calcium, magnesium, zinc, iron and copper can be put in order of their reactivity from their reactions with water and dilute acids

  • non-metals hydrogen and carbon are often included in the reactivity series

• a more reactive metal can displace a less reactive metals from a compound

reactivity series

1. potassium

2. sodium

3. calcium

4. magnesium

5. aluminium

6. carbon

7. zinc

8. iron

9. hydrogen

10. copper

11. silver

12. gold

where are most metals and non-metals found

most metals are extracted from ores found in the Earth’s crust, and unreactive metals are found in the Earth’s crust as the not combined elements

• most metals are found as compounds that require chemical reactions to extract the metal, since they have reacted with other compounds/elements

• gold is found in the Earth as the metal itself as it is very unreactive

explain oxidation and reduction

• oxidation is the GAIN of oxygen

• reduction

how are metals extracted

involves reduction of ores

• metals less reactive than carbon:

  • can be extracted from their oxides by reduction with carbon

    • dont forget: reduction involves the loss of oxygen, so you are reducing the ores to remove the oxygen to obtain the pure metal

explain​ ​ why​ ​ the​ ​ method​ ​ used​ ​ to​ ​ extract​ ​ a​ ​ metal​ ​ from​ ​ its​ ​ ore​ ​ is​ ​ related​ ​ to its​ ​ position​ ​ in​ ​ the​ ​ reactivity​ ​ series​ ​ and​ ​ the​ ​ cost​ ​ of​ ​ the​ ​ extraction​ ​ process, illustrated​ ​ by​ ​ heating​ ​ with​ ​ carbon​ ​ (including​ ​ iron)​ ​ and​ ​ electrolysis​ ​ (including aluminium)​ ​ (knowledge​ ​ of​ ​ the​ ​ blast​ ​ furnace​ ​ is​ ​ not​ ​ required)

• can only be extracted by reduction of carbon if metal is less reactive so that carbon displaces the metal from the ore…

• if more reactive than carbon, electrolysis can be used (metals less reactive than carbon can also be extracted this way)

• electrolysis is expensive due to the use of large amounts of energy to melt the compounds and to produce the electrical current (so you wouldn’t extract a metal using electrolysis if it could be done cheaper using carbon)

• extraction by heating with carbon (including iron):

  • iron oxide loses oxygen, and is therefore reduced. the carbon gains oxygen, and is therefore oxidised

  • 2Fe2 O3 (s) + 3C(s) -> 4Fe(l) + 3CO2 (g)

  • for iron, this is carried out at high temperatures in a blast furnace

• extraction by electrolysis (including aluminum

  • metals that are more reactive than carbon eg. aluminum are extracted by electrolysis of molten compounds

    • too reactive to be extracted by reduction with carbon

    • aluminum is manufactured by the electrolysis of a molten mixture of aluminum oxide and cryolite using carbon as the positive electrode (anode)

  • metals that react with carbon can be extracted by electrolysis as well

evaluate alternative biological methods of metal extraction

• phytoextraction:

  • some plants absorb metal compounds through their roots

  • they concentrate these compounds into their shoots and leaves

  • the plants can be burned to produce an ash that contains the metal compounds

• bacterial extraction:

  • some bacteria absorb metal compounds

  • produce solutions called leachates which contain them

  • scrap iron can be used to obtain the metal from the leachate

explain how a metal’s relative resistance to oxidation is related to its position in the reactivity series

• relative resistance to oxidation is the same as relative resistance to losing electrons/forming positive metal ions

  • less reactive a metal is, the more resistant it is to oxidation, because for a metal to react, it forms a positive metal ion by losing electrons (loss of electrons = oxidation)

advantages of recycling metals

• recycling is important to achieve sustainable development

  • requires less energy to melt and remould metals than it does to extract new metals from their ores

  • mining ores is bad for the enviroment as large quarries are created, which produce noise pollution and dust

  • also, recycling allows for waste metals to be reused, saving money, helping the enviroment and the supply of valuable raw materials (meaning metal ores will last longer)

what does the life time assesment for a product involve

• these are carried out to assess the enviromental impact of products in each of these stages:

  • extracting and processing raw materials

  • manufactoring and packaging

  • use and operation during its lifetime

  • disposal at the end of its useful life, including transport and distribution at each stage

• use of water, resources, energy sources and production of some waste can be fairly easily quantified

• allocating numerical values to pollutant effects is less straightforward and requires value judgments, so LTA (life time assessment) is not a purely objective process

• selective or abbreviated LTAs can be devised to evaluate a product but these can be misuses eg. in support of claims for advertising purposes

what are chemical reactions

reversible and the direction of some reversible reactions can be altered by changing the reaction conditions

• in some chemical reactions, the products of the reaction can react to produce the original reactants

  • these are called reversible reactions

  • the direction of the reaction can be changed by changing the conditions aka if the forwards reaction takes place in hot conditions, lowering the temperature can allow the reverse reaction to take place

explain what is meant by dynamic equilibrium

• equilibrium:

  • rate of forward reaction = rate of backward reaction

  • concentration of reacting substances stay the same

• dynamic equilibrium:

  • once the forward and backward reaction reach equilibrium, they keep going

describe the formation of ammonia

a reversible reaction between nitrogen (extracted from the air) and hydrogen (obtained from natural gas) and it can reach a dynamic equilibrium

• used to manufacture ammonia, which is used to produce nitrogen-based fertilisers

• the raw materials for the Haber process are nitrogen and hydrogen

• nitrogen is obtained from the air and hydrogen may be obtained from natural gas or other sources

• the purified gases are passed over a catalyst of iron at a high temperature (about 450 °C) and a high pressure (about 200 atmospheres)

• some of the hydrogen and nitrogen reacts to form ammonia

• the reaction is reversible so ammonia breaks down again into nitrogen and hydrogen

conditions for Haber process

450 degrees Celsius, pressure 200 atmospheres and iron catalyst

predict​ ​ how​ ​ the​ ​ position​ ​ of​ ​ a​ ​ dynamic​ ​ equilibrium​ ​ is​ ​ affected by​ ​ changes​ ​ in:​ ​ temperature,​ ​ pressure​ ​ and​ ​ concentration

• the relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction

• if a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change

  • effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle

how is the position of dynamic equilibrium affected by changes of concentration

• if the concentration of one of the reactants of products is changed, the system is no longer at equilibrium and the concentrations of all the substances will change until equilibrium is reached again

• if concentration of reactants is increased: position of equilibrium shifts towards products (right) so more product is produced until equilibrium is reached again

• if concentration of products is increased: position of equilibrium shifts towards reactants (left) so more reactant is produced until equilibrium is reached again

how is the position of dynamic equilibrium affected by changes of pressur e

• in gaseous reactions, an increase in pressure will favour the reaction that produces the least number of molecules as shown by the symbol equation for that reaction

• if a reaction produces an increase in pressure and more moles on product side, decreases yield of reaction - equilibrium shifts left

• if a reaction produces an increase in pressure and fewer moles on product side, increases yield of reaction - equilibrium shifts right

• if a reaction produced a decrease in pressure and more moles on product side, increase yield of reaction - equilibrium shifts right

• if a reaction produces a decrease in pressure and fewer moles on product side, decrease yield of reaction - equilibrium shifts left

how is the position of dynamic equilibrium affected by changes of temperature

• if temperature is increased: equilibrium moves in the direction of the endothermic reaction (eg if forward reaction is endothermic and temperature is increased, equilibrium shifts right to produce more product)

• if temperature is decreased: equilibrium moves in the direction of the endothermic reaction

• for the forwards being exo/endothermic and yield meaning the amount of product from the forwards reaction:

  • an increase in temperature, in an exothermic reaction decreases yield of reaction - equilibrium moves left

  • an increase in temperature, in an endothermic reaction increases yield of reaction - equilibrium moves right

  • a decrease in temperature, in an exothermic reaction increases yield of reaction - equilibrium moves right

  • a decrease in temperature, in an endothermic reaction decreases yield of reaction - equilibrium moves left