Topic 9 National 4 Chemistry Summary Notes. Metals and Alloys. Materials

Transcription

1 Topic 9 National 4 Chemistry Summary Notes Metals and Alloys LI 1 Materials Materials are all substances and include: metals ceramics plastics natural substances novel substances. Materials can be used for so many different jobs because of their different properties. Some examples of materials and their uses are shown in the table below. Material Use Property/Properties copper (metal) porcelain (fused with metal) electrical wiring dental bridges good conductor of electricity attractive & strong (ceramic) poly(ethene) (plastic) penicillin washing-up bottles antibiotic light unreactive durable i.e. long lasting destroy bacteria (natural substance) Aluminium (metal) aircraft Strong & low density (lightweight) The material we are going to look at most closely is metals.

2 LI 2 Reactivity of Metals The Reactivity Series, which is shown in the table below, is a list which places metals in order of their readiness to react. The most reactive metals are at the top and the least reactive are at the bottom. Metal Reactivity with Oxygen Reactivity with water Reactivity with Acid Potassium Sodium react with oxygen react with water too reactive to try in acid Calcium Magnesium react with acid Aluminium Zinc do not react with water Iron Nickel Tin Lead only forms metal oxide on the surface of the metal Copper Mercury do not react with acid Silver Gold do not react with oxygen An easy way to remember The Reactivity Series is the following sentence: Police Sergeant Charlie MAZINTL Caught Me Stealing Gold i.e. P for potassium etc 2

3 The general word equations if a metal can react with oxygen, water or acid are given below. Metal + Oxygen metal + oxygen metal oxide e.g. magnesium + oxygen magnesium oxide Metal + Water metal + water metal hydroxide + hydrogen e.g. potassium + water potassium hydroxide + hydrogen MAZINTL Metal + Acid MAZINTL Metal + acid salt + hydrogen e.g. zinc + hydrochloric acid zinc chloride + hydrogen 3

4 LI 3 Extracting Metals Less reactive metals can be found uncombined (not joined up with other elements) in The Earth s Crust and consequently were the first to be discovered. More reactive metals are always found combined and have to be extracted (obtained) from ores. (see * below) Metals have to be extracted from their ores by different methods. The method used is shown in the table below and depends on the reactivity of the metal. Metal Potassium Sodium Extraction Method electrical energy required i.e. electrolysis Calcium Magnesium Aluminium *An ore is a compound of a metal that occurs naturally. For example, iron oxide is iron ore. Zinc Iron heat with carbon or carbon monoxide Nickel Tin Lead Copper Extracting a metal from its ore is an example of a reduction reaction. Mercury Silver heat alone Gold 4

5 The more reactive metals hold on more strongly to oxygen than the less reactive metals. Therefore, it is much easier to remove oxygen from compounds where it is joined to less reactive metals. The most reactive metals hold on to oxygen more strongly than carbon does. Heating with carbon or carbon monoxide therefore does not work. Wars and the invention of electricity led to the large scale extraction of more reactive metals. The Blast Furnace Iron is produced from iron ore in The Blast Furnace as shown in the diagram below. At the bottom of the furnace the reaction makes carbon dioxide C + O 2 CO 2 Zone 1 Higher up, the carbon dioxide reacts with carbon to make carbon monoxide CO 2 + C CO Zone 2 Further up the carbon monoxide reacts with iron oxide to make iron and carbon dioxide. Fe 2 O 3 + CO Fe + CO 2 Zone 3 5 Note: the limestone reacts with the impurities in the iron ore to form slag

6 LI 4 Corrosion Corrosion is a chemical reaction which takes place on the surface of a metal. The metal element changes into a compound. For example, iron + oxygen iron oxide (element) (compound) For metals to corrode you need: 1. Water 2. Oxygen (from the air) Not all metals corrode at the same rate. For example, Gold corrodes so slowly that a piece of jewellery made in Egyptian times is still as bright today! Sodium corrodes so quickly that the formation of the oxide can be seen happening LI 5 Rusting The corrosion of iron is called rusting. During rusting the iron atoms initially lose two electrons to form Fe 2+ ions. i.e. Fe (s) Fe 2+ (aq) + 2e (p.10 data book) The electrons lost are accepted by the water and oxygen to from hydroxide ions. i.e. 2H 2 O (l) + O 2(g) + 4e 4OH - (aq) 6 (p.10 data book)

7 We can show that Fe 2+ ions are produced during rusting by using ferroxyl indicator. Fe 2+ ions turn yellow ferroxyl indicator blue the more intense the blue colour the more rusting has occurred. Remember, whenever iron rusts OH - ions (hydroxide ions) are also produced and these also have an effect on ferroxyl indicator. OH - ions turn yellow ferroxyl indicator pink. LI 6 Preventing Corrosion Summary The following prevent corrosion of a metal and are explained in more depth below: Coating Galvanising Connecting a metal to a more reactive metal Coating If oxygen and water can be kept from the surface of a metal, the metal cannot corrode. Many surface barriers are used to prevent corrosion. Coating with paint (metal fence), grease (bicycle chain) or plastic (metal draining rack) protect iron objects we say physical protection is provided. Coating with another metal ( see Galvanising) 7

8 Galvanising Galvanising in this process the iron object is dipped into molten (liquid) zinc. This stops oxygen and water from reaching the surface of the iron. Even if the zinc coating is scratched it continues to protect the iron since the electrons flow from the more reactive zinc on to the iron. For this reason objects which take rough handling, such as wheelbarrows, are often galvanised. Sacrificial protection is a method for protecting a metal from corrosion by attaching it to a metal which is higher up The Electrochemical Series. ( similar to the Reactivity series, pg 10 of the data booklet) In galvanising, both physical and sacrificial protection are provided. Connecting A Metal To A More Reactive Metal For a metal to corrode it must lose electrons. Connecting the metal to a more reactive metal stops the metal losing electrons. Electrons flow from the more reactive metal on to the metal preventing the loss of electrons by the metal atoms. Here are some examples, Iron/Magnesium Cell 8

9 Ships have zinc plates bolted to their hull. Electrons flow from the zinc on to the ship, slowing down the corrosion of the iron. Since the zinc plates are losing electrons rapidly they have to be replaced regularly. Underground pipes are often connected to magnesium scraps. This slows down the corrosion of the pipe since electrons flow from the magnesium onto the pipe. Once again the scrap magnesium has to be replaced regularly since it rapidly corrodes away. These are examples of sacrificial protection. 9

10 LI 7 Cells/Batteries Note: whenever you see the word cell in these notes it can be replaced with the word battery. A cell is an arrangement which converts chemical energy into electrical energy (electricity). Electricity can be produced by connecting different metals together and dipping them in an electrolyte (see note below) to form a cell. Example The Zinc/Copper Cell The purpose of an electrolyte is to complete the circuit. Note: A solution or a molten compound which conducts electricity is known as an electrolyte i.e. an ionic compound when in solution or when molten. Acids and ammonium chloride solution are examples of electrolytes. 10

11 LI 8 The Electrochemical Series We can use the equipment shown below to compare the voltage produced by different pairs of metals sandwiched between an electrolyte. The results obtained are given in the table below. Metal Pair Voltage Reading (millivolts) copper and copper 0 copper and tin 10 copper and iron 40 copper and zinc 50 copper and magnesium 60 copper and silver -10 These results show that different pairs of metals give different voltages and this leads to The Electrochemical Series which is shown on p.10 of The Data Booklet. The Electrochemical Series places metals in order of their ability to supply electrons (it is very similar to The Reactivity Series but not exactly the same) The metals at the top of The Series supply electrons most easily. Electrons always flow from the metal higher up The Electrochemical Series to the metal lower down. The further apart the metals are the higher the voltage they produce. 11

12 LI 9 More Complicated Cells The Zinc/Copper Cell Again! In the above set-up: Electrons flow from the zinc rod to the copper rod through the wires and the meter. Electrons always flow through the wires and the meter. The purpose of the ion bridge is to complete the circuit it is the movement of ions in the ion bridge which completes the circuit. Ions flow through the ion bridge. Electrons flow through wires. Electrons always flow through the wires and meter from the metal higher up The Electrochemical Series to the metal lower down. When setting up a cell like the zinc/copper cell, for electricity to be produced the metals have to be: 1. different 2. placed in a solution of their own metal ions. For example, zinc has to be placed in a zinc..solution, it cannot be placed in a copper..solution 12

13 LI 10 Alloys An alloy is a mixture of metals or of metals with non-metals. An alloy has physical properties different to those of the elements from which it is made. For example, pure gold is very soft, and if it was used to make jewellery, it would soon wear away. Gold jewellery is actually made from an alloy of gold and copper making it harder i.e. more wear resistant. Some examples of alloys and their uses are shown in the table below. Alloy Elements Present Use brass copper & zinc door Hinges Much harder than either copper or zinc stainless steel The carbon causes the iron to be less brittle. The chromium & nickel causes the iron to have an increased resistance to corrosion iron, carbon, chromium & nickel cutlery solder Melts more easily than lead or tin lead & tin joins components to circuit boards 13

14 Topic 9 Pupil Self Evaluation Metals and Alloys National 4 Number Learning Intention Success Criteria I can: 1 I am going to find out about materials give examples of materials and their uses state the property/properties which are linked to these uses 2 I am going to find out about the reactivity of metals 3 I am going to find out about extracting metals from their ores state the order of metals in The Reactivity Series state if a metal reacts with oxygen, water or acid and write the word equation for the reaction state the definition of an ore state the method of extraction required to extract a particular metal from its ore explain why this method of extraction is required explain why unreactive metals were the first to be discovered give examples of what led to the large scale extraction of the more reactive metals state which metal is produced in the blast furnace state the reactions which take place in the blast furnace 4 I am going to find out about corrosion state the definition of corrosion the necessary conditions for metals to corrode how the speed of corrosion of different metals compare

15 5 I am going to find out about rusting state the metal that undergoes rusting state the two equations involved in rusting state the effect of ferroxyl indicator on Fe 2+ and OH - ions 6 I am going to find out about preventing state ways of preventing corrosion, giving everyday examples corrosion explain what happens in these processes 7 I am going to find out about cells describe how electricity can be produced using metals draw a set-up of how electricity can be produced using metals state the purpose of an electrolyte 8 I am going to find out about The Electrochemical Series explain The Electrochemical Series state the direction of electron flow if two different metals are connected in a cell relate the size of voltage produced to the position of metals in The Electrochemical Series 9 I am going to find out about more complicated cells draw a set-up of how electricity can be produced using metals and solutions of their own ions state where electrons flow in this set-up state where ions flow in this set-up state the purpose of an ion bridge 10 I am going to find out about alloys state the definition of an alloy give examples of alloys and their uses name the elements present in these alloys 15

16 Topic 9 Metals and Alloys Experiments Nat 4 1. Materials - a selection of different materials -??? the ones mentioned in the notes???? 2. Reactivity of Metals Alkali Metal demo SGrade Topic 11 Metals & Water/Acid/Oxygen - could test for hydrogen if released. 3. Extraction of Metals -??????????? do we have anything? 4. Corrosion SGrade Topic 12 nails expt in water etc 5. Rusting ferroxyl indicator and Fe 2+ ion and OH - ion solutions 6. Preventing Corrosion iron/magnesium cell set up in a u-tube with salt water and ferroxyl indicator 7. Cells/Batteries zinc rod copper rod dilute sulphuric acid voltmeter wires lemon, wires, voltmeter, zinc & copper rods????????? 8. The Electrochemical Series SGrade Topic 10 Electrode Potential 9. The Zinc/Copper Cell Again! zinc rod copper rod voltmeter wires filter paper salt solution zinc chloride solution

17 copper sulphate solution 10. Alloys circuits boards a selection of different alloys -??? the ones mentioned in the notes???? 17