Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Topic 10 - Making electricity A redox reaction occurs to produce products and energy is given out because it is an exothermic reaction, BUT the energy is released as electrical energy NOT heat energy so the system shouldn't heat up. A simple cell can be made by dipping two different pieces of metal (of different reactivity) into a solution of ions e.g. a salt or dilute acid. If you use the same metal for both strips, they 'cancel' each other out, so no voltage, so no current of electrical energy. All you need is a solution of charged positive and negative particles called ions e.g. sodium Na+, chloride Cl-, hydrogen H+, sulphate SO42etc. The greater the difference in reactivity, the bigger the voltage produced. BUT a simple demonstration cell can be made by dipping strips of magnesium and copper into a salt solution and connecting them via a voltmeter (e.g. as in diagram) and a voltage is readily recorded. The electrons flow from the more reactive to the less reactive metal. The reactivity of the metal can be found on the ElectroChemical Series (ECS) in your data book. REDOX Oxidation Is Loss of electrons Reduction Is Gain of electrons Easily remembered by the phrase OIL RIG. In a cell the more reactive metal loses electrons – it is oxidised. The less reactive metal gains electrons – it is reduced. As both oxidation and reduction happen at the same time in a cell, it is called a REDOX reaction. Half Cells Cells can also be set up using two half cells as shown in the diagram below. In the above, Zinc, the more reactive metal, loses electrons. It is oxidised to form positive zinc ions: Zn (s) Zn2+(aq) + 2eThe copper ions in the blue solution gain these electrons. The positive copper ions are reduced to copper metal atoms: Cu2+(aq) + 2e- Cu(s) The overall redox equation is: Zn (s) + Cu2+(aq) + 2e- Cu(s) + Zn2+(aq) + 2eAs there are 2 electrons on both sides of the arrow they cancel each other out, leaving: Zn (s) + Cu2+(aq) Cu(s) + Zn2+(aq) Batteries Cells or batteries are useful and convenient portable sources of energy but they are expensive compared to what you pay for 'mains' electricity. We use batteries as portable forms of energy. The energy comes from a chemical reaction. Batteries stop working when one or both of the chemicals involved in the reaction run out. Some batteries however are rechargable, eg a car battery which is a lead /acid accumulator Displacement reactions These reactions occur when one metal reacts with a compound and takes the place of that metal in the compound. The important rule to remember is that a metal will only displace another metal from a compound if it is more reactive than that metal. The list below shows the order of displacement for the metals in the reactivity series. Potassium Sodium Lithium Calcium Magnesium Aluminium Carbon Zinc Iron Tin Lead Hydrogen Copper Silver Gold Platinum To demonstrate the principle of displacement, consider three metals, iron (most reactive), copper and silver (least reactive). Predict what will happen when a piece of iron is placed in a beaker of copper sulphate. The iron displaces the copper from the copper sulphate solution, leaving iron sulphate and copper metal. iron + copper sulphate iron sulphate + copper This could have been predicted since iron is more reactive than copper. This is the reaction. Next, what happens when a piece of silver is placed in the copper sulphate solution? In this situation, no reaction takes place at all because silver is less reactive than copper. silver + copper sulphate No reaction The displacement reactions can be written as ionic equations. In the example using iron and copper(II) sulphate: iron + copper(II) sulphate iron sulfate + copper. Fe(s) + CuSO4(aq) FeSO4(aq) + Cu(s) Copper(II) sulphate and iron sulphate are ionic compounds. When they are dissolved in water the ions become separated by the water molecules. If we write the equation showing the ions we have Fe(s) + Cu2+(aq) + SO42-(aq) Fe2+(aq) + SO42-(aq) + Cu(s) In going from reactants to products iron metal - Fe(s) has become iron ions - Fe2+(aq) copper ions - Cu2+(aq) have become copper metal - Cu(s) sulphate ions - SO42-(aq) are not changed during the reaction. Sulphate ions are the same on the left and the right side of the arrow. Ions which do not change during the reaction are called spectator ions. Spectator ions can be left out of the equation, giving Fe(s) + Cu2+(aq) Fe2+(aq) + Cu(s) This is the ionic equation for the reaction between iron and copper(II) sulfate. Iron is oxidised and copper is reduced. Similarly, the reaction between tin and lead chloride may be written as Sn(s) + Pb2+(aq) Tin is oxidised and lead is reduced Sn2+(aq) + Pb(s)