Presentation on theme: "(including electrolysis) ALWAYS BRINGS YOU BACK HERE"— Presentation transcript:
1 (including electrolysis) ALWAYS BRINGS YOU BACK HERE Working Out Formulae& Balancing Equations(including electrolysis)ValencyWriting EquationsUsing ValencyBalancing EquationsElectrolysis half equationsENDALWAYS BRINGS YOU BACK HERE
2 VALENCY tells you how many BONDS an atom can form You work the valency out like this…Work out how many OUTER ELECTRONS the atom has(That’s it’s GROUP NUMBER in the Periodic Table)If it has 4 or less then THAT’S THE VALENCYIf it has 5 or more then VALENCY = 8 – OUTER ELECTRONSElementGroupOuter ElectronsValencySodium1Chlorine78 – 7 = 1Magnesium2Nitrogen58 – 5 = 3Carbon4Oxygen68 – 6 = 2
3 Here’s another method that works for ions… Sodium forms Na+ ionIt has valency 1For IONIC BONDS, the valency is just the size of the charge on the ionCalcium forms Ca2+ ionIt has valency 2Oxygen forms O2- ionIt has valency 2Chlorine forms Cl- ionIt has valency 1Some transition metals can form more than one ion, so they have more than one valency.Iron forms Fe2+ and Fe3+So it can have valency 2 or 3You can use the same idea for ions made of more than one atomAmmonium ion NH4+has valency 1Nitrate ion NO3-has valency 1Sulphate ion SO42-has valency 2
4 USING VALENCIES TO WORK OUT FORMULAE Write down the two elements (or ions) in the compoundWrite down the valency of each as a “small number” next to the other one. Put brackets round any complicated ions, like sulphate, nitrate etc.Cancel if necessary, and remove any 1sSodium sulphate Na SO4Valency of sodium: 1Valency of sulphate: 2So Na2(SO4)1Remove 1s: Na2SO4Iron(II) chloride Fe ClValency of iron : 2Valency of chloride : 1So Fe1 Cl2Remove 1s: Fe Cl2Magnesium nitrate Mg NO3Valency of magnesium : 2Valency of nitrate : 1So Mg1 (NO3)2Remove 1s: Mg (NO3)2Calcium oxide: Ca OValency of calcium: 2Valency of oxygen: 2So Ca2O2Cancel: CaODO NOT “MULTIPLY OUT THE BRACKETS” !Write (NO3)2 not N2O6
5 WRITING EQUATIONS Step 1: Write a WORD EQUATION Reactants Products The chemicals you start withWhat you make in the reactionReactantsProductsStep 2: Write down the FORMULA of each of the chemicals in your word equationStep 3: Put in the STATE SYMBOLS(s) for solid, (l) for liquid, (aq) for solution, (g) for gasEg: reaction of calcium carbonate with hydrochloric acid:calcium chloride+ water +carbondioxidecalciumcarbonatehydrochloricacid+The PRODUCTSCaCO HClThe REACTANTSCaCl H2O CO2(s)(aq)(aq)(l)(g)Make sure you get the formulae right!No marks for balancing if the formulae are wrong!Now on to the harder bit – balancing the equation!Calcium chloride is soluble, so as there is water present, it is a solutionHydrochloric acid, like other acids, is always a solutionCalcium carbonate is an insoluble solidWater is a liquid(not a solution!)Carbon dioxide is a gasIt forms in bubbles
6 Balancing Equations 2Na + 2H2O 2NaOH + H2 WHAT DOES IT MEAN? Balancing an equation means making sure the numbers of each type of atom are the same on each sideLet’s look at this balanced equation:2Na + 2H2O 2NaOH + H2AtomNumber on LeftNumber on RightNaHave 2Na, so 21 in each NaOH2NaOH. So 2H2 in each water2 waters2 2 = 42 from 2NaOH+ 2 from H2 makes 4O1 in each water2 waters. So 21 in each NaOH2NaOH. So 2They all match. So it’s balanced!
7 How to balance an equation Step 1: Write down the unbalanced formula equation.Step 2: Work out how many of each atom there are on each side. (in your head, if it’s easy)Step 3: Look for any atoms where there aren’t the same number on each sideStep 4: Choose the “unbalanced atom” that’s in the smallest number of different formulae.Step 5: Balance it by putting a number IN FRONT of one of the formulae (don’t change the actual formula!)Step 6: Recalculate numbers of atoms – and repeat if needed!NaOH + H2SO4 Na2SO4 + H2O22Left RightNa 1 2O 5 5H 3 2S 1 12There are different numbers of O and HNa is only in one chemical each sideThere are different numbers of Na and H66H is in fewer different formulae.44We can balance H by putting 2 in front of H2OWe can balance them by putting 2 in front of NaOHTHEY ALL MATCH! IT’S BALANCED!Now recalculate the numbers of atoms…Now recalculate the numbers of atoms…
8 Other examples… 2 2 2 CaCO3 + HCl CaCl2 + H2O + CO2 balancedLeft RightCa 1 1CO 3 3H 1 2Cl 1 22Let’s balance the hydrogens:We can do this by putting 2 in front of HClCaCO3 + HCl CaCl2 + H2O + CO222Eureka ! That has automatically balanced the chlorines too.Left RightCl 2 1KI 1 22Unfortunately, this has also unbalanced the potassium.However putting 2 in front of KI balances both K and I2Let’s balance the chlorines :We can do this by putting 2 in front of KClCl2 + KI KCl + I22222balanced
9 THEY ALL MATCH! IT’S BALANCED! Another example…Al2O3 + HCl AlCl3 + H2O623Left RightAl 2 1O 3 1H 1 2Cl 1 32Now let’s balance the hydrogens:We can do this by putting 6 in front of HClNow let’s balance the oxygens:We can do this by putting 3 in front of H2O3None of the atoms are balanced!They all occur in just two chemicalsChoose one to balance…6666Recalculate…Recalculate…THEY ALL MATCH! IT’S BALANCED!We can balance Al by putting 2 in front of AlCl3Recalculate…
10 THEY ALL MATCH! IT’S BALANCED! An awkward one!2Al + Cl2 AlCl332Left RightAl 1 1Cl 2 322Now we must balance the aluminiumsWe can do this by putting 2 in front of AlChlorines aren’t balanced.66But how can we do the balancing?We haven’t got “nice” numbers!Recalculate…THEY ALL MATCH! IT’S BALANCED!This is like finding the “lowest common denominator” in fractions.We have two chlorines on one side, and three on the other.We find the smallest number two and three go into – that’s six.So we need to aim for six chlorines on each sideIf you like maths, you could try balancing ones like this using fractions instead. You’d need to use 1½To do that, we put 3 in front of Cl2 and 2 in front of AlCl3Recalculate…
11 Here are Cu2+ ions moving to the negative electrode. ELECTROLYSISHere are Cu2+ ions moving to the negative electrode.Positive ions in the solution are attracted to negative electrode – opposite charges attract.When electrons are gained by a positive ion, the name of the chemical change is REDUCTION.REDUCTION IS THE GAIN OF ELECTRONS.THE COPPER ION HAS BEEN REDUCED2e- + Cu2+ CuThe electrode is negative because it has too many electronsAs they get close, the ions gain electrons from the electrode and the Cu2+ is neutralised.e- go to ionCuThis makes copper the element, which covers the electrode.e- go to ion.TWO ELECTRONS FROM THE CATHODEA NEUTRAL ATOM OF THE ELEMENT COPPER.ARE ADDED TO THE COPPER ION
12 When electrons are lost by a negative ion, the name of the chemical change is OXIDATION. OXIDATION IS THE LOSS OF ELECTRONSTHE CHLORIDE ION HAS BEEN OXIDISEDe- go to cellCl-This is what happens at the positive electrode when chloride ions, Cl- are present in the electrolyteHere are negative chloride ionsattracted towards the positiveelectrode. Opposite charges attract.This electrode is positive because some electrons have been removed by the cell.As they get close, each Cl- ion loses an electron which goes onto the electrode.The ion becomes electrically neutralthe ionloses an e-Cl2We have made chlorine the element. The neutral atoms join in pairs to make chlorine molecules, Cl2 which bubble off as a gas.2Cl- - 2e Cl2TWO CHLORIDE IONS, EACH WITH AN EXTRA ELECTRONTHE TWO ELECTRONS LOST BY THE IONS GO TO THE ELECTRODEA NEUTRAL CHLORINE MOLECULE
13 2 3 2 2 2 2 2 4 2 Cu2+ + e Cu Al3+ + e Al Ag+ + e Ag 1 Half equations for reduction at the negative electrode.Here are different ions that might be in a solution.You need to be able to balance the half equations.the ion GAINS electrons from the electrodeCu2+ + e Cu2Al3+ + e Al3click for solutionAg+ + e Ag1Pb2+ + e Pb2In these cases, metallic elements would appear at the negative electrode.2H+ + e H2click for solutionH+ is present in all acids and hydrogen gasis evolved from the electrode.Half equations for oxidation at the positive electrode.Here electrons are lost by the ion.When gaseous elements are produced, they bond together in pairs to make a molecule.The balancing needs to include this.2Cl e Cl22Br e Br224O e O2Cu - e Cu2+2In a special case, a positive copper electrode dissolves in a solution of copper sulphate. Electrons are lost by the copper metal.