Junior cycle science chemistry patterns of chemical change. By Theresa Lowry-Lehnen. Science Teacher.
29/05/13Junior Cycle ChemistryJunior Cycle ChemistryPatterns of Chemical ChangePatterns of Chemical ChangeEdited and Reproduced byTheresa Lowry-LehnenRGN, BSc (Hon’s) Specialist Clinical Practitioner (Nursing), Dip Counselling, Dip Adv Psychotherapy, BSc(Hon’s) Clinical Science, PGCE (QTS) , H. Dip. Ed, MEd, Emotional Intelligence (Level 9) MHS Accredited
29/05/13Rates of ReactionRates of ReactionChemical reactions occur whendifferent atoms or molecules collide:For the reaction to happen the particles must have acertain amount of energy – this is called theACTIVATION ENERGY.The rate at which the reaction happens depends on four things:1) The temperature of the reactants,2) Their concentration3) Their surface area4) Whether or not a catalyst is used
29/05/13Measuring rate of reactionMeasuring rate of reactionTwo common ways:1) Measure how fast theproducts are formed2) Measure how fast thereactants are used up
29/05/13Rate of reaction graphRate of reaction graphAmount ofproductformedTimeSlower reactionFast rateof reactionhereSlower rate of reaction heredue to reactants being used up
29/05/13Enzymes are biological catalysts. They help the reactionsthat occur in our bodies by controlling the rate of reaction.Enzymes aredenaturedbeyond 40OCEnzymesEnzymesYeast is an example of an enzyme. It is used to help a process calledfermentation:Sugar Alcohol + carbon dioxideThe alcohol from this process is used inmaking drinks and the carbon dioxide canbe used to make bread rise.Enzymes work best in certain conditions:Enzyme activityTemp pH pH400CCould beprotease (foundin the stomach)Could be amylase(found in theintestine)
29/05/13Uses of enzymesUses of enzymes1) Enzymes are used in washingpowders to help digest food stains.Biological washing powders will onlywork on 400C or lower.2) Enzymes are used in baby foods to“pre-digest” the proteins.3) Enzymes are used to convert starchinto sugar which can then be used infood.4) Conversion of glucose into fructose– glucose and fructose are “isomers”(they have the same chemical formula),but fructose is sweeter.
29/05/13Endothermic and exothermic reactionsEndothermic and exothermic reactionsStep 1: Energy must beSUPPLIED to break bonds:Step 2: Energy is RELEASEDwhen new bonds are made:A reaction is EXOTHERMIC if more energy is RELEASEDthen SUPPLIED. If more energy is SUPPLIED then isRELEASED then the reaction is ENDOTHERMIC
29/05/13Energy level diagramsEnergy level diagramsEnergylevelReaction progressActivationenergyEnergy givenout byreactionUsing a catalystmight lower theactivation energy
29/05/13Exothermic vs endothermic:Exothermic vs endothermic:EXOTHERMIC – moreenergy is given out than istaken in (e.g. burning,respiration)ENDOTHERMIC –energy is taken in butnot necessarily given out(e.g. photosynthesis)
29/05/13Reversible ReactionsReversible ReactionsSome chemical reactions are reversible. In other words, theycan go in either direction:A + B C + DNH4Cl NH3 + HCle.g. Ammonium chloride Ammonia + hydrogen chlorideIf a reaction is EXOTHERMIC in one directionwhat must it be in the opposite direction?For example, consider copper sulphate:Hydrated coppersulphate (blue)Anhydrous coppersulphate (white)+ Heat + WaterCuSO4 + H2OCuSO4.5H2O
29/05/13Reversible ReactionsReversible ReactionsWhen a reversible reaction occurs in a CLOSED SYSTEM (i.e. no reactantsare added or taken away) an EQUILIBRIUM is achieved – in other words,the reaction goes at the same rate in both directions:A + B C + DEndothermic reactionsIncreased temperature:Decreased temperature:A + B C + DA + B C + DMore productsLess productsExothermic reactionsIncreased temperature:Decreased temperature:A + B C + DLess productsMore productsA + B C + D
29/05/13Making AmmoniaMaking AmmoniaNitrogen + hydrogen AmmoniaN2 + 3H2 2NH3•High pressure•450OC•Iron catalystRecycled H2 and N2NitrogenHydrogenMixture of NH3, H2 andN2. This is cooledcausing NH3 to liquefy.Fritz Haber,1868-1934Guten Tag. My name is Fritz Haber and I won the NobelPrize for chemistry. I am going to tell you how to use areversible reaction to produce ammonia, a veryimportant chemical. This is called the Haber Process.To produce ammonia from nitrogen and hydrogenyou have to use three conditions:
29/05/13Uses of AmmoniaUses of AmmoniaNitrogenmonoxideHotplatinumcatalystAmmonia gasOxygenCooledWater andoxygenNitrogenmonoxideNitricacidAmmonia + nitric acid Ammonium nitrateNH3 + HNO3 NH4NO3Ammonia is a very important chemical as it can beused to make plant fertilisers and nitric acid:More ammonia can then be used to neutralise the nitric acid toproduce AMMONIUM NITRATE (a fertiliser rich in nitrogen).The trouble with nitrogen based fertilisers is that they can alsocreate problems – they could contaminate our drinking water.
29/05/13Haber Process: The economicsHaber Process: The economicsA while ago we looked at reversible reactions:A + B C + DEndothermic, increased temperatureA + B C + DExothermic, increase temperatureExothermicEndothermic1) If temperature was DECREASED the amount of ammonia formed would__________...2) However, if temperature was INCREASED the rate of reaction in bothdirections would ________ causing the ammonia to form faster3) If pressure was INCREASED the amount of ammonia formed wouldINCREASE because there are less molecules on the right hand side ofthe equationNitrogen + hydrogen AmmoniaN2 + 3H2 2NH3
29/05/13Haber Process SummaryHaber Process Summary•200 atm pressure•450OC•Iron catalystRecycled H2 and N2NitrogenHydrogenMixture of NH3, H2and N2. This iscooled causing NH3to liquefy.To compromise all of these factors, these conditions are used:A low temperature increases the yield of ammonia but istoo slowA high temperature improves the rate of reaction butdecreases the yield too muchA high pressure increases the yield of ammonia but costs a lotof money
29/05/13Burning MethaneBurning MethaneCH4 + 2O2 2H2O + CO2To burn methaneyou have to breakall of these bonds:And then youhave to makethese ones:
29/05/13Drawing this on an energy diagram:Drawing this on an energy diagram:2734 Kj3462 KjMore energy is given out (3462) than is given in (2734) –the reaction is EXOTHERMIC. The total (“nett”) energychange is –728 Kj. An endothermic reaction would have apositive energy change.-728 Kj
29/05/13Burning MethanolBurning MethanolTotal for breaking bonds = 6x435(C-H) + 2x360 (C-O) + 2x464 (O-H)+ 3x497 (O=O) = 5749 KJ/mol2CH3OH + 3O2 2CO2 + 4H2OTotal for making bonds =4x803 (C=O) + 8x464 (O-H) =6924 KJ/molEnergy change = 5749-6924 (divide this by two as we aredealing with two molecules of methanol) = -587.5 KJ/mol
29/05/13Atomic massAtomic massSYMBOLPROTON NUMBER = number ofprotons (obviously)RELATIVE ATOMIC MASS, Ar(“Mass number”) = number ofprotons + number of neutrons
29/05/13Relative formula mass, MRelative formula mass, MrrThe relative formula mass of a compound is blatantly the relativeatomic masses of all the elements in the compound added together.E.g. water H2O:Therefore Mr for water = 16 + (2x1) = 18Work out Mr for the following compounds:1) HCl2) NaOH3) MgCl24) H2SO4H=1, Cl=35 so Mr = 36Na=23, O=16, H=1 so Mr = 40Mg=24, Cl=35 so Mr = 24+(2x35) = 94H=1, S=32, O=16 so Mr = (2x1)+32+(4x16) = 98Relative atomic mass of O = 16Relative atomic mass of H = 1
29/05/13Calculating percentage massCalculating percentage massIf you can work out Mr then this bit is easy…Calculate the percentage mass of magnesium in magnesium oxide, MgO:Ar for magnesium = 24 Ar for oxygen = 16Mr for magnesium oxide = 24 + 16 = 40Therefore percentage mass = 24/40 x 100% = 60%Percentage mass (%) =Mass of element ArRelative formula mass Mrx100%Calculate the percentage mass of the following:1) Hydrogen in hydrochloric acid, HCl2) Potassium in potassium chloride, KCl3) Calcium in calcium chloride, CaCl24) Oxygen in water, H2O
29/05/13Calculating the mass of a productCalculating the mass of a productE.g. what mass of magnesium oxide is produced when 60g ofmagnesium is burned in air?Step 1: READ the equation:2Mg + O2 2MgOIGNORE theoxygen in step 2 –the questiondoesn’t ask for itStep 3: LEARN and APPLY the following 3 points:1) 48g of Mg makes 80g of MgO2) 1g of Mg makes 80/48 = 1.66g of MgO3) 60g of Mg makes 1.66 x 60 = 100g of MgOStep 2: WORK OUT the relative formula masses (Mr):2Mg = 2 x 24 = 48 2MgO = 2 x (24+16) = 80
29/05/13Work out Mr: 2H2O = 2 x ((2x1)+16) = 36 2H2 = 2x2 = 41. 36g of water produces 4g of hydrogen2. So 1g of water produces 4/36 = 0.11g of hydrogen3. 6g of water will produce (4/36) x 6 = 0.66g of hydrogenMr: 2Ca = 2x40 = 80 2CaO = 2 x (40+16) = 11280g produces 112g so 10g produces (112/80) x 10 = 14g of CaOMr: 2Al2O3 = 2x((2x27)+(3x16)) = 204 4Al = 4x27 = 108204g produces 108g so 100g produces (108/204) x 100 = 52.9g of Al O1) When water is electrolysed it breaks down into hydrogen and oxygen:2H2O 2H2 + O2What mass of hydrogen is produced by the electrolysis of 6g of water?3) What mass of aluminium is produced from 100g of aluminium oxide?2Al2O3 4Al + 3O22) What mass of calcium oxide is produced when 10g of calcium burns?2Ca + O2 2CaO
29/05/13Another methodAnother methodTry using this equation:Mass of product IN GRAMMESMass of reactant IN GRAMMESMr of productMr of reactantQ. When water is electrolysed it breaks down into hydrogen and oxygen:2H2O 2H2 + O2What mass of hydrogen is produced by the electrolysis of 6g of water?Mass of product IN GRAMMES6g436So mass of product = (4/36) x 6g = 0.66g of hydrogen
29/05/13Calculating the volume of a productCalculating the volume of a productAt normal temperature and pressure the Relative Formula Mass (Mr)of a gas will occupy a volume of 24 litrese.g. 2g of H2 has a volume of 24 litres32g of O2 has a volume of 24 litres44g of CO2 has a volume of 24 litres etcQ. When water is electrolysed it breaks down into hydrogen and oxygen:2H2O 2H2 + O2What VOLUME of hydrogen is produced by the electrolysis of 6g ofwater?• On the previous page we said that the MASS of hydrogen producedwas 0.66g• 2g of hydrogen (H2) will occupy 24 litres (from the red box above),• So 0.66g will occupy 0.66/2 x 24 = 8 litres
29/05/13Example questionsExample questions1) What volume of hydrogen is produced when 18g of water iselectrolysed?2H20 2H2 + O21) Marble chips are made of calcium carbonate (CaCO3). What volumeof carbon dioxide will be released when 500g of CaCO3 is reactedwith dilute hydrochloric acid?CaCO3 + 2HCl CaCl2 + H2O + CO21) In your coursework you reacted magnesium with hydrochloric acid.What volume of hydrogen would be produced if you reacted 1g ofmagnesium with excess acid?
29/05/13Empirical formulaeEmpirical formulaeEmpirical formulae is simply a way of showing how many atoms are in amolecule (like a chemical formula). For example, CaO, CaCO3, H20 andKMnO4 are all empirical formulae. Here’s how to work them out:A classic exam question:Find the simplest formula of 2.24g of ironreacting with 0.96g of oxygen.Step 1: Divide both masses by the relative atomic mass:For iron 2.24/56 = 0.04 For oxygen 0.96/16 = 0.06Step 2: Write this as a ratio and simplify:0.04:0.06 is equivalent to 2:3Step 3: Write the formula:2 iron atoms for 3 oxygen atoms means the formula is Fe O
29/05/13Example questionsExample questions1) Find the empirical formula of magnesium oxide whichcontains 48g of magnesium and 32g of oxygen.2) Find the empirical formula of a compound that contains42g of nitrogen and 9g of hydrogen.3) Find the empirical formula of a compound containing 20gof calcium, 6g of carbon and 24g of oxygen.
29/05/13ElectrolysisElectrolysisMolecule of solidcopper chlorideCuCl2 (s)Molecule of solidcopper chloride afterbeing dissolvedCuCl2 (aq)ChlorideionCopperion
29/05/13ElectrolysisElectrolysisElectrolysis is used to separate a metal from its compund.= chloride ion= copper ionWhen we electrolysedcopper chloride the _____chloride ions moved to the______ electrode and the______ copper ions movedto the ______ electrode –OPPOSITES ATTRACT!!!
29/05/13Electrolysis equationsElectrolysis equationsWe need to be able to write “half equations” to show whathappens during electrolysis (e.g. for copper chloride):2 22At the negative electrode thepositive ions GAIN electrons tobecome neutral copper ATOMS. Thehalf equation is:Cu2++ e-CuAt the positive electrode thenegative ions LOSE electrons tobecome neutral chlorineMOLECULES. The half equation is:Cl-- e-Cl2
29/05/13Calculating masses and volumes from electrolysisCalculating masses and volumes from electrolysisConsider those two half equations again:Cu2++ 2e-Cu 2Cl-- 2e-Cl21) Write down the relative atomic mass:Copper = 632 molecules of chlorine (Cl2) = 2x35 = 70Example question: How much chlorine is released at the positiveelectrode if 2g of copper is collected at the negative electrode?3) Work out the volume: 70g of chlorine would occupy 24 litres, so 2.22gof chlorine would occupy a volume of (2.22/70) x 24 = ______ litres2) Follow the steps:63g of copper makes 70g of chlorine……so 1g of copper would make (70/63) g of chlorine… (=________g)…so 2g of copper would make (70/63) x 2g of chlorine (=________g)