Unit 4 - Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.Unit 4 - Chemical Reactions4.1Introduction for Reactions4.2Net Ionic Equations4.3Representations of Reactions4.4Physical and Chemical Changes4.5 Stoichiometry4.6Introduction to Titration4.7Types of Chemical Reactions4.8Introduction to Acid-Base Reactions4.9Oxidation-Reduction (Redox) ReactionsISPS Chemistry Mar 2022page 1Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.This logo shows it is a Topic Question - itshould only require knowledge included inthis Topic and it should be giving practicein the Science Practice associated with thisTopic.ISPS Chemistry Mar 2022page 2Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.BY:Credit must be given to me, the creator.AP Chemistry Notes 2020 by Gordon Watson are licensedunder Attribution-NonCommercial-ShareAlike 4.0 International.NC:Only noncommercial use of my work is permitted.To view a copy of this license, visitNoncommercial means not primarily intended for or directedtowards commercial advantage or monetary y-nc-sa/4.0/ISPS Chemistry Mar 2022SA:page 3Adaptations must be shared under the same terms.Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.4.1Introduction for ReactionsPhysical or Chemical ?At times, the difference can be clear and obvious but there are also many times when it is far fromclear-cut.Heating of an iron bar:Rusting of an iron bar:Fe(s) Fe(s) Fe(s) Fe(s) Fe2O3(s)greyredgreygreyreddy orangecolour changecolour changeISPS Chemistry Mar 2022page 4Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.The key difference between these processes (best illustrated by the equations) is that there is nochange in composition (metallic network) when an iron bar is heated/cooled wheras the iron haschanged completely (metallic network ionic network) during rustingOne definition of a chemical change is that existing bonds should be breaking and new bondsshould be forming.metallic bondsbreakingcovalent bondsbreakingionic bondsformingHowever, we could argue that metallic bondswere breaking when we heated our iron barand would have to break if we melted and thenboiled the iron.What would prevent us from ever consideringthis as a chemical process would be the factthat there is no change in composition.Fe(s) Fe(l) Fe(g)In changes of state, we consider any bondsbeing broken as intermolecular attractions and,since no new bonds are forming, would confidently label this as a Physical change.metallic bonds breaking during melting Physical changemetallic bonds breaking during rusting Chemical changeSimilarly, when water melts or boils,only intermolecular attractions are being broken.Only the hydrogen bonds between water moleculesare broken, the covalent bonds within the moleculesremain intact.H2O(s) H2O(l) H2O(g)There is no change in the composition - Physical.ISPS Chemistry Mar 2022page 5Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.By contrast, when water is electrolysed bubbles of gas form just likewhen water boils.However, this time covalent bonds within the watermolecules are breaking and new covalent bonds areforming in the oxygen and hydrogen molecules.2 H2O(l) 2 H2(g) O2(g)The composition has changed completely Chemical change.What about dissolving?One can certainly argue that there are bondsbreaking (ionic bonds & hydrogen bonds) andnew attractions (ion-dipole) taking their place.We are also losing the ionic network so changein composition?NaCl(s) NaCl(aq)Our equation makes clear that, in reality, therehas been no change in composition - our Na and Cl- ions remain - so Physical Change.However, the formation of a solution can be spontaneously exothermic or endothermic, and inlater units (particularly in the Equilibrium Unit) we will treat them in the same way as 'normal'reactions so, as I've said, not always clear-cut.What about precipitation?Can depend, if salt precipitates out of a saturatedsolution:NaCl(aq) NaCl(s)then there is no change in composition so it wouldbe considered a Physical Change.If we mix KI(aq) with Pb(NO3)2(aq) and precipitate outsome yellow PbI2(s) :2KI(aq) Pb(NO3)2(aq) PbI2(s) 2KNO3(aqthen the change in composition means it wouldbe considered a Chemical Change.ISPS Chemistry Mar 2022page 6Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.Sometimes both changes are taking place.The combustion of the candle wax is aChemical change. It is an exothermic process.There is a change in composition.C15H32(s) 23 O2(g) 15 CO2(g) 16 H2O(l)Meanwhile. all around the flame, some of thecandle wax is melting in a Physical Change.C15H32(s) C15H32(l)This is an endothermic process but there isno change in composition.The melting of candle wax was endothermic because energy is needed to overcome the LondonDispersion attractions between the wax molecules and to make them move faster in order tochange from solid to liquid - intermolecular attractions only are broken.The dehydration of copper(II) sulfateis considered a Physical change:CuSO4. 5H2O(s) CuSO4(s) 5H2O(l)since the fundamental composition ofthe copper(II) sulfate remains the same:one Cu2 ion for every one SO42-.We would consider the attractions brokenbetween the ions and the water molecules to be intermolecular attractions.If a few drops of water are then added to the colourless anhydeous CuSO4(s), the blue colour ofhydrated CuSO4. 5H2O(s) returns. A characteristic of Physical changes is that they are often easy toreverse.The decomposition of copper(II) carbonate alsoinvolves a colour change (green black) butis a Chemical change:CuCO3(s) CuO(s) CO2(g)There is a change in the composition and ionic andcovalent bonds will be breaking and forming.The chemical change can be reversed, but not easily.Copper carbonate is also hydrated so water vapourwill also be observed due to the Physical change:H2O(s) H2O(l)ISPS Chemistry Mar 2022page 7Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.The decomposition of hydrogen peroxide is a Chemical change:2 H2O2(aq) 2 H2O(l) O2(g)involving the breaking and forming of covalent bonds anda clear change in composition.However, it is a very slow reaction so is usually catalysed bythe addition of manganese (IV) oxide (MnO2 ).When added, it is often in the form of lumps but changes topowder during the reaction. this would not be considered a change in composition.MnO2(s) MnO2(s)Fom lump to powder is a Physical change.A chemical change occurs when substances are transformed into new substances,typically with different compositions.Evidence for a 'new substance with different composition' can include: formation of a gas,formation of a solid precipitate, a colour change and release (exothermic) of energy which can belight, heat or electricity. Some of these can, however, also be seeen in Physical changes so it isimportant to look carefully at the context within which these changes are happening.ISPS Chemistry Mar 2022page 8Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.4.2Net Ionic EquationsISPS Chemistry Mar 2022page 9Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.The illustration above gives us a clear idea of the Chemical process involved in automotivecombustion.We can identify Reactants (octane and oxygen) and Products (carbon dioxide and water) and usethem to write a Word Equation:Reactants Productsoctane oxygen carbon dioxide waterThe illustration also makes clear the composition of each chemical making it easy to write thecorrect Formulae Equation:C8H18 O2 CO2 H2OAn important skill is the ability to ensure that the same number of each type of atom is presentthroughout the reaction to produce the correct Balanced Equation:so,8 C atoms,18 H atoms,C8H18 C8H18 8 CO2 8 CO2 H2O9 H2O25 O atoms,C8H18 12½ O2 8 CO2 9 H2OO2O2The use of 12½ does not imply 'half an oxygen molecule' which is impossible. Instead, it implies' 12½ moles ' which is part of the stoichiometry of the reaction:C8H18 12½ O2 8 CO2 9 H2O162 gStoichiometry1 mol12½ mol8 molMasses (simplified)114 g400 g352 g9 molConservation of Matter (the same number of each type of atom is present throughout the reaction)also leads to Conservation of Mass:C8H18 12½ O2 8 CO2 9 H2OMasses (simplified)114 g400 gReactants 514 gISPS Chemistry Mar 2022page 10352 g162 g514 g ProductsChemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.Similarly with Physical processes such as meltingand evaporation:Word Equation:ice water steamFormula Equation:H2O(s) H2O(l) H2O(g)Matter :1 mol 1 mol 1 molMass:18g 18g 18gIn our Balanced equations, we tend to represent everything as if they were molecules or singleatoms. For example,Fe2O3(s) 2 Al(s) Al2O3(s) 2 Fe(s)makes no attempt to represent the ionic network structure of Fe2O3 and Al2O3 or the metallicnetwork structure of Al and Fe. We sometimes refer to these as Balanced Molecular Equations.When dealing with ionic solutions, however, it is oftenbetter to make the dissociation of the ions clearer, so2KI(aq) Pb(NO3)2(aq) PbI2(s) 2KNO3(aq)becomes2K (aq) 2I-(aq) Pb2 (aq) 2NO3-(aq) PbI2(s) 2K (aq) 2NO-3(aq)This is called a Complete Ionic Equation and makes iteasier to identify spectator ions - ions that have notchanged during / not taken part in the reaction.Removing the spectator ions allows us to see what was actually happening and we write the NetIonic Equation:2I-(aq) Pb2 (aq) PbI2(s)the precipitation of lead(II) iodideThis works for most ionic compounds where we can assume that they dissociate 100% in water.They are referred to as strong electrolytes (strong acids if they release H ions and strong bases ifthey release OH- ions or accept H ions).So weak acids such as hydrofluoric acid and ethanoic acid are often represented as:HF(aq)rather thanH (aq) F-(aq)CH3COOH(aq)rather thanH (aq) CH3COO-(aq)Some of our bases such as magnesium hydroxide and calcium hydroxide are quite insoluble anddissociate 1% so:Mg(OH)2(s)rather thanMg2 (aq) 2OH-(aq)ISPS Chemistry Mar 2022page 11Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.4.3 Representations of ReactionsParticulate representations of chemical equations will normally have:reactants at startproducts at endcorrect formulaecorrect quantities(balanced)conservation ofmatterCH4or2 CH4 2 O2 4 O2 CO2 2 CO22 H2O 4 H2OC2H4O H2O C2H6O2However, expect to seerepresentations where oneof the reactants was inexcess so there will besome of these moleculesmixed in with the newproducts.ISPS Chemistry Mar 2022page 12Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.In this case, Fe and S reacttogether in equimolar quantities.In terms of relative masses, thiswould be,32g S : 56g FeIn this representation, the sulfuris in excess so there will be sulfurleft over.If initial masses are known thenmass of S left can be calculated.Conservation of matter would allowus to deduce the correct formula forthe starting hydrocarbon.3 CO2 3 C atoms so x 34 H2O 8 H atoms so y 8soC3H8 - propaneIf yellow S and red O, which of the drawingsrepresents sulfur dioxide molecules?d) looks OK in terms of numbers of sulfur atoms andoxygen atoms but structure of SO2 isso b) is actually correct answerWhich drawing represents a mixture?a) and d) are mixturesWhich drawing represents a productmixture consistent with the law ofmass conservation?only d) is 'balanced with a)ISPS Chemistry Mar 2022page 13Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.4.4Physical and Chemical ChangesThis was all covered in Lesson 4.1.Reread the notes and then try the following questions.ISPS Chemistry Mar 2022page 14Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.4.5StoichiometrySo far .Molar Mass, M atomic or formula weight in grammesAvogadro's constant 6.023 x 1023 mol-1ISPS Chemistry Mar 2022page 15Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.n PV / RTM n /Lmol l-1Mstock x Lstock Mnew x LnewStoichiometry - using Balanced EquationsUsing the coefficients that were used to balance a chemical equation provides us with molerelationships that can be used in a variety of calculations.For example,3 mol of H22 mol of NH3Which can then be converted into a mass relationship using Molar Masses:6g of H234g of NH3If Pressure and Temperature are known then actual volume relationship can be calculated.However, since they are likely to be under same Pressure and Temperature, we can often derive asimple volume relationship:3 vol of H22 vol of NH3Sometimes, it will be necessary to identifying limiting reactant /reactant in excess to ensure thecorrect mole relationship is used in further calculations:In theory, the 3 mol N2 should beable to produce 6 mol NH3.But there would need to be 9 mol H2,wheras there is only 6 mol H2, so H2is the limiting reactant and should beused to calculate amount of NH3.3 mol H26 mol H212g H2ISPS Chemistry Mar 2022page 162 mol NH34 mol NH368g NH3If 3 mol N2 had been used we wouldhave wrongly calculated 102g NH3.Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.Example 1:MgCl2(aq) 2NaOH(aq) 2NaCl(aq) Mg(OH)2(s)A 100mL sample of 0.1M MgCl2(aq) and a 100mL sample of 0.2MNaOH(aq) were combined,and Mg(OH)2(s) precipitated, as shown by the equation above.What changes would double the amount of Mg(OH)2(s) produced?To double the number of moles of Mg(OH)2(s) would require us to double the numberof moles of both of the reactants - if they were of equivalent amounts to begin withi.e. neither was in excess.so this needs to be checked:MgCl2NaOHn MxL0.1 L x 0.1 M 0.01 mol MgCl20.1 L x 0.2 M 0.02 mol NaOHthe balanced equation tells us: 1 mol MgCl2so:0.01mol MgCl2needs2 mol NaOHneeds0.02mol NaOHso they are of equivalent amounts - just enough to react 100% - neither is in excessso we couldorororn MxLdouble volume of MgCl2anddouble volume of NaOHdouble Molarity of MgCl2anddouble Molarity of NaOHdouble volume of MgCl2anddouble Molarity of NaOHdouble Molarity of MgCl2anddouble volume of NaOHExample 2:According to the equation above, how many moles of potassium chlorate, KClO3, must bedecomposed to generate 1.0 L of O2 gas at standard temperature and pressure?the balanced equation tells us: 3 mol O2would be produced fromthe significance of standard temperature and pressure is that:so:so:so:3 x 22.4 L O2would be produced from1 L O2would be produced from67.2 L O2would be produced from2 mol KClO31mol of any gas 22.4 L2 mol KClO32 mol KClO32/67.2 mol KClO3 0.0298 molA variation on this question might have asked us to calculate the mass of KClO3 required.(Molar Mass 122.5 g mol-1)so:so:so:3 x 22.4 L O2would be produced from1L O2would be produced from67.2 L O2ISPS Chemistry Mar 2022would be produced frompage 172 x 122.5 g KClO3225 g KClO3225/67.2 KClO3 3.35 gChemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.4.6Introduction to TitrationVolumetric AnalysisTitration is the most common application of VolumetricAnalysis - accurate measurements of volumes usingpipettes and burettes.Titration is the slow addition of one solution of a knownconcentration (called a titrant) to a known volume ofpotassiumhydrogenanothersolution of unknown concentration until thephthalate (KHP) is areaction reaches the equivalence point, whichpopular choice for anisoften indicated by a color change.acid.sodium carbonate is aThesolution of known concentration will usually bepopularchoiceforaprepared by accurate weighing and careful transferbase.to a volumetric flask before carefully makingup to the mark.This can often be considered as the PrimaryStandard.ISPS Chemistry Mar 2022page 18Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.Acid-Base TitrationThere are a wide variety of acids and bases (whichwill be dealt with in more detail later) but thenet ionic reaction is:H (aq) OH-(aq)H2O(l)The equivalence point is reached when [H ] [OH-]and pH 7.The titration will stop when the indicator changescolour - this is called the end-point. It may requirean extra drop of OH- solution as indicatorsrarely change colour at exactly pH 7.For example, phenolpthalein will change(colourless to pink) from pH 8 to pH 10.Acid-base titrations are often carried out using a pHmeter to plot a pH curve as shown opposite.The pH changes rapidly around the equivalencepoint so if an indicator was used instead, its colourchange should be completed within a single drop.The equivalence point is not always at pH 7 as thesalts produced can themselves be basic or acidic.However, the equivalence point is always themidpoint in the region of rapid change, allowing thevolume added to be determined.ISPS Chemistry Mar 2022page 19Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.Red-Ox TitrationThere are an even wider variety of chemicalsthat can take part in Red-Ox reactions.The equivalence point is reached whenelectrons lost electrons gained(oxidation)(reduction)The titration will stop when there is anobvious colour change at the end-point.For example, the prescence of starch (anindicator) will cause a blue-black colour toappear once iodine (I2(aq) 2e- 2I-(aq))has completed reacting with the reductant.The first drop of excess I2(aq) will then be freeto react with the starch.Many red-ox reactants are coloured and canbe self-indicating. The managanate ion(MnO4-(aq)) is strongly purple and reacts toform the colourless Mn2 (aq) ion:MnO4-(aq) 8H (aq) 5e-Mn2 (aq) 4H2O(l)Again, once the MnO4-(aq) has completed reacting with the reductant, the first drop of excessMnO4-(aq) will produce a permanent pale pink colour.In Red-Ox reactions we can either use ion-electron ½ equations or combine them to make theoverall balanced net ionic equation:Reduction:MnO4-(aq) 8H (aq) 5e-Oxidation:H2C2O4(aq)Mn2 (aq) 4H2O(l)2CO2(g) 2H (aq) 2e-To balance these ½ equations it is necessary to ensure that electrons lost electrons gained. Thiscan be achieved by multiplying the two equations to reach 10e- so:Reduction:2MnO4-(aq) 16H (aq) 10e-Oxidation:5H2C2O4(aq)2Mn2 (aq) 8H2O(l)10CO2(g) 10H (aq) 10e-After adding the two equations (and cancelling down):Net Ionic Equation:2MnO4-(aq) 6H (aq) 5H2C2O4(aq)2 moles5 moles2Mn2 (aq) 8H2O(l) 10CO2(g)As usual, from the balanced equation we can get our mole relationships for calculations.ISPS Chemistry Mar 2022page 20Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.Stoichiometry - Titration Calculationsn MxLM n/Lx molExample 1:y molHCl(aq) NaOH(aq) NaCl(aq) H2O(l)The end point in a titration of a 50.00-mL sample of aqueous HCl wasreached by addition of 35.23 mL of 0.250 M NaOH titrant.What is the molarity of the HCl?Step 1: determine moles of titrant addedn MxL 0.250 x 0.03523 0.00881 moles NaOHStep 2: determine moles of analytex moly molHCl(aq) NaOH(aq) NaCl(aq) H2O(l)1 mol 1mol0.00881 moles NaOHStep 3: determine molarity of analyteM0.00881 moles HCl n/L 0.00881 / 0.05000 0.176 mol l-1 HClThe accuracy of a titration calculation is usually limited bythe ability to read the burette.Most burettes are marked off to nearest 0.1 ml butexperienced operators are often able to estimate to thenearest 0.01 ml. So probably 6.23 ml rather than 6.2 mlif reading to nearest line.ISPS Chemistry Mar 2022A single drop from a burette 0.05 ml.page 21Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.Example 2:2MnO4-(aq) 6H (aq) 5H2C2O4(aq)A 20.00-mL sample of aqueous oxalic acid, H2C2O4, was titrated with a0.09113-M solution of potassium permanganate.2Mn2 (aq) 8H2O(l) 10CO2(g)A volume of 23.24 mL was required to reach the end point. What is the oxalicacid molarity?Step 1: determine moles of titrant addedn MxL 0.09113 x 0.02324 0.002118 moles MnO4-(aq)Step 2: determine moles of analytex moly mol2MnO4-(aq) 6H (aq) 5H2C2O4(aq)2 mol5mol0.002118 moles MnO4-(aq)Step 3: determine molarity of analyteM5/2 x 0.002118 0.005295 moles H2C2O4(aq) n/L 0.005295 / 0.02000 0.2648 mol l-1 H2C2O4(aq)Titration MethodPipette and burette should be washed severaltimes with water.Pipette should then be washed out, at leasttwice, with the analyte solution.Burette should be washed out, at leasttwice, with the titrant solution.Burette readings should be taken ateye-level and to two decimal places.Titrations should be repeated until threeconcordant results are obtained.ISPS Chemistry Mar 2022page 22Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.4.7ISPS Chemistry Mar 2022Types of Chemical Reactionspage 23Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.Types of ReactionsThe labels synthesis, decomposition anddisplacement are important but we canusually do better!For example, all of the following reactionsare displacements but they are verydifferent in important ways and it is morehelpful to identify them as Acid-Base,Red-Ox or Precipitation reactions.Reactions in which H (aq) from an acidreact with OH-(aq) from a base arebetter labelled as Acid-Base reactions.2HCl(aq) Mg(OH)2(s) MgCl2(aq) 2H2O(l)Removing spectator ions reveals thenet ionic equation:2H (aq) 2OH-(aq) 2H2O(l)2H (aq) O2-(s) H2O(l)2H (aq) CO32-(s) H2O(l) CO2(g)ISPS Chemistry Mar 2022page 24Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.Reactions in which electrons are lost(oxidation) by one species andelectrons are gained (reduction) byanother species.These are better labelled as Red-Oxreactions.2AgNO3(aq) Cu(s) Cu(NO3)2(aq) 2Ag(s)Removing spectator ions reveals thenet ionic equation:2Ag (aq) Cu(s) Cu2 (aq) 2Ag(s)Reactions in which an ion from onesolution reacts with a different ion fromanother solution to form an insolublesolid.These are better labelled asprecipitation reactions.2KI(aq) Pb(NO3)2(aq) 2KNO3(aq) PbI2(s)Removing spectator ions reveals thenet ionic equation:2I-(aq) Pb2 (aq)) PbI2(s)Acids & BasesOur simplest acids are all polar covalent molecules that releasehydrogen ions H (aq) when dissolved in water.H𝛿 — Cl𝛿-(g)The hydrogen ion is essentially just a proton and is too unstable to existalone so will become attached to a water moleculeH (aq) Cl-(aq)H H2O(l)H3O (aq) hydroxonium orhydronium ionISPS Chemistry Mar 2022page 25Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.Similarly, for nitric acid,HNO3(l) H2O(l)NO3 (aq) H3O (aq)the resonance structure helpsstabilise the nitrate ionAnd for ethanoic acid,CH3COOH(l) H2O(l)CH3COO-(aq) H3O (aq)the resonance structurehelps stabilisethe ethanoate ionAcids, like HCl and HNO3, that dissociate 100%are described as strong acids.Other acids, like CH3COOH will dissociate 1%and are described as weak acids.The proportion of dissociated molecules in adiagram can be used to identify the strongestacid:HY HZ HXISPS Chemistry Mar 2022page 26Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.Bases can be more difficult to identify and we need better definitions, which will be developedfurther in the next lesson.Often bases are defined aschemicals that produce hydroxide ions, OH-(aq) when they dissolve in water.Ionic hydroxide compounds, such as NaOH, KOH, and Ca(OH)2, are among the most commonbases, mainly because they are the most soluble so can be 'relied' upon to produce 100% of thepossible OH-(aq) - strong bases.Less obvious bases include covalent ammonia, NH3 ,as well as many of the organic amines such asmethylamine, CH3NH2 .Notice that the use of the reversible arrows is an indication that ammonia will only partiallyionise so it would be a weak base.Other bases include oxides, such as Na2O, and carbonates, such as K2CO3 , which do notobviously contain hydroxide ions, though, similarly to ammonia, we can usually write equationsthat show the formation of hydroxide ions:Na2O(s) H2O(l)2Na (aq) 2OH-(aq)However, these equations can be challenging and not particularly helpful:K2CO3(s) 2H2O(l)Instead we can also define bases as2K (aq) 2OH-(aq) H2CO3(aq)2H (aq) 2OH-(aq) 2H2O(l)chemicals that accept (react with) H (aq) ions.H (aq) NH3(aq) NH4 (aq)2H (aq) O2-(s) H2O(l)2H (aq) CO32-(s) H2O(l) CO2(g)Since our acids start as covalent molecules, acid strength is largely to do with their tendency toionise when dissolved in water. The Equilibrium Unit will provide us with numbers (Ka values)that will make life much easier.Since our bases start as ionicnetworks, base strength is largelyto do with their ability todissolve in water.Group I (alkali metals) andGroup II (alkali earth metals)provide our strongest bases.Ksp values will come later.ISPS Chemistry Mar 2022page 27Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.Oxidation & ReductionOriginally, the term oxidation only referredto reactions with oxygen:Mg(s) O2(g)MgO(s)Combustion was simply an extreme form ofoxidation:C2H5OH(l) 3O2(g)2CO2(g) 3H2O(l)The term reduction referred to the 'reductionin mass' that accompanied reactions inwhich oxygen was lost - typically thedecomposition of metal oxides.2HgO(s)2Hg(s) O2(g)Oxidation - an increase in oxygenReduction - a decrease in oxygenAs our understanding of atomic structure increased, our definitions changed:Oxidation - is loss of electronsReduction - is gain of electronsA substance cannot lose electrons (Oxidation) unless another substance is able to gain thoseelectrons (Reduction) so we will increasingly refer to these reactiona as REDOX reactions.Redox reactions involve the transfer of elecrons between chemical speciesISPS Chemistry Mar 2022page 28Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.Biochemists will often recognise oxidationby an increase in the oxygen content.C2H5OH(l)ethanolCH3COOH(l)ethanoic acidHowever, less obvious oxidation can also beaccompanied by a decrease in hydrogen.C2H5OH(l)ethanolC2H4O(l)ethanalOxidation - an increase in theoxygen: hydrogen ratioReduction - a decrease in theoxygen: hydrogen ratioOxidation NumbersNot surprisingly, we need a system thatmakes it easier to determine oxidation andreduction, regardless of the context.One method is to assign 'notional' oxidationnumbers to the individual species found in achemical.The -4 oxidation number assigned to C in CH4 allows us to work out changes taking place when,for example, methane burns to produce CO2, in which the C would now be 4.The C atom is experiencing a change that is 'equivalent to' the loss of 8 electrons (-4 to 4) whichis clearly oxidation.However, CH4 is a covalent molecule and there is definitely not a C4- ion present in CH4. Similarly,there are no C4 ions present in covalent CO2 molecules.ISPS Chemistry Mar 2022page 29Chemical Reactions
AP Chemistry Notes 2020 by Gordon Watson is licensed under Attribution-NonCommercial-ShareAlike 4.0 International.Not surprisingly, in reactions involvingionic compounds, we see a closer matchbetween oxidation numbers assignedand actual charges.Mg(s)2H (aq) 2e-Mg2 (aq) 2eH2(g)Assigning an oxidation number of 7 to Mn in KMnO4 and knowing that theMn2 ion is produced during at titration involving KMnO4 leads to:Mn7 (aq) 5e-Mn2 (aq)gain of 5 electrons reductionIf the titration was with Fe2 , we would now know that the Fe2 must be losingelectrons (oxidation):Fe2 (aq)Fe3 (aq) eTherefore the molar relationship would be:Again, this is a 'notional' oxidation number. The energy needed to remove 7electron and form a 7 ion make it unlikely/impossible that this is howthe Mn atom in the MnO4- ion exists in reality.1 mol KMnO4 5 mol of Fe2 Instead, the negative charge from the oxygens will 'dampen down' andstabilise the Mn meaning that it's actual charge is significantly less than 7 .However, it definitely react
metallic bonds breaking during melting Physical change metallic bonds breaking during rusting Chemical change Similarly, when water melts or boils, only intermolecular attractions are being broken. Only the hydrogen bonds between water molecules are broken, the covalent bonds within the molecules remain intact. H 2 O (s) H 2 O (l .
Chemical Reactions Slide 3 / 142 Table of Contents: Chemical Reactions · Balancing Equations Click on the topic to go to that section · Types of Chemical Reactions · Oxidation-Reduction Reactions · Chemical Equations · Net Ionic Equations · Types of Oxidation-Reduction Reactions · Acid-Base Reactions · Precipitation Reactions
Chemical reactions called _ reactions give off heat. 5. Other reactions called _ reactions absorb heat and cool the immediate environment. 6. True or False. During all chemical reactions, a chemical change takes place that produces new substances with properties different than those of the original substances. .
Lesson Overview Chemical Reactions and Enzymes Chemical Reactions and Enzymes. Lesson Overview Chemical Reactions and Enzymes Chemical Reactions A chemical reaction is a process that changes , or transforms, one set of chemicals into another by changing the
The Major Classes of Chemical Reactions. 4.6 Elements in Redox Reactions 4.1 The Role of Water as a Solvent 4.2 Writing Equations for Aqueous Ionic Reactions 4.3 Precipitation Reactions 4.4 Acid -Base Reactions. 4.5 Oxidation -Reduction (Redox) Reactions 4.7
Types of Reactions There are five main types of chemical reactions we will talk about: 1. Synthesis reactions 2. Decomposition reactions 3. Single displacement reactions 4. Double displacement reactions 5. Comb
C.P. Chemistry Test Unit 8 Study Guide Chemical Equations and Reactions Recognize evidence of chemical change. Identify the reactants and products in a chemical reaction. Represent chemical reactions with equations. Know what is represented by the symbols: s, l, g, and aq. Categorize chemical reactions by type (synthesis/combination, decomposition,
ii. acid–base neutralization reactions iii. oxidation–reduction or redox reactions. Q.3. What are the important aspects of redox reactions? Ans: Almost every element participate in redox reactions. The important aspects of redox reactions are as follows: i. Large number of natural, biological and industrial processes involve redox reactions .
Topic 8: Chemical Reactions Chemical Equations & Reactions (Chapter 8 in Modern Chemistry) Describing Chemical Reactions A chemical reaction is the process by which one or more substances are changed into one or more different substances. In any chemical reaction, the original substances are known as the
