Chemical change Endothermic and Exothermic Reactions Rate of Reactions Collision Theory Reaction Rate... 4

Transcription

1 Page 1 CHEMISTRY (PAPER 2) Chemical change... 3 Endthermic and Exthermic Reactins... 3 Rate f Reactins... 4 Cllisin Thery... 4 Reactin Rate... 4 Measuring Rate... 4 Bltzmann Distributin f energy... 5 Equilibrium... 6 Open and clsed systems... 6 Reversable Reactins... 6 Dynamic Equillibrium... 6 Le Chatelier s Principal... 6 Equilibrium Cnstant (KC)... 6 KW... 7 KA... 7 Electrchemical Reactins... 8 Vltaic r Galvanic... 8 Electrlytic... 9 Definitins... 9 Standard Hydrgen Electrde Organic Naming Organic Mlecules (IUPAC Names) Dienes Alchls Aldehydes Ketnes Carbxylic Acid Halalkanes Esters Additin Reactins Hydrhalgenatin Halgenatin Hydratin Hydrgenatin Eliminatin Reactins Dehydrhalgentain... 15

2 Dehydratin Cracking Substitutin Reactins Intercnversin The Chemical Industry SASOL Hydrcracking Steam cracking Fractinal distillatin The Chlralkali Industry Uses The Fertiliser Industry Manufacturing Batteries Leclanché dry cell Zinc chlride dry cell Alkaline dry cell Mercury Battery Lithium In Battery Lead-acid strage battery Data bklet Page 2

3 CHEMICAL CHANGE ENDOTHERMIC AND EXOTHERMIC REACTIONS Fr an endthermic reactin: The temperature f the surrundings decreases because the kinetic energy is being transferred t break bnds. The temperature decreases because it takes in energy. Temperature: T Bnd energy: E Δ H > 0 (Psitive) Here the graph shws that it takes mre energy t break bnds than t make bnds. Fr an exthermic reactin: Temperature f the surrundings increase because making bnds create energy E T The activated cmplex is the nature when all bnds between the reactants have been brken, befre any bnds f the prducts have been created. The activatin energy (E A ) is the amunt f energy required t break all bnds befre a chemical reactin can take place. The reactants need sufficient E K t bring upn a chemical reactin. Page 3

4 RATE OF REACTIONS COLLISION THEORY Particles can nly react when they are in cntact with ne anther. The reactin rate is therefre determined by the number f and frequency f the cllisins that is made REACTION RATE The reactin rate is the speed at which a reactin takes place Factrs that affect rate f reactins: O Surface area (f a slid) A greater surface area increases the likelihd f a mlecule clliding and reacting S.A - reactin rate O Cncentratin (f a slutin) Higher cncentratin means there are mre reactant mlecules per unit f vlume, which means that mlecules find it easier t cllide with the substance Cncentratin reactin rate O Pressure (f a gas) Easier t cllide Cncentratin reactin rate O Temperature An increase in temperature means the mlecules speed increase and will increase the number f cllisins per secnd O Additin f a catalyst The catalyst hlds the mlecule int place until anther mlecule reacts with is. Adding a catalyst reactin rate O Nature f reactants The dtted line shws the increased reactin rate. MEASURING RATE Change in temperature Change in clur Change in mass f the reactin vessel Measuring gas vlumes (Pressure gauge) Turbidity (precipitate frmed) vs. time Page 4

5 CHANGE IN MASS CHANGE IN VOLUME Measured using a scale Measure the mass f the reactin vessel. Measured using a gas syringe r pressure gauge BOLTZMANN DISTRIBUTION OF ENERGY The very ht particles (at the end) will escape 1 and the E K will decrease because nly the cler (particles with less E K will be left ver. 1 Change state Page 5

6 EQUILIBRIUM OPEN AND CLOSED SYSTEMS A system in which bth energy and matter can be changed with surrunding is called an pen system. A system in which nly energy but nt matter can be exchange with the surrunding is called a clsed system. REVERSABLE REACTIONS A + B C + D In a chemical equilibrium, the cncentratins f reactants and prducts d nt change. But the frward and reverse reactins have nt stpped - they are still ging n at the same rate as each ther. DYNAMIC EQUILLIBRIUM A dynamic equilibrium is a chemical equilibrium between a frward reactin and the reverse reactin where the rate f the reactins are equal. LE CHATELIER S PRINCIPAL When the cnditins (temperature, cncentratin r pressure) under a clsed system is disturbed, the system acts in such a way as t ppse the change. The equilibrium shifts in the directin that ppsed it. If the temperature increases, the system will try t get clder Three pssible changes can upset the equilibrium: Temperature Decrease in temperature will favur the exthermic reactin Cncentratin If the cncentratin is increased, the equilibrium shifts in the ppsite directin (away frm) Pressure A decrease in pressure will favur the reactin which has an increase in vlume EQUILIBRIUM CONSTANT (KC) Fr the reactin: aa + bb cc + dd a N. f Mles (balancing number) A Mlecule K C = [C]c [D] d [A] a [B] b [A] means the cncentratin f A N H2O Only mlecules in the liquid phase i.e. n slids If KC > 1 then it Favurs Prduct If KC < 1 then it favurs the Reactant Page 6

7 SOLVING KC PROBLEMS 1. Write ut full balanced equatin 2. Cnstruct n start, n reacted, n equilibrium, cncentratin table` 3. Fill in all knwn values 4. See what the questin is asking and slve. aa +bb cc +dd N. f mles a b c d n start n react n equilibrium Cncentratin This ges with the rati cefficients C = n v n start n react If the prducts (C and D) are higher, i.e. K C > 1, it will favur the prducts K C is affected by: Temperature Pressure (gas) NH3 used as fertilizers and cleaning prducts Prductin can be sped up by lwering temperature r increasing H2 r N2 KW K W measures hw easily H2O splits up int H + and OH - ins H 2 O H + + OH - (hydrgen In) (Hydrxide In) 2H 2 O H 3 O + + OH - (Hydrnium) K W = [H 3 O + ][OH - ] = [H + ][OH - ] = [H + ] = lg( ) = 7 = ph f H + in H 2 O KA Acid Dissciatin Cnstant If a chemical releases mre H + ins int a slutin, it is an ACID [H + ] ph f 3 Acidic KW > 7 Alkali Adding OH - will shift the equilibrium t the left, prducing less H + ins All acids dnate H + ins Mntrpic Can nly dnate ne hydrgen Because this area has a big area f negative charge, the H + in des nt dissciate easily, which means it is nt a strng acid. Page 7

8 ELECTROCHEMICAL REACTIONS VOLTAIC OR GALVANIC Cnverts chemical energy int electrical energy Dipped in an electrlyte CALCULATE E 0 CELL: Zn Zn 2+ Ag + Ag Ande first Oxidatin is written n the left Zn Zn e - Reductin is n the right Ag + + e - Ag Zn + Ag + + e - Ag + Zn e - 1. Identify tw half reactins 2. Write the mst psitive ne abve the ther 3. Draw arund them Tp is frward reactin (ANODE) Bttm is reverse reactin (CATHODE) 4. Identify xidatin r reductin 5. Slve Slid Aqueus Aqueus Slid Page 8

9 STANDARD CONDITIONS: 1 atm (gasses) 1 M (slutins) 25 C (temp) (298K) Otherwise pv = nrt ELECTROLYTIC Cnverts electrical energy int chemical ptential energy Electrde reactins sustained by a supply f electrical energy Fr the electrde t becme psitive i.e. if yu want the negatively charged in t be attracted t the electrde, attach it t the psitive terminal O e - gets sucked away frm the electrde attached t the +ve terminal. O O A aq + 2e A s Reductin B (s) B (aq) + 2e Oxidatin Electrrefining DEFINITIONS OXIDATION: LOSS OF ELECTRONS happens at the ANODE REUCTION: GAIN OF ELECTRONS happens at the CATHODE Mass f cathde Mass f ande AN OX RED CAT OIL RIG Ande Oxidatin REDuctin CAThde Oxidatin if Lst / Reductin if Gain Oxidant (Oxidising agent) causes anther t be xidised, which means it gains electrns Reductant (Recuing agent) causes anther t be reduced, which means is lses electrns. Page 9

10 STANDARD HYDROGEN ELECTRODE H 2 O H + + OH - 2H + + 2e - H 2(g) OH - + Na + NaOH Page 10

11 ORGANIC CHEMISTRY The chemistry f carbn NAMING ORGANIC MOLECULES (IUPAC NAMES) *Alphabetical Preference N. f Carbn Name Structure 1 Meth- 2 Eth- 3 Prp- 4 But- 5 Pent- 6 Hex- 7 Hept- 8 Oct- 9 Nn- 10 Dec- Single Bnds between Carbn ane Duble bnds between Carbns Triple bnds between Carbns ene yne Alkanes C n H 2n+2 Alkenes C n H 2n Alkynes C n H 2n 2 Page 11

12 When finding the name f a mlecule, 1. Identify the lngest chain f carbns Lcate 3 any duble r triple bnds 3. Lcate any additinal substituents 4 There s nly ne Carbn Meth- There s 2 chlrines dichlr- It s single bnds -ane Dichlrmethane Alkane + O2 H2O + CO2 Δ H < 0 (exthermic) DIENES Cnjugated Separated by a single bnd Cumulated Tw duble bnds t ne carbn hex-2,4-diene Islated but-2,3- diene Mre than ne saturated carbn bnds between tw duble bnds but-1,4-diene Hmglus Series a grup f cmpunds with the same general frmula 2 Gives yu the rt (meth-, eth- ) 3 Numbered psitin 4 Smething extra (clhr-) Page 12

13 Ismers different structure, but same empirical frmula The lnger the chain f Carbn, the higher the fixed pints (melting and biling pints) Strnger inter-mlecular frces will have high biling pints (mre energy t break bnds) Functinal series duble r triple bnds are functinal grups. They are add-ns that change the chemistry f a carbn chain ALCOHOLS Suffix -l Methanl Ethanl Hex-3,3-dil TYPES OF ALCOHOLS Primary Alchl Secndary Alchl Tertiary Alchl The number f carbns surrunded by the hydrxide determines the type f alchl. Page 13

14 ALDEHYDES Methanal Butanal Suffix -al KETONES Prp-2-ne Hex-2,3-dine Suffix -ne CARBOXYLIC ACID Methanic acid Pentanic acid Suffix: -ic acid HALOALKANES Prefix: hal- chlr- flur- ESTERS Where X is any halgen (Chlrine, Flurine, Brmine, etc) An ester is the prduct f an alchl and a carbxylic acid Dne under acidic cnditins Flurethane 2-brm-1,3-dichlrpentane Tetrachlrmethane Alchl + carbxylic acid ester + water Ethanl + Butanic acid butyl ethanate + water Methanl + Hexanic acid hexyl methanate Saturated alkane bnds, where carbns make ne bnd nly Unsaturated carbns have mre than ne bnd Page 14

15 ADDITION REACTIONS Unsaturated cmpunds underg additin reactins HYDROHALOGENATION HX + alkene Hydrchlric acid + but-1-ene butyl-1-chlride + HALOGENATION X2 + alkene F2 + But-1-ene butyl-1,2-difluride + HYDRATION H2O + alkene Alchl But-1-ene + water butan-1-l + HYDROGENATION H2 + alkene Alkane But-1-ene + Hydrgen gas butane + ELIMINATION REACTIONS Saturated cmpunds underg eliminatin reactins DEHYDROHALOGENTAION DEHYDRATION Eliminatin f HX frm a halalkane Heat under reflux in a cncentrated slutin f NaOH r KOH Eliminatin f H2O frm an alchl Heating f alchl with excess cncentratin Or lw pressure and temperature with catalyst SUBSTITUTION REACTIONS INTERCONVERSION Tertiary alchls nly Just add HX t prduce a halalkane CRACKING H2SO4 Breaking up large hydrcarbn mlecules int smaller and mre useful bits High pressure and temperature withut catalyst Reactins f bases with halalkanes (hydrlysis) prduces alchls Halalkanes frm alkanes Page 15

16 THE CHEMICAL INDUSTRY SASOL Use Cal Gasificatin t prduce liquid fuels HYDROCRACKING Hydrcracking is a cracking prcess that is assisted by the presence f an elevated partial pressure f hydrgen gas. It prduces chemical prducts such as ethane, LPG, isparaffins, jet fuel and diesel. STEAM CRACKING Steam cracking ccurs under very high temperatures. During the prcess, a liquid r gaseus hydrcarbn is diluted with steam and then briefly heated in a furnace at a temperature f abut 850. Steam cracking is used t cnvert ethane t ethylene. Ethylene is a chemical that is needed t make plastics. Steam cracking is als used t make prpylene, which is an imprtant fuel gas. FRACTIONAL DISTILLATION Fractinal distillatin is the separatin f a mixture int its cmpnent parts, r fractins. Since air is made up f a number f gases (with the majr cmpnent being nitrgen), fractinal distillatin can be used t separate it int these different parts THE CHLORALKALI INDUSTRY Prductin f Chlrine and Sdium Hydrxide Thrugh the electrlysis f sdium chlride (NaCl) saturated in the frm f brine. USES Chlrine is used t purify watrer Cleaning agent The membrane cell: Page 16

17 THE FERTILISER INDUSTRY A fertiliser is a cmpund that is given t a plant t prmte grwth. Fertilisers usually prvide the three majr plant nutrients and mst are applied via the sil s that the nutrients are absrbed by plants thrugh their rts. NPK Rati is the percentage f nitrgen, phsphrus and ptassium in the fertiliser MANUFACTURING Ammnia Haber Prcess Nitric Acid Ostwald Prcess Sulphuric Acid Cntact prcess Pg 195 Everything Science. BATTERIES LECLANCHÉ DRY CELL Zinc-carbn dry cell The electrlyte cnsists f an ammnium chlride paste Electrdes are made f zinc and carbn Zinc frms the ande Manganese frms the cathde Pwdered carbn is used in the cathde t imprve cnductivity and retain misture Ande: Zn Zn e Cathde: 2NH MnO 2 + 2e Mn 2 O 3 + 2NH 3 + H 2 O ZINC CHLORIDE DRY CELL Zinc-carbn dry cell Electrlyte is zinc chlride Manganese frms the cathde ALKALINE DRY CELL Ptassium hydrxide as the electrlyte Lasts much lnger than dry cells MERCURY BATTERY Small battery used in watches Zinc ande Steel cathde LITHIUM ION BATTERY Lithium ins mve back and frth between psitive and negatie electrdes Can be recharged LEAD-ACID STORAGE BATTERY 12V used in cars Cntains six cells cnnected in series Led ande Lead xide cathde Page 17

18 DATA BOOKLET Standard pressure P θ 1,013 x 10 5 Pa Mlar gas vlume at STP Vm 22,4 dm 3 ml -1 Standard temperature T θ 273 K m n M n c r V m c MV q = I t E θ cell E θ cathde E θ ande E θ cell E θ reductin E θ xidatin W = Vq E θ cell E θ xidisingagent E θ reducing agent Page 18