National 5 Chemistry. Unit 2 Nature s Chemistry

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1 Natinal 5 Chemistry Unit 2 Nature s Chemistry Tpic 1 Hydrcarbns and hmlgus series Page 1 Minitest 8 2 Energy frm Fuels 9 Minitest 13 3 Cnsumer Prducts 14 Minitest 18 Material surced frm BBC Bitesize Revisin Website

2 1 Hydrcarbns and hmlgus series A hmlgus series is a family f hydrcarbns with similar chemical prperties wh share the same general frmula. We will lk at three series: alkanes, alkenes and the cyclalkanes. a) Alkanes The first hmlgus series is the alkanes. Their names all end in ane. The alkanes have many uses: methane (natural gas) cking, heating prpane used in gas cylinders fr BBQ etc ctane used in petrl fr cars The general frmula f the alkanes is. The names, mlecular frmula and the structural frmula f the first eight alkanes must be learned. Using a mnemnic can make learning the names easier. Methane CH4 Mnsters Ethane C2H6 Eat Prpane C3H8 Pupils Butane C4H10 But Pentane C5H12 Prefer Hexane C6H14 Hairy Heptane C7H16 Haggis Octane C8H18 Occasinally Natinal 5 Chemistry Revisin Page 1 Unit 2 Nature s Chemistry

3 Naming branched chain alkanes There are sme general rules which yu shuld remember when naming rganic cmpunds: Example The lngest unbranched chain cntaining the functinal grup is the parent mlecule, r simply the lngest unbranched chain fr alkanes. Remember that, the lngest chain can g rund a bend. Indicate the psitin f the functinal grup with a number, numbering frm the end nearest the functinal grup. Name the branches, and indicate the number f branches. Methyl indicates there is ne carbn atm in the branch. Ethyl indicates there are tw carbn atms in the branch. The prefix 'di' indicates there are tw branches. The prefix 'tri' indicates there are three branches. Indicate the psitin f the branches with a number, numbering frm the end nearest the functinal grup. Fr mre than ne branch, the branches are identified in alphabetical rder, ignring any 'di', 'tri', etc, prefixes. Each branch needs t be numbered individually, even if they are attached t the same carbn atm. The rule is a cmma between numbers, and a dash between numbers and letters. The alkanes dn't cntain a functinal grup and s the branches are numbered frm the end that gives the lwest set f psitin numbers fr the branches. Use the abve rules t see hw the names f the alkanes belw are built up. Natinal 5 Chemistry Revisin Page 2 Unit 2 Nature s Chemistry

4 b) Alkenes The secnd hmlgus series is the alkenes. Their names all end in ene. Alkenes all cntain a carbn t carbn duble bnd which makes them mre reactive than the alkanes. The alkenes have the general frmula. The alkenes are unsaturated. This means that they have a carbn t carbn duble bnd. The alkanes are saturated because they nly cntain single bnds. An unsaturated hydrcarbn declurises brmine water quickly. Alkanes d this slwly. Alkenes can be used as fuels, slvents and starting materials fr everyday prducts such as plastics and vinegar. Naming alkenes Naming alkenes has the same rules as alkanes. Hwever, the psitin f the carbn t carbn duble bnd must be identified. Example 3-methylbut-1-ene Nte that the functinal grup (in the case f alkenes, the duble bnd) is given the lwest pssible number first, befre any branches are numbered. Natinal 5 Chemistry Revisin Page 3 Unit 2 Nature s Chemistry

5 c) Cyclalkanes The third hmlgus series is the cyclalkanes. Their names all end in ane and begin with cycl-, fr example cyclprpane. The cyclalkanes have the general frmula. Even thugh they have the same general frmula as the alkenes, the cyclalkanes are saturated and cntain nly single bnds. This means that they d nt quickly declurise brmine slutin. Many cyclalkanes are used in mtr fuel, natural gas, kersene, diesel and ther heavy ils. Natinal 5 Chemistry Revisin Page 4 Unit 2 Nature s Chemistry

6 d) Cmbustin reactins All hydrcarbns (including alkanes, alkenes and cyclalkanes) can underg cmbustin reactins with xygen t give the same tw prducts. Hydrcarbn fuels burn when they react with xygen in the air. As all hydrcarbns nly cntain the elements carbn and hydrgen, the nly prducts will be xides f these elements. S as lng as enugh xygen is present fr cmplete cmbustin, the tw prducts frmed will be carbn dixide (CO2) and water (H2O). The wrd equatin fr this reactin is shwn belw. The prducts frm the cmbustin f hydrcarbn fuels can be identified with the fllwing set up in the lab. Carbn dixide gas turns limewater frm clurless t milky white. In the abve experiment, the limewater in the biling tube changes clur indicating carbn dixide is prduced by cmbustin f the hydrcarbn fuel. If water is prduced it will cndense in the U-tube cled by the ice water. This results in cbalt chlride paper changing the clur frm blue t pink. Incmplete cmbustin (when there is an insufficient supply f xygen) culd lead t txic carbn mnxide (CO) being prduced instead f carbn dixide. Natinal 5 Chemistry Revisin Page 5 Unit 2 Nature s Chemistry

7 e) Additin reactins Alkenes are mre reactive than alkanes and cyclalkanes because they have a duble bnd. While alkanes, alkenes and cyclalkanes underg cmbustin reactins with xygen, nly alkenes can participate in additin reactins. In an additin reactin, the duble bnd f the alkene partially breaks when the reactant mlecule attacks and adds n acrss it. Example Additin f brmine water can be used as a test fr unsaturatin. Alkenes quickly turn brmine water clurless, while alkanes and cyclalkanes d nt. The experiment shwn belw is an example f hw t distinguish between alkanes and alkenes by adding brmine water. The additin f brmine t an alkene is called brminatin. Other small mlecules can be added acrss duble bnds in alkenes. If hydrgen is added, then the crrespnding alkane mlecule is frmed. Example The additin f hydrgen t an alkene is called hydrgenatin. If water is added t an alkene, then an alkanl (alchl) is prduced. The additin f water t an alkene is called hydratin. Natinal 5 Chemistry Revisin Page 6 Unit 2 Nature s Chemistry

8 f) Ismers Ismers are cmpunds with the same mlecular frmulae but different structural frmulae. They have the same number f each type f atm but may have different physical and chemical prperties. This diagram shws butane and methylprpane. (Methylprpane used t be called is-butane, but the crrect name is methylprpane). Bth have a mlecular frmula f. The structural frmulae are different thugh, s they are ismers f each ther. Natinal 5 Chemistry Revisin Page 7 Unit 2 Nature s Chemistry

9 Hydrcarbns and hmlgus series Minitest 1 Which f the fllwing is a hydrcarbn? 2 What is the frmula fr ctane? 3 Which f the fllwing culd be the frmula fr an alkene? 4 Which f the fllwing hydrcarbns wuld rapidly declurise brmine slutin? Prpene Prpane Cyclprpane 5 Which f these mlecules is saturated? 3-methyl pentane hex-2-ene 2-methylbut-2-ene 6 What is the name fr the fllwing mlecule? 3-methylpentane 3-methylhexane 2-ethylbutane 7 What is the general frmula fr cyclalkanes? 8 Which f the fllwing mlecules is an ismer f heptane? 9 What type f reactin des this equatin represent? Cmbustin Hydrlysis Additin 10 Which f the fllwing is the shrtened structural frmula fr butene? Natinal 5 Chemistry Revisin Page 8 Unit 2 Nature s Chemistry

10 a) Cmbustin reactins 2 Energy frm fuels Many different cmpunds can be used as fuels, mst cmmnly alkanes and alchls. When these fuels burn, they release heat energy and light energy t the surrundings in exthermic reactins. Endthermic reactins take in heat energy. These reactins are knwn as cmbustin reactins and can be represented by different equatins. Wrd equatin Fr example, prpane gas tanks are cmmnly sld n petrl statin frecurts. These gas bttles have a wide variety f uses including prtable heating and cking fr caravans and barbecues. When prpane burns in a plentiful supply f xygen, it prduces bth carbn dixide and water. This change can be represented by a wrd equatin r a frmula equatin. Balanced frmulae equatin A balanced frmulae equatin, alng with the frmula triangles yu learned in Unit One can be used t calculate infrmatin n the quantities f reactants r prducts invlved in a particular reactin. Natinal 5 Chemistry Revisin Page 9 Unit 2 Nature s Chemistry

11 Examples Example ne Calculate the number f mles f xygen required fr the cmplete cmbustin f 132g f prpane. Step 1 Start with the balanced equatin. Step 2 Use the infrmatin frm the questin t calculate hw many mles f the knwn reactant r prduct are actually used. Step 3 Use the balanced equatin t wrk ut hw many mles f the reactant r prduct yu are asked t calculate this equates t. Frm the equatin, 1 mle f prpane will react with 5 mles f xygen. Natinal 5 Chemistry Revisin Page 10 Unit 2 Nature s Chemistry

12 Example tw If 32g f methane gas are burned in a plentiful supply f xygen, calculate the mass f carbn dixide prduced. Step 1: Start with the balanced equatin Step 2: Use the infrmatin frm the questin t calculate hw many mles f the knwn reactant r prduct are actually used. Calculating the frmula mass f methane Step 3: Use the balanced equatin t wrk ut hw many mles f the reactant r prduct yu are asked t calculate this equates t. Frm the equatin, 1 mle f methane will prduce 1 mle f carbn dixide. S; Step 4 Be careful - this questin asked fr the mass f carbn dixide, nt the number f mles. Use the frmula triangle nce mre t cnvert this number f mles t a mass. Natinal 5 Chemistry Revisin Page 11 Unit 2 Nature s Chemistry

13 b) Calculating the energy released when fuels burn Sme fuels are better than thers at releasing energy during cmbustin reactins. The energy released (smetimes called enthalpy) is calculated using the fllwing equatin: = the heat energy released (in kj r kj ml -1 ) = the specific heat capacity f water. It is a cnstant, 4.18 kj kg -1 C -1 = the mass f water (in kg) (100cm 3 = 0.1kg) = the change in temperature ( C) This frmula is very easy t recall if yu remember that "Every huse can't make triangle tast!" Example Questin : A spirit burner was filled with ethanl (C2H5OH) and used t heat a cpper can cntaining 200 cm 3 f water. The temperature f the water rse frm 22 C t 27 C. Calculate the heat energy released frm this cmbustin f ethanl. Natinal 5 Chemistry Revisin Page 12 Unit 2 Nature s Chemistry

14 Energy f fuels Minitest 1 In the equatin, what des c represent? The mass f liquid The specific heat capacity The change in temperature 2 What are the prducts f cmbustin reactins when hydrcarbns are burned in a plentiful supply f xygen? Carbn dixide and water Carbn mnxide and water Carbn (st) and water 3 If 48 grams f methane (frmula mass 16 g) is burned, what mass f carbn dixide (frmula mass 44 g) will be prduced? 132 grams 88 grams 44 grams 4 When a sample f methane was burned, the heat prduced raised the temperature f 100 cm 3 f water frm 20.2 C t 29.3 C. Calculate the heat energy released by this reactin kj 3.8 kj 3803 kj 5 A sample f methanl was burned, and the heat used t warm 100cm 3 f water kj f heat energy was prduced by the reactin. What will be the temperature increase in the water? 15 C 17 C 19 C 6 Which equatin shws the crrectly balanced equatin fr the cmbustin f pentane? 7 If ne mle f prpane is burned, hw many mles f water will be prduced? One Tw Fur 8 Methanl was burned in a spirit burner and raised the temperature f 800cm 3 f water frm 20 C t 85 C. Use these values t calculate hw much heat energy was released? 2.17 kj kj kj 9 What is the temperature increase when butane is burned, releasing kj f heat energy that is used t heat 1500cm 3 f water? 20 C 25 C 30 C 10 In an experiment, prpan-1-l was burned, releasing 6.688kJ f heat energy. The heat released was used t heat 200 cm 3 f water which was initially 23 C. What was the temperature f the water after the experiment? 8 C 25 C 31 C Natinal 5 Chemistry Revisin Page 13 Unit 2 Nature s Chemistry

15 a) Alchls 3 Cnsumer prducts Alchls, als knwn as alkanls, have the hydrxyl functinal grup OH. They are a hmlgus series and have the general frmula. Their names all end in l. The rules fr naming an alchl are the same as befre. The hydrxyl functinal grup is the mst imprtant part f the mlecule and the carbn chain is numbered t give it the lwest branch number. Example Butan-2-l The psitin f the functinal grup and any branches must be identified. Alchls have a wide variety f uses. While ethanl is mst famus fr its presence in alchlic drinks, many ther alchls exist and are cmmnly used as fuels and slvents. Prpan-2-l is widely used in hand gels and disinfectant wipes. Alchls are particularly useful as fuels due t their high flammability and the fact that they burn cleanly. Natinal 5 Chemistry Revisin Page 14 Unit 2 Nature s Chemistry

16 b) Carbxylic acids Carbxylic acids, als knwn as alkanic acids, all cntain the carbxyl functinal grup COOH. Their names all end in ' ic acid'. When naming carbxylic acids, the psitin f the carbxyl grup des nt need t be identified with a number, as it is always fund at the end f the carbn chain, and s is n the first carbn. Butanic acid One f the mst widely used carbxylic acids is vinegar. Als knwn as ethanic acid, it has far mre uses than simply being added t chips, and is cmmnly used in chemicals used t treat limescale in bathrms r inside kettles. Many fds are als preserved in vinegar (pickled) t make them last fr lnger perids f time. Lts f ther carbxylic acids have less pleasant smells than vinegar and sme are highly crrsive s must be handled with care. Natinal 5 Chemistry Revisin Page 15 Unit 2 Nature s Chemistry

17 c) Esters Esters are frmed by the cndensatin reactin between an alchl and a carbxylic acid. This is knwn as esterificatin. Esters have characteristic smells and are insluble in water. They are cmmnly used as fragrances and perfumes. ( means a reversible reactin) They have the functinal grup: (R means rest f the mlecule.) The functinal grup (-COO) is knwn as the ester link. Naming esters T name an ester: Change the alchl name t end in yl. Change the acid name t end in ate. Alchl name ges t the frnt, acid name t the back. Example The ester prpyl ethanate which is respnsible fr the distinct pear drp smell assciated with the sweet can be made by the cndensatin reactin between prpanl and ethanic acid. Esters can be made in the lab using apparatus illustrated in the diagram belw. Natinal 5 Chemistry Revisin Page 16 Unit 2 Nature s Chemistry

18 Cnsumer Prducts Minitest 1 What is the name f this carbxylic acid? 5 Which f these mlecules is an ismer f butan-2-l? Prpanic acid Ethanic acid Pentanic acid 2 Which f the fllwing is nt a use f alchls? Fuels Flavurings Slvents 3 Which f these structures shws the ester ethyl methanate? 6 Which f these reasns is nt a cnsideratin when handling many carbxylic acids? They are strng acids They are flammable They are crrsive 7 What is the catalyst used t prduce esters in the lab? Dilute hydrchlric acid Sdium hydrxide Cncentrated sulfuric acid 4 What type f reactin is this an example f? Additin Hydrlysis Cndensatin 8 What is the name f the ester frmed when butanl and ethanic acid react tgether? ethyl butanic acid ethyl butanate butyl ethanate 9 Ethanl can be dehydrated using a catalyst. Which f the fllwing culd be prduced during this reactin? Ethane Ethene Ethanic acid Natinal 5 Chemistry Revisin Page 17 Unit 2 Nature s Chemistry