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Chemistry Revision 5

Chemistry Revision 5. Developing Fuels 1. A couple of gas calculations. What volume of water vapour would be formed if I reacted 10cm 3 of oxygen with excess hydrogen? 2H 2 (g) + O 2 (g)  2H 2 O (g). The answer. 20cm 3 !! Remember equal amounts of gases occupy equal volumes .

meredith-horne
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Chemistry Revision 5

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  1. Chemistry Revision 5 Developing Fuels 1

  2. A couple of gas calculations • What volume of water vapour would be formed if I reacted 10cm3 of oxygen with excess hydrogen? • 2H2(g) + O2 (g) 2H2O (g)

  3. The answer • 20cm3!! • Remember equal amounts of gases occupy equal volumes. • Thus if all the substances in an equation are gases then because 1 mole oxygen makes 2 moles water the amount has doubled so the volume must have doubled as well!

  4. Now the other type of gas calc. • What volume of hydrogen will be made if I react 4.6g of sodium with water? • Ar Na = 23 • 1 mole of gas occupies 24dm3 at room temperature and pressure • 2Na (s) + 2H20 (l) 2NaOH (aq) + H2 (g)

  5. The answer • Now not all chemicals are gases so • Work out moles of sodium (0.2) • How many moles of hydrogen made (0.1) • Multiply moles of gas by 24 • 0.1 x 24 = 2.4dm3

  6. Next bond enthalpy calculations • Calculate the enthalpy change of combustion for the following reaction • Ethane + oxygen  water + carbon dioxide • Bond enthalpies (all kJ/Mol): • C-H +413 O=O +498 C=O +805 • C-C +347 H-O +464 • You must write a balanced equation first

  7. Method • C2H6 + 3.5O2 2CO2 + 3H2O • Bonds Broken Bonds Formed • 1(C-C) 1x347 4(C=O) 4x805 • 6(C-H) 6x413 6(H-O) 6x464 • 3.5(O=O) 3.5x498 • Total=4568 Total=6004

  8. The final answer • Enthalpy = +(Bonds + -(Bonds made) change broken) Enthalpy Change = (+4568) + (-6004) =-1436kJ Mol-1 Note it must be negative because it is a combustion reaction

  9. More on Bond enthalpies • Remember that bond breaking is endothermic and bond forming is exothermic • Comment on the relative strength and length of the C-C bond compared to C=C • (C-C = +347 kJ/Mol C=C = +612 kJ/mol)

  10. Hess cycles • Draw a Hess Cycle to link the ΔHf of methane and the ΔHc of methane and the combustion of its elements. • Use this Hess cycle to calculate for ΔHf of methane • ΔHc (CH4) = -890 kJ/mol • ΔHc (C) = -393 kJ/mol • ΔHc (H2) = -286 kJ/mol

  11. Your Hess Cycle

  12. So… • ΔHf = ΔH1 + -ΔH2 • ΔHf = (-393 + 2(-286)) + -(-890) • ΔHf = -75kJ/mol • See me for more help if needed!

  13. Measuring enthalpy changes • Draw labelled diagram of the apparatus you could use in the lab to measure the enthalpy change of combustion of ethanol

  14. The answer • 1 mark for a spirit burner, 1 mark for a copper can, with water and thermometer, and 1 mark for a draft excluder! • Give 2 reasons why the value you obtain from this experiment differs from the data book value

  15. The answer • 1- cooling losses • 2- incomplete combustion of the fuel

  16. Identify families of compounds • A) C2H5OH D) C4H10 • B) C6H6 E) CH3OCH3 • C) C6H12 F) C6H5Cl • Which is/are aromatic? • Which could be a cycloalkane? • What else could this compound be? • Which is the ether & which the alkane?

  17. The answers • A) C2H5OH D) C4H10 • B) C6H6 E) CH3OCH3 • C) C6H12 F) C6H5Cl • Which is/are aromatic? B & F • Which could be a cycloalkane? C • this compound could also be an alkene • The ether is E & the alkane is D

  18. Isomers • Define the term isomer • Draw the full structural formula of ethanol • Draw the skeletal formula of pentane • Draw and name an isomer of pentane

  19. That’s it • More in two weeks time • As you leave hand in your folder of revision notes for me to check. • Please ensure it is NAMED!!

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