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. 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!

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 24dm 3 at room temperature and pressure 2Na (s) + 2H 2 0 (l)  2NaOH (aq) + H2 (g)

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 x 24 = 2.4dm 3

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 +413O=O +498 C=O +805 C-C +347H-O +464 You must write a balanced equation first

Method C 2 H O 2  2CO 2 + 3H 2 O Bonds BrokenBonds Formed 1(C-C)1x3474(C=O) 4x805 6(C-H)6x4136(H-O) 6x (O=O) 3.5x498 Total=4568Total=6004

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

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)

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 (CH 4 ) = -890 kJ/mol ΔHc (C) = -393 kJ/mol ΔHc (H 2 ) = -286 kJ/mol

Your Hess Cycle

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

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

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

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

Identify families of compounds A)C 2 H 5 OH D) C 4 H 10 B)C 6 H 6 E) CH 3 OCH 3 C) C 6 H 12 F) C 6 H 5 Cl Which is/are aromatic? Which could be a cycloalkane? What else could this compound be? Which is the ether & which the alkane?

The answers A)C 2 H 5 OH D) C 4 H 10 B)C 6 H 6 E) CH 3 OCH 3 C) C 6 H 12 F) C 6 H 5 Cl 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

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

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!!