(a) describe the meaning of enthalpy change in terms of exothermic (ΔH negative) and endothermicbr (ΔH positive) reactionsbr (b) *represent energy changes by energy profi le diagrams, including reaction enthalpy changes andbr activation energies (see 6.1(c))br (c) describe bond breaking as an endothermic process and bond making as an exothermic processbr (d) *explain overall enthalpy changes in terms of the energy changes associated with the breaking andbr making of covalent bondsbr (e) describe combustion of fuels as exothermic, e.g. wood, coal, oil, natural gas and hydrogenbr (f) describe hydrogen, derived from water or hydrocarbons, as a potential fuel for use in future, reactingbr with oxygen to generate electricity directly in a fuel cell (details of the construction and operation of abr fuel cell are not required) and discuss the advantages and disadvantages of thisbr (g) name natural gas, mainly methane, and petroleum as sources of energybr (h) describe petroleum as a mixture of hydrocarbons and its separation into useful fractions by fractionalbr distillationbr (i) name the following fractions and state their usesbr (i) petrol (gasoline) as a fuel in carsbr (ii) naphtha as feedstock for the chemical industrybr (iii) paraffi n (kerosene) as a fuel for heating and cooking and for aircraft enginesbr (iv) diesel as a fuel for diesel enginesbr (v) lubricating oils as lubricants and as a source of polishes and waxesbr (vi) bitumen for making road surfacesbr (j) describe photosynthesis as the reaction between carbon dioxide and water in the presence ofbr chlorophyll, using sunlight (energy) to produce glucose and explain how this can provide a renewablebr energy source.