Focus 5.10 Reactions

Chemical reactions>>>12 Ethane gas burns according to the chemical equationbelow:C 2 H 4 (g) + 3O 2 (g) → 2CO 2 (g) + 2H 2 O (g)a Write a word equation for this reaction.b What is meant by the symbol ‘(g)’ found after thereactants and products?c Suggest a reason for the numbers (coefficients) foundin front of the oxygen, carbon dioxide and water inthis equation.Rates of chemical reactions13 What is meant by the term ‘rate of chemical reaction’?Explain your answer by giving an example of a chemicalreaction that occurs very quickly and one that occursquite slowly.14 List the five ways of increasing the rate of a chemicalreaction.Exothermic and endothermic chemicalreactions15 Describe how you would recognise an exothermic andan endothermic chemical reaction.Use your head16 Identify the products produced by the followingchemical reactions:a strontium carbonate (SrCO 3 ) is heatedb hydrochloric acid is added to strontium carbonatec nitric acid is reacted with strontium hydroxide(Sr(OH) 2 )d hydrochloric acid is reacted with strontium hydrogencarbonate (Sr(HCO 3 ) 2 )e pentane (C 5 H 12 ) is burnt.17 Explain the following observations in terms of reactionrate:a If iron filings are sprinkled into a Bunsen burner flamethey will burn rapidly but if you place an iron nail intoa flame it will simply get hotter and glow red.b In cars the pistons are designed to compress thepetrol and oxygen mixture together in the cylinderbefore it is ignited by the spark plug.c An oven cleaner product has the followinginstructions on the spray pack tin: ‘If the oven iswarm, spray the cleaner on the grease and leave for5 minutes before wiping off. If the oven is cold, leavefor 30 minutes before wiping off.’d Some students are producing hydrogen gas byreacting zinc metal with hydrochloric acid. Theynotice that hydrogen gas is produced faster whenthey use a more concentrated acid.Investigating questions18 Water at 20°C was used to dissolve two differentcompounds. The initial temperature of the water wasrecorded and as soon as the compounds dissolvedthe final temperature of the solution was recorded. Theresults are shown in the table below:CompoundInitial watertemperatureTemperature (°C)Final solutiontemperatureNH 4 Cl 20 12CaCl 2 20 35a Many ionic compounds ‘dissociate’ into their ionswhen they are dissolved in water. This is called a‘dissociation reaction’ and can be shown as anequation. The dissociation equation of sodiumchloride is:NaCl (s) → Na+ (aq) + Cl – (aq)Write a dissociation equation for the two ionicsubstances in this investigation.b Which compound gave out heat energy when itdissolved, and how do we know?c Would dissociation of NH 4 Cl beexothermic or endothermic?d Which compound absorbed heat energywhen it dissolved and how do we know?e Would dissociation of CaCl 2 be exothermic DYOor endothermic?f You might like to try the activity yourself.Explore the effects of variables such asvolume of water or amount of solute.DYO19 In this Focus the chemical decomposition ofhydrogen peroxide to oxygen gas and water, usinga manganese dioxide catalyst, was mentioned.2H 2 O 2 (l) catalyst MnO 2 → O 2 (g) + 2H 2 O (l)a Investigate the use of this catalyst by setting upa catalysed and a non-catalysed reaction of a10 per cent solution of hydrogen peroxide.b Other substances can also catalyse this reaction.Once such substance is a solution of iodide ion(I – ). Explore this by designing an appropriateinvestigation. Check this with your teacher first.c An enzyme called catalase is found in vegetablessuch as potato. Explore the action of the catalaseenzyme on the decomposition of hydrogen peroxideby adding a small slice of potato to your hydrogenperoxide solution.266