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What is a chemical reaction?

What is a chemical reaction?. Can you define a ‘chemical reaction’? What do the following words mean: ENDOTHERMIC EXOTHERMIC. Can you define a ‘chemical reaction’? A  chemical reaction  is a process that leads to the transformation of one set of  chemical substances  to another.

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What is a chemical reaction?

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  1. What is a chemical reaction?

  2. Can you define a ‘chemical reaction’? • What do the following words mean: • ENDOTHERMIC • EXOTHERMIC

  3. Can you define a ‘chemical reaction’? • A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. • What do the following words mean: • ENDOTHERMIC – takes in heat • EXOTHERMIC – gives out heat

  4. Examples • Endothermic reaction citric acid + sodium bicarbonate sodium citrate + carbon dioxide + water • Exothermic reaction magnesium + hydrochloric acid magnesium chloride + hydrogen

  5. Collision theory • For a chemical reaction to occur, the reactant particles must collide. But collisions with too little energy do not produce a reaction. • The particles must have enough energy for the collision to be successful in producing a reaction. • The rate of reaction depends on the rate of successful collisions between reactant particles. The more successful collisions there are, the faster the rate of reaction.

  6. Chemical Reactions – whatneedsto happen? • First of all chemicalbondsneedtobebroken. • Breakingbondsisendothermic – theprocessneedsenergy. • Thennewbondsareformed • Making bondsisexothermic – theprocessgives out energy.

  7. CombustionReactions Whenmethanereactswithoxygenthe total energygiven out isgreaterthanthe total energytaken in. So overalthereactiongives out energy. Itis an exothermicreaction. The bonds in theproductsarestrongerthanthebonds in thereactants. The productsaremorestable.

  8. ExampleofEndothermicReactions • Endothermicreactionsarelesscommon. • Energyisabsorbedfromthesurroundings. • The reactionofnitrogenandoxygenisendothermic. • The bondingoftheproductsisweakerthanthatofthereactants. • Overalenergyistaken in. • Photosynthesisis also endothermic.

  9. Exothermic reactions • Most reactions require some energy to get started (activation energy); • Exothermic reactions give out more energy than is put in:

  10. Endothermic reactions • Endothermic reactions take in more energy than they give out:

  11. HeatofReaction • The energychange in goingfromthereactantstotheproducts in a chemicalreactionisknownastheheatofreaction ∆H (kJ) 1 kilojoule (1KJ) = 1000 joules (1000J) The ∆H of a reactionisusuallycalcualted per mole(KJ/mol)

  12. HeatofReaction∆H • Forexothermicreactions∆H is negative, heatisgiven out. • Forendothermicreactions∆H is positive, heatistaken in.

  13. Bond Energy • Experiments havebeencarried out to find out howmuchenergyisneededto break variousbonds. Bond Energy

  14. Using Bond Energytocalculate∆H Bond Breaking Bond Making 2 C=O bonds 2 x 803 = 1606 4 O-H bonds 4 x 464 = 1856 Total = 3462 KJ/mol 4 C-H bonds 4 x 435 = 1740 2 O=O bonds 2 x 498 = 996 Total = 2736 KJ/mol ∆H = energydifference ∆H = energyto break bonds – energytomakebonds ∆H = 2736 – 3462 ∆H = -726 KJ/mol

  15. ActivationEnergyEA • Even thoughmostreactionsareexothermiconly a fewarespontanious. • Eg. Na or K reactionwithwater • Most reactionsneedsomeenergytogetthemgoing. • Fuelsburningneed a sparktoignitethem. • Itisrequiredassomebondsneedtobebrokenbeforethereactioncantakeplace. • ThisistheactivationenergyEA

  16. Task: • The reaction between oxygen and hydrogen is extremely exothermic. • Write the balanced symbol equation for the reaction. • Why isn’t there an explosion when the 2 gases are mixed? • In Wales it is cold and wet; in Borneo it is hot and wet. Where do you think a car will rust faster, Wales or Borneo – explain your answer. • Do questions on page 204

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