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Double bonds or not

Double bonds or not. A saturated fat has no C=C double bonds (alkene functional groups) and is usually a solid fat like margarine or animal fat. An unsaturated fat has C=C double bonds and is usually an oil like vegetable oil . Example question. Mark scheme. Making margarine.

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Double bonds or not

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  1. Double bonds or not • A saturated fat has no C=C double bonds (alkene functional groups) and is usually a solid fat like margarine or animal fat. • An unsaturated fat has C=C double bonds and is usually an oil like vegetable oil.

  2. Example question

  3. Mark scheme

  4. Making margarine • To make margarine we have to saturate vegetable oil by bubbling hydrogen gas through the oil. • This process is called hydrogenation

  5. Is a fat or oil saturated or not? • We can test for this by adding bromine water. • If there are double bonds present the bromine water changes from orange/brown to colourless.

  6. Example question

  7. Mark scheme

  8. Hydrolysis • When an ester is hydrolysed it goes back to an acid and alcohol • We can hydrolyse by adding acid or alkali (NaOH).

  9. Example question

  10. Mark scheme

  11. Energy changes in chemistry C7.2

  12. Quiz • When a chemical reaction takes place heat may be given out or taken in. • Can you remember the word we use when heat is given out? • Can you remember the word we use when heat is taken in?

  13. What do I need to know? • Recall and use the terms ENDOTHERMIC and EXOTHERMIC • Describe examples of ENDOTHERMIC and EXOTHERMIC reactions. • Use simple energy level diagrams to represent ENDOTHERMIC and EXOTHERMIC reactions.

  14. Change in energy • Chemical reactants have a certain amount of chemical energy stored within them. • When the reaction has taken place they have either more or less energy stored within them than before.

  15. Definitions When heat is given out (exothermic) then the products have less energy than they did before. They have lost it to the surroundings. When heat is taken in (endothermic) then the products have more energy than they had before. They have taken it from the surroundings.

  16. Energy level diagrams Which diagram do you think is endothermic and which is exothermic? Heat given out Heat taken in

  17. Energy level diagrams Endothermic Exothermic Heat given out Heat taken in Energy level of products is higher than reactants so heat taken in. Energy level of products is lower than reactants so heat given out.

  18. Example question

  19. Mark scheme

  20. Bond enthalpies C7.2

  21. Quick quiz • Reactions where the products are at a lower energy than the reactants are endothermic (TRUE/FALSE) • Activation energy is the amount of energy given out when a reaction takes place (TRUE/FALSE) • A reaction which is exothermic transfers heat energy to the surroundings (TRUE/FALSE) • How can we tell if a reaction is exothermic or endothermic? • Sketch the energy profile for an endothermic reaction. • When methane (CH4) burns in oxygen (O2) bonds between which atoms need to be broken?

  22. Answers • Reactions where the products are at a lower energy than the reactants are endothermic (TRUE/FALSE) • Activation energy is the amount of energy given out when a reaction takes place (TRUE/FALSE) • A reaction which is exothermic transfers heat energy to the surroundings (TRUE/FALSE) • How can we tell if a reaction is exothermic or endothermic? • Sketch the energy profile for an endothermic reaction. • When methane (CH4) burns in oxygen (O2) bonds between which atoms need to be broken? FALSE FALSE TRUE Measure the temperature change C—H bonds and O=O bonds

  23. What do I need to know? 1. Recall that energy is needed to break chemical bonds and energy is given out when chemical bonds form 2. Identify which bonds are broken and which are made when a chemical reaction takes place. 3. Use data on the energy needed to break covalent bonds to estimate the overall energy change for a reaction.

  24. Activation energy revisited • What is the activation energy of a reaction? • The energy needed to start a reaction. • BUT what is that energy used for and why does the reaction need it if energy is given out overall? • The activation energy is used to break bonds so that the reaction can take place.

  25. Burning methane Consider the example of burning methane gas. CH4 + 2O2 CO2 + 2H2O This reaction is highly exothermic, it is the reaction that gives us the Bunsen flame. However mixing air (oxygen) with methane is not enough. I need to add energy (a flame).

  26. What happens when the reaction gets the activation energy? C H H H H O O O O Bond Breaking Bond Forming H O Energy in chemicals O C O O O H H H H H O C O O H H Progress of reaction

  27. Using bond enthalpies By using the energy that it takes to break/make a particular bond we can work out the overall enthalpy/energy change for the reaction. Sum (bonds broken) – Sum (bonds made) = Energy change

  28. BIN MIX Breaking bonds is ENDOTHERMIC energy is TAKEN IN when bonds are broken Making bonds is EXOTHERMIC energy is GIVEN OUT when bonds are made.

  29. Bond enthalpies

  30. Can you work out the energy change for this reaction? CH4 + Cl2 CH3Cl + HCl Tip: Draw the reactants and products and work out the bonds you are breaking and the ones you are making.

  31. The answer is -120 kJ

  32. Example question part 1

  33. Question part 2

  34. Question part 3

  35. Mark scheme

  36. Challenge question • The true value for the energy change is often slightly different from the value calculated using bond enthalpies. • Why do you think this is?

  37. Example question The calculated value is 120 kJ

  38. Mark scheme

  39. Definitions Write each of these phrases in your book with a definition in your own words: • Exothermic reaction • Endothermic reaction • Activation energy • Catalyst • Bond energy/enthalpy

  40. How did you do? Exothermic reaction A reaction which gives energy out to the surroundings. Endothermic reaction A reaction which takes in energy from the surroundings. Activation energy The energy required to start a reaction by breaking bonds in the reactants Catalyst A substance that increases the rate of a reaction by providing an alternative pathway with lower activation energy. It is not used up in the process of the reaction Bond energy/enthalpy The energy required to break a certain type of bond. The negative value is the energy given out when that bond is made.

  41. Popular exam question • Explain why a reaction is either exothermic or endothermic? -------------------------------------------------------------------- -------------------------------------------------------------------- -------------------------------------------------------------------- -------------------------------------------------------------------- --------------------------------------------------------------------

  42. Popular exam question • Explain why a reaction is either exothermic or endothermic? • In a chemical reactions some bonds are broken and some bonds are made. • Breaking bonds takes in energy. • Making bonds gives out energy. • If the energy given out making bonds is higher than the energy needed to break them the reaction is exothermic. • If the energy needed to break bonds is higher than the energy given out making them the reaction is endothermic.

  43. Chemical Equilibria C7.3 Reversible Reactions & Dynamic Equilibria

  44. What do I need to know? • State that some chemical reactions are reversible • Describe how reversible reactions reach a state of equilibrium • Explain this using dynamic equilibrium model.

  45. Reversible or not reversible Until now, we were careful to say that most chemical reactions were not reversible – They could not go back to the reactants once the products are formed.

  46. Example In the case of the vast majority of chemical reactions this is true, the reaction of methane and oxygen for example: CH4(g) + O2(g)   CO2(g) + 2H2O(l) It is almost impossible to return the carbon dioxide and water to the original methane and oxygen.

  47. Reversible • Some chemical reactions, however, will go backwards and forwards depending on the conditions. • CoCl2·6H2O(s)  CoCl2(s) + 6H2O(l) pinkblue

  48. How do we write them down? • This is the symbol for used for reversible reactions. CoCl2·6H2O(s) CoCl2(s) + 6H2O(l)

  49. What is equilibrium? • Reversible reactions reach a balance point, where the amount of reactants and the amount of products formed remains constant. • This is called a position of equilibrium.

  50. Dynamic Equilibrium. • In dynamic equilibrium the forward and backwards reactions continue at equal rates so the concentrations of reactants and products do not change. • On a molecular scale there is continuous change. • On the macroscopic scale nothing appears to be happening. The system needs to be closed – isolated from the outside world.

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