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Exothermic and endothermic reactions

Exothermic and endothermic reactions. Chemical Reactions usually involve a temperature change (heat is given out or taken in). Law of conservation of energy. Energy cannot be created or destroyed , but only changed from one form into another. Magnesium reacting with acid.

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Exothermic and endothermic reactions

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  1. Exothermic and endothermic reactions

  2. Chemical Reactions usually involve a temperature change (heat is given out or taken in)

  3. Law of conservation of energy • Energy cannot be created or destroyed, but only changed from one form into another

  4. Magnesium reacting with acid Thermit reaction Exothermic Reactions Exothermic reactions increase in temperature. • Examples include: • Burning reactions including the combustion of fuels. • Detonation of explosives. • Reaction of acids with metals.

  5. 45o C 25o C magnesium Hydrochloric acid Exothermic Reactions • Magnesium + Hydrochloric acid Heat energy given out Gets hot

  6. Exothermic Reactions 45o C 25o C Reactants convert chemical energy to heat energy. The temperature rises.

  7. Exothermic Reactions • Almost immediately the hot reaction products start to lose heat to the surroundings and eventually they return to room temperature. 45o C 25o C

  8. Reactants have more chemical energy. reactants Some of this is lost as heat which spreads out into the room. Energy / kJ) Products now have less chemical energy than reactants. products Progress of reaction (time) Energy Level Diagram for an Exothermic Reaction

  9. Energy / kJ) Progress of reaction (time) Energy Level Diagram for an Exothermic Reaction reactants products

  10. Energy / kJ) Progress of reaction reactants products Exothermic Reaction - Definition Exothermic reactions give out energy. There is a temperature rise and H is negative. His negative

  11. Heat changes also happen when substances change state.

  12. An exothermic reaction • When hydrocarbons burn in oxygen they produce carbon dioxide and water vapour. • The reaction also involves the loss of heat so it is an exothermic reaction.

  13. Objectives for today.. • Define endothermic reaction • Draw an energy profile diagram for an endothermic reaction • Bond energy definition • Mandatory experiment Homework – Write up today’s experiment in experiment copy

  14. Endothermic reaction • An endothermic reaction is when heat is taken in in a reaction.

  15. Endothermic Reactions • Endothermic chemical reactions are relatively rare. • A few reactions that give off gases are highly endothermic - get very cold.

  16. Heat energy taken in as the mixture returns back to room temp. Ammonium nitrate Water Endothermic Reactions Endothermic reactions cause a decrease in temperature. Cools Starts 25°C Cools to 5°C Returns to 25°C

  17. Endothermic Reactions • The cold reaction products start to gain heat from the surroundings and eventually return to room temperature. The reactants gain energy. 25o C 5o C 25o C This comes from the substances used in the reaction and the reaction gets cold. Eventually heat is absorbed from the surroundings and the mixture returns to room temperature. Overall the chemicals have gained energy.

  18. Energy / kJ Progress of reaction products reactants Endothermic Reaction Definition Endothermic reactions take in energy. There is a temperature drop and H is positive. H=+

  19. Homework • Revision CH 1,2,3 HL 2015 Q4a,b Q5a,c 2014 Q4b, Q10b OL 2015 Q4a 2013 Q5b 2011 Q4a, 2010 Q4a 2009 Q4b 2008 Q11a

  20. Heat of reaction • The heat of reaction is the heat change when the number of moles of reactants indicated in the balanced equation react completely. For an exothermic reaction: the heat of reaction is always negative e.g ∆H = -34kJ For an endothermic reaction: the heat of reaction is always positive e.g ∆H = +34kJ

  21. Measuring the heat of reaction of hydrochloric acid and sodium hydroxide A polystyrene cup is used as it is an insulator of heat – it doesn’t let the heat escape ( has negligible heat capacity) The equation for the reaction is HCl + NaOH → NaCl + H2O

  22. Other precautions to ensure an accurate result! • Make sure both solutions are at the same temperature before you start! • Wash the thermometer and dry it before switching solutions. • Stir the mixture slowly and make sure none of the mixture is splashed out of the cup.

  23. Results

  24. Calculations – What is the heat change in our reaction: Heat change = mc ∆ T Temperature rise Mass in kilograms Specific heat capacity

  25. How many moles were reacted in our experiment?

  26. Question: What is the heat of reaction? Balanced equation: HCl + NaOH NaCl + H2O 1 1 1 1 0.1 moles of HCl released ________of heat 1 mole of HCl would release_________ of heat The heat of reaction = - J (or – kJ) The negative sign is because the reaction is exothermic ( heat is given out – the temperature went up!)

  27. Q260. Calculations for the heat of reaction • 261. Calculate the heat of reaction (using the formula ΔH = mcΔT) for the reaction between and nitric acid sodium hydroxide from the following experimental results: • Volume of nitric acid = 100 cm3 of 1.0 M • Volume of sodium hydroxide = 100 cm3 of 1.0 M • Initial temperature of the solutions = 17.5 oC • Final temperature of the solutions = 24.4 oC • Specific heat capacity of the mixture = 4080 J Kg—1oC—1

  28. 1. What is the heat change in the reaction: Heat change = mc ∆ T Temperature rise Heat change= (.2kg) x (4080Jkg-1K-1) x 6.9 oC Heat change = 5630J Mass in kilograms Specific heat capacity

  29. 2. How many moles of Nitric acid were reacted?: • 100cm3 of a 1M solution of HNO3 was reacted 1 x 100 = 0.1 moles 1000 Number of moles of HNO3reacted = 0.1moles

  30. Question: What is the heat of reaction? Balanced equation: HNO3 + NaOH NaNO3 + H2O 1 1 1 1 0.1 moles of HNO3 = 5630J of heat 1 mole of HNO3 = 5630 X10 = 56300 The heat of reaction = - 56300J (or – 56.3kJ) The negative sign is because the reaction is exothermic ( heat is given out – the temperature went up!)

  31. Example from notes

  32. 1. What is the heat change in the reaction: Heat change = mc ∆ T Temperature rise Heat change= (.4kg) x (4060Jkg-1K-1) x 6.9 oC Heat change = 11205.6 J Mass in kilograms Specific heat capacity

  33. 2. How many moles of Nitric acid were reacted?: (1) = 0.2 MOLES 1000 200cm3 of a 1M solution of HNO3 was reacted Number of moles of HNO3reacted = 0.2moles

  34. Question: What is the heat of reaction? Balanced equation: HNO3 + NaOH NaNO3 + H2O 1 1 1 1 0.2 moles of HNO3 = 11205.6 J of heat 1 mole of HNO3 = 11205.6 x 5 = 56028 kJ The heat of reaction = -56025kJ (or – 56.025kJ) The negative sign is because the reaction is exothermic ( heat is given out – the temperature went up!)

  35. Q261. Calculations for the heat of reaction • A student carried out an experiment to measure the heat of reaction (neutralisation) of nitric acid by sodium hydroxide in a container made of plastic of negligible heat capacity. She used 100 cm3 of 1.0 M nitric acid and 100 cm3 of 1.0 M sodium hydroxide. The initial temperature of the solutions was 15.6 oC and the final temperature of the solution was 22.4 oC. Given that specific heat capacity of the solution is 4080 J Kg—1 K—1, calculate the heat of reaction. • (Assume that the density of the solution is 1 g cm—3)

  36. 1. What is the heat change in the reaction: Heat change = mc ∆ T Temperature rise Heat change= (.2kg) x (4080Jkg-1K-1) x 6.8 oC Heat change = 5548.8J Mass in kilograms Specific heat capacity

  37. 2. How many moles of Nitric acid were reacted?: • 100cm3 of a 1M solution of HNO3 was reacted • 1000cm3 of solution = 1 mole in it. • 100cm3 of solution = x moles (1) = x 10 Number of moles of HNO3reacted = 0.1moles

  38. Question: What is the heat of reaction? Balanced equation: HNO3 + NaOH NaNO3 + H2O 1 1 1 1 0.1 moles of HNO3 = 5548J of heat 1 mole of HNO3 = 5548 x 10 = 55480kJ The heat of reaction = -55480kJ (or – 55.48kJ) The negative sign is because the reaction is exothermic ( heat is given out – the temperature went up!)

  39. Question from WS: . What is the heat change in the reaction: Heat change = mc ∆ T Temperature rise Heat change= (.5kg) x (4060Jkg-1K-1) x 3.4 oC Heat change = 6902J Mass in kilograms Specific heat capacity

  40. 2. How many moles of Hydrochloric acid were reacted?: • 250cm3 of a 0.5M solution of HCl was reacted 1000cm3 of solution = 0.5 mole in it. 250cm3 of solution = (0.5/ 1000) x 250 = 0.125moles Number of moles of HCl reacted = 0.125moles

  41. Question: What is the heat of reaction? Balanced equation: HCl + NaOH NaCl + H2O 1 1 1 1 0.125 moles of HCl = 6902J of heat 1 mole of HNO3 = 6902 x 8 = 55216 The heat of reaction = -55216J (or – 55.216kJ) The negative sign is because the reaction is exothermic ( heat is given out – the temperature went up!)

  42. Q266 (d) In the experiment 50 cm3 of 1 M hydrochloric acid (HCl) and 50 cm3 of 1M sodium hydroxide (NaOH) were mixed. The temperature rise was recorded as 6.8 K. Assuming the densities and heat capacities of both solutions are the same as that of water, calculate the heat produced by the reaction. • [Density of water is 1g /cm3 specific heat capacity of water is 4.2 kJ kg―1 K—1.] (e) How many moles of hydrochloric acid were used in the experiment? Calculate the heat of reaction (ΔH) when 1 mole of each solution is used.

  43. 1. What is the heat change in the reaction: Heat change = mc ∆ T Temperature rise Heat change= (.1kg) x (4.2kJkg-1K-1) x 6.8K Heat change = 2.856J Mass in kilograms Specific heat capacity

  44. Q266 (d) In the experiment 50 cm3 of 1 M hydrochloric acid (HCl) and 50 cm3 of 1M sodium hydroxide (NaOH) were mixed. The temperature rise was recorded as 6.8 K. Assuming the densities and heat capacities of both solutions are the same as that of water, calculate the heat produced by the reaction. • [Density of water is 1g /cm3 specific heat capacity of water is 4.2 kJ kg―1 K—1.] (e) How many moles of hydrochloric acid were used in the experiment? Calculate the heat of reaction (ΔH) when 1 mole of each solution is used.

  45. How many moles of Hydrochloric acid were reacted?: • 50cm3 of a 1M solution of HCl was reacted • 1000cm3 of solution = 1 mole in it. • 50cm3 of solution = x moles (50)(1) = x(1000) (1) x (50) = x 1000 Number of moles of HCl reacted = 0.05moles

  46. Q266 (d) In the experiment 50 cm3 of 1 M hydrochloric acid (HCl) and 50 cm3 of 1M sodium hydroxide (NaOH) were mixed. The temperature rise was recorded as 6.8 K. Assuming the densities and heat capacities of both solutions are the same as that of water, calculate the heat produced by the reaction. • [Density of water is 1g /cm3 specific heat capacity of water is 4.2 kJ kg―1 K—1.] (e) How many moles of hydrochloric acid were used in the experiment? Calculate the heat of reaction (ΔH) when 1 mole of each solution is used.

  47. Question: What is the heat of reaction? Balanced equation: HCl + NaOH NaCl + H2O 1 1 1 1 0.05 moles of HCl = 2.856kJ of heat 1 mole of HNO3 = 2.856 /20 = 57.12 J of heat The heat of reaction = -57.12kJ The negative sign is because the reaction is exothermic ( heat is given out – the temperature went up!)

  48. Q267 • (f) Calculate the number of moles of acid neutralised in this experiment. • In an experiment to measure the heat of reaction for the reaction between sodium hydroxide with hydrochloric acid, a student added 50 cm3 of 1.0 M HCl solution to the same volume of 1.0 M NaOHsolution in a polystyrene foam cup.Taking the total heat capacity of the reaction mixture used in this experiment as 420 J K–1, calculate the heat released in the experiment if a temperature rise of 6.7 ºC was recorded. • Hence calculate the heat of reaction for NaOH + HCl → NaCl + H2O

  49. How many moles of Hydrochloric acid were reacted?: • 50cm3 of a 1M solution of HCl was reacted • 1000cm3 of solution = 1 mole in it. • 50cm3 of solution = (( 1/1000) x 50) = 0.05 moles Number of moles of HCl reacted = 0.05moles

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