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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.

elizabeth-kirk
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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 reactants products Exothermic Reaction - Definition Exothermic reactions give out energy. There is a temperature rise and H is negative. His negative

  10. Heat changes also happen when substances change state.

  11. 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.

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

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

  14. 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

  15. 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.

  16. 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=+

  17. 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

  18. 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

  19. 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.

  20. Results

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

  22. 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

  23. 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

  24. 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

  25. 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!)

  26. 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)

  27. 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

  28. 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

  29. 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!)

  30. Q262. Calculations for the heat of reaction • A student mixed 250 cm3 of 0.5 M HCl with an equal volume of 0.5 M NaOH in a plastic container. The original temperature of both solutions was 14.8 oC and the final temperature was 18.2 oC. Calculate the heat of reaction (neutralisation) of hydrochloric acid and sodium hydroxide. • Assume that the density of the final solution is 1 g cm—3 and its specific heat capacity is 4060 J Kg—1 K—1.

  31. 1. 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

  32. 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

  33. 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!)

  34. 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.

  35. 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

  36. 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.

  37. 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

  38. 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.

  39. 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!)

  40. 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

  41. 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

  42. 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

  43. 1. What is the heat change in the reaction: Heat change = mc ∆ T Temperature rise Heat change= (.1kg) x (420Jkg-1K-1) x 6.7 oC Heat change = 281.4J Mass in kilograms Specific heat capacity

  44. Question: What is the heat of reaction? Balanced equation: HCl + NaOH NaCl + H2O 1 1 1 1 0.05 moles of HCl = 281.4J of heat 1 mole of HCl = (281.4 x 20) = 5628 J of heat The heat of reaction = -5628J ( - 5.628kJ) The negative sign is because the reaction is exothermic ( heat is given out – the temperature went up!)

  45. Bond energy

  46. Energy in chemicals Energy needed Breaking chemical bonds • Breaking chemical bonds requires energy – is an endothermic process. Heat taken in Energy needed to overcome the bonding between the atoms

  47. Activation Energy. • Before new bonds can be formed we need to break some existing chemical bonds. • This requires an energy input –known as the activation energy (EaorEact) • Activation energy is the energy input into a reaction to allow chemical bonds to be broken.

  48. Energy in chemicals Energy given out Making chemical bonds • Energy will be given out in an exothermic process when bonds are formed. Heat given out Energy given out as bonds form between atoms

  49. Bond energy This is the energy needed to break 1 mole of covalent bonds or The energy released when 1 mole of covalent bonds are made

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