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Heat from Reactions

Heat from Reactions. In this presentation you will:. explore how energy is produced and needed for reactions. identify how to determine whether a reaction is endothermic or exothermic. apply thermochemical energy equations. Next >. Reactants. Products. Introduction.

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Heat from Reactions

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  1. Heat from Reactions In this presentation you will: • explore how energy is produced and needed for reactions • identify how to determine whether a reaction is endothermic or exothermic • apply thermochemical energy equations Next >

  2. Reactants Products Introduction When a chemical reaction takes place, matter is not lost or gained. Therefore, the mass of the reactants used up is always equal to the mass of the products formed. Energy can never be created or destroyed in a chemical reaction. Next >

  3. Reactants Products Introduction However, energy can join or leave the reactants when the products are formed. This energy normally takes the form of heat, which is either given to, or taken from the surroundings. Next >

  4. Chemical Reactions Chemical reactions involve the making and breaking of chemical bonds. Energy Energy A chemical bond is a mechanism that chemically combines atoms. Energy is needed to break chemical bonds. Energy is given out when chemical bonds are made. Next >

  5. Chemical Reactions Energy Energy During a chemical reaction there are energy changes. The difference between the energy taken in to break the bonds in the reactant molecules, and the energy given out when the new bonds are formed, gives us the net energy change. The overall result is either energy being given out or energy being taken in. Next >

  6. Question 1 Chemical reactions involve the making and breaking of chemical bonds. Which of the following statements is NOT true? • Energy is given out when chemical bonds are made. B) Energy is needed to break chemical bonds. C) Energy is given out when chemical bonds are broken. Next >

  7. Question 1 Chemical reactions involve the making and breaking of chemical bonds. Which of the following statements is NOT true? • Energy is given out when chemical bonds are made. B) Energy is needed to break chemical bonds. C) Energy is given out when chemical bonds are broken. Next >

  8. Energy changes during a reaction can often be detected as a temperature change. Chemical Reactions Energy can be given out as heat, causing a rise in temperature, or the temperature could fall due to energy being taken in. Next >

  9. Chemical Reactions Therefore the products of a reaction can have more or less energy than the original reactants. These reactions can be classified as: • Exothermic – heat given out • Endothermic – heat taken in Next >

  10. Bond Energies If we know which bonds have been broken and formed, then it is possible to look up the bond energies involved. Tables have been created and can be examined to show the bond energy in kJ/mol. Next >

  11. Bond Energies – Example 1 Let’s look at the reaction: 2HI→ H+ I (g) 2(g) 2(g) Bonds broken = 2HI = 2 × 299 = 598 kJ/mol Bonds created = H2 + I2 = H-H + I-I = 436 + 151 = 587 kJ/mol Energy difference = energy released by breaking bonds – energy to create bonds = 598 – 587 = +11 kJ/mol Next >

  12. When burned with oxygen, methane gas gives out heat and light. Bond Energies – Example 2 Methane is acting as a fuel. The chemical reaction taking place is: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) Next >

  13. Bond Energies – Example 2 CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) Bonds broken = 4C-H + 2O=O = (4 × 412) + (2 × 496) kJ/mol = 1,648 + 992 = 2,640 kJ/mol Bonds created = 2C=O + 4O-H = (2 × 743) + (4 × 463) = 3,338 kJ/mol Energy difference = energy released by breaking bonds – energy to create bonds = 2,640 – 3,338 = -698 kJ/mol Next >

  14. Reaction Profiles Energy changes during a chemical reaction are usually shown by a reaction profile. Exothermic Energy Heat given out Reactants Exothermic and endothermic reactions can be determined from a reaction profile. Products Course of reaction Endothermic Energy This is done by calculating the energy contained in the reactants and the energy contained in the products of the reaction. Products Reactants Heat is taken in (absorbed) Course of reaction Next >

  15. Reaction Profiles From our two examples: Example 1 is endothermic because less energy is needed to form the new bonds than is released by breaking the old bonds. It will absorb 11 kJ/mol. Endothermic Energy Products Reactants Heat is taken in (absorbed) Course of reaction Next >

  16. Reaction Profiles Exothermic Example 2 is exothermic since more energy is needed to form the new bonds than is released by breaking the old bonds. It will release 698 kJ/mol. Energy Heat given out Reactants Products Course of reaction Next >

  17. Enthalpy – ΔH The amount of energy absorbed or released is given the symbol ΔH. Exothermic Energy Heat given out Reactants +ΔH Products Δ is the symbol used to indicate “change of ” and H refers to the energy or heat produced or released. Course of reaction Endothermic Energy Products -ΔH Reactants Heat is taken in (absorbed) Course of reaction ΔH is called the enthalpy. Next >

  18. Enthalpy – ΔH The enthalpy, ΔH, will be negative for an exothermic reaction and positive for an endothermic reaction. Exothermic Energy +ΔH Heat given out Reactants Products Course of reaction Endothermic Energy -ΔH Products Reactants Heat is taken in (absorbed) ΔH = energy released by breaking bonds – energy to create bonds Course of reaction Next >

  19. Thermochemical Equations A thermochemical equation shows not only the products formed from the reactants, but also the amount of energy released or absorbed. Reactants Products Next >

  20. Thermochemical Equations Reactants Products There are two ways in which this can be done: Method 1: 2H2(g) + O2(g) → 2H2O, ΔH = -484 kJ/mol Method 2: 2H2(g) + O2(g) → 2H2O + 484 kJ/mol Method 1 is the approved method as it is dimensionally correct, but you may see method 2 in older texts. Next >

  21. Question 2 What does a positive enthalpy mean? A) Heat is given out in a reaction. B) More energy is produced by breaking old bonds than by making new ones. C) The energy level diagram will show a step-down. D) The reaction is exothermic. Next >

  22. Question 2 What does a positive enthalpy mean? A) Heat is given out in a reaction. B) More energy is produced by breaking old bonds than by making new ones. C) The energy level diagram will show a step-down. D) The reaction is exothermic. Next >

  23. Question 3 What does a negative enthalpy mean? A) Heat is absorbed in a reaction. B) More energy is produced by breaking old bonds than by making new ones. C) The energy level diagram will show a step-up. D) The reaction is exothermic. Next >

  24. Question 3 What does a negative enthalpy mean? A) Heat is absorbed in a reaction. B) More energy is produced by breaking old bonds than by making new ones. C) The energy level diagram will show a step-up. D) The reaction is exothermic. Next >

  25. Exothermic Reactions Reactants Energy Products An exothermic reaction is when the products contain less energy than the reactants. Energy in the form of heat is released or given out to the surroundings. This causes a rise in the temperature of the surroundings. Next >

  26. Exothermic Reactions Examples of exothermic reactions: • combustion reactions (burning of fuel) • neutralization reactions (acid reacting with alkali) • hydrogen cell reactions (fuel cell) Next >

  27. Energy Exothermic Reactants Heat given out (-ve) Products Course of reactions Exothermic Reactions During exothermic reactions, the energy given out when bonds form is greater than the energy needed to break bonds. This is shown by this reaction profile. Next >

  28. Energy Exothermic Reactants Heat given out (-ve) Products Course of reactions Exothermic Reactions In exothermic reactions, the heat of reaction is negative (-ve), showing that the energy content of the system has decreased. Some energy in the form of heat has left the system and entered the surroundings. Next >

  29. Question 4 ''An exothermic reaction is when the products contain less energy than the reactants. Energy in the form of heat is released to the surroundings.'' Is this statement true or false? Answer True or False. Next >

  30. Question 4 ''An exothermic reaction is when the products contain less energy than the reactants. Energy in the form of heat is released to the surroundings.'' Is this statement true or false? Answer True or False. True Next >

  31. An endothermic reaction is when the products contain more energy than the reactants. Products Energy Reactants Endothermic Reactions Energy in the form of heat is absorbed from the surroundings. This causes a decrease in the temperature of the surroundings. Next >

  32. Products Energy Reactants Endothermic Reactions Examples of endothermic reactions: • reaction of sodium hydrogen carbonate with citric acid • formation of hydrogen iodide from hydrogen and iodine Next >

  33. Endothermic Reactions During endothermic reactions, the energy given out when bonds form isless than the energy needed to break bonds. Endothermic Energy Products Heat taken in (+ve) Reactants Course of reactions This is shown by this reaction profile. Next >

  34. Endothermic Reactions Endothermic Energy Products Heat taken in (+ve) Reactants Course of reactions In endothermic reactions, the heat of reaction is positive (+ve), showing that the energy content of the system has increased. Some energy has entered the system after being absorbed from the surroundings. Next >

  35. Some candies produce a fizzing sensation when you eat them. Endothermic Reactions This is because a chemical reaction occurs when they meet with the moisture in your mouth. The candy is filled with a mixture of sodium hydrogen carbonate and citric acid. When water is added to this mixture, the chemicals react to release carbon dioxide. Next >

  36. Endothermic Reactions If you have ever tried this candy, you may have noticed that the fizzing sensation makes your mouth feel cold. This is because the reaction takes heat from your mouth. This is an endothermic reaction – it needs heat to take place. Next >

  37. Question 5 In an endothermic reaction, the heat of reaction is... A) neutral B) positive C) negative Next >

  38. Question 5 In an endothermic reaction, the heat of reaction is... A) neutral B) positive C) negative Next >

  39. Summary In this presentation you have seen: • the energy changes involved in reactions • thermochemical equations • both exothermic and endothermic reactions • the different reaction profiles for endothermic and exothermic reactions End >

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