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Understanding Energy Changes in Chemical Reactions

Learn about activation energy, heat of reaction, endothermic, and exothermic reactions. Calculate heat absorbed or released in reactions with simple steps and examples.

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Understanding Energy Changes in Chemical Reactions

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  1. Chapter 6 Chemical Reactions and Quantities 6.9Energy Changes inChemical Reactions

  2. Reaction Conditions Reaction conditions for a chemical reaction require • collisions between reacting molecules • collisions with sufficient energy to break the bonds in the reactants • the breaking of bonds between atoms of the reactants • the forming of new bonds to give products

  3. Chemical Reactions In the reaction H2(g) + I2(g) 2HI(g), • the reactants H2 and I2 collide • the bonds of H2 and I2 break • the bonds for HI form H2 + I2 collision bonds break HI new bonds form

  4. Activation Energy Activation energy • is the minimum energy required upon collision for a reaction to take place

  5. Heat of Reaction The heat of reaction • is the amount of heat absorbed or released during a reaction • is the difference in the energy of the reactants and the products • Has the symbol ΔH ΔH = ΔHproducts − ΔHreactants

  6. Exothermic Reactions In an exothermic reaction, • the energy of the products is less than the energy of the reactants • heat of reaction is released • heat is a product C(s) + O2(g) CO2(g) + 394 kJ ΔH = –394 kJ/mole (heat released)

  7. Endothermic Reactions In an endothermic reaction, • heat is absorbed • the energy of the products is greater than the energy of the reactants • heat is a reactant N2(g) + O2(g) + 180 kJ 2NO(g) ΔH = 180 kJ (heat added)

  8. Summary Reaction Energy Heat Sign of Type Change in ReactionΔH Endothermic Heat absorbed Reactant side + Exothermic Heat released Product side –

  9. Learning Check Identify each of the following reactions as: EX) exothermic or EN) endothermic A. N2(g) + 3H2(g) 2NH3(g) + 22 kcal B. CaCO3(s) + 133 kcal CaO(s) + CO2(g) C. 2SO2(g) + O2(g) 2SO3(g) + heat

  10. Solution EX A. N2(g) + 3H2(g) 2NH3(g) +22 kcal EN B. CaCO3(s) +133 kcal CaO(s) + CO2(g) EX C. 2SO2(g) + O2(g) 2SO3(g) + heat

  11. Guide to Calculations Using Heat of Reaction ( H)

  12. Heat Calculations for Reactions In the reaction N2(g) + O2(g)2NO(g), ΔH = 180 kJ. If 15.0 g of NO are produced, how many kJ were absorbed? STEP 1Given: 15.0 g of NO produced ΔH = 180 kJ/2 moles of NO Need: kJ absorbed STEP 2 Plan: g of NO moles of NO kJ

  13. Heat Calculations for Reactions (continued) STEP 3 Write the conversion factors: 2 moles NO = 180 kJ 180 kJ and 2 moles NO 2 moles NO 180 kJ 1 mole NO = 30.0 g of NO 1 mole NO and 30.0 g NO 30.0 g NO 1 mole NO STEP 4 Set up the problem to calculate kJ: 15.0 g NO x 1 mole NO x 180 kJ = 45 kJ 30.0 g NO 2 moles NO

  14. Learning Check How many grams of O2 reacted if 306 kcal are released in the following reaction? CH4(g) + 2O2(g) CO2(g) + 2H2O(l) + 213 kcal 1) 91.9 g of O2 2) 46.0 g of O2 3) 2.87 g of O2

  15. Solution 1) 91.9 g of O2 STEP 1 Given: 306 kcal released ΔH = 213 kcal /2 moles of O2 Need: g of O2 reacted STEP 2 Plan: kcal moles of O2 g of O2 STEP 3 Write the conversion factors: 2 moles of O2 = 213 kcal 213 kcal and 2 moles O2 2 moles O2 213 kcal

  16. Solution (continued) STEP 3 (continued) 1 mole of O2 = 32.0 g of O2 32.0 g O2 and 1 mole O2 1 mole O2 32.0 g O2 STEP 4 Set up the problem to calculate g of O2: 306 kcal x 2 moles O2 x 32.0 g O2 = 91.9 g of O2 (1) 213 kcal 1 mole O2

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