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Enthalpy of Chemical Reactions

Enthalpy of Chemical Reactions. CHEMISTRY 11 Feb. 16, 2010. Chemical Bonds. A molecule is composed of atoms held together by chemical bonds For a chemical reaction to occur, bonds must be broken, and new chemical bonds must form

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Enthalpy of Chemical Reactions

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  1. Enthalpy of Chemical Reactions CHEMISTRY 11 Feb. 16, 2010

  2. Chemical Bonds • A molecule is composed of atoms held together by chemical bonds • For a chemical reaction to occur, bonds must be broken, and new chemical bonds must form • To break a chemical bond, energy must be put into molecule. (Surroundings feel cold) • When chemical bonds are formed, the molecule releases energy. (Surroundings feel hot)

  3. Chemical Bonds • Every reaction contains many chemical bonds breaking and new bonds forming • The total energy of a chemical reaction is the net sum of the energy released and energy absorbed by molecules • A reaction is either exothermic(heat releasing)or endothermic (heat absorbing)

  4. Enthalpy • If molecules absorb heat from their surroundings, the surroundings feel cold • If molecules release heat into the surroundings, the surroundings feel hot • Enthalpy = “heat energy” in molecules • Can only measure the change in enthalpy • ∆H = HPRODUCTS - HREACTANTS

  5. Enthalpy Changes (∆H) in Endothermic Reactions H + Cl Products ∆H > 0 Change in enthalpy is positive You can write change in enthalpy (∆H) in two ways: Add energy HCl + energy (kJ)  H + Cl 431 kJ HCl Reactants HCl  H + Cl ∆H = +energy (kJ) 431 kJ Reaction proceeding

  6. Endothermic Chemical Reactions HCl + 431 kJ  H + Cl Endothermic reactions need more heat energy in order to start the reaction, so energy appears on the reactants side H + Cl  HCl ∆H = +431 kJ Molecules or atoms absorb energy from the surroundings, so ∆H is positive

  7. Enthalpy Changes (∆H) in Exothermic Reactions H + Cl Reactants ∆H < 0 Change in enthalpy is negative You can write change in enthalpy (∆H) in two ways: Give off energy H + Cl  HCl + energy (kJ) 431 kJ HCl H + Cl  HCl ∆H = -energy (kJ) 431 kJ Products Reaction proceeding

  8. Exothermic Chemical Reactions H + Cl  HCl + 431 kJ Exothermic reactions produce heat energy, so energy appears on the products side H + Cl  HCl ∆H = -431 kJ Molecules or atoms lose energy to the surroundings, so ∆H is negative

  9. Change in Enthalpy in Chemical Reactions H(g) + Cl(g) HCl(g) ∆H = -431 kJ HCl(g) H(g) + Cl(g) ∆H = +431 kJ H2(g) + F2(g) 2HF(g) ∆H = -542 kJ C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(l) ∆H = -1418 kJ

  10. Exercises • Energy & Enthalpy in Chemical Reactions Handout • HEBDEN: pp. 120-122, q. 68-80 • Test February 22, 2010

  11. Enthalpy Changes (∆H) in Chemical Reactions

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