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What you have done so far…

What you have done so far…. Explained what ΔH is. Describing the difference between exothermic and endothermic. Enthalpy diagrams The definitions for ΔH r , ΔH f , ΔH c How to calculate the energy change using experimental data. Q= mc Δ T. Bond Energy and Calculations. OBJECTIVES

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What you have done so far…

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  1. What you have done so far… Explained what ΔH is. Describing the difference between exothermic and endothermic. Enthalpy diagrams The definitions for ΔHr, ΔHf,ΔHc How to calculate the energy change using experimental data. Q=mcΔT

  2. Bond Energy and Calculations OBJECTIVES Understand that bonds are broken and made in chemical reactions Learn how to calculate the energy needed and released

  3. Bond energy and calculations ENERGY IS NEEDED TO BREAK A BOND ENERGY IS NEEDED TO MAKE A BOND In a chemical reaction, bonds are BROKEN in the REACTANTS and MADE in the PRODUCTS Click here to see an example

  4. Average Bond energy The bond energy is not exact, as it changes for each bond. The bond enthalpy is an average.

  5. Example 1 • Hydrogen + Chlorine Hydrogen Chloride • H2(g) + Cl2(g) 2HCl(g) • H-H + Cl-Cl H-Cl + H-Cl

  6. H HClCl 678 436 242 H-H and Cl-Cl Reactants 862 431 431 H-Cl and H-Cl 678 - 862 = 184 kJ/mol Products

  7. The total energy (ΔH) = Energy to make bonds – Energy to break bonds = 678 –862 = -184 kJ mol-1 The answer is NEGATIVEso this is an EXOTHERMIC REACTION

  8. Example 2 • Hydrogen Bromide Hydrogen + Bromine • 2HBr(g) H2(g) + Br2(g) • H-Br + H-Br H-H + Br-Br

  9. H H Br Br 732 629 436 193 H-H and Br-Br Products 366 366 H-Br and H-Br Reactants 732 - 629= 103 kJ/mol

  10. The total energy (ΔH) = Energy to make bonds – Energy to break bonds = 732 - 629 = 103 kJ mol-1 The answer is POSITIVEso this is an ENDOTHERMIC REACTION

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