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Energetics

Energetics. Energy changes in chemistry. Thermochemistry. What is thermochemistry? Thermochemistry is the study of energy changes associated with chemical reactions. Enthalpy. What is enthalpy?

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Energetics

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  1. Energetics Energy changes in chemistry.

  2. Thermochemistry • What is thermochemistry? • Thermochemistry is the study of energy changes associated with chemical reactions.

  3. Enthalpy • What is enthalpy? • Enthalpy (H) is the total energy of system. It is in the molecules, the bonds, and stored chemically. • Enthalpy change (Δ H) is the measure of the change in the potential energy of the bonds.

  4. Endothermic vs exothermic • What is the difference between the two? • Endothermic reactions have more energy stored in the new bonds (products) than were in the old bonds (reactants) which takes in energy. • Exothermic reactions have more energy in the old bonds (reactants) than in the new bonds (products).

  5. Enthalpy change • What is the value of enthalpy change for exothermic reactions? • Exothermic reaction enthalpy change is negative due to Δ H = enthalpy products – enthalpy reactants. • Endothermic reaction would be the opposite because of the same equation.

  6. Enthalpy change • How do we show enthalpy change of a reaction? • It is usually written with the equation. • + represents endothermic, - represents exothermic • It is given in the units of kJ/mol (or just kJ) because it changes with the amount of reactants and limiting reactants. • This is at thermochemical standard conditions which are 25 degrees C, 1 atm and solution concentrations of 1 mol/dm3.

  7. Enthalpy change • What is the difference between heat and temperature? • Temperature is the measure of the kinetic energy of the particles and does not depend on the amount present. • Heat is the measure of the total energy of a substance that increases and decreases with amount of substance.

  8. Enthalpy change • How do we find heat energy? • Heat energy = mass x specific heat capacity x change in temperature (m x c x ΔT). • Specific heat capacity depends on the specific material involved.

  9. Calorimetry • What is Calorimetry? • This is a technique to measure the energy change in a reaction. • Total heat = (m c ΔT)liquid + (m c ΔT)solid

  10. Calorimetry • When 8.00 g of ammonium nitrate dissolves in 100.0 cm3 of water, the temperature dropped from 19.0 C to 14.5 C. What is the enthalpy of ammonium nitrate? • 100 g x 4.18 J/g K x 4.5 K • 1881 J • 8.00g/80.06 g/mol • .100 mol • 1881 J/ .100 mol • 18.8 kJ/mol

  11. Hess’s Law • What does Hess’s Law say? • It states that if you can add 2 or more different equations to produce the final equation, you can add the individual enthalpy changes to find the total. • This means that if you break down the steps involved, the overall change will be the same.

  12. Hess’s Law • 2H2O2(l) 2H2O(l) + O2(g) • 2H2(g) + O2(g) 2H2O(l)ΔH= -572 kJ • H2(g) + O2(g)  H2O2(l)ΔH= -188 kJ • Where do we start?

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