Energetics part 1 enthalpy change

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  • Boardworks AS Chemistry Energetics Photo credit: Jupiterimages Corporation Teacher notes More precisely, H = U + pV U is the internal energy of the system p is the pressure of the system V is the volume of the system The internal energy is the total kinetic energy due to the motion of molecules, plus the potential energy associated with their interactions. pV is the work done by the system.
  • Boardworks AS Chemistry Energetics
  • Boardworks AS Chemistry Energetics Teacher notes Heat is energy transferred from one body to another due to a temperature difference.
  • Boardworks AS Chemistry Energetics
  • Teacher notes The standard state for a particular substance is the state that substance is in under standard conditions (298 K and 100 kPa).
  • Boardworks AS Chemistry Energetics

Transcript

  • 1. 1 of 9 © Boardworks Ltd 2009
  • 2. 2 of 9 © Boardworks Ltd 2009 What is enthalpy? The enthalpy, H, of a system is a measure of the energy stored in (or heat content of) a system. It cannot be measured directly. The enthalpy change for a reaction is usually observed as a change in temperature, which can be measured or calculated. During reactions, the enthalpy of the reactants and the products is not the same. This results in energy being either given out or taken in during the reaction. This energy is the enthalpy change, ∆H (‘delta H’).
  • 3. 3 of 9 © Boardworks Ltd 2009 Enthalpy changes The enthalpy change of a reaction is the heat energy exchange with its surroundings at constant pressure. Enthalpy is the energy content of the reactants and is given the symbol H. Standard enthalpy changes are measured at a standard pressure of 100kPa and temperature of 298K. Standard enthalpy changes are represented by ∆Hө 298 but this is usually shortened to ∆Hө . Therefore, enthalpy change is represented by ∆H. It has the units kilojoules per mole (kJmol-1 ). In science, change is represented by the upper case Greek letter delta, ∆.
  • 4. 4 of 9 © Boardworks Ltd 2009 Exothermic reactions
  • 5. 5 of 9 © Boardworks Ltd 2009 Endothermic reactions
  • 6. 6 of 9 © Boardworks Ltd 2009 Exothermic and endothermic reactions
  • 7. 7 of 9 © Boardworks Ltd 2009 Types of enthalpy change
  • 8. 8 of 9 © Boardworks Ltd 2009 Standard enthalpies: examples The standard enthalpy of formation of methane can be represented by: C(s, graphite) + 2H2(g) → CH4(g) By definition, the standard enthalpy of formation of an element, in its standard state, must be zero. ∆Hf ө = -74.9kJmol-1 The standard enthalpy of combustion of methane can be represented by: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ∆Hc ө = -890kJmol-1
  • 9. 9 of 36 © Boardworks Ltd 20099 of 36 © Boardworks Ltd 20099 of 9 © Boardworks Ltd 2009 Enthalpy change summary
  • 10. 9 of 36 © Boardworks Ltd 20099 of 36 © Boardworks Ltd 20099 of 9 © Boardworks Ltd 2009 Enthalpy change summary