enthalpy

• enthalpein (to warm)
• enthalpy Represented by the symbol, H.
• enthalpy refers

  to the energy transferred under constant pressure.
• enthalpy Often referred to as the heat content
• enthalpy cannot be measured but it is possible to measure the

  change in enthalpy or heat content, ΔH
• Enthalpy change, ΔH, that accompanies chemical

  reaction is called heat of reaction.
• enthalpy is the net energy change resulting from the breaking

  and making of bonds.
• ΔH = Hproducts - Hreactants
• The sign of ΔH indicates the direction of heat

  transfer.

  A positive value indicates that the system

  absorbs heat from the surroundings and is,

  endothermic.

  A negative value indicates that heat is released

  by the system, hence, the process is exothermic.
• Enthalpy also indicates the amount of heat stored as

  potential energy.
• in an exothermic reaction, the

  products will have a lower energy than the reactants.
• A balanced equation in which the value of ΔH, with the

  appropriate sign, is usually written at the right side.
• Balanced chemical equation:

  CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) + 890.4 kJ

  Thermochemical equation:

  CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ΔH = −890.4kJ

  it is negative because heat is at the product side meaning the system produced heat meaning it is exothermic
• Balanced chemical equation:

  CaCO3(s) + 177.8 kJ → CaO(s) + CO2(g)

  Thermochemical equation:

  CaCO3(s) → CaO(s) + CO2(g) ΔH = 177.8 kJ

  it is positive because the caco3 is heated resulting to the product meaning the substance is heated by the surrounding meaning it is endothermic
• Calorimetry

  Concerned with the measurement of heat

  changes or heat flow in a chemical reaction.

  The device used to measure heat changes is

  a calorimeter.
• hesss law states that ΔH = ΔH1 + ΔH2 +
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