HEAT IN CHEMICAL REACTIONS

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HEAT IN CHEMICAL REACTIONS

• Lesson Outcome
• To understand the concepts of endothermic and
  exothermic reactions.
• To be able to calculate the heat given off or
  needed by a chemical reaction.

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Exothermic and Endothermic Reactions

• Joule The SI unit for energy and heat.
• Bond breaking vs. bond forming.
• Combustion is a process in which a
  self-sustaining chemical reaction occurs at
  temperatures above those of the surroundings.

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EXOTHERMIC

• Exothermic reactions release heat.
• EXAMPLES
• direct combination reactions
• reactions of metals with water and acids
• reactions of acids with bases

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ENDOTHERMIC

• Endothermic reactions absorb heat.
• EXAMPLES
• decomposition reaction
• some precipitation reactions

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Enthalpy (H)

• ENTHALPY is a measure of the total energy
  possessed by a substance or group of substances.
• The change in enthalpy for a chemical reaction,
  ?H, is defined as the heat absorbed (per mole of
  specified reactant or product) when the reaction
  occurs at constant pressure.

?H enthalpy of products enthalpy of reactants
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• Heat transfer and the sign of the enthalpy
  change
• When ?H is Positive
• Endothermic
• Heat absorbed
• Eg 95 KJ/mol
• When ?H is Negative
• Exothermic
• Heat released
• Eg 56KJ/mol

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Enthalpy of Reactions ?Hrxn Hproducts -
Hreactants

• The enthalpy change (?H) for a reaction is equal
  to the heat absorbed or gained during the
  reaction.
• The amount of heat absorbed / released is
  dependent on the quantity (mole).
• Standard enthalpy change (?Ho) at 1atm and 250C.

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EXAMPLE OF ENTHALPY CALCULATIONS
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EXAMPLE OF ENTHALPY CALCULATIONS
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