Enthalpy changes

• Enthalpy H is a measure of the heat energy in a chemical system.
• The chemical system refers to the atoms, molecules, or ions making up the chemicals.
• Enthalpy is sometimes thought of as the energy stored within bonds. Enthalpy cannot be measured, but enthalpy changes can.
• In a chemical reaction, the reactants and products are likely to have different enthalpies. The difference in enthalpies is the enthalpy change:

  ∆H = H(products) - H(reactants)
• ∆H can be positive or negative, depending on whether the products contain more or less energy than the reactants.
• When a chemical reaction involving an enthalpy change takes place, heat energy is transferred between the system and the surroundings
• In a chemical reaction:

  • The system is the chemicals - the reactants and products
  • The surroundings are the apparatus, the laboratory, and everything that is not the chemical system
  • The universe is everything, includes both system and surroundings
• Energy transfer from the system to the surroundings = an exothermic change
• Energy transfer from the surroundings to the system = an endothermic change
• EXOTHERMIC REACTION

  The conservation of energy means that:

  • The chemical system releases heat energy to the surroundings
  • Any energy loss by the chemical system is balanced by the same energy gain by the surroundings
  • ∆H is negative
  • The temp of the surroundings increases as they gain energy
• In the enthalpy profile diagram of an exothermic reaction, the reactants are higher than the products
• ENDOTHERMIC REACTION

  The conservation of energy means that:

  • The chemical system takes in heat energy from the surroundings
  • Any energy gain by the chemical system is balanced by the same energy loss by the surroundings
  • ∆H is positive
  • The temp of the surroundings decreases as they lose energy
• In the enthalpy profile diagram of an endothermic reaction, the reactants are lower than the products
• The energy input required to break bonds in the reactants acts as an energy barrier to the reaction, known as the activation energy
• Activation energy is the minimum energy required for a reaction to take place
• Reactions with small activation energies take place very rapidly because the energy needed to break the bonds is readily available from the surroundings. Very large activation energies may present such a large energy barrier that a reaction may take place extremely slowly or even not at all
• Chemists use standard conditions for measurements such as enthalpy changes
• The units of enthalpy change are usually KJmol-1
• Standard pressure is 100KPa.
• Standard temp is usually 298K
• Standard concentration is 1 moldm-3 (relevant for solutions only)
• Standard state is the physical state of a substance under standard conditions
• The standard enthalpy change of reaction is the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states
• The standard enthalpy change of formation is the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states
• The standard enthalpy change of combustion is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states
• The standard enthalpy change of neutralisation is the energy change that accompanies the reaction of an acid by a base to form one mole of water, under standard conditions, with all reactants and products in their standard states
• Neutralisation involves the reaction of H+(aq) with OH-(aq) to form one mole of H2O. The value of ∆neutH is the same for all neutralisation reactions