enthalpy and reactions

• Chemical energy of substances - the process when chemicals react to form new substances + bonds break in the reactants + new bonds are formed as products are made
• enthalpy →thermal energy that is stored in a chemical system

  →impossible to measure directly the enthalpy of reactant or product so we can measure the energy absorbed or released to surroundings during chemical change

  the form of this energy can vary but chemists usually measure energy changes in reactions by monitoring thermal energy } monitored during temperature changes
• Systems and surroundings

  • the system is the actual chemical reaction, atoms + bonds involved
  • surroundings are everything else

  HCI + NaOH → NaCl + H2O

  → System would be the atoms + bonds shown + surroundings are everything else, including aqueous solution that all substances are dissolved in

  [temperature you take a reaction is usually taking temperature of surroundings]
• law of conservation of energy - energy cannot be created or destroyed, only moved from one place to another
• if heat is released or absorbed, we can easily measure the effect by recording the temperature change with a thermometer

  this means that :

  • heat loss in a chemical system = heat gain to the surroundings (accompanied by temperature increase)
  • heat gain in a chemical system = heat loss from the surroundings (accompanied by temperature decrease)
• An enthalpy change is :

  • the heat exchange with the surroundings during a chemical reaction, at constant pressure
  • the difference between the enthalpy of products and enthalpy of reactants

  energy is measured in kilojoules (kJ)

  negative enthalpy change - exothermic reaction

  positive enthalpy change - endothermic reaction
• Thermodynamics -

  branch of physical chemistry that focuses energy in a chemical system,which is the reactants and products
• Overall enthalpy change (∆H) can be calculated as follows:

  ΔH = energy to break bonds (+ve) + energy released by making bonds (-ve)