Enthalpy Changes

• Enthalpy changes involve the transfer of energy between the system (chemicals) and the surroundings
• In an exothermic change, energy is transferred from the system to the surroundings, resulting in products with less energy than the reactants
• Exothermic reactions have a negative ∆H
• In an endothermic change, energy is transferred from the surroundings to the system, requiring an input of heat energy
• Endothermic reactions have a positive ∆H
• Common oxidation exothermic processes include the combustion of fuels and the oxidation of carbohydrates like glucose in respiration
• Activation Energy is the minimum energy particles need to collide to start a reaction
• Enthalpy changes are usually quoted at standard conditions, which include: 100 kPa pressure, 298 K temperature, solutions at 1 mol dm-3, and all substances in their normal state at 298 K
• The standard enthalpy change of formation is the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions
• The enthalpy of formation of an element is 0 kJ mol-1
• The standard enthalpy change of combustion is the enthalpy change when one mole of a substance is combusted completely in oxygen under standard conditions
• Incomplete combustion results in soot, carbon monoxide, and water, being less exothermic than complete combustion
• The standard enthalpy change of neutralisation is the enthalpy change when solutions of an acid and an alkali react under standard conditions to produce 1 mole of water
• The calorimetric method is used to measure the enthalpy change for a reaction experimentally
• In the calorimetric method, the energy change is calculated using the equation: Q (J) = m (g) x cp (J g-1K-1) x ∆T (K)
• To counteract cooling during slow reactions, readings are taken at regular intervals and the temperature curve is extrapolated back to when the reactants were mixed
• Errors in the calorimetric method include energy transfer from surroundings, approximation in specific heat capacity, neglecting the specific heat capacity of the calorimeter, incomplete or slow reactions, and assuming the density of the solution is the same as water
• In the practical application of the calorimetric method, it's important to carefully follow the method, draw graphs with extrapolation, and describe the experiment accurately
• Calorimetric method for calculating the enthalpy change of reaction:

  • Read the question carefully
  • Describe the method
  • Draw a graph with extrapolation
  • Describe the calculation
• General method for calculating the enthalpy change of reaction:

  1. Use q= m x cp x ΔT to calculate energy change for quantities used

  2. Work out the moles of the reactants used

  3. Divide q by the number of moles of the reactant not in excess to give ΔH

  4. Add a sign and unit (divide by a thousand to convert Jmol-1 to kJmol-1)
• Assumptions in calorimetry:

  • The heat capacity of water is 4.18 J g-1K-1
  • In reactions where the reactants are dissolved in water, assume the heat capacity is the same as pure water
  • Assume the solutions have the density of water, which is 1g cm-3
• Mean Bond Energies:

  • Definition: Enthalpy change when one mole of gaseous covalent bonds is broken
  • Use average values for bond energies in calculations
  • Calculation: ΔH = Σ bond energies broken - Σ bond energies made
• In endothermic reactions, more energy is absorbed when breaking bonds than is released when making bonds
• Hess's Law states that the total enthalpy change for a reaction is independent of the route by which the chemical change takes place
• Hess's Law is a version of the first law of thermodynamics, which states that energy is always conserved
• On an energy level diagram, the directions of the arrows can show the different routes a reaction can proceed by
• Hess's Law can be used to work out the enthalpy change to form a hydrated salt from an anhydrous salt
• Using Hess's Law to determine enthalpy changes from enthalpy changes of formation
• Remember, elements have standard enthalpy of formation (ΔHf) of 0
• Hess's Law cycles are used to measure the enthalpy change for a reaction that cannot be measured directly by experiments
• Enthalpy change in a reaction is the enthalpy change when the number of moles of reactants as specified in the balanced equation react together
• What is the definition of mean bond energies

  The enthalpy change when one mole of bonds of gaseous covalent bonds is broken