Enthalpy

• Enthalpy


  A measure of the heat energy in a chemical system
• Chemical system


  The atoms, molecules, or ions making up the chemicals
• Enthalpy cannot be measured, but enthalpy changes can
• Enthalpy change


  The difference in the enthalpies of the reactants and products in a chemical reaction
• Enthalpy change can be positive or negative, depending on whether the products contain more or less energy than the reactants
• Conservation of energy


  Energy cannot be created or destroyed
• System


  The chemicals - the reactants and products
• Surroundings


  The apparatus, the laboratory, and everything that is not the chemical system
• Universe


  Everything, including both system and surroundings
• Exothermic change


  Energy transfer from the system to the surroundings
• Endothermic change


  Energy transfer from the surroundings to the system
• In an exothermic change, the chemical system releases heat energy to the surroundings, and the temperature of the surroundings increases
• In an endothermic change, the chemical system takes in heat energy from the surroundings, and the temperature of the surroundings decreases
• Activation energy


  The minimum energy required for a reaction to take place
• Reactions with small activation energies take place very rapidly, while reactions with very large activation energies may take place extremely slowly or not at all
• Standard conditions


  Standard pressure of 100 kPa, standard temperature of 298 K, and standard concentration of 1 mol dm^-3 for solutions
• Standard enthalpy change of reaction

  The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions
• Standard enthalpy change of formation


  The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions
• All elements have an enthalpy change of formation of 0 kJ mol^-1
• Standard enthalpy change of combustion

  The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions
• Standard enthalpy change of neutralisation

  The energy change that accompanies the reaction of an acid by a base to form one mole of H2O(l) under standard conditions
• The value of the standard enthalpy change of neutralisation is the same for all neutralisation reactions
• Kelvin scale of temperature

  The temperature scale used in science, where 0 K is absolute zero and 1 K rise is the same as a 1°C rise
• Mass


  The quantity measured by weighing the materials that are changing temperature
• Specific heat capacity


  The energy required to raise the temperature of 1 g of a substance by 1 K
• For water, the specific heat capacity is 4.18 J g^-1 K^-1
• Temperature change


  The change in temperature of the surroundings, measured using a thermometer
• Energy change of the surroundings

  Calculated from three quantities: mass, specific heat capacity, and temperature change
• Mass of the surroundings (m)


  Measured by weighing the materials that are changing temperature
• Specific heat capacity of the surroundings (c)

  The energy required to raise the temperature of 1 g of a substance by 1K
• Good conductors of heat, such as metals, have small values of c. Insulators of heat such as foam plastic, have large values of c.
• In most experiments, you will be measuring the temperature change of water or aqueous solutions. For water, c=4.18J/g·K
• Temperature change of the surroundings (ΔT)

  Determined from the thermometer readings: ΔT=T(final)-T(initial)
• Heat energy (q)


  Calculated using the equation: q=m·c·ΔT
• Calculating an energy change

  • Demonstrate knowledge, understanding and application of calculating energy changes from experiments
  • Determination of enthalpy changes directly
  • Practical techniques for measuring mass, specific heat capacity, and temperature change
• The data book value for ΔH of methanol combustion is -726 kJ/mol, while the experimental value is -514 kJ/mol. This difference is due to factors like heat loss, incomplete combustion, and non-standard conditions.
• Techniques like using draught screens and oxygen gas input can minimize errors in enthalpy change experiments.
• Average bond enthalpy


  The energy required to break one mole of a specified type of bond in a gaseous molecule
• Bond enthalpies are always endothermic and have positive enthalpy values.
• The actual bond enthalpy can vary depending on the chemical environment of the bond.