2.1 - Thermochemistry

• calorimetry - a technique used to measure the amount of heat energy absorbed or given out by a reaction
• deltaH - symbol used to represent enthalpy change
• endothermic reaction - a chemical reaction during which heat energy is taken in from the surroundings. delta H for the reaction is positive.
• enthalpy - a meausre of energy stored in (or heat content of) a system under constant pressure. Given symbol H
• exothermic reaction - a chemical reaction during which heat energy is given out to the surroundings. delta H for the reaction is negative
• Hess’s Law - a rule stating the overall enthalpy change for a reaction is independent of the route the reaction takes
• standard enthalpy of combustion - the enthalpy change when 1 mole of a substance undergoes complete combustion in oxygen under standard conditions, with all substances being in their standard states
• What is the standard enthalpy of formation?

  The enthalpy change when 1 mole of a substance is formed from its elements under standard conditions with all substances being in their standard states
• Under what conditions is the standard enthalpy of formation measured?

  At 298 K and 100 kPa
• What is the definition of standard enthalpy of reaction?

  The enthalpy change for a reaction under standard conditions with all substances being in their standard states
• What are the conditions for measuring standard enthalpy of reaction?

  At 298 K and 100 kPa
• What are the standard states of substances in enthalpy measurements?

  • Elements in their most stable form
  • Gases at 1 atm
  • Liquids at their boiling point
  • Solids at room temperature
• For the energy transferred to be an 'enthalpy change' what must the reaction be carried out at?

  A constant pressure
• Energy is conserved
• Reacting system is assumed by convention to be the...

  contents of the reaction vessel
• The surroundings refers to...

  Wherever the energy is being transferred to
• The First Law of Thermodynamics:

  The Law of Conservation of Energy - energy is never destroyed or created, only transferred from one form to another
• In endothermic reactions, the reactants have a greater enthalpy than the products
• In exothermic reactions, reactants have less enthalpy than the products
• In an exothermic reaction energy is transferred to the surroundings and temperature increases
• In an endothermic reaction energy is transferred from the surroundings and temperature decreases
• Delta H in an exothermic reaction is...

  negative
• Delta H in an endothermic reaction is...

  positive
• The standard state of oxygen is

  a gas
• The standard state of bromine is

  a liquid
• The standard state of carbon is

  in the form of graphite
• Experimental enthalpy changes can be calculated using...

  q = mc$\Delta$T

  q = enthalpy change in joules

  m = mass being heated in grams

  c = specific heat capacity in J/K/g

  T - temperature
• Enthalpy change per mole:

  $\Delta H$= -q/n

  (minus sign used if exothermic)
• Why is Hess's Law useful to chemists?

  Allows enthalpy changes to be calculated where reactions didn't occur under standard conditions or are difficult to perform in practise.
• Hess's questions involving formation enthalpies, which side of the equation typically must change its sign?

  The left side ( always where arrows don't flow in the same direction)
• Hess's questions involving combustion enthalpies, which side of the equation typically must change its sign?

  The right side (always where the arrows do not flow in the same directions)
• Hess's cycle drawing for formation changes, arrows from elements are:

  up
• Hess's cycle drawing for combustion changes, arrows to H2O and CO2 are:

  down
• Cheat formation equation:

  Enthalpy of formation = enthalpy of products - enthalpy of reactants
• Cheat combustion equation:

  Enthalpy of combustion = enthalpy of reactants - enthalpy of products