8.6 - Enthalpy Changes & Direction

• what sort of reactions usually happen?

  reactions that give out energy to the surroundings
• in what direction does change happen in?

  direction that energy is spread around and dissipated to the surroundings
• what's enthalpy change a guide to?

  likely direction of change
• why enthalpy change may not be a good guide to direction of change

  direction of change may depend on temperature & pressure

  some endothermic reactions happen under normal conditions

  some exothermic reactions don't happen because rate of reaction is too slow and reactants are inert
• example of how direction of change depends on temperature

  below 100 degrees steam condenses into water - exothermic

  above 100 degrees mater evaporates into steam - endothermic
• example of endothermic reaction that happens under normal conditions

  citric acid and sodium hydrogencarbonate react

  mixture fizzes and cools rapidly
• example of exothermic reaction that doesn't happen

  change from diamond to graphite