Energy Changes in Reactions (DP IB Chemistry: SL)

• What is the difference between heat and temperature?
• This can be illustrated using a beaker of boiling water and a pipette:

The effects of boiling water can be quite different depending on the quantity of water involved!

• You would get a very nasty burn if a whole beaker of boiling water spilled onto to your hand, but a drop of boiling water would cause little problem
• The water is at the same temperature in the pipette as in the beaker, but the beaker has a much higher heat content
• We say that temperature is a measure of the average kinetic energy of the particles whereas heat is a measure of the energy content of a substance
• The particles have kinetic energy because they are moving
  • The faster they move the more energy they have and the higher the temperature of the substance

• Energy is a measure of the ability to do work
• There are many different types of energy and heat is only one of them
• During chemical reactions energy flows in and out of the reaction vessels
  • Inside the reaction vessel is known as the system
  • Outside the reaction vessel is known as the surroundings

• Systems come in three types: open, closed and isolated:

Three types of systems

• Isolated systems are very rare; most chemical reactions are open systems
• Open systems are very important when thinking about chemical equilibrium which is covered in Topic 7
• Although energy can be exchanged between open and closed systems and the surroundings, the total energy of the process cannot change
• This is known as the Law of Conservation of Energy and is a cornerstone to understanding how chemical changes affect the energy flow in and out of systems

• The total chemical energy inside a substance is called the enthalpy (or heat content)
• When chemical reactions take place, changes in chemical energy take place and therefore the enthalpy changes
• An enthalpy change is represented by the symbol ΔH (Δ= change; H = enthalpy)
• An enthalpy change can be positive or negative

Exothermic reactions

• A reaction is exothermic when the products have less enthalpy than the reactants
• Heat energy is given off by the system to the surroundings
  • The temperature of the surroundings increases
  • The temperature of the system decreases

• There is an enthalpy decrease during the reaction so ΔH is negative
• Exothermic reactions are thermodynamically possible (because the enthalpy of the reactants is higher than that of the products)
• However, if the rate is too slow, the reaction may not occur. In this case the reaction is kinetically controlled

The enthalpy change during an exothermic reaction

Endothermic reactions

• A reaction is endothermic when the products have more enthalpy than the reactants
• Heat energy is absorbed by the system from the surroundings
  • The temperature of the surroundings decreases
  • The temperature of the system increases

• There is an enthalpy increase during the reaction so ΔH is positive

The enthalpy change during an endothermic reaction