• 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?ΔHis?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?ΔHis?positive

The enthalpy change during an endothermic reaction