Topic 1 - Energy in reactions

ENERGY IN CHEMICAL REACTIONS

• The scientific definition of energy is ‘the capacity to do work’. Energy can be stored and changed from one form to another, but it is never created or destroyed.
• In chemistry, energy changes are almost always movements of HEAT.
• Light or sound produced in a chemical reaction are also forms of energy being released.
• Many reactions give out a lot of heat. The combustion reactions of hydrocarbons give out heat. Other examples are the reaction of sodium or potassium with water. 

• exothermic reactions = reactions that give out heat.
• Reactions generally have some kind of energy barrier between the start and the finish. You might represent the energy change in an exothermic reaction by means of an energy level diagram
• activation energy= the minimum quantity of energy which the reacting species must possess in order to undergo a specified reaction.

• endothermic reactions= reactions in which the substance absorbs energy from its surroundings. Heat must be supplied all the time to keep the reaction going.

• Exothermic compounds are stable and are formed with the evolution of heat.
• Endothermic compounds are unstable and are formed with the absorption of heat.

REVERSIBLE REACTIONS

• reversible reactions= a chemical reaction where the reactants form products that, in turn, react together to give the reactants back.

QUANTIFYING THE ENERGY CHANGE

• The heat evolved or absorbed in a chemical reaction is often called delta-H or ▲H and expressed in kilojoules per mole.
• ▲H = the enthalpy change of the reaction.
• The joule is the unit of…