Endothermic & Exothermic Reactions Energy transfers during chemical reactions

1. When a chemical reaction occurs… bonds between the atoms are broken and new and different bonds are formed a) The old bonds in the reactants are broken b) Atoms rearrange themselves and form new products. c) New bonds are formed in the products

2. Energy is required to break any bond The reaction continues until one/all of the reactants are used up Bond breaking is usually initiated by heat, light, electricity, agitation, etc.

Not this kind of Agitation…

2. Energy is required to break any bond Enthalpy is a measure of the heat content within a system (∆H) - A system is where the reaction occurs (could be a beaker is the system) - Measured in units of kJ/mol (kilojoules per mole)

Exothermic reactions 1) Heat is given off during the reaction 2) Heat energy is measured in kilojoules (kJ), but the heat of reaction is measured in kJ/mol 3) Because heat energy is released, the heat content or enthalpy (∆H) is less after the reaction than before 4) The reactants contain more heat energy than the products

Exothermic reactions ∆H of the system is negative as you have lost heat energy to the surroundings from the system. (∆H = -80kJ) The products are more stable than the reactants (because the products have less energy) “When bonds are broken, energy is absorbed but when bonds are formed, energy is released”. The energy released by the bond formation in the products is greater than the energy needed to break the bonds in the reactants (so the excess is released as heat or light!)

Exothermic reactions This reaction will continue on its own (with no additional input of energy) until one or all reactants is used up. When writing exothermic reactions, the energy is written as part of the products Since the surroundings are absorbing the released energy – the surroundings get warmer Ex: __________________________ Making ice cubes = exothermic!

Activation Energy – energy needed to start reaction

How would we write this equation? ∆H = -20kJ When 1 mole of A reacts with 1 mole of B A + B  C + D +20 kJ How much enthalpy if 2 moles of it decompose? -40kJ!

A very EXOTHERMIC Reaction… Combustion: Burning Methane (Natural Gas CH4 + O2  CO2 + H2O

Endothermic reactions Energy is absorbed from the surroundings (e.g. air, container, hand, etc.) The ∆H is positive since energy is taken in (∆H =+80kJ). The products have more heat content (enthalpy) than the reactants. Once initiated, energy must be continuously supplied in order for the reaction to go on (as it is being used up by the reaction itself).

How would a catalyst change things?

Endothermic reactions The new bonds formed do not produce enough energy to break enough of the old bonds in the reactants. The products are less stable than the reactants as they have more heat content. (e.g. dynamite, TNT) Since the surroundings are losing energy – the surroundings get colder. When endothermic reactions are written, the energy is included as part of the reactants. Ex: The decomposition of sulfur dioxide is endothermic. 8 SO2(g) + Energy  S8(s) + 8 O2(g) Melting ice cubes = endothermic!

How would we write this equation? K+ ClO3- ∆H = +50kJ KClO3 When 1 mole of KClO3 decomposes… KClO3 + 50kJ  K+ + ClO3-

Do Hebden Pg. 120 #69-79 (odds)