Why changes take placeSpontaneous process Takes place ‘naturally’ with no apparent cause or stimulus.Nonspontaneous process Requires that something be done in order for it to occur. Spontaneous Nonspontaneous 6-1
When will a reaction be spontaneous?Spontaneity of a reaction can be determined by a study of thermodynamics. Thermodynamics can be used to calculate the amount of useful work that is produced by some chemical reactions. The two factors that determine spontaneity are enthalpy and entropy. entropy 6-2
EnergyEnergy - the ability to do work.Work - when a force is applied to an object.There are several types of energy:• Thermal - heat• Electrical• Radiant - including light• Chemical• Mechanical - like sound• Nuclear 6-3
EnergyEnergy can be classified as: Potential energy Stored energy - ability to do work. Kinetic energy Energy of motion - actually doing work. Energy can be transferred from one object to another. It can also change form. 6-4
Kinetic vs. potential energyPotentialPotential Energy Energy 6-5
Energy and chemical bondsDuring a chemical reaction • Old bonds break. • New bonds are formed. • Energy is either absorbed or released. Exothermic Energy is released. New bonds are more stable. Endothermic Energy is required. New bonds are less stable. 6-8
Exothermic ReactantsEnergy Products Since excess energy is released, the products are more stable. 6-9
Endothermic ProductsEnergy Reactants Additional energy is required because the products are less stable. 6 - 10
EntropyEntropy - a measure of the ‘disorder’ or randomness of a system. Disorder is favored over order and may account for reaction occurring spontaneously even if it is endothermic. Increased entropy solid gas 6 - 11
Rate of changeNot all spontaneous changes take place in a useful time period. Some may require some initial energy to get them started. spark 2H2 (g) + O2 (g) 2H2O (l) Others can be made faster by adding a catalyst. I- 2H2O2 (l) 2H2O (l) + O2 (g)Kinetics - the study of the rate of a reaction. 6 - 12
Temperature, energy and heatTemperature. An intensive property of a material.Thermal energy. Energy of motion of molecules, atoms or ions. All materials have this energy if at a temperature above 0 K.Heat. Thermal energy transfer that results from a difference in temperature. Thermal energy flows from warm objects to cool ones. 6 - 13
Law of conservation of energy“Energy cannot be created or destroyed in a chemical reaction.” During a reaction, energy can change from one form to another.Example. Combustion of natural gas. Chemical bonds can be viewed as potential energy. So during the reaction: 2CH4 (g) + 3O2 (g) 2CO2 (g) + 2H2O (l) + thermal energy + light some potential energy is converted to thermal energy and light. 6 - 14
A particular slide catching your eye?
Clipping is a handy way to collect important slides you want to go back to later.