2 The First Law of Thermodynamics Energy can neither be created or destroyedThe energy of the universe is constant, but it can change forms.
3 Energy book keeperFirst Law accounts for energy, but it does not tell us why a particular process occurs in a given direction
4 Spontaneity DOES NOT MEAN FAST!!! Means that the process occurs without any outside intervention
5 Energy released or absorbed during a chemical reaction (heat of reaction) is equal to the difference between the potential energy of the products and the potential energy of the reactants.In a chemical reaction; reactants --> productsΔPE = PE products - PE reactantsPE can be thought as heat energy (H)Therefore, ΔH (kJ) = H products - H reactants
6 When ΔH is negative H products < H reactants and the reaction is exothermic.
7 When ΔH is positive H products > H reactants and the reaction is endothermic.
8 Energy released or absorbed by a chemical reaction can be represented by a potential energy diagram.
9 Activation EnergyThe activation energy is the minimum energy required to start a chemical reaction by providing colliding molecules with enough energy for effective collisions to occur.The activated complex is the short-lived and unstable intermediate species located at the highest of the activation energy.
10 CatalystsA catalyst provides an alternate reaction pathway, which has a lower activation energy than an uncatalyzed reaction.
11 Look at Table A-6 Notice the following: Substances are in alphabetical orderΔHf (enthalpy of formation) is in kJ/moleFree elements have a ΔHf = 0(they are not compounds formed from elements)The enthalpy of reaction is equal to the sum of the enthalpies of formation for the products – the sum of the enthalpies of formation for the reactants Σ ΔHr = ΔHf products - Σ ΔHf reactants
15 Which has more entropy? 1. Solid or gaseous phosphorus 2. CH4(g) or C3H8(g)3. NaCl(s) or NaCl(aq)
16 Second Law of Thermodynamics In any spontaneous process there is always an increase in the entropy of the universeThe entropy of the universe is constantly increasing
17 ENTROPY CALCULATIONSIf the reaction increases entropy, ∆S is positive and the reaction is said to be ENTROPY-FAVOREDCalculate the entropy change(∆S) for the following reactionCH4(g) O2(g) CO2(g) H2O(l)
19 Suniv If it is +, the entropy of the universe is increasing Process is spontaneousIf it is negative, the process is not spontaneous
20 Change of state H2O(l) H2O(g) What happens to the S of the water? Ssys= +
21 What about surroundings? Heat is flowing from the surroundings to the systemRandom motion of particles decreasesSsurr = -
22 Which S controls the situation? DEPENDS ON TEMP Is it spontaneous?Need to look at SunivWhich S controls the situation?DEPENDS ON TEMP
23 Exothermic Process Always increases entropy of surroundings But, its significance depends on the temp at which the process occursEnergy transfer will be more significant at lower temps
24 GIBBS FREE ENERGY CH4(g) + 2O2(g) CO2(g) + 2H2O(l) Gibbs free energy (∆G) is a measure of the chemical reaction potential of a systemIf ∆G is negative, the reaction is spontaneousIf ∆G is positive, the reaction is not spontaneousCalculate the change in free energy for the following reactionCH4(g) O2(g) CO2(g) H2O(l)
25 Gibbs Free Energy ∆H ∆S ∆G Enthalpy and Entropy can be combined to predict reaction spontaneity∆G = ∆H - T∆S∆H∆S∆GComments on Reaction-+Always spontaneous+ or -Spontaneous at high temperaturesSpontaneous at low temperaturesNever spontaneous