ENERGY *FORMS *KINDS *PHASE DIAGRAMS *PE DIAGRAMS Kenneth E. Schnobrich

Energy: Physical & Chemical Changes In our discussion of matter and its changes we discussed both physical and chemical change. As we convert a solid to a liquid or a liquid to a gas there is an energy change that takes place. In our discussion of chemical changes it is important to remember that bonds are being made and broken as we form new substances. This making and breaking of bonds involves energy.

FORMS OF ENERGY Mechanical Chemical Thermal (Heat) Radiant (Light) Electrical Nuclear* Interconvertibility *Nuclear changes are not a normal part of the chemical reactions we will study.

FORMS OF ENERGY Kinetic Energy – energy of motion (temperature is a measure of average kinetic energy) Potential Energy – stored energy or energy of position. The energy a system has as a result of forces interacting

KINETIC ENERGY On a macroscopic level: On a molecular or atomic level A car rolling down a hill A skier skiing downhill A book falling off a desktop On a molecular or atomic level Translational motion Rotational motion Vibrational motion

POTENTIAL ENERGY On an everyday level On atomic level Stretching a rubber band Falling off a ladder Jumping down a series of stairs On atomic level The interaction of the protons & electrons The energy stored in a chemical bond Changing phase solid to liquid to gas

PHASE DIAGRAMS Phase diagrams are used to show the energy changes as we convert from one phase to another phase Gas Liquid Gas Temperature (oC) Liquid Solid Liquid Solid Time

PHASE DIAGRAMS Here we are showing changes in potential and kinetic energy D KE Endothermic D PE Temperature (oC) D KE D PE Exothermic D KE Time

PHASE DIAGRAMS Energy calculations taken from the phase diagram Q = m(C)DT Q = m(Hv) Temperature (oC) Q = m(C)DT Q = m(Hf) Q = m(C)DT Time

Some Calculations: Q = m(C)DT Q = (50g)(2.09 J/goC)(10oC) Calculate the energy in Joules to convert 50 grams of ice at -10oC to 0oC? The specific heat capacity (C) of ice is 2.09 J/goC The mass (m) is 50 grams The change in temperature (DT) is 10oC Q = m(C)DT Q = (50g)(2.09 J/goC)(10oC) Q = 1045.00 J

Some Calculations: Q = m(Hf) Q = (50g)(336.0J/g) Q = 16,800 J Calculate the energy in Joules to convert 50 grams of ice at 0oC to water at 0oC? The heat of fusion of ice is 336.0 J/g The mass (m) is 50 grams There is no change in temperature (DT) – phase change Q = m(Hf) Q = (50g)(336.0J/g) Q = 16,800 J

Here is one for you: What is the total amount of energy needed to convert 10g of water from -20oC to 120oC? Cp of H2O(s) = 2.09 J/goC C pof H2O(g) = 2.09 J/goC Cp of H2O(l) = 4.18 J/goC Hf = 336 J/g Hv = 2260 J/g MP = 0oC BP = 100oC *Think about the phase diagram

POTENTIAL ENERGY DIAGRAMS In cases where there is an actual chemical reaction, PE diagrams are useful tools to help understand what is happening energywise as we convert reactants to products. Heat Content (Enthalpy) of Reactants Heat Content (Enthalpy) of Products Activation Energy (forward & reverse rx) Activated complex (Intermediate) Heat of Reaction (DH) * Reactions generally do not occur in a single step

PE Diagram(Exothermic) Activated Complex Activation Energy (forward) Potential Energy Activation Energy (reverse) Heat of Reactants Heat of Rx (DH) = - Heat of Products Reaction Pathway

PE Diagram(Exothermic) Activated Complex Activation Energy (reverse) Activation Energy (forward) Heat of Products Potential Energy Heat of Rx (DH) = + - Heat of Reactants Reaction Pathway

Driving Forces Remember, there are two fundamental driving forces for changes that occur – ENERGY ENTROPY Energy – if it is at all possible, systems like to change from a higher energy state to a lower energy state. From an energy standpoint exothermic processes are favored. Entropy – entropy describes the relative disorder of a system. It is natural for systems to seek a state of higher entropy (greater disorder). Entropy is temperature dependent, so we frequently speak of T∆S If T∆S is + it means that the entropy of the system (relative disorder) is increasing – this is a favored change If T∆S is – it means that the entropy of the system (relative disorder) is decreasing – this is not a favored change

Driving Forces For energy changes – if ∆H = - it means that the change is exothermic and that represents a favored change if ∆H = + it means the change is endothermic and that does not represent a favored energy change Remember that ∆H represents the change in energy and is also called the Enthalpy change If both the change in Entropy (T∆S) and the change in Enthalpy (∆H) are favorable the change will be spontaneous (T∆S = + and ∆H = -) For other cases it is helpful to calculate what we call the change in Free Energy (∆G). ∆G = ∆H – T∆S If ∆G = - it means the reaction will still be spontaneous If ∆G = + it means that energy will be required to bring about the change

Driving Forces if ∆G = 0 it means the system has reached a point where the two driving forces are equal and the system has reached equilibrium. Entropy like Enthalpy has units associated with it and they are generally J/K or J/mol•K (they are sometimes called eu’s – entropy units) If the ∆H for a reaction is – 5000 J and the value for T∆S is -7000 J the value for ∆G will be +2000 J, this means that the reaction (change) is not spontaneous ∆G = ∆H – T∆S = (-5000 J) – (-7000 J) = +2000 J There are times when the driving forces work in opposition to one another.