2 Definitions Energy is the ability to do work. Energy is conserved (Law of Conservation of Energy).Energy is made up of two parts: Heat and Work.State function: independent of the path, or how you getfrom point A to B.Examples of state functionsvolume, pressure, temperature, density, refractiveindex .
3 Definions Path functions: properties that depend on the path (e.g. work).Work is a force acting over a distance.Work = Force (N) x Distance (m)Heat is energy transferred between objectsbecause of temperature difference
4 Heat Energy Every energy measurement has three parts. a unit ( Joules or calories).a number ( how many).a sign to tell direction.negative - exothermicpositive- endothermic
5 The Universe is divided into two halves. the system and the surroundings.The system is the part you are concerned with.The surroundings are the rest outside the system.Exothermic reactions release energy to the surroundings.Endothermic reactions absorb energy from the surroundings.
6 Quantity of Heat Heat- energy transferred between objects because of temperature difference isexpressed as: q = m x c xHeat lost = Heat gained-q metal = + q water
7 Calorimetry The amount of heat evolved or absorbed in a chemical reaction is meaured by anapparatus known as a calorimeter.
8 Calorimetry q = specific heat x mass x change in temperature q = c x m xwhere q = heat flowC = specific heatm = mass of the substance in grams= change in temperature =Tfinal - Tinitial
9 Specific HeatThe specific heat is the amount of heat required to raise the temperature of one gram of a substance by one degree Celsius.Molar Heat capacity is the amount of heat required to raise the temperature of one mole of a substance by one degree Celsius.
10 The Enthalpy Change The heat content of a material is called enthalpy. It is given the symbol H.The heat flow is exactly equal to thedifference between the enthalpy of theproducts and that of the reactants.
11 Example@Exothermic reaction is associated with a decrease in enthalpywhile endothermic reaction increases the enthalpy
12 Thermochemical Equations A balanced equation with the states specificed and the heat flow listed.Note: coefficients = # of molesstate, l, s, or gspecify temperature
13 Laws of Thermochemistry is directly proportional to the amount ofsubstance that reacts or is produced in areaction.2. for a reaction is equal in magnitute but opposite in sign to for the reverse rxn..3. If a reaction can be regarded as the sum of two or more other reactions, for the overall reaction must be the sum of the enthalpy changes for the other reactions.
14 Hess's Law The third statement above is known as Hess's Law Add________________________
15 Heats of FormationThe molar heat of formation of a compound, , is equal to the enthalpy change, , when one mole of the compound is formed from the elements in their stable forms at and 1 atm.for any reaction is equal to the sum of the heats of formation of the products compounds minus the sum of the heats of formation of the reactant compounds.Note: Any elementary substance in its stable form has zero heat of formation.S(s) +
16 Example Note that the for Al and Fe is 0 respectively since they are elements.Calculate
17 Spontaneous Reactions/Processes A physical or chemical change that occurs without outside influence or intervention. That is, it occurs without obvious reason.CH4(g) + 2O2(g)4Fe(s) + 3O2(g)The above reactions are exothermic and spontaneous.
18 Spontaneous Reactions/Processes H2O(s) H2O(l);NOT exothermic but spontaneous.So, is NOT the sole determinant of spontaneityEntropy plays an important role indetermining whether a reaction or process isspontaneous or not
19 Free Energy Change, GThe capacity of a spontaneous reaction, at constant temperature and pressure, to produce useful work is known as the free energy, G.
20 Free Energy Change, G1. If G is , the reaction at constant temperature and pressure is capable of producing useful work and hence is spontaneous.2. If G is +, work must be supplied to carry out the reaction at const T and P and it is non spontaneous. The reverse reaction is spontaneous.3. If G = 0, the reaction system is at equilibrium.
21 Entropy, S a measure of the disorder or randomness in a system. the tendency towards randomness or disorder Law of Disorderspontaneous processes tend to always proceed to increase the entropy of the universe.
23 Exampleb. Dissolving a gas in a solvent Solution
24 as the number of mol gas decreases Examplec. Chemical reactionas the number of mol gas increases.as the number of mol gas decreases2SO3(g)= # mol gas products - # mol gas reactantsTherefore,(increases)
25 Example d. Solution Formation Solution formation is always accompanied by increase in entropye.Temperature EffectIncrease in temperature is always accompanied by increase in entropy since molecular motion always increase with increase in temperature.
26 The Gibbs-Helmholtz Equation G = H TS1. A negative value of H. Exothermic reaction H 0, will tend to be spontaneous in as much as they contribute to a negative value of G.2. A positive value of S. If the entropy change is positive, S 0, the term TS will make a negative contribution to G. Reactions tend to be spontaneous if the products are less ordered than the reactants.