Chapter 17: Energy and Kinetics

Transcription

1 Pages S K K Chapter 17: Energy and Kinetics Thermochemistry: Causes of change in systems Kinetics: Rate of reaction progress (speed) Heat, Energy, and Temperature changes S J J

2 Heat vs Temperature Heat measure of energy change in a system. Temperature measure of the average kinetic energy (movement) of the particles in a system. Exothermic System loses energy to surroundings Endothermic System gains energy from surroundings K = C Bires, 2010-b Slide 2

3 Specific Heat Specific Heat measure of how a substance reacts to heat energy changes. Think thermal inertia is the heat energy required to raise one gram of a pure substance one degree Celsius. is a property of matter; different species have different Specific Heats. The symbol we use is c p. The p stands for constant pressure while heat is added or lost. Bires, 2010-b Slide 3

4 Specific Heat Capacity Metals have very low c p, which is why metals often feel cold to the touch. Water has a very high c p, J/g 0C Substances with lower c p will rise in temperature faster and require less energy to do so than do substances with high c p. Substance Water (0 o C to 100 o C) J/g/ o C or J/g/K cal/g/ o C or cal/g/k Zinc Ice (-10 o C to 0 o C) Steam (100 o C) Brass Wood (typical) Soil (typical) Air (50 o C) Aluminum Tin Glass (typical) Iron/Steel Copper Silver Mercury Gold Lead Bires, 2010-b Slide 4

5 Specific Heat Capacity Cp (H 2 O) = C p units are J/g 0 C) Q Change in heat (joules, J) = Change in temperature (degree, 0 C) x Mass (mass, g) x = ΔT m 1 calorie = Joules Specific Heat Capacity (4.184 for water) c p Bires, 2010-b Slide 5

6 Specific Heat Example Exercise Q = ΔT m c p Determine the specific heat of 34 grams of an unknown material if 485 J of heat are absorbed to change the temperature by 20.0 o C. If 950 J of heat are added to 5.4 ml of water at 280 K, what will be the resulting temperature of the water? (hint: ml g) Bires, 2010-b Slide 6

7 The Calorimeter The Calorimeter (shown) Heat energy is transferred from a reaction inside the calorimeter to the water in the calorimeter. The temperature change of the water is observed. Text page 519 When two objects are in contact, they eventually obtain Thermal Equilibrium; their temperatures become equal. Bires, 2010-b Slide 7

8 Enthalpy, ΔH Enthalpy heat energy transferred for a specific change to take place. We specify enthalpy with ΔH. Δ means change in. Exothermic reaction negative enthalpy (-ΔH ) Endothermic reaction Positive enthalpy (+ΔH). The universe favors LOW energy states -if the products have lower energy reaction is favored. Elements in their standard (elemental) state have a ΔH of zero. O 2, Fe, Cu, N 2, He, etc are have H f = 0 kj/mol Bires, 2010-b Slide 8

9 Enthalpy, ΔH Some common changes involving ΔH: ΔH fus = heat of fusion ΔH vap = heat of vaporization ΔH cond = heat of condensation ΔH sub = heat of sublimation ΔH rxn = heat of reaction ΔH f = heat of formation ΔH sol = heat of solution ΔH comb = heat of combustion State change to/from? Sign of ΔH? Changes of State.mov Bires, 2010-b Slide 9

10 Phase Changes 1. Solid + heat = temp 2. Solid + heat = phase change 3. Liquid + heat =? 4. And then? 5. Gas + heat =? temperature temperature Heat added Heat added ΔQ = cmδt ΔQ = ml Bires, 2010-b Slide 10

11 Reaction Enthalpy If ΔH is negative, the reaction is exothermic. C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O kJ ΔH rxn = kj/mol energy If ΔH is positive, the reaction is endothermic. energy 2H 2 O kJ 2H 2 + O 2 ΔH rxn = kj/mol Bires, 2010-b Slide 12

12 Spontaneous Spontaneity A reaction that will proceed on its own once started. Sometimes, all the reaction needs to get going is the kinetic energy of nearby colliding atoms. Kinetic Molecular Theory: All Matter is made of particles in constant motion Some collisions are more energetic than others. Why? Spontaneous combustion occurs when the kinetic energy of colliding oxygen molecules striking a fuel have enough energy on their own to start the combustion reaction. Bires, 2010-b Slide 13

13 ΔH rxn Exothermic Reaction: products have lower energy than do the reactants. What if endothermic? Bires, 2010-b Slide 14

14 In the diagram, the hump is called a activation energy barrier - the amount of energy required for the reaction to begin. Activated complex We can reduce the activation energy with a catalyst. All reactions require some sort of activation energy, E a. Bires, 2010-b Slide 15

15 Hess s Law: If two reactions begin with the same reactants in the same condition and end with the same products in the same condition, they must have the same enthalpy change. It doesn t matter if you perform a reaction in several steps or produce your final product in one step, the enthalpy change will be the same. Consider the reaction A + B D : A + B kj C then C + 50 kj D Must be the same as A + B kj D Bires, 2010-b Slide 16

16 Hess s Law Enthalpy of Reaction ΔH rxn = H products H reactants = ΣH f, all the products ΣH f, all the reactants Sum of Enthalpy of Formation Bires, 2010-b Slide 17

17 Hess s Law Example Exercise ΔH rxn = H products H reactants Calculate the heat of reaction when 350 grams of methane, CH 4 are burned in excess oxygen. H f book values for each species are: CH 4(g) = kj/mol O 2(g) =? H 2 O (g) CO 2(g) = kj/mol = kj/mol Bires, 2010-b Slide 18

18 Entropy Entropy, ΔS is a measure of relative disorder. Thermodynamics tells us that the universe tends towards disorder or entropy. Temperature affects entropy (why?) Entropy calculations are very similar to enthalpy calculations: EntropyandTemperature.swf ΔS rxn = ΣS products ΣS reactants Entropy has the unit J/K*mol Bires, 2010-b Slide 19

19 Entropy, ΔS The universe tends towards entropy entropy plays a part in predicting whether or not a reaction will be spontaneous. Solids have very low entropy Gases have very high entropy Solutions also have high entropy Bires, 2010-b Slide 20

20 Qualitative Entropy Values We can make generalizations about a reaction s entropy; 2KClO 3(s) 2KCl (s) + 3O 2(g) 2 solids 2 solids + 3 gases Entropy appears to increase in this reaction. Bires, 2010-b Slide 21

21 Quantitative Entropy Values 2KClO 3(s) 2KCl (s) + 3O 2(g) S of KClO 3(s) S of KCl (s) S of O 2(g) = J/mol*K = 82.6 J/mol*K = J/mol*K Using ΔS rxn = S products S reactants, the reaction has a total entropy change of J/mol*K Bires, 2010-b Slide 22

22 Entropy Values A positive ΔS = increase in entropy A negative ΔS = decrease in entropy Do not confuse entropy and enthalpy! Tending toward spontaneity: Negative Enthalpy (-ΔH) Positive Entropy (+ΔS) Bires, 2010-b Slide 23

23 Free energy, ΔG: Free Energy, ΔG allows us to assign a value to an entire reaction to predict whether a reaction is spontaneous, product favored. or nonspontaneous, reactant-favored. Named for American Chemist, J. Willard Gibbs ΔG ΔH TΔS Free Energy kj/mol Enthalpy kj/mol Entropy J/mol K temperature in Kelvin Bires, 2010-b Slide 24

24 Gibbs Free Energy, ΔG rxn Negative Gibbs Energy (-ΔG rxn ) Spontaneous, Product favored Positive Gibbs Energy (+ΔG rxn ) Nonspontaneous, Reactant favored A ΔG of zero means that neither the products nor reactants are favored-the reaction is in equilibrium. Bires, 2010-b Slide 25

25 ΔG = ΔH - TΔS Bires, 2010-b Slide 26

26 Reaction Rates Reaction rates how fast a reaction proceeds. Some factors will affect reaction rate: Temperature of reactants: higher = faster Concentration of reactants: greater = faster Surface area of reactants: greater = faster (powders react faster than chunks) Pressure of gaseous reactants: greater = faster Catalyst presence: catalysts make rxns faster reduce activation energy! are not used up (not reactants) Bires, 2010-b Slide 27

27 Rate Laws For any reaction: aa + bb cc + dd The rate is based on the [reactants]: Rate = k[ A][ B ] [X] : 1 st order : 2x [A], 2x rate [X] 2 : 2 nd order : 2x [A], 4x rate [X] 3 : 3 rd order : 2x [A], 8x rate Rate = k[a] 2 Rate = k[a] 3 Rate = k[a] End of C17, conclusion follows Bires, 2010-b Slide 28

28 In conclusion Recall that K = C Specific Heat Capacity, c p (J/gK) the amount of heat energy required to raise 1 gram, 1 degree Enthalpy, ΔH (kj/mol) the heat energy transferred in a reaction Entropy, ΔS (J/mol-K) the change in disorder of the species in a reaction Gibbs Free Energy, ΔG (kj/mol) measure of spontaneity; how product favored or reactant favored a reaction is Bires, 2010-b Slide 29

29 CCSD Syllabus Objectives 16.1: Thermodynamics, definition 16.2: Exothermic/Endothermic 16.3: Changes in Enthalpy 16.4: Thermochemical Calculations 16.5: Energy Diagrams 16.6: Enthalpy-Entropy-Free Energy 17.1: Kinetics Definition 17.2: Factors that Affect Reaction Rate Bires, 2010-b Slide 30

30 Aligned Labs and Demos Lab: Flaming Cheeto Calorimetry Lab Lab: Metals Calorimetry Lab Lab: NaOH-HCl Enthalpy of Reaction Lab Lab: Ba(OH) 2-8H 2 O w/ NH 4 NO 3 and H 2 O 2 with a catalyst Free Energy Lab Lab: KI-H 2 O 2 Kinetics Lab Demo: Carbon Snake with powdered vs granular sugar (Kinetics) Bires, 2010-b Slide 31