Chapter 17 Energy Heat, Energy, and Temperature changes Pages 510-547 Thermochemistry: Causes of Change in Systems Thermodynamics: Movement of energy in/out of a system Heat, Energy, and Temperature changes Standard unit of heat is the Joule, J Standard unit of temperature is Kelvin, K

Heat vs Temperature Heat Temperature measure of energy change in a system. Temperature measure of the kinetic energy (movement) of the particles in a system. Gaining or losing heat energy in a substance can change its temperature. Exothermic System loses energy to surroundings Endothermic System gains energy from surroundings

Specific Heat Specific Heat The symbol we use is cp. Specific Heat… measure of how a substance reacts to heat energy changes. The symbol we use is cp. The “p” stands for constant pressure while heat is added or lost. Specific Heat… is the heat energy required to raise one gram of a pure substance one degree Celsius. is a property of matter, and different species have different Specific Heat.

Specific Heat Capacity Substance J/g/oC or J/g/K cal/g/oC or cal/g/K Water (0 oC to 100 oC) 4.186 1.000 Zinc .387 0.093 Ice (-10 oC to 0 oC) 2.093 0.500 Steam (100 oC) 2.009 0.480 Brass .380 0.092 Wood (typical) 1.674 0.400 Soil (typical) 1.046 0.250 Air (50 oC) Aluminum .900 0.215 Tin .227 0.205 Glass (typical) .837 0.200 Iron/Steel .452 0.108 Copper 0.0924 Silver .236 0.0564 Mercury .138 0.0330 Gold .130 0.0310 Lead .128 0.0305 Metals have very low cp, which is why metals often feel cold to the touch. Table on page 513 Water has a very high cp, 4.184 J/g·0C Substances with lower cp will rise in temperature faster and require less energy to do so than do substances with high cp. 1 calorie=4.184 Joules

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). Elements in their standard (unbonded) state have a ΔH of zero. O2  ΔH=0 kJ/mol, Fe(s)  ΔH=0 kJ/mol

Enthalpy, ΔH ΔHfus = heat of fusion ΔHvap = heat of vaporization Some common changes involving ΔH: ΔHfus = heat of fusion ΔHvap = heat of vaporization ΔHcond = heat of condensation ΔHsub = heat of sublimation ΔHrxn = heat of reaction ΔHf = heat of formation ΔHsol = heat of solution ΔHcomb = heat of combustion State change to/from? Sign of ΔH?

Devices to Measure ΔHcomb 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. A Bomb Calorimeter uses a chamber of pure oxygen to measure heats of combustion to the 1/1000 of a joule. When two objects are in contact, they eventually obtain Thermal Equilibrium; their temperatures become equal. Text page 519

Reaction Enthalpy If ΔH is negative, the reaction is exothermic. C6H12O6 + 6O2 6CO2 + 6H20 + 2870kJ ΔHrxn = -2870 kJ/mol If ΔH is positive, the reaction is endothermic. 2H2O + 571.6kJ  2H2 + O2 ΔHrxn = +571.6 kJ/mol energy

Spontaneity Spontaneous A reaction that will proceed on its own once started. Sometimes, all the reaction needs to get going is the kinetic energy of nearby atoms. Spontaneous combustion occurs when the kinetic energy of oxygen molecules striking a fuel have enough energy on their own to start the combustion reaction.

Spontaneity In the diagram, the hump is called a reaction energy barrier - the amount of energy required for the reaction to begin. We can reduce this barrier with an appropriate catalyst. All reactions have some sort of reaction barrier (sometimes very small) The energy needed to overcome this barrier is the activation energy

DHrxn Exothermic Reaction: products have a lower energy state than do the reactants. How are things different in an endothermic reaction?

Hess’s Law 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. Consider Species A, B, C, D : A + B + energy  C then C + energy D Must be the same as A + B + 2energy  D 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.

Hess’s Law Enthalpy of Reaction ΔHrxn = Hproducts – Hreactants = ΣHf, all the products – ΣHf, all the reactants “ Sum of ” Enthalpy of formation Example on page 330 Do # 16, p332

ΔSrxn = ΣSproducts – ΣSreactants Entropy, ΔS Entropy is a measure of relative disorder. Thermodynamics tells us that the universe tends towards disorder or entropy. Entropy calculations are very similar to enthalpy calculations: ΔSrxn = ΣSproducts – ΣSreactants Entropy has the unit J/K*mol

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

Temperature and Entropy Recall: Kinetic Molecular Theory (Particle Model): All matter is made up of particles in constant and random motion. Temperature is a measurement of the kinetic energy (movement) of particles in a system. Increasing temperature increases particle movement, increasing the disorder in a system, increasing its entropy.

Entropy Values We can make generalizations about a reaction’s entropy; 2KClO3(s)  2KCl(s) + 3O2(g) 2 solids  2 solids + 3 gases Entropy appears to increase in this reaction.

143.7 J/mol*K  82.6 J/mol*K + 205.1 J/mol*K Entropy Values We can assign values to a species’ entropy of formation, called standard entropy, and calculate a reaction’s entropy quantitatively. 2KClO3(s)  2KCl(s) + 3O2(g) 143.7 J/mol*K  82.6 J/mol*K + 205.1 J/mol*K Using ΔSrxn = Sproducts – Sreactants, the reaction has a total entropy change of +493.1J/mol*K

Negative Enthalpy (-ΔH) Positive Entropy (+ΔS) 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)

FREE ENERGY !!!! Free Energy: A measurement of a system’s ability to change. Or, a measurement of the force required for a system to change. We can link both enthalpy and entropy into one value, called the Gibbs Free Energy value, ΔG, named in honor of American chemist, J. Willard Gibbs, who developed an equation to relate enthalpy and entropy to whether or not a reaction was likely to occur.

ΔG = ΔH -TΔS 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. The basic equation for free energy is: ΔG = ΔH -TΔS Free Energy Enthalpy Entropy temperature in Kelvin

Gibbs Free Energy, ΔGrxn Negative Gibbs Energy (-ΔGrxn) Spontaneous, Product favored Positive Gibbs Energy (+ΔGrxn) Nonspontaneous, Reactant favored A ΔG of zero means that neither the products or reactants are favored-the reaction is in equilibrium.

DG = DH - TDS

Some factors will affect reaction rate: 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) Catalyst presence: catalysts make rxns faster Catalysts reduce activation energy! End of C17, conclusion follows

In conclusion… Specific Heat Capacity, cp Enthalpy, ΔH Entropy, ΔS the amount of heat energy required to raise 1 gram, 1 degree Enthalpy, ΔH the heat energy transferred in a reaction Entropy, ΔS the change in disorder of the species in a reaction Gibbs Free Energy, ΔG measure of spontaneity; how product favored or reactant favored a reaction is

In conclusion… Specific Heat Capacity, cp Enthalpy, ΔH Entropy, ΔS Hey! 1 calorie=4.18 joules! In conclusion… Specific Heat Capacity, cp Joules / gram * degree Enthalpy, ΔH kilojoules / mole Entropy, ΔS Joules / mole*Kelvin Gibbs Free Energy, ΔG Although you can use degrees Celsius or Kelvin for the first three, you MUST use Kelvin with Gibbs Free Energy Recall that K = C + 273.15 End of Chapter 17

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

Aligned Labs and Demos Lab: Flaming Peanut Calorimetry Lab Lab: Metals Lab Lab: NaOH-HCl Enthalpy of Reaction Lab Lab: KI-H2O2 Kinetics Lab Demo: Carbon Snake (Kinetics)