Chemistry 30 Review Sessions THERMOCHEMISTRY

Chemistry 30 Review Sessions THERMOCHEMISTRY

Curriculum bullets Use Q = mcδt to analyze heat transfer. Use calorimetry data to determine the enthalpy changes in chemical reactions.

Calorimeters rely on the principle of energy conservation: Heat Gained = Heat Lost Q cal = Δ r H mcδt = nδ r H m A Bomb Calorimeter Enthalpy Change in Bomb shows up as Temperature Change in Calorimeter

Typical Heat Lost=Heat Gained Problem Wikipedia gives the molar enthalpy of combustion for methane as 890 kj/mol. What minimum mass of methane must be burned to warm 4.00 L of water from 22.4ºC to 87.6ºC, assuming no heat losses?

Heat Lost = Heat Gained Water increased in temperature heat gained Q Burning Methane (CH 4 ) enthalpy change - ΔH Q = ΔH mcδt= nδ r H m n = mcδt/ Δ r H m n = (4.00 kg)(4.19kj/kg C)(87.6-22.4 C)/(890kJ/mol) n = 1.2278 mol m = nm m = (1.2278 mol)(12.01 +4.04 g/mol) = 19.7 g

Curriculum Bullet design a method to compare the molar enthalpy change when burning two or more fuels evaluate the economic and environmental impacts of different fuels by relating carbon dioxide emissions and the heat content of a fuel provide examples of personal reliance on the chemical potential energy of matter, such as the use of fossil fuels

Comparing Fuels Compare propane with methane in terms of a. energy per gram of fuel b. energy per mole of CO2(g) produced Molar Enthalpies of Combustion propane Δ c H m = -2220 kj/mol methane Δ c H m = -890.4 kj/mol

Energy per mass of Fuel Calculation: molar enthalpy /molar mass = Δ c H m /M Propane: Methane: -2220 kj/mol/44.11g/mol = - 50.33 kj/g -890.4 kj/mol/16.05g/mol = -55.47 kj/g Methane is a little better, i.e. more energy/gram

Comparing Fuels: Energy/CO2 produced CO2(g) contributes to global warming. Write the balanced equation for 1 mole of fuel C3H8(g) + 5 O2(g) 3 CO2(g) + 4 H2O(g) CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g) Calculate : Δ c H m /n CO2 propane Δ c H m /n CO2 = - 2220 kj/mol/3 = -740 kj/mol methane Δ c H m /n CO2 = -890.4 kj/mol/1 = -890.4 kj/mol Methane gets you more energy per CO2 - a good thing!

Curriculum Bullet Explain, in a general way, how stored energy in the chemical bonds of hydrocarbons originated from the sun. Hydrocarbon combustion releases energy, so HCs are storing energy, but Hydrocarbons are decomposed living things! d all living things rely upon photosynthe which just converts light to chemical energy and light comes from the sun so light from the sun provides the energy which builds organic molecules

Curriculum bullet Define enthalpy and molar enthalpy Write balanced equations for chemical reactions that include energy changes. Classify endothermic or exothermic, including : photosynthesis, cellular respiration and hydrocarbon combustion. Use and interpret ΔH notation

One of these ain t like the other ones, one of these just ain t the same State the molar enthalpy 2 Na(s) + Cl 2 (g) 2 NaCl(s) Δ f H m = -411.2 kj/mol NaCl Include the absolute value of the energy in the balanced equation. 2 Na(s) + Cl 2 (g) 2 NaCl(s) + 822.4 kj State the enthalpy change at the end of the reaction. 2 Na(s) + Cl 2 (g) 2 NaCl(s) ΔH= - 822.4 kj

What s the energy change here? The molar enthalpy of formation of sodium chloride solid is -411.2 kj/mol. This reaction is balanced if i kj of energy is included at position ii. 2 NaCl(s) + x 2 Na(s) + Cl 2 (s) + y i ii a 411.2 x b 411.2 y c 822.4 x d 822.4 y

Hess Law Predict the enthalpy change for chemical equations using standard enthalpies of formation. Explain and use Hess law to calculate energy changes for a net reaction from a series of reactions.

Solve for Δ r H by summation SO 3 (g) + H 2 O(g) H 2 SO 4 (l) ΔH =? Calculate the enthalpy of the above reaction using: H 2 S(g) + 2O 2 (g) H 2 SO 4 (l) ΔH= -78.5 KJ H 2 S(g) + 2O 2 (g) SO 3 (g) + H 2 O(l) ΔH= -69 KJ H 2 O(l) H 2 O(g) ΔH= +11 KJ

Solve for Δ r H with formation reactions George Bush has made corn very expensive because he plans to make automobile fuel out of ethanol which is made from corn. 1. Write the balanced chemical equation for the combustion of liquid ethanol. C 2 H 5 OH(l) + 3 O 2 (g) 3 H 2 O(g) + 2 CO 2 (g) 2. Calculate the enthalpy change for the balanced reaction. Δ r H = [Sn Δ f H m ] prod [Sn Δ f H m ] reac

Δ r H = [Sn Δ f H m ] prod [Sn Δ f H m ] reac Δ r H = [Sn Δ f H m ] prod [Sn Δ f H m ] reac Δ r H =[ 3 x h20+ 2 x co2 ] [1 x ethanol] Δ r H =

Curriculum Bullet Identify that liquid water and carbon dioxide gas are reactants in photosynthesis and products of cellular respiration gaseous water and carbon dioxide gas are the products of hydrocarbon combustion in an open system. 6 H 2 O(l) + 6 CO 2 (g) + 2802.5 kj 6 O 2 (g) + C 6 H 12 O 6 (s) CH 4 (g) + 2 O 2 (g) 2 H 2 O(g) + CO 2 (g) + 890 kj

Curriculum Bullet Define activation energy as the energy barrier that must be overcome for a chemical reaction to occur. draw and interpret enthalpy diagrams for chemical reactions

Curriculum Bullet Explain the energy changes that occur during chemical reactions, referring to bonds breaking and forming and changes in potential and kinetic energy. Analyze and label energy diagrams of a chemical reaction, including reactants, products, enthalpy change and activation energy.

Enthalpy Diagrams Use the enthalpy diagram to determine the molar enthalpy of reaction for: S 8 (s) and Cl 2 (g) SCl 4 (l) SCl 2 (g) + Cl 2 (g) ΔH (kj/mol) 19.7 SCl 2 (g) 56.1 SCl 4 (l)

The Effect of a Catalyst 1 How does the graph show the enthalpy change is not affected by the catalyst? 2. What is the benefit of the catalyst? 3. Where is the activation energy of the forward reaction. 4. Where is the activation energy of the reverse reaction? 5. Is the forward reaction endo or exo?

Curriculum Bullet Explain that catalysts increase reaction rates by providing alternate pathways for changes, without affecting the net amount of energy involved; e.g., enzymes in living systems.