Thermochemistry Ch. 20

1. Thermochemistry Ch. 20

2. Chemical Energy • Thermochemistry- study of energy released/absorbed during chemical reactions • Transfer of heat between the system and its surrounding • System- the reaction being observed • Surroundings- everything the system is in contact with • Universe Systems + Surroundings

3. What is Energy? • Types of energy: • Potential vs. Kinetic • Radiant • Thermal • Chemical • Energy- ability to do work/supply heat • Heat (q)- movement of energy from higher concentrations (Hot) to lower concentrations (Cold)

4. Enthalpy • Enthalpy (H)- potential heat contained in a substance or a system • ΔH- transfer of heat energy under constant pressure; measured in kilojoules (kJ) • +ΔH absorption of energy by the system from the surroundings Endothermic • -ΔH release of energy by the system to the surroundings Exothermic

5. Endothermic Reactions • A + B + Energy C +D; +ΔH • Products have more energy than the reactants • Products need to absorb energy to form; energy supply must be constant • Electrolysis of H2O: 2H2O 2H2 + O2 • H2 and O2 have more energy than H2O • Once the energy is cut off, the reaction stops ΔH +572 kJ

6. Exothermic Reactions • A + B  C + D + Energy; -ΔH • Reactants have more energy than products • As reactants breakdown to form produces, excess energy is released (heat or work) • Hydrogen-Oxygen Fuel Cell 2H2 + O2 2H2O • Redox reaction powers battery • Electricity is used to do work; some energy lost as heat ΔH -572 kJ What does the ΔH of the forward and reverse reaction prove? Law of Conservation of Energy

7. Starting a Reaction • All reactions need additional energy to occur • Activation energy- energy required to start a reaction • Endothermic: AE  energy supplied till product is formed • Exothermic: AE energy supplied till reaction is self-sustaining If exothermic reactions have an AE, why are the exothermic? Overall ΔH is negative; energy released is larger than AE

8. Lowering AE • High AE can limit when or how a reaction can occur • Catalysts: • Lower the AE by making the reaction more efficient • Lowers the “randomness” factor 3H2 + N2 –Fe 2NH3 How do catalysts support complex life on Earth? Enzymes (biological catalysts) speed up reactions in a cell

9. Order vs. Disorder • All reactions are controlled by two simple rules: 1) Systems move from high energy to low energy ex. Movement of heat 2) Systems move from order to disorder ex. Smashed glass • Entropy (S): degree of disorder in a system • Is NOT conserved; lost order is not recovered • Motivates reactions to happen spontaneously • Increases with # mols, # molecules formed, phases changes (melting or evaporation)

10. Spontaneous vs. Non-Spontaneous • Both exothermic and endothermic reactions can be spontaneous • Spontaneity determined by level of entropy or energy:

11. Measuring Energy • Calorimeter: measures change in temperature of a liquid surrounding a thermochemical reaction H20 4.184 J/g∙oC • Specific Heat Capacity (C): the amount of heat needed to raise the temp of 1 g of substance 1oC; J/g∙oC • Low C substance heats up/cools down quickly • High C substance heats ups/cools down slowly

12. Specific Heat Capacity (C) • Metals have low C; Non-metals have high C • C= q / (m)(ΔT) q= heat absorbed by the substance; J m= the mass of the substance; g ΔT= change in temp of the substance; oC q= 794 J m= 89.1 g ΔT=51.1oC-22.0oC= 29.1oC C= 794/(89.1)(29.1)= 0.306 J/g∙oC

13. Using a Calorimeter • Heat (q) from the reaction will be absorbed by the water; so using the Specific Heat Capacity of water, we can calculate the energy of the reaction • qwater= (m)(ΔT)(Cwater) m= mass of the water; g ΔT= temp change of the water; oC Cwater= 4.184 J/g∙oC • The heat absorbed by the water is the energy released by the reaction qreaction = -(qwater) +qreaction -qreaction Exothermic Endothermic

14. Calorimeter Practice • A 1.75 g sample of acetic acid, CH3CO2H, was burned in oxygen in a calorimeter. The calorimeter contained 925 g of water its contents increased from 22.2oC to 26.5oC. What is the molar heat of combustion of acetic acid? • qwater= (mwater)(ΔT)(Cwater)  (925g)(26.5-22.2)(4.184) • qwater= 16,641.86 J • qreaction= - qwater  -16,641.86 J -16.6 kJ qreaction= -16.6 kJ__________ = 60.5g CH3CO2H 575.3 kJ/mol 1.75 g CH3CO2H 1mol CH3CO2H

15. Energy in Food • Body burns certain amount of energy everyday • Height/Weight • Activity Level • Male/Female • calorie: 4.184 J • kilocalorie: 1000 calories • Calorie: energy unit for food; 4.184 kJ • If you are going for a long hike what food would you bring?

16. Economics of Energy • Redox reaction electricity • Thermochemistry heat • No energy system is 100% efficient; most energy lost through heat • Modern systems are based on fossil-fuels which are only 63% efficient • Each stage of energy captures lowers that 63% more through their own inefficiencies • Modern coal plant is only 36% efficient

17. Homework • Group PPTs will look into the modern methods of conserving energy and alternative energy sources: 1) Recycling 2) Clean Coal Burning 3) Solar Energy 4) Geothermal 5) Wind Energy 6) Nuclear Power -Present on the pros and cons of the technology -How does the energy production compare to Fossil Fuels -How easily does the technology fit into society