Thermochemistry Ch. 20

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

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)

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

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

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

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

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

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)

Spontaneous vs. Non-Spontaneous Both exothermic and endothermic reactions can be spontaneous Spontaneity determined by level of entropy or energy: Reaction Type Entropy Spontaneous? Exothermic (losing energy) increasing Yes decreasing Yes at low temps Endothermic (gaining energy) Yes at high temps Decreasing No

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

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

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

Calorimeter Practice 575.3 kJ/mol qreaction= -16.6 kJ__________ = 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

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?

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

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