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 surroundingsExothermic

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 Cell2H2 + O2 2H2O

– Redox reaction powers battery– Electricity is used to do work; some

energy lost as heatΔH -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 heat2) 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

Exothermic (losing energy) decreasing Yes at low temps

Endothermic (gaining energy) increasing Yes at high temps

Endothermic (gaining energy) 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; Jm= the mass of the substance; gΔT= change in temp of the substance; oC

q= 794 J m= 89.1 g ΔT=51.1oC-22.0oC= 29.1oCC= 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 -qreactionExothermicEndothermic

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__________ =1.75 g CH3CO2H

60.5g CH3CO2H

1mol CH3CO2H575.3 kJ/mol

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) Recycling2) Clean Coal Burning3) Solar Energy4) Geothermal5) Wind Energy6) 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