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:
Reaction Type
Entropy
Spontaneous?
Exothermic (losing energy)
increasing
Yes
decreasing
Yes at low temps
Endothermic (gaining energy)
Yes at high temps
Decreasing No

11. Measuring Energy
Calorimeter: measures change in temperature of a liquid surrounding a thermochemical reaction
H20 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)= 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= 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 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)( )(4.184)
qwater= 16, J
qreaction= - qwater  -16, J kJ
qreaction= 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: J
kilocalorie: 1000 calories
Calorie: energy unit for food; 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