2. Section 6.1
The Nature of Energy
• Define energy, work, potential energy, kinetic energy, system and
surroundings, endothermic and exothermic.
• Be able to identify each of the above.
• Understand ΔE = q + w and use the equation to find the sign of E.
• Know ΔH = H products – H reactants.
• Use calorimetry to solve for a variable or estimate a temperature of a
system.
• Use Hess’s law to calculate enthalpy of a reaction.
• Use Standard enthalpy of formation to solve for enthalpy of reaction: ΔH
= ΣnHreactants – ΣnHproducts.
Chapter 6: Thermochemistry
Objectives
3. Section 6.1
The Nature of Energy
Chapter 6: Thermochemistry
Table of Contents
Unit 6: Thermodynamics
Energy
Types of Energy
Kinetic Energy
Potential Energy
Chemical Energy
Energy Changes in Chemical Reactions
Thermodynamics
First Law of Thermodynamics
Heat vs. Work
Enthalpy
Exothermic and Endothermic Processes
Change in Enthalpy (ΔH)
Enthalpy of Reactions (Hrxn)
System vs. Surroundings in a Chemical Reaction
Calorimetry
Specific Heat and Heat Capacity
Calorimeters
Bomb Calorimeters
Standard Enthalpy of Formation and Enthalpy of Reactions
Standard States and Standard Enthalpy Changes
Calculating ΔH Directly
Hess’ Law
Heat of Solution
Energy Use and the Environment
Present Sources of Energy
Energy Consumption
Environmental Problems Attributed to Fuel Consumption
21. Section 6.1
The Nature of Energy
21
Work = P × A × Δh = PΔV
P is pressure.
A is area.
Δh is the piston moving
a distance.
ΔV is the change in
volume.
Work
42. Section 6.3
Hess’s Law
Example
Multiply reactions to give the correct numbers of
reactants and products:
4( ) 4( )
3( ) 3( )
Desired reaction:
2 2 2 32 N ( ) 6 H O( ) 3 O ( ) 4 NH ( )+ → +g g g g
2 2 3
2 2 2
1 3
2 2
N ( ) H ( ) NH ( ) H = 46 kJ
2 H O( ) 2 H ( ) O ( ) H = +484 kJ
+ → ∆ −
→ + ∆
g g g
g g g
Exothermic process because you must remove energy in order to slow the molecules down to form a solid.
Exothermic (heat energy leaves your hand and moves to the ice)
Endothermic (heat energy flows into the ice)
Endothermic (heat energy flows into the water to boil it)
Exothermic (heat energy leaves to condense the water from a gas to a liquid)
Endothermic (heat energy flows into the ice cream to melt it)
System – methane and oxygen to produce carbon dioxide and water; Surroundings – everything else around it; Direction of energy transfer – energy transfers from the system to the surroundings (exothermic)
System – water drops; Surroundings – everything else around it; Direction of energy transfer – energy transfers from the surroundings (your body) to the system (water drops) (endothermic)
Water is lower in energy because a lot of energy was released in the process when hydrogen and oxygen gases reacted.
They both perform the same amount of work. w = -PΔV
a) w = -(2 atm)(4.0-1.0) = -6 L·atm
b) w = -(3 atm)(3.0-1.0) = -6 L·atm
a) q is positive for endothermic processes and w is negative when system does work on surroundings; first condition – ΔE is negative; second condition – ΔE is positive
b) q is negative for exothermic processes and w is positive when surroundings does work on system; first condition – ΔE is positive; second condition – ΔE is negative
(5.00 g C3H8)(1 mol / 44.094 g C3H8)(-2221 kJ / mol C3H8)
ΔH = -252 kJ
The correct answer is b).
The correct answer is c). The final temperature of the water is 23°C.
- (100.0 g)(4.18 J/°C·g)(Tf – 90.) = (500.0 g)(4.18 J/°C·g)(Tf – 10.)
Tf = 23°C
The correct answer is c). The final temperature of the water is 18°C.
- (50.0 g)(0.45 J/°C·g)(Tf – 90.) = (50.0 g)(4.18 J/°C·g)(Tf – 10.)
Tf = 18°C