Class notes for Chapter 9: Thermochemistry in the class General Chemistry: Macroscopic Investigations and Reaction Principles (CHEM 130) at the University of Michigan. Topics covered include laws of thermodynamics, heat and work, constant-volume and constant-pressure calorimetry, enthalpy, and latt...
The Nature of Energy
●Thermochemistry: the study of the relationship between chemistry and energy
●Energy: capacity to do work or cause heat flow
○Transferred by work (w): force acting through a distance
○Transferred as heat (q): energy transfer due to temperature difference
●Energy is something that an object possesses, heat and work are ways that objects exchange energy
●Kinetic Energy: energy associated with motion of an object
●Thermal Energy: energy associated with temperature of an object
○Form of kinetic energy
●Potential Energy: energy associated with the position or composition of an object
●Chemical Energy: energy associated with relative positions of electrons and nuclei in atoms and molecules
○Form of potential energy
●Units: 1 Joule (1kg m2/s2) = 4.184 cal = 10-3 Cal
●Law of Conservation of Energy: energy is always conserved
●System is the thing we define and focus on, surroundings is everything else
○System and surroundings exchange energy
The First Law of Thermodynamics
●Thermodynamics: general study of energy and its interconversions
●First Law of Thermodynamics: law of energy conversion – the total energy of the universe is constant
●Internal Energy (E): sum of kinetic and potential energies of all the particles that compose a system
○Change in internal energy = heat + work
■∆Esystem = q + w
●q: when a system absorbs thermal energy, q is positive; when a system releases thermal energy, q is negative ●w: ○Change in internal energy is also difference between final and initial states
■In chemical system, reactants are initial and products are final
●State Function: function whose value depends only on the state of the system, not on how the system got to that state
○The state something is in currently, no matter how it got there
■Temperature, pressure, concentration, phase of matter, internal energy
■Change in state function = final – initial
○EX: When you climb a 10,000 ft mountain, at then end your elevation is always 10,000ft no matter what route you take up → elevation is a state function
■Distance traveled is not a state function because different paths could have been different lengths
○Value of change of state function is the difference between final and initial values
●Just like we can portray the changes of climbing a mountain as diagram of altitude, we can portray energy changes during a chem reaction with an energy diagram
●Energy lost by reactants flows out of the system (the reactants and products in chemical reactions) and into the surroundings
○System absorbs energy from surroundings: CO 2 (g) → C(s) + O2(g)
■∆Esystem = +
■∆Esurroundings = –
○System releases energy to surroundings: C(s) + O 2(g) → CO2(g)
○Vertical axis of the diagram is internal energy ■Increases as we move up the diagram
○Because reactants in this reaction are higher than products, change in E (E products – Ereactants) is negative
●The change in internal energy of the system (ΔE) is the sum of the heat transferred (q) and the work done (w)
●EX: The air in an inflated balloon (defined as the system) warms over a toaster and absorbs 115J of heat. As it expands, it does 77kJ of work. What is the change in internal
energy for the system?
○GIVEN:
■q= +115J
■w= -77kJ = –77,000J
■∆Esystem = q+w
○FIND: ∆Esystem
○SOLVE: ∆Esystem = 115J + (–77,000J) = –76,885
Quantifying Heat and Work
●Heat (q) is thermal energy flow caused by temperature difference
○Temperature is a thermal energy measurement
●Heat flows from higher temperature to lower temperature substance until thermal equilibrium (when system and surroundings share the same temperature) is reached
Temperature Changes and Heat Capacity
●When a system absorbs heat (q), its temperature changes by ∆T where ∆T = Tfinal – Tinitial
●q = C x ∆T
○Heat absorbed = heat capacity x temperature change
○Specific Heat Capacity (C s): heat required to increase temperature by 1C (J/C)
■Depends of quantity of substance being heated
●The higher the heat capacity of a system, the smaller the change in temperature for a given amount of absorbed heat ●Molar Heat Capacity: amount of heat required to raise the temperature of 1mol of a substance by 1C
○Depends on kind of substance being heated, not amount
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