Test 1 CHEM 1412

• Hess’s law : in going from reactants to products, the change in enthalpy of the reaction is the same whether it takes place in one step or in a series of steps
• If a reaction is reversed, the sign of ΔH is also reversed
• Magnitude of ΔH is directly proportional to the quantities of reactants and products in a reaction
• Hess’s law — in going from reactants to products, the change in enthalpy of the reaction is the same whether it takes place in one step or in a series of steps
• Standard formation enthalpies (∆𝐻𝐻𝑓𝑓 ° ) : Change in enthalpy from the formation of one mole of a compound from its elements with all substances in their standard states ; Degree symbol (superscript) indicates standard conditions
• Standard state — precisely defined reference state ( 1 atm, 1 M , 25C exactly)
• Thermodynamics — study of energy and its interconversions
• First law of thermodynamics — the total energy of the universe is constant
• Law of conversation of energy — energy can be converted from one form to another, but can neither be created nor destroyed
• System — the part of the universe that one wishes to focus his/her attention
• Surroundings — includes everything else in the universe that is not the system
• Exothermic — results in the evolution of heat energy
• Endothermic — results in the absorption of heat energy from the surroundings
• Calorimetry — science of measuring heat transfer
• Heat capacity (C) — defined by heat absorbed per degree increase in temperature
• Specific heat capacity (s) — the energy required to raise the temperature of one gram of a substance by one Celsius (or kelvin) degree
• ▪ Molar heat capacity — energy required to raise the temperature of one mole of a substance by one Celsius (or kelvin) degree
• Coffee-cup calorimeter — contains two nested styrofoam cups with a cover through which a stirrer and thermometer can be inserted
• Constant pressure calorimetry : Atmospheric (ambient) pressure remains constant during the process ; ΔH = qp (at constant pressure) ; Heat transferred between two objects in a calorimeter
• Constant pressure calorimetry : Determines change in enthalpy for any process including reactions that occur in solution
• Constant volume calorimetry — used to study the energy changes in reactions under conditions of constant volume
• Spontaneous process — occurs without external intervention (can be fast or slow)
• Thermodynamics of a reaction — provides information on whether a reaction is spontaneous as predicted through state functions
• Kinetics of a reaction — rate of a reaction that depends on the pathway from the reactants to products
• Entropy — a state function that describes the number of arrangements available to a system (disorder of a system / molecular randomness )
• A set number of molecules increases in entropy as the volume increases
• The larger volume has the highest positional probability
•  Entropy in changes of state — the volume occupied by a constant number of molecules is smallest as a solid, larger as a liquid, and significantly larger as a gas
• Second law of thermodynamics — in any spontaneous process, there is always an increase in the entropy of the universe
• Free energy (G) — can directly determine spontaneity
• H : enthalpy
• S : entropy
• In a case where ∆𝐻soln(stand) is positive, it is not expected to be large in magnitude to overwhelm the expected positive value of ∆𝑆,soln(stand) in the process
• To cause solutions to mix, ∆𝐺soln(stand) must be negative to make it a spontaneous process
• Positional entropy can influence changes in a chemical system
• Therefore, Δn is negative and the entropy (ΔS) change is expected to be negative
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• Third law of thermodynamics — the entropy of a perfect crystal at 0 K is 0 J/mol K
• Entropy of a substance increases as the temperature increases
• Standard free energy change (ΔG°) : Change in free energy that will occur if the reactants in their standard states are converted to the products in their standard states at 25°C