GenChem6 (Thermochemistry) Flashcards
Aspect of chemistry that helps determine whether a reaction is spontaneous
Thermodynamics/thermochemistry
A reaction that can occur under a given set of conditions without outside assistance is said to be
Spontaneous
A system that cannot exchange energy or matter with the surroundings, like an insulated bomb reactor
Isolated system
A system that can exchange energy but not matter with the surroundings, like a steam radiator
Closed System
A system that can exchange both matter and energy with the surroundings, like boiling water
Open System
a process that occurs when the temperature of the system remains constant
Isothermal
A process that occurs when no heat exchange occurs
Adiabatic
A process that occurs when the pressure of a system remains constant
Isobaric
A form of energy that can easily transfer to or from a system; the result of a temperature difference between the system and its surroundings
Heat
Reactions that absorb heat energy are
Reactions that release heat energy are
Endothermic
Exothermic
This measures heat changes
Calorimetry
What is the equation for the heat absorbed or released in a given process
q=mcΔT
m=mass, c=specific heat, ΔT=change in temperature
Type of calorimetry where the volume remains constant
Type of calorimetry where the pressure remains constant
Constant-volume calorimety
Constant-pressure calorimetry
What is the equation for measuring the q of a reaction with constant volume-calorimetry?
qrxn=-(qwater+qsteel)
q=mcΔT
Note that q of the entire system =zero (for review, page 352)
Properties whose magnitude depends only on the initial and final states of the system, and not on the path of the change, are known as
State Function
What are the state functions? (7)
Pressure, temperature, volume, enthalpy (H) entropy (S), free energy (G) and internal energy (E or U)
What are standard conditions for measuring the enthalpy, entropy, and free energy of a reaction?
25 degrees C and 1 atm
A substance in its most stable form under standard conditions is said to be in its
Standard State
The term to express heat changes at constant pressure (typically 1 atm)
Enthalpy
What is the equation for the enthalpy change of a reaction using standard heat of formation?
ΔH(rxn)=H(prod) - H(react)
A positive ΔH corresponds to anexothermic/endothermic process
A negative ΔH corresponds to an endothermic/exothermic process
endothermic
exothermic
Defined as the enthalpy change that would occur if one mole of a compound were formed directly from its elements in their standard states
ΔH(f), enthalpy of formation
What is the ΔH(f) of an element in its standard state
0
Defined as the hypothetical enthalpy change that would occur if the reaction were carried out under standard conditions
ΔH°(rxn)
What is the formula for ΔH°(rxn) using standard heat of formation?
ΔH°(rxn)=sum of ΔH°f(prod) - sum of ΔH°f(react)
What value is used for Hess’s law
heat of vaporization, ΔH°(vap)
an average of the energy required to break a particular type of bond in one mole of gaseous molecules
Bond breakage is always exothermic/endothermic
Bond Dissociation energy
endothermic
There is one more ΔH° value, what is it.
Heat of combustion
Defined as a measure of the disorder, or randomness, of a system.
What are the units of it?
Entropy
J/K or cal/K (energy/temperature)
A system reaches its maximum entropy at ______, a state in which no observable change takes place as time goes on. What is the equation for ΔS(universe)
ΔS(universe) is _______ for a reversible process
ΔS(universe) = ΔS(system) + ΔS(surroundings) = 0
ΔS(universe)=0
The thermodynamic state function that combines the two factors that affect to spontaneity of a reaction (ΔH and ΔS), it represents the maximum amount of energy released by a process, occurring at constant temperature and pressure, that is available to perform useful work.
G (Gibbs free energy)
What is the equation for ΔG
ΔG=ΔH-TΔS
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What does TΔS represent?
The total amount of heat absorbed by a system when its entropy increases reversibly
Is free energy at a minimum or maximum at equilibrium?
What range of G can a reaction occur spontaneously?
Free energy is at a minimum in the equilibrium state
ΔG<0 (negative values)
If ΔG is zero, the system is in a state of
What equation can be used in the circumstance
If ΔG is positive, the reaction is
equilibrium
ΔH=TΔS
non-spontaneous, but spontaneous in the reverse direction
If ΔH is negative and ΔS is positive, what can be said about the spontaneity of the reaction?
Spontaneous at all temperatures
If ΔH is positive and ΔS is negative, what can be said about the spontaneity of the reaction?
Non-spontaneous at all temperature
If ΔH is positive and ΔS is positive, what can be said about the spontaneity of the reaction?
Spontaneous at high temperatures
If ΔH is negative and ΔS is negative, what can be said about the spontaneity of the reaction?
Spontaneous at low temperatures
What does the rate of a reaction depend upon?
Activation energy, not ΔG
What are the parameters of standard free energy, or ΔG°
25°C, 1atm, all concentrations at 1M
The free energy change that occurs when 1 mol of a compound in its standard state is formed from its elements in their standard states under standard conditions is
ΔG°(f), standard free energy
The standard free energy of formation of any element in its most stable state/form is
0
The free energy change that occurs when a reaction is carried out under standard state conditions, ie when th ereactants in their standard states are converted to produts in their standard states at standard conditions of T and P
ΔG°(rxn) or the standard energy of a reaction
ex: conversion of C(diamond) to C(graphite) is spontaneous, but very slow.
What is the equation for ΔG°(rxn)?
ΔG°(rxn) = sum of ΔG°(f) of products- sum of ΔG°(f) of reactants
The equation for ΔG°(rxn) derived from the equilibrium constant is
ΔG°(rxn) = -RT(lnKeq)
Once a reaction commences, the standard state conditions no longer hold. Keq must be replaced by another parameter… the
reaction quotient
What is the equation for the reaction quotient
Q= ([C]^c[D]^d)/([A]^a[B]^b)
for equation aA + bB —> cC + dD
What is the equation for ΔG when standard state conditions no longer hold?
ΔG= ΔG° +RT in Q
For the equation
2C6H6 (l) + 15O2 (g)—> 12CO2(g) + 6H2O(g) + heat
is the reaction spontaneous? Which side has greater entropy?
The reaction is spontaneous because heat is released, and there are more moles of gas on the right side of the equation.
