Thermodynamics Flashcards
the laws of thermodynamics underlie any even in which ______ is transformed
energy
the Zeroth Law
if two systems are both in thermal equilibrium with a third system, then the two initial systems are in thermal equilibrium with one another
if systems are in thermal equilibrium, what does this imply about their temperatures?
their temperatures must be the same
the First Law
the total energy of the universe is constant, energy may be transformed from one form to another, but it cannot be created or destroyed
an isolated system has a constant ______
amount of matter and ENERGY: no transformation of the energy is possible
how can a system increase its overall energy?
if work is put into its system
what happens when energy flows into a system from the surroundings?
the energy of the system increases (+) and the energy of the surroundings decrease
what happens when energy flows out of a system into the surroundings?
the energy of the system decreases (-) and the energy of the surroundings increase
enthalpy, H
the measure of the heat energy that is released or absorbed when bonds are broken and formed during a reaction that’s run at constant pressure
when a bond is formed, energy is:
released, enthalpy change is negative
when a bond is broken, energy is:
put into the bond, enthalpy change is positive
heat of reaction
enthalpy change, delta H
when is enthalpy change negative, exergonic?
when the products of a chemical reaction have stronger bonds than the reactants, so that more energy is released in the making of product bonds than was put in to break chemical bonds
how do you calculate enthalpy change?
enthalpy of products - enthalpy of reactants
exothermic
energy is released overall from the system, enthalpy change is negative
endothermic
energy is absorbed into the system, enthalpy change is positive
standard conditions
298K (25 degrees Celsius), pressure is 1 atm, all solids and liquids are assumed to be pure, solutions are at a concentration of 1 M
standard heat of formation
the amount of energy required to make one mole of a compound from its constituent elements in their natural or standard state
what does it mean if standard heat of formation is a positive value?
an input of heat is required to make the compound from its constituent elements
what does it mean if standard heat of formation is a negative value?
making the compound from its elements gives off energy
Hess’s law
if a reaction occurs in several steps, then the sum of the energies absorbed or given off in all the steps will be the same as that for the overall reaction
what does it mean that enthalpy is a state function?
the enthalpy change is independent of the pathway of reaction
what happens to the value of enthalpy change if reaction is reversed?
the sign of enthalpy change is reversed too
what happens to the value of enthalpy change if an equation is multiplied by a coefficient?
the enthalpy change must be multiplied by that same value
enthalpies can be viewed as the energy stored in the chemical bonds of a compound that denotes how much energy is required to:
break the bond homolytically
how do you calculate the enthalpy change of reaction?
sum of BDE of bonds broken - sum of BDE of bonds formed
second law of thermodynamics
the disorder of the universe increases in a spontaneous process
entropy
disorder or randomness, S
what is the sign of entropy change if randomness increases and order decreases in a reaction?
positive
what is the sign of entropy change if randomness decreases and order increases in a reaction?
negative
what is the value of enthalpy change in a reverse reaction?
the same magnitude as that of the forward reaction but with opposite signs
can reactions in which entropy decreases occur?
yes, but it would require either an input in energy or else it must gain energy from making more stable bonds
The Third Law
absolute zero is a state of zero-entropy
Gibbs free energy (delta G)
the energy that’s available (free) to do work from a chemical reaction
what is the spontaneity of a reaction determined by?
changes in enthalpy and in entropy, included by G
what is the formula for Gibbs free energy
G=H-TS
what does it mean if Gibbs free energy is negative?
it is spontaneous in the forward direction
what does it mean if Gibbs free energy is zero
the reaction is at equilibrium
what does it mean if Gibbs free energy is positive
it is nonspontaneous in the forward direction
if delta G is positive for a reaction, then what is the value of delta G for the reverse reaction?
it would be the same magnitude but opposite sign, so that the reverse reaction is spontaneous
what does entropy depend on?
temperature; increased temperature=entropy has greater significance
the universe tends towards…
increasing disorder (positive S) and stable bonds (negative H), a favourable condition of S and H will result in spontaneous processes
what is the delta G of the overall reaction?
the difference between the energy of the products and the energy of the reactants
the activation energy determines:
the kinetics of the reaction: it is the extra energy the reactants are required to overcome
thermodynamics predicts:
the spontaneity and the equilibrium of reaction, not the rates (kinetics)
microscopic reversibility
the reverse reaction has the same magnitude for all thermodynamic values but of the opposite sign, and the same reaction pathway but in reverse (mirror images)
what is different for forward and reverse reactions?
activation energy
