4. Thermodynamics Flashcards
What is the zeroth law of thermodynamics?
Heat flows from areas of increased temperature to areas of decreased temperature, until thermodynamic equilibrium is achieved.
What is the first law of thermodynamics?
Total energy is the universe is constant. This law also states that energy cannot be created or destroyed, only transferred.
In an isolated system, energy must be constant because it cannot be transferred.
t or f, temperature is a measure of average kinetic energy.
True, this is exactly temperature.
What is ΔH?
ΔH = H (products) - H (reactants)
A measure of the heat of a reaction. Note: bond formation is ΔH < 0, destruction ΔH > 0.
t or f, a catalyst does not affect Keq, ΔH, ΔS, or ΔG in any-way.
true, catalysts only decrease activation energy.
What are standard conditions? What is STP (standard pressure and temperature?
standard conditions: T = 298K (25C°), P = 1atm, [] = 1M. liquids and solids are assumed to be pure. Values under SC have the ° symbol.
STP is 273K (0C°) and 1atm
What is standard heat of formation (ΔH°f) ?
Standard heat of formation is the amount of heat needed to form 1 mole of a compound from its constituent elements in their natural state !!
What is the equation for standard heat of formation of a given reaction (ΔH°f) ?
ΔH°f = (sum(n) x ΔH°f, products) - (sum(n) x ΔH°f, reactants)
n = stoichiometric coefficients
what is the ΔH°f of O2?
0 because O2 exists in its natural state (diatomic)
how does each halogen exist in their natural state?
All diatomic,
F and Cl = gases
Br = liquids
I = solid
What is Hess’s Law?
If a reaction occurs in multiple steps, we can sum all of the ΔH°f reactions together to get the overall ΔH°f of the entire process.
Hess’s Law, what if
- a reaction is reversed
- an equation is multiplied by some coefficient
- if the rxn reverses, the sign of ΔH°f flips
2. if the rxn is multiplied by n, the ΔH°f is multiplied by n too.
fill in the blank: Energy is needed to ___ a bond. Energy is released to ____ a bond.
Energy is needed to break a bond. Energy is released to form a bond.
How do we use bond dissociation energy (BDE) to calculate ΔH of a reaction?
ΔHrxn = Σ(BDE bonds broken) - Σ(BDE bonds formed)
note: we account for stoichiometry still. Thus, we multiple by the actual number of bonds
What is the second law of thermodynamics?
Nature has a tendency to become increasingly disorganized (random).
The second law states that all processes increase disorder (entropy).
Explain the entropy of gases, liquids, solids, and aqueous solutes.
g > aq > liq > solids
What is the third law of thermodynamics?
The third law of thermodynamics explains the least thermally energetic state. This is a state of zero-entropy which occurs at absolute zero (K = 0)
0 kelvin = the most highly ordered state possible
What is the formula for Gibbs free energy ΔG?
ΔG = ΔH - TΔS
thus negative ΔH and positive ΔS define a spontaneous reaction
Explain the role of Temperature in Gibbs free energy when ΔH and ΔS have the same sign.
ΔG = ΔH - TΔS
If ΔH is + and ΔS is +
At high temperatures, the rxn will occur
If ΔH is - and ΔS is -
At low temperatures, the rxn will occur
t or f, enthalpy is often given in KJ anc entropy is often given in J.
True - always convert units before doing calculations
t or f, activation energy and gibbs free energy change in magnitude for the forward and reverse reaction.
False, gibbs free energy only changes its sign. In contrast, the activation energy will be different depending which direction the reaction goes (unless ΔG = 0)
t or f, the internal energy of an object is proportional to its temperature.
true
If ΔG = 0, what is true?
the reaction is at equilibrium
