Chapter 8 -Enthalpy + Thermochemistry Flashcards
What is the equation for enthalpy?
H = E + PV
△H = △H (products) - △H (reactants)
Where △H = enthalpy (change in heat) at constant pressure
E = internal energy
P = pressure
V = volume
Define Exothermic and Endothermic
Exothermic: heat is produced by the system, heat EXITS therefore enthalpy is negative
Endothermic: heat is absorbed by the system, heat ENTERS therefore enthalpy is positive
Define standard enthalpy of formation
△Hf° = standard enthalpy of formation
is what would occur if 1 mole of a substance is formed.
Enthalpy of standard states like H gas and C solid have a standard enthalpy of zero since no change in formation
What is bond dissociation enthalpy and what is formed?
Bond dissociation enthalpy is a measure of bond strength in the homolytic cleavage. 2 free radicals are formed.
Bond energy and dissociation are always positive. It takes energy to pull apart bonds
Define entropy and its relation to energy and spontaneity
Disorder. A measure of randomisation of energy.
Gas < liquid < gas (gas most disordered)
△S° = △S° (products) - △S° (reactants)
Entropy SI unit is J/k and cal/k
Reactions needing LESS energy = LARGE disorder = most spontaneous
Reactions needing MORE energy = SMALL disorder = less spontaneous
*entropy also dependent on molecule number, if there are 2 moles of reactants and 4 moles of products more disorder has resulted hence increase in entropy
Define calorimetry, its two equations and its units
Heat evolved or exchanged.
Q = mC (T2 - T1)
where m = mass
C = heat capacity (constant)
T2, T1 = temperature
Q = nC (T2 - T1)
where n = number of moles
Units of heat capacity = Jkg^-1 k^-1
*note heat can be released or absorbed without a change in temperature
Law of conservation of energy equation
Q = mL m = mass L = latent heat (constant)
Q = force x distance force = mass x accel
What is the Gibbs free energy equation and what is it used to calculate
Spontaneity (direction)
if G = -ve is right (exothermic) SPONTANEOUS
if G = +ve is left (endothermic) NOT SPONTANEOUS
G = 0 is equilibrium
△G° = △H° - T△S°
where H is heat
T is temperature K
S is entropy
Define heat capacity
The quantity of heat needed to change the temperature of a substance 1 degree
What is the equation for thermal conductivity
heat transferred x thickness
K = _______________________
arear x △T
units of conductivity K = W/mK where W is power
What is the equation for thermal resistance and its SI unit
thickness
R = __________
area x conductivity
SI unit thickness is m, area is m^2 and conductivity is W/mK so:
R = m/m^2(W/mK) = 1/m x W/mK = 1/(W/k) = K/W
Where would the equilibrium shift if the temperature is increased of a -△H°/-△G° system?
a -△H°/-△G° system is exothermic, meaning the equilibrium is shifted to the right, but adding heat makes the system more positive meaning the equilibrium will shift left to compensate
P → C + p △G° = -43kj/mol
AT → AD + p △G° = -30.5kj/mol
What is the △G° for:
P + AD → C + AT? justify
Hess’s Law, first equation cannot be changed.
Therefore second equation must be inverted to get AT on right and AD on left sides of equation. When inverting the sign of △G changes. so instead of -30.5jk/mol it is now 30.5kj/mol. So we add -43 + 30.5 and get the △G = -12.5kj/mol
Why is a steam burn of 100°C more severe than a water burn of 100°C?
Steam has more heat energy than water. The steam gives off a large amount of heat when it condenses
