C10. Thermodynamics Flashcards
what is thermodynamics/
study of the transfer of energy- ALLOWS US TO PREDICT IF REACTION WILL HAPPEN
what is energy?
capacity to do work
what is work
motion against an opposing force
what is kinetic energy and potential energy?
-Kinetic energy the energy something possess due to its motion.
-Potential energy the energy something possess due to orientations / positions of its components (gravitational / coulombic).
what are the four laws of thermodynamics?
-The Zeroth Law: Thermodynamic equilibrium
-The First Law: The conservation of energy
-The Second Law: Principles of entropy
-The Third Law: Absolute zero temperature
what is a system and surrounding?
-System the bit we’re interested in (e.g. chemical
reaction).
-Surroundings everything else and where
we make our observations (e.g. in air / in a
water bath)
system+ surrounding=
universe
Describe open system
In an open system, both matter and energy
can be transferred between the system and
the surroundings
Describe closed system
In a closed system, only energy can be
transferred between the system and the
surroundings (i.e. only energy changes)
Describe isolated system
In an isolated system, no matter and no
energy can be transferred
What is the internal energy, U?
The Internal Energy, U is the sum of all the kinetic energy (KE) and
potential energy (PE) contributions of all the components in the
system. This cannot be calculated as it is impossible to measure individual KE and PE contributions from
atoms, electrons etc
change in internal energy equation?
triangleU= U final - U initial
what is the internal energy of a system changed by?
-heat or work
-Work (w) and heat (q) are done on or done by the system
-Work and heat are what transfer energy into and out of our system.
Equation to calculate change in internal energy from work and heat?
triangleU = q + w
-q is +ve if heat is absorbed by the system, and -ve if given off.
-w is +ve if work is done on, and -ve if work is done by the system
when does change in internal energy = q?
At constant volume, where there is no expansion work ΔU = q.
Describe oxidation of fat, work of expansion example
-The oxidation of fat is exothermic (q = -ve).
-But work (w) has been done by the atmosphere on the system due to the reduction in volume (compression).
-ΔU > q because some of the total energy of the system has been
gained by the atmosphere doing work on the system
EXAMPLES ON ONE NOTE
Describe enthalpy, EQUATIONS?
-Enthalpy, H takes in account pressure and volume
-Equation for change in internal energy: ΔU = q + w
-Substitute definition of w: ΔU = q – PΔV as
w=-PatmΔV
qp= H
Rearrange: qp = ΔU + PΔV
ΔH = ΔU + PΔV
ONE NOTE
What is enthalpy?
Enthalpy is the heat content of an open system at constant pressure.
-Exothermic processes, heat is released ΔH < 0.
-Endothermic processes, heat is absorbed ΔH > 0
what is enthalpy change of reactions?
-The enthalpy change of a reaction, ΔHrxn is
-used to refer to energy changes during
chemical reactions.
-It is the enthalpy change between the
products and the reactants
what are the different types of enthalpy changes of reaction?
ΔHcom = enthalpy change of combustion.
ΔHfus = enthalpy change of melting.
ΔHvap = enthalpy change of vaporisation.
ΔHsol = enthalpy change of solubilisation
enthalpies are reported in?
kJ mol-1. The
total enthalpy change is governed by the
stoichiometry of the reaction
what does ° mean
° means “at
standard conditions”
(298.15 K, 1 atm)
What does Hess’s state?
Hess’s law states that the
standard reaction enthalpy is the sum of all the standard reaction enthalpies into which
the overall reaction is divided.
ΔH°rxn = ΔH°1 + ΔH°2 +ΔH°3
ONE NOTE
what is enthalpy of formation?
Standard enthalpy of formation ΔH°f of a substance is standard enthalpy (per mol)
for its formation from its elements in their
reference state
ΔH°rxn= ?
ΔH°rxn = ∑ v ΔH°
f (products) - ∑ v ΔH°
f (reactants)
ONE NOTE
v?
v = stoichiometry
How to calculate enthalpy of reaction using bond enthalpies?
ΔH°rxn = ∑ ΔΗ°
bonds broken (reactants) - ∑ ΔH°bonds formed (products)
-Bond breaking is endothermic (energy must be put in)
-Bond making is exothermic (energy is released)
Describe second law of thermodynamics, principles of entropy
-In an isolated system entropy increase is a spontaneous change
-In an open/closed system, you have to take into account the entropy
of the universe i.e the system and its surroundings.
-Spontaneous process are irreversible
What is entropy?
-The disorder of a system
is defined as entropy, S
-Systems tend to move from
states of order to states of
disorder / chaos.
Describe entropy of surroundings
-Upon transitioning from liquid à solid, the chemical process gives out
a large amount of heat (exothermic)
-The surroundings become hotter (q increases) and thermal motion is
greater
-Dispersion of energy is greater therefore the entropy of the
surroundings increases.
-The total entropy of the surroundings and the system (i.e. the
universe) is increasing.
-ΔS of the surroundings ∝ q surroundings
Describe changes in entropy
-From cold ->hot ΔS»_space; 0
-From warm -> hot ΔS >0
-Entropy is still increasing, but the increase in entropy is inversely proportional to the temperature.
-Therefore ΔS proportional to 1/T
ONE NOTE
ΔS is proportional to what?
ΔS is proportional to q
ΔS is proportional to T
Therefore ΔS is proportional to q/T
T is in Kelvin ONE NOTE
ΔG equation?
ΔG= ΔH - TΔS
If ΔG<0 reaction is…
spontaneous
if ΔG>0 reaction is…
not spontaneous
how to predict spontaneity?
one note
How does Gibbs free energy relate to ATP in biology?
The dephosphorylation of ATP is exothermic. Cells use ATP’s negative gibbs free energy to power reactions which require energy making it more energetically favourable
