Introduction to spectroscopy, Energy and Bonding Spring Semester Flashcards
How do thermodynamics determine chemical reactivity?
Thermodynamics refers to the transfer of heat and work, meaning thermodynamic factors control whether a reaction will take place or not. Work is done as a result go of motion against an opposing force (molecules vibrating).
How do kinetics determine chemical reactivity?
Kinetic factors determines the rates of a reaction, a reaction may take place but on an unreasonable timescale.
What is the systems and surroundings in a thermodynamic problem?
System > the part of the Universe we want to study
Surroundings > the part of the universe outside of the system
A change in energy can determined by measuring energy transfer to and from the surroundings.
What is the equipartition Theory?
This states that energy is shred equally amongst all energetically accessible degrees of freedoms of a system. This means each degree of freedom contains energy 1/2kT or 1/2RT per mole.
How to calculate the number of degree of freedom?
linear molecules = 3N-5
Non-linear molecules = 3N-6
What is the definition of internal energy?
The sum of all the kinetic energy contributions and potential energy contributions (U). it cannot be measured by ΔU and be thought about the capacity to do work.
U = total kinetic energy + total potential energy
What is the molecular origin of internal energy?
- Molecules contain potential energy, because of their position relative to one another
- Molecules are neutral, and extent attractive and repulsion forces on one another
How do you calculate the average kinetic contribution to the molecular internal energy?
- With a linear molecule is can rotate about two axis so rotational contribution is 2x(1/2)RT
- A nonlinear molecule can rotate about three axis so has a rotational contribution is 3x(1/2)RT
- A gas has a translation contributions of 3x(1/2)RT
it is then calculated add of the kinetic contribution together
e.g. for a non linear molecule
(3x1/2kT) + (3x1/2kT) = 3kT
then multiplying by k or R and T
T in kelvin = stated in question
Then R (idea gas constant) used to calculate energy per mole and K (Boltzmann constant) used to calculate energy per molecule.
What causes changes to internal energy?
Internal energy changes by transferring energy to or from the system as heat
- If work is done BY a system then its internal energy decreases
- If work is done ON a system then internal energy increases and it capacity to do work increases
What is the first law of thermodynamics?
In a isolated system, the change in internal energy is the sum of the energy change due to transfer of heat (q) and work (w) both in joules. energy cannot be created or destroyed, only transferred.
ΔU = q + w
w is positive if work is done ON the system and negative if work in door BY the system
q is positive is heat is supplied to the system and negative if heat if released form the system
What are two types of work a system can do?
- expansion (change in volume of the system by a reaction with gaseous products)
- non-expansion (work that does not involve a change in volume)
How do you calculate the work done in reversible expansions?
Reversible expansion
w = nRTln(Vf/Vi)
w = work doen (joules)
n = moles
R = ideal gas
T = Temp (k)
Vf = final volume (dm^3)
Vi = initial volume (dm^3)
How do you calculate the work done in irreversible expansions?
Irreversible expansion
w = -PexΔV
Pex = pressure (Pa)
Δv = change in volume (m^3)
What are the properties of state functions in calculations?
internal energy is a state function, the means that the value depends only on the initial and final sates of the system but not on the route taken (volume, temp and pressure are also state functions)
How do you use the properties of state functions in calculations?
The change in the state of the system is independent of how the change was brought about.
How can internal energy changes be measure with a calorimetry?
Calorimeters are used to measure the internal energy change of a reaction initiated inside a constant volume. Any rise and fall in the temperature os the water bath can be equated to heat released by the reaction. It happens without transferring heat or mass between the system and surroundings.
As the reaction occurs at a constant volume the flow of heat corresponds to the change in internal energy and be calculated as
ΔU = -CcalΔT
Ccal = experimental constant (heat capacity/calorimeter constant)
T = temperature (k)
per mole is workout by dividing by the number of moles of the substance
What is the definition of enthalpy?
Enthalpy (H) is the heat transferred by a chemical reaction at a constant pressure (in an open system atmospheric pressure does not change), its a state function.
H = U +pV
p = pressure
V = volume
U = work (j)
As it take place a constant pressure more frequently written as
ΔH = q
if the enthalpy is negative the reaction is exothermic and if its positive endothermic.
ΔH = H(products) - H(reactants)
What is the definition of heat capacity?
Heat Capacity (C) is a measure of how much heat must be supplied to raise the temperature by a given amount. Specific heat capacity normalise to mass
Cs - q/ mΔT
Moor heat capacity is normalised to moles
Cm = q/nΔT
What is the definition of standard enthalpy changes?
The enthalpy change which occurs when one mole of a compound is formed from its elements under standard conditions, and with everything in its standard state.
How do you estimate enthalpy changes from internal energy changes?
For reactions involving solids the volume of product and recant is very similar change is volume is equal to 0 and enthalpy change equivalent to change in internal energy.
ΔrH = ΔrU
For gases volume differs dramatically so assuming ideal behaviour
ΔrH = ΔrU + ΔngasRT
Δn = n of products - n of reactants
R = ideal gas
T = temp (K)
How do you calculate standard reaction enthalpies form Hess’ law?
The overall enthalpy changer is the sum of the enthalpy changes of the steps into which the reaction can be divided.
Creating a hess diagram of formations
ΔH = ΔH reactants to elements + ΔH elements to products
How do you estimate reaction enthalpies using average bond enthalpies?
Bond enthalpies is a measure of the bond strength, its the standard enthalpy change for the dissociation of a molecule in the gas phase into atoms in the gas phase. it can be used as an alternative to calculate reaction enthalpy.
XY (g) –> X(g) + Y(g)
How do you use Kirchhoff’s law to calculate standard reaction enthalpies at another temperature?
The enthalpy of reactant and products increases with temperature. The rate of increase in enthalpy with temperature is the heat capacity at constant pressure as can be written as
ΔCp = total nCp (products) - total nCp (reactants)
What is the second law of thermodynamics?
The entropy of an isolated system increases during any spontaneous process, increasing total entropy of the universe. Entropy is change is grater than 0.
What is the third law of thermodynamics?
The entropy of a perfect crystal is 0 at T=0K. This means there is no thermal motion of molecules and the non zero entropy arises form the number if possible orientations a molecule can take in the crystal.
What is the definition of entropy?
Entropy (S) is a measure of how energy is dispersed in a system.
How do you calculate entropy change?
Entropy change is measured at
ΔS = q rev/ T
s = entropy (J k-1)
q = quantity of heat added reversibly (J)
T = temperature (K)
if a lot of energy is transferred there is a large increase in entropy, its a state function.
How do you calculate entropy change when volume changes in gas?
ΔS = nRTln(Vf/Vi)
n = number of moles
R = ideal gas
T = temperature (K)
V = Volume (m^3)
How do you calculate entropy change when temperature is allowed to change?
ΔS = Cvln(T2/T1)
Cv = molar heat capacity
T = temperature (K)
How do you calculate entropy change for changes in physical state?
For a phase transition the temperature of a substance remains constant while a phase transition takes place at a constant pressure is can be equated by
ΔS = ΔH/T
Entropy increases form solid to liquid to gas
What is the Gibbs equation?
Gibbs gives the total entropy change of the Universe expressed as ΔH system and ΔS system.
ΔG = ΔH system - TΔS system
a spontaneous processes is a decrease in the Gibbs free energy
How do you determine the direction of phase change using physical equilibrium?
Physical equilibrium inclines process such as state transitions which occur without changes in chemical composition, this transition is related to a decrease in Gibbs free energy.
What is a phase diagram?
A graph of P against T showing regions in which different phase of a substance is stable. It is separated by phase boundaries. The triple point is where all three phase boundaries intersect and all three phases coexist.
What is the vapour pressure and how is it calculated?
Vapour pressure is the pressure at the gas-liquid boundary its a measure of force between liquid molecules. it equated use the Clausius-Clapeyron equation
lnP2/P1 = -(ΔvapH/R)(1/T2 - 1/T1)
ΔvapH = standard mole of vaporisation (KJ mol-1)
R = ideal gas constant
T - temperature (K)
the to calculate the final vapour pressure
e^(answer from above) = x bar
What is a dynamic equilibrium?
This is when the rate of the forward and revise reaction is equal and there is thus no change in composition. Thermodynamic can be used to calculate the position of the equilibrium and kinetics asses how quickly the equilibrium is achieved.
How does Gibbs energy change with composition?
If the reaction mixture contains more reactants than the equilibrium composition the spontaneous direction of change is more towards the products (reaction will take place) and ΔG is negative, when equilibrium is reached there is no further net change as ΔG is 0.
if ΔG is negative change happen towards products
if ΔG is 0 then its at equilibrium
if ΔG is positive then the reaction is non-spontaneous.
for ΔG to be negative ΔH need to be negative and ΔS is positive.
What is the molar Gibbs free energy and how is it calculated? (WILL BE IN EXAM)
The molar Gibbs free energy (the actual Gibbs energy) varies from the standard molar Gibbs free energy (ideal) by an amount dependent on the partial pressure (Pa = mole fraction x total pressure)
G(A) = G(A) + RTln Pa/p
Pa = partial pressure of pure A, calculated by mole fraction
P = standard pressure (1 bar)
What is the reaction quotient?
The reaction quotient (Q) tells us the progress of a reaction at any point as it determine the composition of a mixture.
Q = products (Px/P)^n / reactants (Px-P)^n
What is the equilibrium constant?
K is the equilibrium constant and tel us where the equilibrium lies its equivalent to Q (at equilibrium K is replaced by Q)
ΔrG = -RTlnK
- When ΔG is negative, K is large then 1 and products are favoured
- When ΔG is positive K is lower than 1 and reactants are favoured
What is a heterogeneous equilibrium>
This is when the equilibrium consists of substances in different states of matter. Only the gaseous substances will appear in the equilibrium constant.
k = p(Ni(CO)4 / p(CO)4
What equation is used to calculate Gibbs at equilibrium?
ΔrG = -RTlnK
What is Le Chanteliers Principle?
When a change is made to a system in dynamic equilibrium, the equilibrium responds so as to minimise the effect of the change.
How does a change in compositions (concentration) affect an equilibrium?
It will move to restore the decreased concentration.
e.g. N2 + 3H2 –> 2NH3
the NH3 is constantly removed through dissolving in water to shift the equilibrium right.
Q < K and ΔG < 0 so direction of spontaneous change in towards the products.
How does a change in pressure affect the equilibrium?
The equilibrium responds to pressure increase by shifting towards the side with fewer gaseous molecules and vis versa.
- Standard Gibbs energy is a fixed values and only varies with temp so equilibrium constant doesn’t dependent on pressure however the composition does depend on pressure.
How does change in temperature affect equilibrium?
Increasing temp will shift the equilibrium in the exothermic direction.
How does a catalyst affect equilibrium?
Increase the rate at which equilibrium is reached as both forward and backwards reaction rate increased/. It can also be used to lower the temperature of a reaction shifting it endothermically.
How does kinetics and thermodynamic control affect product formation?
Some reactions have two possible routes of reactions initially the kinetic product will form (the one with lowest activation energy) but given enough thermal energy the thermodynamic product will dominate (lowest Gibbs)
- low temps favour kinetic product and high temps favour the thermodynamic product
What is an electrochemical cell?
This is the interconversion of electrical energy and chemical energy producing electricity through a chemical reaction or through forcing an unfavourable reaction to occur through application of a voltage.
How do batteries work?a
Reactants are operated form each other and connect via a wire, electrons are then forced through the wire. The travelling electrons do the electrical work (We).
What is an electrochemical cell and hat are the different types?
An electrochemical cell comprises of two half cells in each of which a particular redox reaction is happening (oxidation in one and reduction in the other). The two types are Galvanic cells and electrolytic cells
What is the difference between the electrolytic and galvanic cells?
Galvanic cells > the electrochemical cell process electricity as a result of a spontaneous reactions. Fuel cells are galvanic cells in which reagents are supplied continuously.
Electrolytic cell > a non-spontaneous reaction is forced through an external source of current (an applied voltage0
What are the different quantities used in electrochemistry?
An electrical current (coulomb) flows if their is a potential difference (voltage) between two points, the rate of this charge (I) is measured in amps (A).
V (voltage) = I (current speed) x R (resistance)
1 Amp = 1 C s^-1
What happens in the Daniell cell?
Galvanic ell
In one compartment, a zinc electrode is submerged in Zn2+ ions here the Zn is being oxidised
Zn(s) –> Zn^2+ + 2e^-
the electrons then travel through the wire creating a current to
a copper electrode in Cu2+ solution the Cu2+ is being reduced
Cu2+ (aq) + 2e- –> Cu(s)
SO4^2- entering the Zn2+ solution and 2Na+ entering the Cu2+ solution via the salt bridge.
How are electrochemical notation written?
Electron flow –>
(anode) metal/inert electrode I solution fo metal ions II solution of metal ions I metal electrode/inert electrode (cathode)
Anode = oxidation half cell
cathode = reaction half cell
I = phase boundary
II = salt bridge
if two type so ions are in the solution they are operated by a comma
What is the usefulness of standard potential and standard hydrogen electrode?
A Standard Hydrogen Electrode has a standard electrode potential value is zero meaning it is used as a reference on all half-cell potential reactions allowing cell potentials for different electrodes to be calculated.
Stronger oxidising agents have more positive E values and reducing agents have more negative E values.
How can you find thermodynamic quantities (equilibrium constant) from electrochemical measurements?
During flow of e- form the anode to the cathode against a resistance results in work being done. This can be calculated as
We = - E x Q
E = Size of the potential difference
Q = size of the changed passed
a voltmeter can ,aware the potential difference in a a cell at a constant temp and pressure wok is calculated by
W max = ΔrG = -zFE
z = no. of transferred electrons
F = faraday constant
E = electrode potential
a postive emf indicates that a reaction is spontaneous at ΔG is less than 0.
How does standard potential relates to equilibrium constant?
lnK = - (zF/RT)E(of cell)
if K>1 favours products
K<1 reactions favoured
What is the concept of activity?
Ions do no tomb independently of one another the ionic atmosphere (electron cloud) around then create ion interactions. Its determined by the activity coefficient (y)
if y < 1 ion sin a real solution are less active than ideal
Whats the Nernst equation?
The Nernst equation tell su amount the effect of concentration on cell potential
Ecell = E (standard)cell - (RT/zF) lnQ
Ecell = cell potential under non standard conditions
E (standard)cell = cell potential under standard conditions
R = gas constant
T = temperature (K)
z = number of moles of electrons transferred
F = Faraday constant
Q = reaction quotient
How do you add and multiply cell potentials (E)?
- multiplying E doesn’t work as cancelled out by the differing Z
- cannot add standard potentials as the number of electrons transferring are different
How does E depend on T?
As the temperature increases the cell potential also increases.
What are the key examples of electrolytic processes?
The electrolysis of water. It is the decomposition of water into oxygen and hydrogen gas due to a electrical current being passed through it.
Anode = 2H2O –> 4H+ + O2 + 4e-
Cathode = 2H2O + 2e- –> H2 + 2OH-
What is the definition of chemical kinetics?
Chemical kinetics is the study of the speed of chemical reactions and the factors that affect that speed
What is an elementary step?
Any chemical process can be broken down into one or ore elementary steps, this is the collision between two molecules or a dissociation of one molecule.
How do you track a rate of reaction?
Once a reaction is intimated the concentration of the reactants will fall and the products will rise. This means a groan can be plotted will time on the y-axis and concentrations if reactant/products on the x axis.
d[A]/dt
What is the rate equation?
Reaction rate (v) is
v = 1/2 d[products)/dt
Why is concentration important to the rate?
Concentration allows for K (rate constant) to be found via
Rate = k[A]^n[B]^m
K is the rate constant and n and m are the orders with respect to the concentration of the species, rate constant is a measure of the speed of the reaction is it is high the rate is fast and if it low the reaction is slow.
What are the different orders of reaction?
zero order > v = k[R]^0 when n = 0 is is a zero order reaction and the rate is independent of the concentration.
First order > v = k[R]
Second order > = k[R]^2
How do you work out the differential rate equation?
The rate equation is first written as normal
v = k[CH3]^2
Then it is differentiated to give e
-d[CH3]/dt = 2k[CH3]^2
how do you determine rate laws for complex reactantions?
This can be found experimentally, by measuring how the rate changes with changes made to the concentration, which is then plotted.
If its a straight line it is zero order, if its a straight diagonal line its first order and if its is a curved lien its second order.
- The equation os the straight line has the intercept as 0 and the slope as k
- If a straight line is not obtained you plot then initial rate versus the initial concentration^2 and if that line is straight its second order
- A log plot can be down plotting log[initial A] VS logV with the gradient m order and intercept k
log(rate) = log[A] + log[A]^m
What does the integrated rate equation give us?
The rate equation feel us the rate of a reaction at a given point and the integrated rate equation is an expression that gives the concentration of a species as a function of time allowing a prediction of contrition of a species at any time and to find the order of reaction.
Which graph give a straight Lin for each order?
Order straight line plot
0 [A] against T
1 ln[A] against T
2 1/[A] against T
What is the reaction half life?
The half-life (t1/2) is the time it takes for the concentration of the reactant to decrease by half of its original value.
[A]t1/2 = [A]0/2
- for a zero order the half life is inversely proportional to k
t1/2 = [A]0/2k
- for first order = -kt1/2, it falls half it values at each successive period of time
t1/2 = ln2/k
- second order is depends on the initial concentration and k t1/2 gets bigger
t1/2 = 1/[A]0k
What is the reaction mechanism?
The reaction mechanism is the sequences of elementary steps accounted for the overall stoichiometry. The species found in the rate equation is the rate determining step and must be placed in the 1 elementary step.
What is molecularity?
This is the number of reactant molecules or atoms taking part in an elementary reaction.
unimolecular > 1 reactant
bimolecular > 2 reactants
What are the key ideas for the kinetics of a reversible reaction?
- At equilibrium the net rate of consumption of the reactant and products is 0 as rates are equal
- rate order will be same for both the forward and revise reaction
The rate equilibrium constant is calculated by
keq = k1 (forward reaction) /k-1 (backwards reaction)
What are the kinetics for when two reactions are taking place?
This takes place when the same reactant is being consumed in two reactions happening simultaneously with different rate equations are combined by
d[A]/dt =-(k1 + k2)[A][B] ???????
What is a consecutive reaction and its associated kinetics?
This is when the product from the frost step becomes the reactant for the second step and so on.
d[A]/dt = -k1[A]
d[B]/dt = k1[A] - k2[B]
d[C]/dt = k2[B]
How do intermediates affect rate of reaction?
It given by the steady state approximation that the concentration of all the intermediates remain constant and small throughout the reaction and if is behave ‘ideally’ is equated as
d[I]/dt = 0
What is the steady sate approximation equation?
[intermediates] which is equal to 0 = k1[product]/ (k-1 + k2[reactant]0
What is the rate determining step?
The slowest elementary step in a multistep reaction which given the rate of the overall reaction. Higher the stiochiochemisty more likely to be rate determining.
How does temperature affect reaction rate?
Arrhenius discovered that he temperature dependence of many reactions can be described
k = Ae^-Ea/RT
with temperature increasing the rate of reaction (k) also increases
Ea = activation energy (minimum kinetic energy needed for a reaction to take place
A = pre-exponential factors (rate at which collisions occur, independent of T)
How do you apply Arrhenius equation at two temperatures?
- Work out
lnK1 and lnK2 for each temperatures - work out
ink(k2/k1) = Ea/R (1/T1 - 1/T2) - Rearrange these two equation to get the component needed
How is the spread if energies shown?
The energies of the reactant are spread over wide range which is dependant on temperatures is shown with number of particles on the y axis and kinetic energy on the x axis (Boltzmanns)
What are the two main theory’s for the occurrence of a reaction?
Collison theory > molecules behave as hard spheres with collision between them resulting in a reaction if energy is above Ea and they are in the correct orientation.
Transition state theory >the transition state is the highest point of the erg profile showing particle bonding between reactants and product
How does catalyst affect reaction rates?
The rate of the reaction is increased through the addition of a catalyst as they offer a different reaction route with a lower activation energy.
How can you determine the activation energy for a reaction?
- plot lnk (y axis) against 1/T in kelvin (x-axis) should give a straight lien exhibiting Arrhenius behaviour.
- work out the gradient in K
Δ(lnk)/Δ(1/T) - work out Ea
Ea = - gardaient x R - convert to Kj mol-1
