Introduction to spectroscopy, Energy and Bonding Spring Semester

Question 1

How do thermodynamics determine chemical reactivity?

Answer 1

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).

Question 2

How do kinetics determine chemical reactivity?

Answer 2

Kinetic factors determines the rates of a reaction, a reaction may take place but on an unreasonable timescale.

Question 3

What is the systems and surroundings in a thermodynamic problem?

Answer 3

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.

Question 4

What is the equipartition Theory?

Answer 4

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.

Question 5

How to calculate the number of degree of freedom?

Answer 5

linear molecules = 3N-5
Non-linear molecules = 3N-6

Question 6

What is the definition of internal energy?

Answer 6

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

Question 7

What is the molecular origin of internal energy?

Answer 7

• Molecules contain potential energy, because of their position relative to one another
• Molecules are neutral, and extent attractive and repulsion forces on one another

Question 8

How do you calculate the average kinetic contribution to the molecular internal energy?

Answer 8

• 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.

Question 9

What causes changes to internal energy?

Answer 9

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

Question 10

What is the first law of thermodynamics?

Answer 10

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

Question 11

What are two types of work a system can do?

Answer 11

• expansion (change in volume of the system by a reaction with gaseous products)
• non-expansion (work that does not involve a change in volume)

Question 12

How do you calculate the work done in reversible expansions?

Answer 12

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)

Question 13

How do you calculate the work done in irreversible expansions?

Answer 13

Irreversible expansion

w = -PexΔV
Pex = pressure (Pa)
Δv = change in volume (m^3)

Question 14

What are the properties of state functions in calculations?

Answer 14

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)

Question 15

How do you use the properties of state functions in calculations?

Answer 15

The change in the state of the system is independent of how the change was brought about.

Question 16

How can internal energy changes be measure with a calorimetry?

Answer 16

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

Question 17

What is the definition of enthalpy?

Answer 17

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)

Question 18

What is the definition of heat capacity?

Answer 18

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

Question 19

What is the definition of standard enthalpy changes?

Answer 19

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.

Question 20

How do you estimate enthalpy changes from internal energy changes?

Answer 20

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)

Question 21

How do you calculate standard reaction enthalpies form Hess’ law?

Answer 21

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

Question 22

How do you estimate reaction enthalpies using average bond enthalpies?

Answer 22

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)

Question 23

How do you use Kirchhoff’s law to calculate standard reaction enthalpies at another temperature?

Answer 23

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)

Question 24

What is the second law of thermodynamics?

Answer 24

The entropy of an isolated system increases during any spontaneous process, increasing total entropy of the universe. Entropy is change is grater than 0.

Question 25

What is the third law of thermodynamics?

Answer 25

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.

Question 26

What is the definition of entropy?

Answer 26

Entropy (S) is a measure of how energy is dispersed in a system.

Question 27

How do you calculate entropy change?

Answer 27

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.

Question 28

How do you calculate entropy change when volume changes in gas?

Answer 28

ΔS = nRTln(Vf/Vi) n = number of moles R = ideal gas T = temperature (K) V = Volume (m^3)

Question 29

How do you calculate entropy change when temperature is allowed to change?

Answer 29

ΔS = Cvln(T2/T1) Cv = molar heat capacity T = temperature (K)

Question 30

How do you calculate entropy change for changes in physical state?

Answer 30

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

Question 31

What is the Gibbs equation?

Answer 31

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

Question 32

How do you determine the direction of phase change using physical equilibrium?

Answer 32

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.

Question 33

What is a phase diagram?

Answer 33

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.

Question 34

What is the vapour pressure and how is it calculated?

Answer 34

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

Question 35

What is a dynamic equilibrium?

Answer 35

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.

Question 36

How does Gibbs energy change with composition?

Answer 36

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.

Question 37

What is the molar Gibbs free energy and how is it calculated? (WILL BE IN EXAM)

Answer 37

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)

Question 38

What is the reaction quotient?

Answer 38

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

Question 39

What is the equilibrium constant?

Answer 39

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

Question 40

What is a heterogeneous equilibrium>

Answer 40

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

Question 41

What equation is used to calculate Gibbs at equilibrium?

Answer 41

ΔrG = -RTlnK

Question 42

What is Le Chanteliers Principle?

Answer 42

When a change is made to a system in dynamic equilibrium, the equilibrium responds so as to minimise the effect of the change.

Question 43

How does a change in compositions (concentration) affect an equilibrium?

Answer 43

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.

Question 44

How does a change in pressure affect the equilibrium?

Answer 44

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.

Question 45

How does change in temperature affect equilibrium?

Answer 45

Increasing temp will shift the equilibrium in the exothermic direction.

Question 46

How does a catalyst affect equilibrium?

Answer 46

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.

Question 47

How does kinetics and thermodynamic control affect product formation?

Answer 47

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

Question 48

What is an electrochemical cell?

Answer 48

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.

Question 49

How do batteries work?a

Answer 49

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).

Question 50

What is an electrochemical cell and hat are the different types?

Answer 50

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

Question 51

What is the difference between the electrolytic and galvanic cells?

Answer 51

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

Question 52

What are the different quantities used in electrochemistry?

Answer 52

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

Question 53

What happens in the Daniell cell?

Answer 53

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.

Question 54

How are electrochemical notation written?

Answer 54

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

Question 55

What is the usefulness of standard potential and standard hydrogen electrode?

Answer 55

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.

Question 56

How can you find thermodynamic quantities (equilibrium constant) from electrochemical measurements?

Answer 56

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.

Question 57

How does standard potential relates to equilibrium constant?

Answer 57

lnK = - (zF/RT)E(of cell) if K>1 favours products K<1 reactions favoured

Question 58

What is the concept of activity?

Answer 58

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

Question 59

Whats the Nernst equation?

Answer 59

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

Question 60

How do you add and multiply cell potentials (E)?

Answer 60

- multiplying E doesn't work as cancelled out by the differing Z - cannot add standard potentials as the number of electrons transferring are different

Question 61

How does E depend on T?

Answer 61

As the temperature increases the cell potential also increases.

Question 62

What are the key examples of electrolytic processes?

Answer 62

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-

Question 63

What is the definition of chemical kinetics?

Answer 63

Chemical kinetics is the study of the speed of chemical reactions and the factors that affect that speed

Question 64

What is an elementary step?

Answer 64

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.

Question 65

How do you track a rate of reaction?

Answer 65

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

Question 66

What is the rate equation?

Answer 66

Reaction rate (v) is v = 1/2 d[products)/dt

Question 67

Why is concentration important to the rate?

Answer 67

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.

Question 68

What are the different orders of reaction?

Answer 68

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

Question 69

How do you work out the differential rate equation?

Answer 69

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

Question 70

how do you determine rate laws for complex reactantions?

Answer 70

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

Question 71

What does the integrated rate equation give us?

Answer 71

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.

Question 72

Which graph give a straight Lin for each order?

Answer 72

Order straight line plot 0 [A] against T 1 ln[A] against T 2 1/[A] against T

Question 73

What is the reaction half life?

Answer 73

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

Question 74

What is the reaction mechanism?

Answer 74

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.

Question 75

What is molecularity?

Answer 75

This is the number of reactant molecules or atoms taking part in an elementary reaction. unimolecular > 1 reactant bimolecular > 2 reactants

Question 76

What are the key ideas for the kinetics of a reversible reaction?

Answer 76

- 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)

Question 77

What are the kinetics for when two reactions are taking place?

Answer 77

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] ???????

Question 78

What is a consecutive reaction and its associated kinetics?

Answer 78

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]

Question 79

How do intermediates affect rate of reaction?

Answer 79

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

Question 80

What is the steady sate approximation equation?

Answer 80

[intermediates] which is equal to 0 = k1[product]/ (k-1 + k2[reactant]0

Question 81

What is the rate determining step?

Answer 81

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.

Question 82

How does temperature affect reaction rate?

Answer 82

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)

Question 83

How do you apply Arrhenius equation at two temperatures?

Answer 83

1. Work out lnK1 and lnK2 for each temperatures 2. work out ink(k2/k1) = Ea/R (1/T1 - 1/T2) 3. Rearrange these two equation to get the component needed

Question 84

How is the spread if energies shown?

Answer 84

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)

Question 85

What are the two main theory's for the occurrence of a reaction?

Answer 85

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

Question 86

How does catalyst affect reaction rates?

Answer 86

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.

Question 87

How can you determine the activation energy for a reaction?

Answer 87

1. plot lnk (y axis) against 1/T in kelvin (x-axis) should give a straight lien exhibiting Arrhenius behaviour. 2. work out the gradient in K Δ(lnk)/Δ(1/T) 3. work out Ea Ea = - gardaient x R 4. convert to Kj mol-1