CHEM111 Exam

Question 1

What is the Order of a Reaction rate = k[A][B]

Answer 1

2, as the same of the co efficients is 1+1

Question 2

The rate law shows the dependence in rate on the…

Answer 2

Concentration of reactants

Question 3

What is the unit of k in rate law for a 0, 1st and 2nd order reaction

Answer 3

0: mol L^-1 s^-1
1: s^-1
2: L mol^-1 s^-1

Question 4

If the rate is qaudrupled when the concentration of reactant A is doubled…

Answer 4

The exponential of [A] is 2 (partial order of 2) as if A is doubled [A]^2 is qaudrupled.

Question 5

If the Half life of a substance is independent of its initial concentration the reaction order is

Answer 5

1st Order, as t1/2 = ln(2)/k

Question 6

If a graph is plotted with axes labeled ln[A] by time and the plot looks to be a linear decrease, what order of reaction is it..

Answer 6

1st order as ln[A] = -kt -[A0]

Question 7

What Order of reaction would you expect the 1st and second half life to be the same

Answer 7

1st Order

Question 8

Explain all the terms in the arrhenius equation.
k = A x e^(-E/RT)

Answer 8

K is the rate constant
A is the number of Collison occurring with appropriate orientation per second
E is the activation energy for the reaction
R is the gas constant
T is temperature

Question 9

What is a Mechanism and an Elementary reaction

Answer 9

A Elemnentary reaction occurs in one collision between reacting species, whereas a Mechanism is a set of elementary reactions which detail the bond making and bond breaking process as reactants are consumed to form a product.

Question 10

What does the maxwell boltzman graph show

Answer 10

The possibility (y axis) that a particles will have any amount on energy

Question 11

What do you call an equilibrium in which all species are of the same phase. What about different Phases?

Answer 11

Same phase - homogenous equibrium
Different phase- heterogenous equilibrium

Question 12

The reaction quotient for concentration ignores what species for what reasons?

Answer 12

Solids and pure liquids as the amount of such substances does not affect the concentration of salutes, and solvents such as H2O as their large excess mean their concentration change negligiblely as a consequence of reaction

Question 13

How does increasing the temperature affect the rate of reaction on an equilibrium where the forward direction is exothermic?

Answer 13

The rate of reaction for the forward and reverse reaction both increase. However, the reverse reactions rate increases more as it is endothermic

Question 14

For a reaction gasous A + B -> C + D how would using a higher pressure then atmospheric affect the time take to reach equalibrium

Answer 14

Increasing the pressure increases the concentrations of all the gaseous species and therefore the rate of reaction so the time taken to reach equalibrium decreases

Question 15

What variable should remain unchanged when monitoring the rate of reaction and why?

Answer 15

Temperature as k varies with T, wr should hold T constant in kinetics measurements

Question 16

What is true about rate co-efficient
A) They must be positive in value
B) They are dimensionless
C) They are independent of temperature in value

Answer 16

“A” as you can never have a negative concentration of a substance

Question 17

For a reaction A(g) + 2B(g) -> C(g) + 2D(g), what is a valid rate and why
A) rate = -d[C]/dt
B) rate = k[A][B]²
C) rate = d[D]/dt
D) rate = d[A]/dt

Answer 17

C, As A and D have the wrong signs which would give the negative of the rate of reaction. For B we cannot know the order of the reaction as it is only determined experimentally

Question 18

What is Molecularity of an elementary reaction. What is the molecularity of:
2NO₂2 -> NO + NO₃

Answer 18

The molecularity of an elementary reaction is the number of reactant molecules involved.
Biomolecular as their are two reactant molecules. One reactant would be unimolecular and three reactants would be termolecular

Question 19

What molecualrity of an elementary reaction is very rare?

Answer 19

Termolecular as it is very rare for 3 molecules to collide with the right orientation and energy

Question 20

Ehat is the diffrence between thermodynamics and kinetics?

Answer 20

Thermodynamics = how it happens
Kinetics = how fast it happens

Question 21

What is the is partial order in a rate law. What’s does a 0th partial order suggest

Answer 21

Partial order is the dependency of the rate on each of the concentrations of substances in the reaction. A 0th partial order suggests the rate of the reaction does not depend on the concentration of that moelcualr at all

Question 22

What is the diffrence between rate and rate law?

Answer 22

While the rate -(1/v_A)d[A]/dt is a definition defining the average rate of a concentration of a molecules, rate law k[A]^a[B]^b id a law pinpointing the parameter on which a rate depends

Question 23

What is the difference between a static and dynamic equilibrium?

Answer 23

In a static equilibrium, simply nothing happens, whereas in a dynamic equilibrium, the rate of change of two things in opposite directions (i.e water filling up a bucket with a hole in it) is equal, so that it appears nothing in the system is changing.

Question 24

For an elementary reaction at equilibrium, the rate of the forward reaction divided by the rate of the reverse reaction is…

Answer 24

1, as the rates must be equal and opposite for it to be in equilibrium.

Question 25

For a complex reaction (multistep), the Kc values can be derived from...

Answer 25

The product of the elementary steps Kc values

Question 26

What form/value does the activity of a dilute solute, gas and liquid solids and solvents take on?

Answer 26

for a dilute solute a_X = [X]/(1 mol L-1) for a gas a_X = p/(1 bar) for ilquids solid and solvebnts aa_X = 1

Question 27

write an expression for K for the following reaction CaCO3(s) ⇌ CaO(s) + CO2(g)

Answer 27

K = (a_CO2a_CaO)/a_CaCO3 = p_CO2/bar

Question 28

What property must be satisfied so that Kp = Kc

Answer 28

n = 0, as Kc = kp*1/(RT)^{delta n}

Question 29

if K = 4 for a reaction A + 2B -> C what is the eqaulibrium constant of 3A + 6B = 3C

Answer 29

K = 4^3 = 64

Question 30

If K = x for the forward reaction, what is K equal to for the reverse reaction?

Answer 30

K = 1/x

Question 31

for N2O4(g) ⇌ 2NO2(g), K = 0.113 at 25 °C. what is the Value of K for NO2(g) ⇌ ½N2O4(g).

Answer 31

K = 1/(K^0.5) = 1/0.113^0.5 = 2.975

Question 32

An equilibrium 2A(aq) ⇌ B(aq) is established in aqueous solution. If the solution is then diluted by the addition of pure water while holding the temperature constant, what can be said about the change in the reaction?

Answer 32

The equilibrium will favour the reverse reaction as Q = [B]/[A]^2, and when each concentration is diluted, A will decrease more, and therefore the equilibrium will shift to counteract this change by producing more of A, and therefore the reverse reaction is favoured.

Question 33

What is the Second law of thermodynamics

Answer 33

The total entropy of the universe increases in a spontaneous process and remains unchanged in a process at equilibrium

Question 34

what is S°₂₉₈?

Answer 34

This represents standard enthalpy with conditions of 298K and 1 bar.

Question 35

What are the units for S?

Answer 35

J K_-1

Question 36

What is the third law of thermodynamics?

Answer 36

The entropy of a perfect crystal at absoulute zero(zero kelvin) is 0.

Question 37

What is the change in entropy for the phase change of H20 where delta(H) = +40 Kj/mol at 394K?

Answer 37

delta(s) = 40000/394

Question 38

Under what conditions must a phase change occur to calculate the change in entropy with delta(S) = q/T

Answer 38

The phase change must occur at a constant pressure so that q = delta(H)

Question 39

Calculate delta(S°₂₉₈) for N₂(g) + 3H₂(g) -> 2NH₃(g). Where delta(S) of N₂(g) = 50, H₂(g) = 10 and NH₃(g) = 30

Answer 39

2(30) - (50 + 3(10)) = - 20

Question 40

For any reaction, how would you define the entropy change of the surroundings?

Answer 40

if dS = q_surroundings/T and so ds = -q_system/T, which in turn is ds = -dH_system/T when the entropy change occurs under constant pressure.

Question 41

Would a the change in Entropy of a heat transfer at a lower temperature be greater than the same heat transfer at a higher temprature.

Answer 41

True beacuase dS = dq/T and is inutitive because a transfer of energy from somthing that is has a lot of energy (heat) would seem insignificant to the system. Whereas a system at lower energy (temprature) would view the same energy transfer as more siginifcant.

Question 42

What is the significance of the delta Gibbs free energy when it takes on certain intervals of values?

Answer 42

dG < 0 reaction is thermodynamically favoured/spontaneous. dG = 0 reaction is at equilibrium. dG > 0 reaction is non-spontanous.

Question 43

Calculate delta G°₂₉₈ for H2(g) + ½O2(g) -> H2O(l), where delta formation G°₂₉₈ of H2O is -237.1 kJ/mol

Answer 43

G°₂₉₈ = -237.1 - (0 + 0) = -237.1 Kj/mol therefor the reaction is sponatnous.

Question 44

Define Gibbs Free Energy

Answer 44

Gibbs free energy is the maximum amount of energy present in a thermodynamic system that can be used to perform work at a constant temperature and pressure

Question 45

The second law of thermodynamics is concerned with the Gibbs energy of A. The system only B. The surroundings only C. Both the system and the surroundings D. Neither the system nor the surroundings

Answer 45

A, the system only as gibbs free energy concerns the system and is derived from dS of system + dS of surroundings.

Question 46

3. Which of the following has the lowest value of standard Gibbs energy change of formation at 298 K, DfG°298? A. H2O(s) B. H2O(l) C. H2O(g) D. Require more information to say

Answer 46

B, because H2O (l) is the attack form of water in ther

Question 47

What is the relationship between the Gibbs energy of a system and the entropy of the universe?

Answer 47

dG_sys = –T dS_univ

Question 48

Explain the terms of this equation and its significance: dG = dG° + RT lnQ

Answer 48

dG is the change in Gibbs free energy, while dG° is the change in Gibbs free energy for standard conditions, R is the gas constant, T is the temperature, and Q is the reaction quotient. This equation allows you to find the Gibbs free energy for non-standard conditions.

Question 49

What equation shows the linear relationship between ln(K) and 1/T?

Answer 49

The Van't hoff eqaution

Question 50

What is the significance of the van't Hoff equations for exothermic and endothermic reactions in an equilibrium?

Answer 50

The van't Hoff equation shows the linear relationship between ln(K) and 1/T and so for an endothermic reaction, as 1/T increases (or T decreases) ln(K) (thereby K)decreases so the equilibrium favours the reverse reaction more. For an exothermic reaction, as 1/T increases (or T decreases) ln(K) (thereby K)increases, and so the equilibrium favours the forward reaction.

Question 51

A9. Which of the following statements is incorrect about a catalyst? a) A catalyst increases the rate of the forward reaction. b) A catalyst increases the rate of the reverse reaction. c) A catalyst shifts the equilibrium of the reaction to favour the products. d) The enthalpy change of the process is unaffected by the catalyst.

Answer 51

C is INCORRECT

Question 52

For sublimation of CO2 at –78 °C and atmospheric pressure, the Gibbs energy change is: a) negative; b) zero; c) positive; d) cannot say.

Answer 52

A, as sublimation is when a solid goes to a gas and as CO2 is natrually a gas gibbs free energy change must be negative so that it is spontanous.

Question 53

Why is water less dense as a liquid at 4 degrees Celsius than as ice? Hence why does ice float in water?

Answer 53

When water is in its solid form (ice), the hydrogen bonds hold the molecules further apart from each other than at four degree, where the molecules are less ordered and closer together. As water is more dense, ice typically floats in water.

Question 54

What causes aqueous solutions to form a meniscus when in a glass tube?

Answer 54

water forms weak hydrogen bonds with the molecules that for the glass lattice, and therefor their is a meniscus effect.

Question 55

What causes the surface tension of water, or a droplet?

Answer 55

Water molecules line up to forma chain of hydrogen bonds which causes surface tension

Question 56

What are the different kinds of kinetic energy?

Answer 56

tranlational - going from one place to another Rotational - spinning Vibrations - Bond stretching and compression

Question 57

What substances can dissolve in water and why are solutions favoured?

Answer 57

Polar molecules are attracted to the polar regions of water molecules and therefore can dissolve in water. Solutions spontaneously occur as the entropy of the system increases as waters structure is disrupted and the ionic solutes separate into ions which move around with more kinetic energy.

Question 58

What is the relationship between ion size and water mobility?

Answer 58

The smaller the ion, the more dense its charge is, and therefore it becomes more hydrated (more H2O molecules that surround it), and so there is greater drag. Larger ions have a less dense charge and therefore are less hydrated and have less drag and greater water mobility.

Question 59

Why is H+ a smaller ion than Li+, which has a 9 times greater water mobility?

Answer 59

Due to the Grotthuss Mechanism, where hydrogen can interact with chains of H2O molecules, which creates a rippling effect where the H+ kicks off one of the preexisting H's in H2O, replacing it, and that hydrogen ion does the same to the next molecule. Therefore, it is not actually the same hydrogen that comes out tof he other end.

Question 60

What effect does the addition of an ionic substance to H20 have on its fusion point and why? How about vaporisation?

Answer 60

Ionic substances interact with the water molecules and disrupt the formation of the crystal lattice, decreasing the fusion point. For vaperisation their are more bond within the solutions due to the addion of the ions and therefor more bonds need to broken to form a gas and the vapourisation point is increased

Question 61

What is the difference between solutes, colloids, and suspensions?

Answer 61

A solute completely interacts with water and light can shine right through it. A colloid forms spheres where the hydrophobic parts of the molecules arrange themselves in the center, while the hydrophilic parts arrange themselves in a sphere. When light is shone through a colloid, light scatters. Suspensions, the molecules do not interact with water and are "suspended" in the liquid and no light can be shone through it.

Question 62

what is pH and pOH?

Answer 62

pH = -log₁₀[H3O+] pOH = -log₁₀[OH-]

Question 63

What does amphiprotic mean, and what is a simple example of this type of molecule?

Answer 63

A substance that can either donate or accpet proton and therefor can act as a base or an acid. H2O is amphiprotic as it can both donate and accept a proton.

Question 64

how do you measure the strength of an acid (K_a) or the strength of a base (K_b).

Answer 64

The ratio (or equilibrium constant) of the products (including H+ or OH-) over reactants.

Question 65

What is the relationship between the strength of an Acid and it conjugate base?

Answer 65

The weaker the acid the stronger its conjugate base

Question 66

What requirements and assumptions can be made when calculating pH from Ka?

Answer 66

The reacting Acid must be a weak acid so that the conjugate base concentration of the acid can be estimated to be the same as the initial concentration of the acid. Additionally as H+ and the conjugate base are produced at the same rate [H+] = [conjugate base]

Question 67

How do you decide which buffer system would be best to maintain a pH of 7

Answer 67

pK_a should be aprox 7

Question 68

what is Beer's law and describe the terms in A = Epsolon_lambacl

Answer 68

Beer's law expresses the relationship between absorbance of light and the sample concentration. Epsolon _{lambda = the molar absorptivity at a certain wavelength for a given species A = absorbance c = concentration of solution being measured l = sample thickness}

Question 69

If 5 mL of a 1 mol/L species A is transferred into a 100 mL which was filled up to the 100 mL mark with distilled water, what is the concentration of A in the volumetric flask?

Answer 69

c = n/V => n = CV Therefore as the number of mols in the 5mL pipette and 100mL volumetric flask are equal cV = cV c = (cV)/V c = (1*0.005)/0.1 = 0.0005mol/L

Question 70

Summarise Experiment 4 (Kinetics)

Answer 70

Obtain the rate law of the reaction between a dye and bleach by using Beer's law to measure the concentration of the dye over time.

Question 71

Summarise Experiment 5 (Equilibrium)

Answer 71

Synthesize biodesil from canola oil and investigate how to mole ratio of starting materials (canola oil + methanol) influences the yield of synthesized biodesil.

Question 72

What is the percent yield of biodesil if the theoretical yield mass is 50g and the actual yield mass is 40g?

Answer 72

percentage yeild = 40/50*100 = 80%

Question 73

For a reaction A(g) + B(g) -> 2C(g), the rate coefficient is 3.6 × 10^–2 per hour. What is the overall order of the reaction?

Answer 73

First order as the coefficient is 1/t

Question 74

What is evidence of a complex reaction?

Answer 74

The partial orders of reaction do not match the overall reaction stoichiometry. Additionally, if there is evidence of an intermediate product.

Question 75

Which of the following observations would prove that a reaction is complex? a) The partial orders of reaction match the reaction stoichiometry. b) The reaction rate increases with increasing temperature. c) The reaction rate is increased by the addition of a catalyst. d) The reaction rate is independent of the concentration of one of the reactants.

Answer 75

d, as the partial order would thereby not match that of the stoichometric coefficents

Question 76

what are the three types of catalysts?

Answer 76

Homogeneous catalyst is the same phase as the reactant,s while a heterogeneous catalyst is of a different phase. An enzyme catalyst is a biological catalyst.

Question 77

If an Elementary reaction is carried out in the presence of a catalyst, is it still elementary?

Answer 77

No, as the Catalyst provides an alternate pathway which produces intermediates and hence it must be complex.

Question 78

Which of the following statements is incorrect about the Haber-Bosch process? a) It is a homogeneous catalytic process. b) It is used to synthesise ammonia according to N2(g) + 3H2(g)  2NH3(g). c) It requires substantially elevated temperatures (400 – 600 °C). d) It requires substantially elevated pressures (200 – 400 bar).

Answer 78

a) it uses a solid catalyst and is therefore uses a heterogeneous catalysis

Question 79

At atmospheric pressure, CO2(s) does not melt but undergoes sublimation to become CO2(g) at –78 °C. For sublimation of CO2 at –78 °C and atmospheric pressure, the Gibbs energy change is a) negative b) zero c) positive d) cannot say

Answer 79

b) as at -78 degrees, the solid and gas forms of CO2 are being produced at equal rates, meaning the reaction is at equilibrium. Therefore, Gibbs free energy change is 0.

Question 80

A79. What is the pH of a mixture containing 0.35 mol L–1 formic acid (HCOOH) and 0.25 mol L–1 sodium formate (NaHCOO)? pKa (HCOOH) = 3.75 a) 3.60 b) 3.90 c) 3.75 d) 2.10

Answer 80

a, using the henderson hasselbalch equation for a buffer solution.

Question 81

what is a state function, and what is never a state function?

Answer 81

A state function is best described as a function that depends only on the starting point and ending point of a process. Heat is a common example of something that isn't a state function.

Question 82

what is an intermediate?

Answer 82

A molecule that appears in the mechanism but not the overall reaction.

Question 83

12. What information does a rate constant give? a) The relationship between the concentrations of a species and the rate of the reaction. b) It describes the kinetic theory of gases. c) The time it takes for half the reactant to be converted into products. d) The number of molecules involved in the reaction. e) How fast the reaction proceeds at a certain temperature when all concentrations are 1 mol L –1 .

Answer 83

e) as that is what k is when everything else = 1.

Question 84

Which of the following has the LOWEST value of standard molar entropy at 298 K, S°298 and why? a) C8H18(g) b) C6H6(g) c) CH3OH(g) d) CH4(g) e) H2(g)

Answer 84

e) As H2 is the smallest and simplest molecule and as it has the fewest atoms it has the fewest possible microstates leading to the least amount of disorder

Question 85

What is the entropy change of the universe equal to?

Answer 85

The entropy change of surroudns plus the entrop change of the system.

Question 86

what is the Brønsted-Lowry definition of an acid and a base?

Answer 86

An acid is a proton donor and a base is a proton acceptor

Question 87

If the concentration a base is known how do we find it pH when put in an aqoud solution.

Answer 87

Find its pOH with -log([base]) and then do 14-pOH.

Question 88

What is pK_a and pK_b?

Answer 88

They determined the strength of the acid or base. The lower the pKa or pKb the stronger the acid or base. Additonally pKa + pKb = 14 at 25 degrees celcius.