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Lab Assessment Flashcards

1
Q

First law of thermodynamics

A

Energy cannot be created nor destroyed, only altered
If heat is lost by the system, it is Gained by the surroundings, and vice versa
q system = -q surroundings
q = heat

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2
Q

q>0

A

q is positive
Heat flows in
Object heats up, surroundings cool down
Product-sided
Temperature change does not necessarily accompany heat flow

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3
Q

q<0

A

q is negative
Heat flows out
Object cools down, Surroundings heat up
Reactant-sided
Temperature change does not necessarily accompany heat flow

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4
Q

Heat capacity

A

Specific heat capacity, c, is a measure of a materials resistance to temperature change

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5
Q

Greater heat capacity =…

A

For a constant mass of material (m) and amount of heat exchanged (q), a material with a greater heat capacity is more resistant to temperature change

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6
Q

Formula to find heat lost or gained by a system

A

q = mc(Tf-Ti)
q is heat transferred into or out of material, typically in units of J, can be positive or negative
m is mass of material, typically in units of g
c is specific heat of material, typically in units of J/gCelsius or J/gK
Delta T is temperature change of material due to the gained or lost heat

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7
Q

For two materials, A and B, in thermal conduct in an isolated system, then…

A

For two materials, A and B, in thermal conduct in an isolated system, like a perfect calorimeter, then qA=-qB

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8
Q

Calorimeter

A

Main purpose is to contain the heat transfer between two systems (a chemical reaction and surrounding solution) and limit heat exchanged with the surroundings
Without calorimeter, there is no way to know how much heat is exchanged with the surroundings

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9
Q

Experimental enthalpy change

A

When an aqueous reaction is carried out in a constant-pressure calorimeter, q reaction ~=~ -q solution
Under constant pressure, q reaction = (n)(delta H), where n is moles of reactions that occur
q solution = m solution * c solution * delta T solution
(n)(delta H) = -m solution * c solution * delta T solution
Delta H can be found by (-m solution * c solution * delta T solution)/(n)
Final units = kJ/mol

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10
Q

Enthalpy change

A

Delta H
Represents the heat absorbed or released by the reaction under constant pressure

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11
Q

Delta H > 0

A

Reaction is endothermic and absorbs heat
q > 0

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12
Q

Delta H < 0

A

Reaction is exothermic and releases heat
q < 0

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13
Q

Determining Tf and Ti through calorimeter graph

A

Tf (extrapolated) is the y-intercept, if it were to occur instantaneously

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14
Q

Expected enthalpy change

A

Sum the enthalpies of formation of reactants and subtract that from the sum of enthalpies of formation of products
Ex:
A + B -> C + D
(C+D) - (A+B) = expected delta H
NOTE: make sure to balance equation given, and multiply the each enthalpy by its moles
Ex:
2A + B -> C + D
(C+D) - ((2*A)+B) = expected delta H

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15
Q

Endothermic

A

q > 0
Delta H > 0
Energy in solution increases
Heat absorbed from surroundings
System gets hotter
Beaker would feel cold
Color intensifies with heat

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16
Q

Exothermic

A

q < 0
Delta H < 0
Energy in solution decreases
Heat absorbed from system
System gets colder
Beaker would feel hot
Color fades with heat

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17
Q

Freezing point depression

A

Delta Tf is the freezing point depression of a solution due to a dissolved solute, conventionally reported as a positive number even though the temperature is going down
Delta Tf = |Tf solvent - Tf solution|
Delta Tf = i * m * Kf
Freezing point of pure solvent is higher than that of a solution
More concentrated solutions have lower freezing points
Magnitude of freezing point depression is directly proportional to concentration of solute

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18
Q

i

A

van’t Hoff factor
Represents number of independent solute particles per unit of whole solute in solution
Freezing point depression is a colligative property, so all that matters is the number of solute particles, not the type of solute particles
i = 1 for nonelectrolytes
i = number of dissociated ions for electrolytes
Ex: NaCl -> Na^+ + Cl^- , so i=2

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19
Q

Electrolytes

A

Substances which, when dissolved, break up into cations and anions
Ex: NaCl turns into Na^+ and Cl^-
Can be identified if consists of a metal and a non-metal, or if its one of the strong acids
Most ionic compounds, most acids, most bases

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20
Q

m

A

Molality of solute in solution
m = (mol solute)/(kg solvent)
Special measure of concentration that is independent of temeprature

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21
Q

Kf

A

Freezing point depression constant that depends on the solvent

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22
Q

How to identify freezing point from temperature-v-time data

A

Temperature is constant over time while material is freezing
Average of data points in flat region of graph

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23
Q

Calculate molar mass using freezing point depression

A

From Delta Tf = imKf, m can be found. Then use known mass of dissolved solute and known mass of solvent to find molar mass of solute
m = (delta Tf)/(Kfi)
mol of solute = mkg of solvent
molar mass = (mass of solute in grams)/(mol of solute)

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24
Q

Greater magnitude freezing point depression

A

Means lower freezing point
Means higher freezing point depression

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25
Transmittance
Proportion of light that passes through a sample
26
Absorbance
Proportion of light that does not pass through sample because it is absorbed by solution Increases with increasing concentration of solution Color seen is what is reflected and complimentary color is what is absorbed Ex: Plants - Green is reflected, red is absorbed
27
A lambda
The absorbance of the solution for a specific wavelength of light Unitless
28
a
E lambda Molar absorptivity of solution for given wavelength Has units of M-1 * cm-1 Represents the strength with which the solution absorbs a given wavelength Slope of the line (m) in calibration curve = E lambda
29
b
Pathlength that is distance the light travels through the solution Experimentally, this is the width of the cuvette that holds the solution Typically in cm
30
c
Concentration of solution Typically in M
31
Calibration curve
Linear plot of absorbance (y axis) versus concentration (x-axis) for standard solutions Model that shows how a dependent variable varies across a range of standard solution
32
How to determine concentration of sample using calibration curve
We can use the equation of the best-fit line (y = mx + b) of the calibration curve to determine the concentration (x) of an unknown solution from its measured absorbance (y)
33
Lambda max
Every chemical has one Specific wavelength at which the chemical has maximum absorbance
34
Serial dilution calculation
M1V1 = M2V2 M1 = concentration of original solution V1 = volume taken out from original solution V2 = total volume after dilution M2 = concentration of diluted solution
35
Beer's Law and calibration curve
A lambda = a * b * c Predicts that the y-intercept in a calibration curve should be zero in the ideal case Slope of the line (m) in calibration curve = E lambda Do not force through trendline
36
Corrected absorbance
Corrected absorbance = Lambda max absorbance - Absorbance of region Ex: Chlorophyll-a A corrected = A lambda max - A 750nm
37
Components of an electrochemical cell
Cathode: Electrode where reduction occurs Anode: Electrode where oxidation occurs Electrolyte solution: Allows ions (and charge) to move between electrodes
38
Voltaic/Galvanic cells and electrolytic cells
Voltaic cells: Also known as Galvanic cells; Electrons spontaneously move from the anode to the cathode under standard conditions Electrolytic cells: An external power supply is required to drive electrons from the anode to the cathode under standard conditions
39
Redox reactions
Identified by determining the oxidation numbers of all elements in the reaction If the oxidation numbers for elements change across the reactants and products, a redox reaction has occurred
40
Electroplating
Ions plate onto an electrode driven by an applied current External power supply drives the movement of electrons from the anode to the cathode
41
How many electrons are transferred between the reducing and oxidizing agents in the balanced redox reaction: Zn (s) + Cu^2+ (aq) -> Zn^2+ (aq) + Cu (s)
2 electrons transferred per reaction
42
Total moles of electrons transferred via Faraday's Constant
F = 96500C/mol e^- Ex: (2.00 mol e^-)(F) = 1.93x10^5C
43
Electrical current
Charge/time
44
Anode and canode
Anode: Where the process of oxidation, or loss of electrons, occurs; Zn (s) Cathode: Where the process of reduction, or gain of electrons, occurs; Cu (s) Red cat and an ox Reduction occurs at the cathode and anode is oxidized Don't PANIC Positive is anode, negative is cathode Electrons move from anode to cathode
45
Total charge from experiment in Coulomb
C = current (A) * time (s)
46
Percent difference
((|Calculated charge - Integrated charge|) / Integrated charge) * 100
47
Expected change in mass
1) Write out balanced ionic equation 2) Find the moles of electrons 3) Find the moles of Zn^2+ that is reduced (or Zn is oxidized) 4) Determine the change in mass Ex: Zn (s) -> e^- + Zn^2+ (aq) 1Zn (s) -> 2e^- + 1Zn^2+ (aq)
48
Experimental amount of charge transferred
of mole = change in mass / molar weight of Zn Once calculate number of Zn transferred, will need to determine amount of charge that was transferred in the electrolytic solution mole of e^- = (mol e^- / mol Zn) * mole of Zn Charge = F * mole of e^-
49
Cell efficiency
% cell efficiency = (|Experimental change in mass| / Expected change in mass) * 100 % cell efficiency = (|calculated amount of charge| / integrated value) * 100
50
Faraday's law of electrolysis
m = (Q*M)/(n*F) m = mass of substance plated in grams Q = total charge in Coulombs M = molar mass of substance in g/mol n = number of electrons involved in reaction (for Zn^2+ to Zn, n =2) F = Faraday's constant, or 96485 C/mol
51
The magnitude of the average rate of change in concentration of a species A over a time interval delta t is
delta [A] / delta t
52
For given reaction aA + bB -> cC + dD then
Reaction rate = (-1/a)(d[A]/dt) = (-1/b)(d[B]/dt) = (1/c)(d[C]/dt) = (1/d)(d[D]/dt) d[A] = change in concentration of A dt = change in time Negative sign indicates consumption of reactant and positive sign indicates formation of product
53
Rate law
If aA + bB -> cC, then Rate = k[A]^m[B]^n Rate is instantaneous rate of reaction k is rate constant of reaction at given temperature [A] and [B] are the concentrations of the reactants in the reaction m and n are the corresponding reaction orders for A and B m + n is the overall reaction order
54
Arrhenius equation
k = Ae^(-Ea/RT) ln(k) = -(Ea/R)(1/T) + ln(A) k generally increases with increasing temperature because higher temperature means higher energy, thus speeding up reaction and increasing k k is large, reaction is fast k is small, reaction is slow Can be used for the calculation of activation energy of a reaction with and without a catalyst
55
Catalysts
Increase the rate of reaction Provides lower energy pathway for the reaction
56
Determining reaction order
When comparing trials, always make sure everything else is kept constant except the variable that is being compared Easier to compare any trial number with baseline trial to find reaction order 0th order - if concentration triples, then reaction rate does not change 1st order - if concentration triples, then reaction rate triples 2nd order - if concentration triples, then reaction rate multiplies by 9, or (3)^2
57
Determining reaction order example
Rate 3: [A]3 ^m = 1.0x10^-6 = 0.30^m Rate baseline: [A]baseline^m = 1.0x10^-6 = 0.10^m Rate3/Rate baseline = (0.30/0.10)^m In this case, the rate is not changing, so m = 0 Although the concentration is tripled, since m = 0, then the reaction is in the 0th order, so the reaction rate does not change
58
What influences the rate of a chemical reaction
1) Adding a catalyst 2) Changing the temperature 3) Changing the concentration of the reactants
59
What is needed to determine stoichiometry between titrant and analyte
A balanced reaction
60
Arrhenius acid
Ionizes in water to yield H^+ ions
61
Arrhenius base
Ionizes in water to yield OH^- ions
62
Bronsted-Lowry Acid
Any substance that can donate a proton, H^+ ion
63
Bronsted-Lowry Base
Any substance that can accept a proton, H^+ ion
64
Lewis acid
An electron pair acceptor
65
Lewis base
An electron pair donor
66
Lewis acid and Lewis base in a diagram
Lewis base gives electrons to hydrogen atom that is attached to Lewis acid Hydrogen atom gives electrons to Lewis acid forming conjugate base Hydrogen atom attaches to Lewis base forming conjugate acid
67
Endpoint
When indicator changes color in titration Equivalence point is assumed to be the endpoint
68
How to determine molarity of acid
1) Balance chemical equation 2) Determine which is the one with known concentration, which in this case, is the base 3) Determine number of moles added for BOH 4) Apply molar ratio between base BOH and acid HA 5) Determine number of moles of the acid HA titrated with BOH 6) Determine molarity of the acid HA
69
Keq
State of chemical equilibrium Rate of forward and reverse reactions are equal to each other Keq = [Products]eq / [Reactants]eq Keq > 1: More products at equilibrium Keq < 1: More reactants at equilibrium
70
Le Chatelier's Principle
If the system is at equilibrium and a reactant is added, more product is produced to reach equilibrium again Reaction will shift to the right to get rid of excess reactants If system is at equilibrium and a product is added, more reactants will be produced to shift to equilibrium Reaction will shift to the left to get rid of excess products
71
Equilibrium constants are...
Temperature dependent If the reaction absorbs energy from the surroundings, it is endothermic (Delta H > 0, favor products when temperature increases) If the reaction release energy from the surroundings, it is exothermic (Delta H < 0, favor reactants when temperature increases)
72
If solution becomes more pigmented, then...
More products are being created If color intensifies with heat, the reaction is endothermic If color fades with heat, the reaction is exothermic
73
Relate standard Gibbs free energy change to equilibrium constant for a reaction at a given temperature
Delta G = -R*T*ln(Keq) R = 8.314 J/(mol*k) = 0.008314 kJ/(mol*K)
74
Related Gibbs free energy to entropy change and enthalpy change
Delta Go = Delta Ho - T*Delta So Delta H: Enthalpy Delta S: Entropy
75
Relate Keq to standard entropy change and standard enthalpy change of a reaction
ln(Keq) = -(Delta H) / (R*T) + (Delta S) / R or y = mx + b
76
Point of saturation depends on...
The temperature and the pressure of the solution
77
Ksp
Solubility product constant Ex: NaCl (s) -><- Na^+ (aq) + Cl^- (aq), so Ksp = [Na^+][Cl^-] Don't include solids in Ksp Ksp > 1: Soluble compounds Ksp < 1: Insoluble compounds
78
Standard (Go) vs. Non-Standard (G) Gibbs Free Energy Change
Delta Go > 0, Keq < 1: Majority reactants at equilibrium Delta Go < 0, Keq > 1: Majority products at equilibrium Delta G > 0, Q > Keq: Proceeds spontaneously in reverse to reach equilibrium Delta G < 0, Q < Keq: Proceeds spontaneously forward to reach equilibrium
79
Calculate Delta Go
Delta Go = -RTln(Ksp) Delta Go = Delta Ho - T*Delta So Delta Ho - T*Delta So = -RTln(Ksp) ln(Ksp) = -(Delta Ho/R)(1/T) + (Delta So/R) This equation is in the form of y=mx+b Slope = -(Delta Ho/R) y-intercept = (Delta So/R)
80
Buffer
Designed to neutralize small amount of added acids or bases, allowing for only small pH changes Consists of a conjugate acid/base pair Generally made up of a weak acid and its conjugate base An acid added to the buffer solution reacts with the weak base of the buffer The conjugate base of an acidic buffer will accept hydrogen protons when a strong acid is added to the solution The weak acid of an acidic buffer will donate hydrogen protons when a strong base is added to the solution Greatest degree of buffering is when the desired pH is equal to the pKa
81
pH of buffer depends on...
1) pKa of the acid 2) Ratio of the base: acid concentrations pH can be determined by Henderson-Hasselbach equation Buffer works best when within +/- 1 pH of the pKa of the acid involved
82
Henderson-Hasselbach equation
pH = pKa + log[base]/[acid] pKa = -log(Ka) = -log([H^+][A^-]/[HA])
83
Weak acid titration with strong base
HA is weak acid and A^- is conjugate base Find pH before titration using ICE table for HA Before titration, HA is predominant species At equivalence point, all HA has reacted with the base to form A^- Find pH at Veq: ICE table for A^- Find pH after Veq: Excess [OH^-] Excess OH^- is dominant species
84
How to determine which calculation strategies to use to find pH
1) Excess strong acid/strong base: Use -log method 2) Weak acid and conjugate base: Use HH equation 3) Only weak acid or only conjugate base: Use ICE table

Chem 120 (2 decks)

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