CHM 144 Exam 2
In a single displacement reaction between a metal and an aqueous solution of a strong acid, such as Zn (s) + 2 HCl (aq) → ZnCl2 (aq) + H2 (g), gas bubbles are evidence of a reaction. If this is observed, Zn(s) was oxidized. What is the oxidation number of Zn in ZnCl2 (aq)?
+2
oxidation number of an atom in its elemental form is
0 Ex) Ca
the sum of oxidation numbers for the atoms of a neutral compound is
0 Ex) CaO the sum of +2 and -2 is 0
The spectrophotometer must be calibrated during the experiment. For calibration, the "blank" standard solution will be used. What is the Cu2+ concentration of the "blank" solution?
0.00 g/L
the stoichiometry of the reaction is ____ mole of citric acid to _____ moles of NaOH
1, 3
This lab will make use of a calorimeter to measure the amount of heat given off or absorbed by a reaction. The calorimeter needs to be well-insulated in order to obtain accurate measurements; this is why _____ polystyrene coffee cups are used.
2
In the acid base reaction of aqueous citric acid with aqueous sodium hydroxide. How many moles of base are needed to react with (neutralize) one mole of citric acid?
3
Including both rapid and accurate titrations, how many trials will are conducted in Part I (Traditional Titration Using an Indicator Dye)?
5
For the acid-base reaction of citric acid and sodium hydroxide, the equivalence point occurs at which pH?
8.5
A single displacement reaction in which a metal displaces a second metal in a solution of the second metal ion establishes the relative reactivity of the two metals as reduction agents. Which of the following could be evidence of this type of reaction?
A color change in solution & Deposition of the displaced metal on the surface of the reactive metal.
What are the appropriate units for the Absorbance (y-axis) on the calibration curve?
A.U
endothermic
Absorbs heat
Which was the least reactive metal in experiment 6
Ag
Citric acid is a triprotic acid. Why is it that there is only one observable end point if there are three acidic protons?
All three acidic protons are of similar strength.
Why must a colorless soda be used for the traditional titration method?
Because colorless sodas allow you to see a color change to faint pink.
"no reactions" in experiment 6
Cu (s) + MgSO4 (aq) Sn(s) + Zn(NO3)2 (aq)
In compounds or polyatomic ions the oxidation numbers for certain specific atoms are:
F is -1 group 1A metals are +1 group 2A metals are +2 Al is +3 O is -2 in peroxides and -2 in all other compounds, except when bonded to F H is -1 in compounds with metals or boron, and +1 in compound with all other nonmetals group 7A halogens are -1 when bonded to metals, nonmetals, and other halogens lower in the group
What temperature scale must be used to express the relationships between variables in the gas law experiments?
Kelvin
Which was the most reactive metal in experiment 6
Mg
Which soda(s) cannot be titrated using the traditional (phenolphthalein) method?
Orange Crush & Jones Green Apple
A plot of pressure (P) vs. temperature (T) for an ideal gas shows a linear relationship with a positive slope and an intercept of zero. The equation for this relationship is:
P= mT y=mx where y=P and x=T, P=mT (b is zero). P and T are directly proportional from PV = nRT.
the most common indicator used for titrations of strong acids by strong bases and it changes color from colorless to pink
Phenolphthalein
exothermic
Releases heat
analyte
Substance being analyzed (citric acid in expt. 5)
Titrations that employ an indicator dye such as phenolphthalein have an inherent limitation to their accuracy because:
The endpoint detected by the titration is not identical to the equivalence point.
calorimetry
The precise measurement of heat flow out of a system for chemical and physical processes
Consider the balanced reaction for citric acid and sodium hydroxide (NaOH). Which statement is correct?
The stoichiometry is 1 mol acid to 3 mol base.
Why is it important that these reactions are run in two polystyrene cups nestled in a beaker (instead of only a beaker)?
The two polystyrene cups are needed because they insulate heat much better than the glass beaker would.
T/F: An activity series is useful tool because it allows one to make predictions regarding metal displacement reactions without having to actually conduct the experiments.
True
The instrument used in this experiment measures how much light energy, of a particular wavelength, is absorbed by a sample placed in the light beam of the instrument. What is it?
UV-Vis spectrophotometer
Water should be carefully excluded from the apparatus used in the properties of gases experiment because:
Water vapor behaves non-ideally at ordinary temperatures and pressures.
Why is it important that the penny sample absorbance values be between the highest and lowest absorbance values for the standard solutions?
We don't know the relationship outside of the linear range; therefore, interpolation works because it's based on linear fit of known values
The large excess of Zn present in the penny does not interfere with the measurement of the Cu content of the penny because:
Zn(NH3)42+ does not absorb at the wavelength at which the Cu(NH3)42+ is detected.
calorimeter
a device that can measure the amount of heat given off or absorbed by a reaction
titrant
a solution of known concentration (sodium hydroxide NaOH in expt. 5)
polyprotic acid
acid that has multiple acidic protons that can react with a base.
monoprotic acids
acid that has only one proton available to react with a base. always react with bases with a 1/1 stoichiometry (HCl hydrochloric acid)
A metal will displace any metal ______ it in the activity series from an aqueous salt solution; and a metal ______ H2 will displace H2 from an acid in aqueous solution
below, above
During the experiment, ammonia (NH3) is added to the solution of dissolved Cu and Zn, to form two different complex ions. The species [Cu(NH3)4(H2O)2]2+ is ______ The species [Zn(NH3)4(H2O)2]2+ is _______
blue, colorless
Which device is used to both deliver and measure the volume of sodium hydroxide (NaOH) to the citric acid solution?
buret
a substance that is oxidized (loses electrons)
causes a reduction and is therefore called a reducing agent
a substance that is reduced (gains electrons)
causes an oxidation and is called an oxidizing agent
The absorption of light energy by a molecular species can be of use for quantitative analysis because the amount of light energy absorbed at a particular wavelength by a compound in solution is proportional to its __________________ in that solution.
concentration
complex ion
consists of a central metal ion covalently bonded to two or more anions or molecules called ligands
The [Cu(NH3)4(H2O)2]2+ ion has a _________ color and the [Zn(NH3)4(H2O2)]2+ ion is _______
deep-blue, colorless
absorbance is important because it is ___________ to the concentration of the light-absorbing species in a sample at a particular wavelength
directly proportional
If the temperature of the water decreases when a reaction takes place in aqueous solution, the reaction was ____________.
endothermic
In an experiment, a student measured the initial temperature of 100.0 mL of water to be 22.0oC. By graphical analysis, they found Tf = 16.1oC. Based on the change in temperature, is this an exothermic or endothermic reaction?
endothermic
system absorbs heat change H > 0 Qp > 0
endothermic reaction
Part 1 of the experiment involves qualitative heat studies. You can determine if a particular reaction is endothermic or exothermic by touching the outside of the reaction test tube after a salt is dissolved in water. If the test tube gets warmer upon the dissolution of the salt, the reaction was ______
exothermic
system loses heat change H < 0 Qp < 0
exothermic reaction
In the data analysis of the quantitative studies you will graph your data using Logger Pro. The initial temperature (Ti) is the temperature at time 0 seconds. The final temperature (Tf) cannot be measured directly and must be obtained by ________________ to time zero.
extrapolation
evidence of a reaction will be the
formation and release of a gas (H2 bubbles) and a change in the appearance of the metal as it reacts and goes into the solution
What are the appropriate units for the Concentration (x-axis) on the calibration curve?
g/L
Reduction is the ____ of electrons
gain
reduction
gain of electrons
∆H = qp where ∆H is the change in enthalpy and qp is the _____________ gained or lost by a system at constant pressure.
heat
acid-base reaction
hydrogen ion (proton, H+) is transferred from an acid to a base
The oxidation of both Cu and Zn (both present in a penny) with nitric acid generates complex ions of Cu and Zn in solution. It also produces nitrogen dioxide, a brown, toxic gas. For this reason, where must the dissolving of the penny be carried out?
in the fume hood
If the volume of an enclosed gas at constant temperature is decreased the pressure of the gas will:
increase Pressure and volume and inversely proportional.
pH can be measured indirectly by the use of an ______ that changes color at the desired pH.
indicator dye
the calorimeter needs to be well-insulated for experiment 8. A polystyrene cup with a polystyrene lid sitting _________ polystyrene cup is an effective insulator.
inside another
Students will use the equation of the line from the calibration curve and the absorbance values for each penny solution from the group to ______________ the concentration of Cu (g/L) in each penny sample from the group.
interpolate
________ is used to determine the concentration of [Cu(NH3)4(H2O2)]2+ in the penny sample
interpolation
Microscale Laboratory Techniques
involve much smaller volumes of solutions that allow you to carry the experiments quickly, safely, and with less waste than with traditional laboratory techniques.
if temperature of the surroundings increases, _______. if temperature of the surrounding decreases, ________.
it is exothermic, it is endothermic
purpose of experiment 7
learn how visible and UV absorbance of a compound in solution is related to its concentration learn how to use a UV-visible spectrophotometer to prepare and use a calibration curve based on the beer-lambert law learn about oxidation of less active metals and complex ion formation to experimentally determine the amount of copper in a penny using the absorbance characteristics of Cu(NH3)4 2+
Oxidation is the _____ of electrons
loss
oxidation
loss of electrons
Any titration requires that the reaction be monitored to determine when the equivalence point has been reached. For acid-base titrations and the experiment you will conduct, the most common methods rely on
measuring the pH
What are the units of molarity?
mol/L
three types of equations are used to represent aqueous ionic reactions:
molecular, total ionic, and net ionic equations
modern titration
monitors pH directly with an electrode works with highly colored solutions because it does not require the observation of a visual end point the estimate of the equivalence point can be detected as the point of most rapid pH change as a function of titrant volume in a plot of pH vs. titrant volume
traditional titration
must be possible to see the color change from colorless to light pink at the end point of the titration at the end point a slight pink color indicated that the pH of the solution has reached the point at which the dye begins to change color end point will coincide closely with the equivalence point
if change in T is positive, change of Hrxn is
negative
chemical reaction
occurs when a substance is converted into a different substance
In experiment 5, citric acid has ____ observable end point because all three protons of the acid have similar strengths
one
single-displacement reaction
one element displaces another element from a compound
decomposition reactions
one reactant decomposes into two or more products
A substance that is reduced causes an oxidation and is therefore called a/an ______
oxidizing agent
pH can be measured directly with a _______ that is sensitive to [H+]
pH electrode
equivalence point
point where the number of moles of base added (the titrant) is stoichiometrically equal to the number of moles of acid originally present in the analyte
three most important chemical reactions
precipitation, acid-base reactions, oxidation-reduction (redox) reactions
the physical behavior of a sample of gas can be described by 4 variables:
pressure, volume, temperature, and amount (number of moles)
In Part 2 of the experiment, you will do ______ heat studies. The device you will use to conduct these studies is a calorimeter.
quantitative
A substance that is oxidized causes a reduction and is therefore called a/an _______
reducing agent
All three acidic protons of citrci acid are of __________, so they all react with a base such as NaOH to a similar extent
similar strength
Why was it necessary to find Tf (final temperature) using Logger Pro by extrapolating back to time = 0 seconds?
some of the heat may have been lost throughout the experiment to surroundings.
oxidation number
the charge each atom would have if all shared electrons were transferred to the atom that more strongly attracts the electrons
specific heat capacity (Cs
the energy required to raise the temperature of one gram of a substance by one degree Celsius or one K 4.184
displacement reactions
the number of substances remains the same during the reaction, but atoms or ions exchange places.
end point
the point in a titration at which an indicator changes color
the oxidation number of an atom in a monoatomic ion is
the same as the charge on the ion Ex) Ca2+ = +2
In oxidation-reduction (redox) reactions,
there is a net transfer of electrons from one reactant to another.
Citric acid is triprotic; therefore it needs ____ moles of base to completely neutralize one mole of citric acid
three
Using a solution of known concentration to determine the concentration of another solution through a monitored reaction of known stoichiometry that goes to completion is called a(n) ________________.
titration
In both the traditional titration (Part 1) and the titration using the pH electrode (Part 2), you reached an estimated equivalence point. Which method gives results that align more closely to the true equivalence point?
titration using the pH electrode (Part 2)
Purpose of experiment 6
to explore and compare the relative reactivity as reducing agents of several metals and hydrogen to use microscale techniques to evaluate a series of possible reactions between solid metals and dissolved cations in aqueous solution to develop a limited activity series for a group of metals based on experimental data to write balanced molecular, total ionic, and net ionic equation for displacement reactions
purpose of experiment 10
to learn how to draw lewis structures of molecular species to use the lewis structures of molecular species and VSEPR theory to predict their geometry to learn to represent three-dimensional structures with two-dimensional drawings
purpose of experiment 9
to learn how to perform simple gas phase experiments involving the measurement of volume, temperature, and pressure to develop hypotheses about the pair-wise relationships of 4 variables in the ideal gas law to experimentally test those hypotheses to learn how the variables in the ideal gas equation are related
Purpose of experiment 5
to understand difference between polyprotic and monoprotic acids to learn the basics of the pH scale to learn how to perform an acid-base titration to compare and contrast 2 methods (traditional indicator dye vs. modern pH electrode) to calculate molarity of citric acid in 2 sodas
purpose of experiment 8
to understand the concept of enthalpy (H) to understand how change in H is related to temperature changes during a reaction to learn to use calorimetry to experimentally determine change in H to learn to use thermodynamic data for practical applications
T/F: Aqueous solutions of nitric acid are strong oxidizing agents that are capable of oxidizing a number of less active metals, including Cu.
true
combination reactions
two or more substances react to form one product
spectroscopy
uses the interactions of matter and light to determine the concentration of colored solutions
Avogadro's law
when the number of moles of a gas is increased, I expect the pressure of the gas to increase . V1/n1 = V2/n2
When does the equivalence point occur in an acid-base titration?
when the number of moles of acid is stoichiometrically equal to the number of moles of base added.
Gay-Lussac's Law
when the temperature of a gas is increased, I expect the pressure of the gas to increase . P1/T1 = P2/T2
Charles law
when the temperature of a gas is increased, I expect the volume of the gas to increase V1/T1 = V2/T2
Boyle's Law
when the volume of a gas is decreased, I expect the pressure of the gas to increase . P1V1 = P2V2
Could the titration with the pH electrode be done on colorless sodas?
yes, the titration with the pH electrode can be done on colorless sodas in addition to colored sodas
For the traditional titration method, colorless sodas like Sprite or 7-up had to be used. Could the titration with the pH electrode be done on colorless sodas?
yes, the titration with the pH electrode can be done on colorless sodas in addition to colored sodas
The oxidation number of an atom in its elemental form is _________.
zero