CHM 144 Exam 2

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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


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