CHEM101 LAB FINAL

Why is a catalyst needed in the decomposition of hydrogen peroxide into water and oxygen?

The enzyme catalase acts as a catalyst, which lowers the activation barrier for the reaction. Otherwise the process would occur very slowly, especially as the presence of water increases

does calibration using standards help to improve the accuracy or the precision of the experiment?*

accuracy; All instruments require calibration, using standards, substances with a known true value for comparison.

calorimeters are not perfect insulators. what does that mean? how do scientists account for imperfect insulation?

calorimeters are not perfect insulators because some of the enthalpy change due to the reaction is leaked from the medium to calorimeter itself. to account for imperfect insulation, we calibrating the calorimeter. To calibrate, a known amount of heat energy from a known process is released into the medium, and the temperature change is measured. The difference between the experimental enthalpy change and the reference value defines the amount of heat energy loss, called the calorimeter constant. A second practical limitation is that the enthalpy change of the reaction does not transfer to the calorimeter medium instantaneously; it takes time for energy to move from a hot object to a cold one and to equilibrate throughout the calorimeter medium. To account for this effect, a cooling curve is measured (rather than a single temperature) and the data are used to extrapolate to the exact time that the enthalpy exchange began. The resulting values of these corrections are used to find qrxn, which is used to determine DHrxn. ** Calorimeters will still allow for some heat transfer between the experiment and the environment. We take into account this amount lost to the environment by finding the calorimeter constant and estimating the heat lost to the environment (calibrating).

how to calculate corrected volume and density

corrected volume: C= Vb - Vt - Vd Vb = largest volume increment Vt = volume with thermometer Vd= Vi - Vt Vt = cell containing a recorded volume from the table density: mass / corrected volume

how to calculate end point of a reaction

two trendlines: y1 = m1x1 + b1 and y2 = m2x2 + b2 x=(b2−b1)/(m1−m2)

question 20

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question 27*

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question 36

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question 38

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question 40

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question 41

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question 55*

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Given a photon emitted from a hydrogen lamp with a wavelength of 434 nm, identify the correct transition in the Balmer series. RH= 2.18x10^-18. (hint, it is in the visible region of the electromagnetic spectrum)

*reference notes

What is a decomposition reaction? Named one you learned in 101L

A decomposition reaction involves breaking down a compound into two or more products. Reaction A in experiment 5 was a decomposition of 2 H2O2(aq) to yield 2 H2O(l) + O2 (g) Another Ex: 2KClO3 -> 2KCl + 3O2

how to calculate CV

100 * (standard deviation / mean)

Why does ice float in water?

Ice is less dense than water because the negatively charged oxygen atoms repel one another to prevent the ice from becoming any denser, so density decreases as temperatures continue to fall below 4 degrees celsius

What is the purpose of a calibration curve?

A calibration curve can be used to predict concentration based on absorbance as shown in the linear relationship relationship between the two that is defined by Beer's Law. OR to define the relationship between Allura Red AC concentration and absorbance

what is a precipitate?

"products of a reaction exceed their solubility and form a new phase as either a solid, liquid, or gas" that forms from a solution during a chemical reaction.

What volume of a 3.0x10^-3 M solution is needed to prepare 50.0 mL of a 5.0x10^-5 M; how many moles of solute are in this solution?

(0.003 M) * (x L) = (0.00005 M) * (0.050 L) x = 0.00083 L or 0.83 mL moles of solute = M * L = 0.003 M * 0.00083 L = 2.5 x 10^-6 mol

question 11*

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question 12

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question 16

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Does making a bond require or release energy

making a bond releases energy

After measuring the absorbance vs. concentration (and working with the same spectrophotometer), you realized that you had worked with the wrong maximum wavelength. Which of the following will get affected in the regression line for the calibration curve?

- slope of the absorbance vs. concentration plot * (prob more correct) - % transmittance

1 L to mL

1 L = 1000 mL

1 mL to liters

1 mL = .001 L

1 mL to microliters

1 mL = 1000 microliters

1 microliter to mL

1 microliter = 0.001 mL

question 17

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question 18

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question 22

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question 46

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question 47

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question 54*

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When should you NOT wear gloves in the laboratory setting?

1) when entering data into your ELN on your laptop 2) when writing notes on scratch paper using a pen

how to calculate standard deviation and definition

1. Calculate the mean (simple average of the numbers). 2. For each number: subtract the mean. 3. Square the result. 4. Add up all of the squared results. 5. Divide this sum by one less than the number of data points (N - 1). This gives you the sample variance. Take the square root of this value to obtain the sample standard deviation. - measure of deviation from the mean; The smaller the standard deviation, the less scatter and the more precise the results

Use the trend line A= 17453C + 0.0745 to calculate the concentration of a solution that has an absorption of 1.0, where A and C are absorption and concentration, respectively.

1.0 = 17453 * C + 0.0745 C = 0.000053 M of 5.3x10^-5 M

explain the difference between a stoichiometric reaction and catalytic reaction, give one example for each that was not part of the experiment (be specific with a type of redox, combustion, or acid-base reaction)**

A stoichiometric chemical reaction is one where the quantities of the reactants and products are such that all of the reactants are consumed and none remain after completion of the chemical reaction, while in a catalytic reaction, the catalyst is not consumed during a reaction and therefore does not contribute to the stoichiometry of a reaction (it isn't shown in the products or reactants). catalyst is a substance added to a chemical reaction to increase the rate of the reaction without influencing the energy of the products or reactants. An example of a stoichiometric reaction is the acid-base titration of HCl+NaOH-->NaCl+H2O. Combustion Stoichiometric: 2H2(g) + O2(g) ---> 2H2O(g) Redox Catalyst: 2CO(g) + O2(g) --(Pt)--> 2CO2(g)

Define the variables in Beer's Law

A=εlc A: absorbance (no units) ε: molar extinction coefficient *defines how strongly the analyte absorbs at a particular wavelength* (M-1cm-1) c: concentration of the analyte in solution (M) l: path length (1 cm)

What is the full balanced equation for the neutralization of barium hydroxide with sulfuric acid?

Ba(OH)2 (aq) + H2SO4 (aq) --> Ba(SO4) (s) + 2 H2O (l)

why is enthalpy change considered a state function? how does this relate to hess's law

Enthalpy change is a state function because it is not pathway dependent. It doesn't matter what steps are taken to get there. Hess's Law states that the enthalpy change is the sum of all changes, so it doesn't matter which order the changes were in.

2 H2 + O2 --> 2H2O is an example of what type of reaction?*

Exothermic, combination, redox, combustion

True of False: Yeast was the catalyst of the decomposition of hydrogen peroxide into oxygen gas and water

False, yeast produces the enzyme catalase

For an endothermic reaction, is qcal pos or neg? why?

In an endothermic reaction, the system absorbs heat from the surroundings, and the surroundings lose heat to the system. thus, qcal (the heat lost to the calorimeter) is negative because the calorimeter loses heat. energy is transferred from the calorimeter to the system. Qcal and Qsol (both part of the surroundings) will lose heat to the reaction and will thus be negative in sign. Also, note Qrxn = -(Qcal + Qsol).

Why do we only record absorbance at a wavelength that gives the greatest absorbance value for Allura Red AC (max wavelength)?

Molecules all have a specific wavelength at which they absorb the most. This allows us to make easier comparisons between molecules of the same kind at different concentrations and ensures that e are comparing actual absorbance from this molecule and not background absorbance. Also, it would be impossible to compare at different/ multiple wavelengths.

The Rydberg constant (RH= 2.18x10^-18 J) is a value useful for calculating the change in energy involved in electronic transitions between an excited state and a state where nf = 2 in hydrogen atoms. Can the Rydberg constant be applied to determine changes in energy observed in mercury emission lines using the same relationship? why or why not?

No, because mercury has many electrons with many possible final energy states to come to rest at. The rydberg equation only works for hydrogen/hydrogen like species and the model was not intended for atoms with more than 2 electrons

What causes absorption?

Photons are absorbed to provide the energy that excited the electrons to the next energy.

What experimental components hindered the launch of the pipet rockets in experiment 5?

Presence of water/water vapor, exposure to the atmosphere, letting gas out, etc.

why do we use two cups in a coffee cup calorimeter set up?

Styrofoam acts as an insulator to ensure heat transfer within the system is accurately measured, and that heat from the surrounding environment does not affect the measurements. The second cup is to reinforce the first one so that the insulator is effective.

What is the purpose of the blank in Beer's Law experiment?

The blank is used to calibrate the spectrometer based on a solution with no analyte present so we can compare the solutions that do contain the analyte. (to measure the background absorbance in the cuvet and solution)

After you finish collecting your absorbance readings on the spectrophotometer, you notice a dark smudge on your cuvette you used to blank while while doing a Beer's Law experiment. How would you expect this to have affected your absorbance readings on samples you ran after the blank?

The effect of a dark smudge would be a significant decrease in the calibration measurement of absorbance, and subsequent samples in comparison would have a lower absorbance than they would have against a true blank. Because it is the blank cuvet, the following absorption measurements will be lower because the calibrated spectrometer thinks '0' absorbance is actually higher, say 10. Then, when the others are tested, instead of 20 absorbance, it will only be 10. (Just random numbers)

An electron moves from the ground state (n=1) to an excited state (n=5). Is this emission or absorbance? Is the change in energy positive or negative? Describe the relative stability of the electron after the transition.

The electron absorbs energy to move up in energy state (excitation/ emission) and therefore gains energy (positive), putting it further from the nucleus in a less stable state.

In experiment 5, you were asked to fill up the pipet bulb with water before adding the gases. Why did you need to do this?

The pipette bulb is filled with water because this is used to trap the gases into the bulb for collection and also to indicate how much gas is being collected as the water is displaced.

Explain energy flow in reaction with negative delta H value

The reaction is exothermic because the value of delta H you calculated is negative. Therefore, energy flows out of the reaction into the surroundings, increasing the energy in the surroundings (including the solution, calorimeter, etc.).

Why do you need a stir bar in the calorimeter?

To ensure the reaction goes to completion and the solution is homogeneous; steady stir rate without having to lift lid --> less heat loss and reduction of human error

A solution absorbs 50% of a specific wavelength of light that passes through it in a spectrophotometer. What is the % transmittance and absorbance at this wavelength for this solution? ( absorbance= - log(l / l0) ; %transmittance = (l / l0)

Transmittance = 50% A= -log (.5) = .301

A solution containing an unknown metal ion is sprayed into an open flame, giving rise to an orange color by eye. Upon researching metal ions which burn with this color, you find several candidate ions which also burn orange and are unsure how to identify the unknown. What test(s) would you perform to find the identity of the unknown?

You could try to continue this approach by using a spectrometer to compare with more detail, or you could proceed to test its physical qualities (lustrous, ductile, shiny, density). ** You could use a spectrometer to find the wavelengths corresponding to a peak in %transmittance vs. wavelength curve of the known chemicals that caused an orange flame, and compare them to that of the unknown to see which have the same peaks.

using your spectroscope, you observe a light source and see several strong, distinct lines. Is this source continuous or discrete? Given a line with a wavelength corresponding to 500 nm, calculate the energy of the emitted light.

discrete E = (h*c)/wavelength h: 6.626 x 10^-34 Js c: 2.998 x 10^8 m/s 1 m = 1,000,000,000 nm 500 nm = 0.000000500 m E = ( 2.998 x 10^8 * 6.626 x 10^-34) / 0.0000005 = 3.97 x 10^-19

For endothermic/ exothermic reactions, will the final temp be higher or lower?

endothermic - lower ; exothermic - higher

systematic error

error that arises from a flaw in the equipment or experimental design. this type of error can be detected and corrected, and it represents the limitations in the accuracy of a measurement. EX: a balance that is calibrated incorrectly and always reads 2.3 grams heavier than the material being weighed ; malfunctioning equipment ; if you're recording time for a reaction and your stopwatch reads 100 seconds for an actual time of 99 seconds

random error

error that arrises from uncontrolled, and oftentimes uncontrollable, variables in an experiment which has equal chance of being positive and negative. It affects the precision, or reproducibility, of a measurement. this type of error cannot be detected or corrected. Any measurement between two markings must be estimated to the best ability of the measurer, but cannot be perfect. For example, the length of the object in Figure 1-1 is between 1.4 and 1.5 units. Its length is "read" to be approximately 1.46 units. There is uncertainty in the last digit, 6, and therefore it is an estimated value. Different people reading the scale, or the same person reading it multiple times might report a range of values ; another example, reading a meniscus on a graduated cylinder and the meniscus being in between values... unable to determine exact measurement

Would you expect a bond forming reaction to be endo or exothermic?

exothermic

A reaction is being run in a calorimeter. if DT is 4.6 degrees C, is the reaction endothermic or exothermic?

exothermic because the water is gaining heat to increase the temperature, indicating that the reaction released heat

Looking through your spectroscope, you see several discrete, clearly defined lines. What is a possible source for this spectrum?

fluorescent lamp

why do forming bonds release energy?

forming bonds releases energy because the potential energy is lower than the separate states, therefore it is more stable and more favorable.

when a species get reduced it...

gains electrons

how does temperature affect the density of a solution?

heating a substance causes the molecules to speed up and spread slightly further apart, occupying a larger volume. larger volume --> decrease in density

define accuracy

how close a measurement is to the standard or known value

Define precision

how close a series of measurements are to one another (the repeatability of a measurement)

How do you recognize a combustion reaction?

look for oxygen on the reactant side of the equation and heat on the product side (but heat is not always shown); however, the general form of a combustion reaction is the reaction between a hydrocarbon and oxygen to yield carbon dioxide and water:

when should you be allowed to have food at the bench top in the lab?*

never

describe the behavior of a photon

photons behave both in a wave-like manner as well as acting like particles

After observing light from a fluorescent bulb as a series of colored bands through a spectroscope, a table is compiled with two columns. The left column corresponds to the independent variable -- the literature wavelength of each colored band and the right column is the dependent variable -- the position on the scale of the spectroscope at which each colored band appears. The data is plotted as a scatter plot and fitted to a line with the following equation: y= 0.0084x - 1.237. What is the wavelength that corresponds to a line which appears at 6.57 on the scale of a spectroscope?

plug position (6.57) in for y, since the problem states that scale position is the dependent variable 6.57 = 0.0084x - 1.237 = 929 nm * remember sig figs (3)

How do you calculate Ti and Tf given regression lines and the time of addition of the reactant?

plug time added in for x in the regression line used for extrapolation, which gives Tf. Ti is the first recorded temperature before the reaction begins.

for an endothermic reaction, is qrxn pos or neg? why?

since both qcal and qsoln are negative, and qrxn is the negative sum of qsoln and qcal ( qrxn = - [qsoln + qcal] ), qrxn is positive in an endothermic reaction, as energy is flowing out of the medium (the solution) and the calorimeter into the system. the reaction is endothermic, it gains heat

in an aqueous solution of NaCl, is water the solvent or solute?*

solvent

what is the larger quantity, solvent or solute?

solvent

you make a solution of 0.1 M NaOH and accidentally spill the solution, covering your gloved hand and onto your arm. You wrist was exposed, an the solution came into direct contact with your skin. What should you do?

take of your gloves, let TA know, and go to sink to wash your hands (at least 15 minutes)

for an endothermic reaction, is qsoln pos or neg? why?

the enthalpy change of the medium (qsoln: for example, in experiment 6, it is the aqueous solution in the calorimeter) is negative in an endothermic reaction because heat is transferred from the solution to the system. Qcal and Qsol (both part of the surroundings) will lose heat to the reaction and will thus be negative in sign. Also, note Qrxn = -(Qcal + Qsol).

If the experimenter made a mistake and put sulfuric acid in the beaker and barium hydroxide in the buret, how would the slope of the first and second lines change?

the slope of the first line would be negative and the slope of the second line would be positive. You would still begin with a certain level of conductivity and slowly lose it since the same reaction will still occur, followed by an increase in conductivity after the completion. The idea does not change - the same thing is occuring. All you have done is switched which one you are titrating with.

how to calculate mean

the sum of the measured values divided by the number of measurements

solid KOH is being dissolved in water: identify the system and the surroundings. if this reaction is exothermic, what happens to the heat of each species?

the system is the KOH, and the surroundings is the H2O, the calorimeter, and everything else around the KOH. If the reaction is exothermic, the heat in KOH is lost to the surroundings -- most directly the water

You see a flame that has a yellow color, and an emission line on the spectroscope at 6.03 mm. You have a calibration curve of y= 117.0x - 133.4 where y is the wavelength in nm and x is the position on the spectroscope in mm. What is the energy of this electron as it relaxes and emits a photon?

y = 117.0(6.03) - 133.4 wavelength = 572.1 nm 571.3 nm = .0000005713 E = (h*c) / wavelength E = ((6.626x10^-34)(2.998x10^8)) / 0.0000005713 = 3.47x10^-19 J

In the laboratory, should you add acid to water, or water to acid?

you should add acid to water; adding water to acid makes acid more likely to splash out, more dangerous in general