Chem 101 Lab Final Exam

When should you NOT wear gloves in the lab setting?

a. When weighing our reagents at the balances

b. When entering in data on your laptop

c. When handling clean glassware

d. When writing notes on scratch paper using a pen and paper

b and d

*also should not wear gloves when opening doors

General glove rules

- you should never reuse gloves

- dispose of gloves if they are heavily contaminated

- dispose of gloves if they are torn

- dispose of gloves in the glove hazardous waste container

- gloves must be worn at all times while handling chemicals or glassware

- do not wear gloves outside of lab

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

let your TA know and wash your skin under running water for at least 15 minutes

When should goggles be worn in lab?

The whole time you are in lab

You are taking an evening laboratory session and want to bring a snack to help you make it though the evening. When should you be allowed to have food at the bench top in the lab?

a. If the experimental work is over and cleaned up

b. At the beginning because it is dinner time and being a student is exhausting.

c. Only if the food remains in the students backpack while at the bench top.

d. Never.

e. Never, unless you have a tendency to become hypoglycemic, then you are allowed to eat at the bench to prevent health issues.

d. Never

Accuracy

A measure of how close a measurement is to the true value of the quantity measured.

Precision

A measure of how close a series of measurements are to one another.

Systematic error arises from a flaw in the equipment or in the experimental design. This type of error can be detected and corrected. It leads to predictable results. Provide several examples.

- a scale that is improperly calibrated and always reads 2.3 grams heavier than the object actually is

- a stretched out measuring tape that always reads values lower than the true value.

Random error arises from uncontrolled, often uncontrollable, variables in an experiment which has equal chance of being positive and negative. This type of error cannot be detected or corrected for. It leads to unpredictable results. Results will likely cluster around true value. Provide several examples.

- wind speed might speed up or slow down at unpredictable times

- any measurement between 2 markings because different people will read the measurement differently

EXP 1: Why does ice float in water?

Ice is less dense than water because the negatively charged oxygen atoms repel one another due to Hydrogen bonding. After 4 C (40 F), water begins losing density.

EXP 2: You measure NaOH by difference using an analytical balance. The initial mass of the NaOH bottle is 5.042g. After removing the NaOH, the bottle mass is 4.812g. What is the mass of the NaOH you are using? (use correct sig figs)

0.230 g

Mass is measured to _____ significant figs

4

Volume is measured to _____ significant figures

3

Density equation

mass/volume

Find the density of liquid CO2. Remember that 1 mL=1 cm^3.

Mass - 55.05g, volume - 10mL

55.05g/10mL = 5.505 g/mL

Find the density of Dry Ice (solid CO2). Determine whether the solids will sink of float when put into liquid CO2. Remember that 1 mL = 1 cm^3.

Mass - 15.62g, 10cm^3

15.62g/10cm^3 = 1.562g/cm^3

Find the density of ice. Determine whether the solids will sink or float when put into liquid CO2. Remember that 1 mL = 1 cm^3.

Mass - 9.34g, 10cm^3

9.34g/10cm^3 = 0.934g/cm^3

Mean equation

add up all the data and divide by # of data points

Standard deviation equation

1. find the mean

2. subtract the mean from each data point

3. square the answer ^

4. find the mean of the new data points by adding them up and dividing by 1/N

5. take the square root of that answer

Coefficient of variation

measure of precision as a % of the value measured

Coefficient of variation equation

standard deviation/mean * 100

How does temperature affect the density of a solution?

Density decreases as temperature increases (inversely related)

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

Accuracy

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?

system - KOH

surroundings - water and everything else.

If exothermic, KOH will lose heat and water/surroundings will gain heat.

percent error

(approximate-exact)/exact * 100

percent difference

(higher quality mean-mean)/(higher quality mean) * 100

A reaction is being run in a calorimeter. Find DT given Ti = 24.6, Tf = 29.2. Is this reaction endothermic or exothermic?

DT = Tf-Ti = 4.60. This reaction is exothermic because the change in temperature is positive.

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

acid to water because the solution that forms is very diluted and will not splash as much.

Which is the larger quantity, solvent or solute?

Solvent

In aqueous solution of NaCL, is water the solvent or solute?

solvent because an aqueous solution is one in which the solvent is water

Precipitate

an insoluble solid that emerges from a precipitation reaction

Why do we use two cups in a coffee cup calorimeter setup?

to insulate/isolate the system more and prevent heat exchange to the surroundings

Why do you need a stir bar in the calorimeter?

to ensure that the solute gets completely dissolved in the solvent so that the reaction goes to completion; to create a uniform environment for measurement by the temperature probe

T/F - to calculate change in temperature you use Ti-Tf

false, other way around

Does making a bond release or require energy?

bond formation always releases energy to the surroundings

Does breaking a bond require or release energy?

bond breaking always requires energy from the surroundings

Why do bond forming reactions release/require energy?

when you make a chemical bond, the electrons go to a lower energy states which simultaneously releases energy in the form of heat or light

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

exothermic because energy is released to the surroundings. Endothermic would break bonds because it absorbs energy from the surroundings.

Explain the difference between a stoichiometric reaction and a catalytic reaction, give one example for each that was not part of your experiments.

Catalytic reactions: Hydrogen peroxide (produces foam), benzine. Stoichiometric: Zn and HCL (produces gas), when Oxygen and Hydrogen react with water......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)

What is a decomposition reaction? Name on that you have learned in 101L.

- a chemical reaction that occurs when a single compound breaks down into two or more elements or new compounds

- example: H2O2 into water and oxygen

combustion reaction

a chemical reaction that occurs when a substance reacts with oxygen, releasing energy in the form of heat and light (exothermic)

2H2 + O2 --> 2H2O is what type of reaction

combustion reaction

catalyst

substance added to a chemical reaction to increase the rate of reaction without influencing the energy of the products of reactants. Not consumed during a reaction therefore does not contribute to its stoichiometry

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

to increase the rate of reaction because it is generally very slow

T/F - yeast was the catalyst of the decomposition of hydrogen peroxide into oxygen gas and water

false, yeast produces the catalyst, catalase, nature's catalyst

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

transferring the bulb could have released gas out of the bulb, problems with the spark gun, reactions slowed down over time requiring more reactants to fill the gas, air besides oxygen and hydrogen in the bulb, imprecise measuring of bulb, improper ratio

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?

so that we could see how much gas we were adding by displacement of water

Why is enthalpy change considered a state function? How does this relate to Hess's law?

Enthalpy change is a value that doesn't depend on the path taken. Hess's law is due to enthalpy being a state function, which allows us to calculate the overall change in enthalpy by simply summing up the changes for each step of the way, until product is formed. All steps have to proceed at the same temperature and the equations for the individual steps must balance out.

For an endothermic reaction, what is the sign of qcal, and why?

negative because the calorimeter loses heat

For an endothermic reaction, what is the sign of qsoln and why?

negative because the solution loses heat to the system

For an endothermic reaction, what is the sign of qrxn and why?

positive because the reaction it is the negative sum

Calorimeters are not perfect insulators. What does this mean? How do we, as scientists, account for imperfect insulation?

They are not perfect insulators because heat exchange still occurs with them. We account for this by using 2 cups and a lid. It is a systematic error.

What's the purpose of a calibration curve?

A calibration curve is necessary to determine the concentration in an unknown sample//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 causes absorption?

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

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 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 measuring the absorbance vs. concentration (and working with the same spectrophotometer), you realized you had worked with a wrong wavelength max. Which of the following will get affected in the regression line for the calibration curve? a. concentration b. slope of the absorbance vs. concentration plot c. length of the spectrophotometer d. transmittance

b. Slope of the absorbance vs. Concentration plot

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

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 we are comparing actual absorbance from this molecule and not background absorbance. Also, it would be impossible to compare at different/ multiple wavelengths.

After you finish collecting your absorbance readings on the spectrophotometer, you notice a dark smudge on your cuvette you used to blank 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?

A smudge results in less light being detected and the instrument will think there is a greater absorbance than there actually is. 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).

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

a. Fluorescent lamp

b. Incandescent lightbulb

c. Candle

d. Sunlight

a. fluorescent lamp

An electron moves from the ground stair (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.

Describe the behavior of a photon.

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

The Rydberg Constant (Rh = 2.18*10^-18) is a value useful for calculating 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

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

calculating 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

standard deviation

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

density and temperature

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

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)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)**