unc chem lab final 101L

What are the proper procedures to respond to an accident?

Report to TA- fill out an incident report

What safety practices are standard for working in the lab?

no running, no food or drink, place bags in cabinets

Attire and PPE

attire: eye wear, lab coat, gloves, proper shoes, full coverage of clothes, (PPE=personal protective equipment)

The volume marking at the sealed bottom of a buret segment is 38.60 mL, and has markings for every 0.1 mL. When water is added, the bottom of the meniscus sits about halfway between the line for 18.3 mL and the next mark up. What volume of water is contained in the buret segment?

20.35 mL

Use of equipment

Always add acids, use the hood ventilation, transport chemicals in closed containers.

Define density and the key relationships between variables: mass, volume,

temperature.

density = mass/volume

As temperature increases, density decreases (negative slope value)

How did we control for the variables in this experiment (1) to make a sound

determination and comparison to reference data?

kept the temperature range from 80-25 degrees, and made sure each volume was the same in both tests (5 ml of water). The mass of the buret segment was also constant.

How are dependent and independent variables defined?

Temperature is the independent variable, density is the dependent variable

independent variable is the variable that is within your control.

What were the technical challenges of this experiment? (exp 1)

Heating up the hotplate to 80 degrees. and making sure you could insert and remove beaker without moving the buret segment.

What property was measured and with what technique? (exp 2)

temperature was measured by calorimetry. This experiment was to show temperature rises when sodium carbonate (base) reacts with water, and when ammonium chloride (acid) reacts with water. This was to show exothermic and endothermic reactions.

(acid base reactions) and reactions with acid, base, and neutral.

How was that property used to infer reaction progress? How does this appear in

graphical format? (exp 2)

if the temperature rose, it was an exothermic reaction, because it is releasing heat into its surroundings. If the temperature decreased, then it was an endothermic reaction. This causes a decrease in temperature.

Define reactant and product and identify them from a chemical equation and via

specific experimental observations.

reactant is what goes into the equations (everything before the yield arrow)

for example-sodium bicarbonate (base) and citric acid (acid) yields water, co2, and trisodium citrate. Water, co2 and the citrate are the products.

When calculating the displacement volume of the thermometer, the volume reading on the buret is 28.47 mL without the thermometer, and 27.82 mL with the thermometer in place. How many significant figures should be recorded for the displacement volume of the thermometer?

2

Define solution, solute, solvent, dissolution, and precipitation and identify them in an

experimental setup.

(Exp 2)

Aqueous solutions part 1

dissolution-breaking ionic bonds between solute and solvent

solute-substance of smaller amount (typically solid)

solvent-larger amount (liquids)

precipitate-when a solid is formed as a reactant

Mg(s)+2HCl(aq) -->MgCl2(aq)+H2(g)

Define calorimetry and describe how to set up an experiment.

technique in measuring amount of heat energy evolved or absorbed in chemical process

2 cups to keep heat in. place thermometer in middle of cup. record initial temps and then add solutes.

Relate reaction equations to specific experimental observations. (exp2)

insoluble salts-precipitates

insoluble gas- bubbles form

Define spectator ions and identify them based on a chemical equation or via

experimental observations.

spectator ions are ions that appear in identical form on the product side and on the reactant side. They can be crossed out. Net ionic does not have them.

Define acid-base reactions and neutralization reactions and identify specific reactions of this type via chemical equations or specific experimental observations. (exp 3)

acid base reactions are when an acid and a base form a water and a salt.

2Hcl (aq) + Mg(OH)2 (s)--> 2 water and MgCl2 (salt)

endothermic

t-final is less than t initial (positive delta h)

exothermic

t-final is greater than t initial (negative delta h)

determining the excess reagent experimentally in an acid base neutralization

molar ratio--if solute can dissolve then solvent is in excess

stoichiometry

numerical relationship between quantities of reactants and products in a chemical equation

important because of conservation of matter --limiting reagents.

conductivity (exp 4)

measurement of the ability of a substance to conduct an electrical current (measured in siemens (S))

as number of ions (hence concentration) in solution decreases, conductivity decreases.

reagents in exp 4

reasons they r good

barium hydroxide and sulfuric acid.

Before reacting, Ba(OH)2 and H2SO4 are almost completely dissociated or ionized into their respective ions. Neither of the reaction products, however, is significantly dissociated or ionized. Barium sulfate is a slightly soluble precipitate and water is predominantly molecular.

endpoint of conductivity titration

until the conductivity level reaches a minimum-and you add and additional 4-5 measurements of Barium hydroxide or acid after.

Identify a gas-forming reaction from the chemical equation and with experimental observations.

yields insoluble gas

(zinc and hydrogen chloride)

Zn + 2HCL --> ZnCl2 + H2

insoluble gas- bubbles form.

Identify a combustion reaction from the chemical equation and with experimental observations.

2H2 + O2 --> 2H2O

2 to 1 hydrogen to oxygen ratio is best.

Identify a catalytic reaction from the chemical equation and with experimental observations.

2 hydrogen peroxide (2h2o2)--> water and O2

catalase in this reaction-yeast--to increase the decomposition of hydrogen peroxide

most energetic reactions (experiement 5 with the rockets)

hydrogen to oxygen 2:1

then zinc

then hydrogen peroxide breakdown

measured by distance travelled with rocket

Thermochemistry

study of heat energy--involved in chemical reactions and and changes of physical state

specific heat

the amount of energy required to raise 1 gram of substance 1 degree celsius

heat capacity

magnitude of heat energy flow that accompanies an increase in temperature. depends on mass and identity of substance

Hess's Law

if a reaction is the sum of 2 or more reactions, the delta h for the overall process must be the sum of the delta h for the constituents reactions.

state function

independent of pathway between reactants and products

Beers Law

A = ebc

absorbance = molar exctinction x path length x concentration (no units)(EB is the slope) (b is typically 1) and c = x

y=mx

beers law plot

Absorbance vs concentration

as concentration increases, absorbance increases

purpose of multiple measurements of standard solution (beers law)

to figure out the relationship of concentration and absorbance. The more red there was in a test tube, the less light that was able to shine through it. This is because the concentration was higher, thus it absorbed more light.

plotting individually and not averaging (beers law)

important to see without averaging that it is still extremely linear.

colorimetry

instrumental method based on measurement of light absorption by colored solutions.

transmittance

light passing through sample relative of light passing through the blank

molar extinction coefficient

how strongly the analyze absorbs at a particular wavelength

stock solution

measurements that form the calibration curve

How does one calculate the concentration of a solution prepared by dilution of a stock solution? By combination of a solution with a diluent? By dissolving a solid?

concentration = moles/liters

Describe the electronic processes associated with absorption or emission of radiation. Which process is associated with an electron moving to a higher energy level?

absorption-ground state to excited state

emission-excited to ground

How the energy level of an electron related to the energy of a photon?

they are equal

balmer series

4 spectral lines that appear in the visible light region when a hydrogen atom undergoes a transition from energy levels n>2 to n=2.

rydbergs equation

delta e = constant x ((1/nf^2)-(1/ni^2))

nf has to equal two, and ni can be 3,4,5,or 6in order to produce photos in visible range

The molecular equation for a reaction is: Mg (s) + 2 HCl (aq) →MgCl2 (aq) + H2 (g). Based on this equation and the observations of the reaction, the spectator ions in the ionic equation would be:

Cl

A technique in which the temperature change in a closed system is measured to assess the heat transfer between a reaction and its surroundings is called

calorimetry

The temperature at the start of a reaction was 26.5°C and at the end of the reaction was 18.5°C. Based on these observations the reaction of sodium bicarbonate with citric acid would be:

endothermic, because energy is transferred to the reaction from the surroundings.

For the reaction of 3.5 g of sodium bicarbonate with 30 mL of 0.5 M HCl if the temperature rises from 22.4°C to 32.1°C. The value of ΔT/mol (°C/mol) would be:

233 °C/mol

When observing a candle flame and a fluorescent lamp through a spectroscope, both show a continuous spectrum that appears as a rainbow on the spectroscope scale, but one of them also has bright colored lines superimposed over the rainbow. The best explanation for this behavior is:

The candle produces only a continuous rainbow because its spectrum derives from its temperature, while the fluorescent light has both a continuous spectrum and the lines associated with emission from its mercury source

When viewed through a spectroscope, calcium ions produce three distinct lines in the visible region of the spectrum, at spectroscope positions of 7.30, 7.85, and 8.45. Assuming the linear fit for the calibration of the spectroscope has a positive slope, the most likely red line would be at a position of:

8.45

Based on your observations in Experiment 5, an exothermic reaction may be identified via observation in the lab when it: (select all correct answers)

causes loud popping noise, produces bubbles

The following equations represent reactions used in Experiment 5. Of these reactions, choose the correctly balanced oxidation-reduction reaction.

2 H2 (g) + O2 (g) → 2 H2O (g) (no catalase cause not asking for catalytic reaction)

Potential source(s) of significant error in Experiment 5 include: (gas forming reactions and rockets)

Losing H2 gas because it is much less dense than air.

Collecting gases while the reaction is still hot, resulting in mostly H2O vapor in the bulb.

According to the stoichiometry of the exothermic reaction equation 2 H2(g) + O2(g) →2 H2O(g), the maximum energy that can be released by this reaction would occur at a H2:O2 ratio of

2:1 hydrogen to oxygen

In Experiment 7, the purpose of creating the Beer's Law plot of absorbance vs. concentration of Allura Red AC is best described as:

to determine the mg/serving of Allura Red AC in a solution prepared from a soft drink sample.

A Beer's law calibration curve for a particular analyte has a best-fit equation of A=(11765 M-1)C + 0.197, with an R2 value of 0.9373. If a sample solution has an absorbance reading of 0.624, the concentration of the unknown sample would be:

0.0363 mM

In a determination of the mass of Allura Red AC in one serving of a food sample, the result is found to be 10% too low when compared with another determination. The most likely experimental error to explain this result would be:

The pipet used to dilute the sample measured 10% too little of the sample, so the transmittance was measured 10% lower than the actual sample transmittance.

During a conductivity titration, 15.00 mL of 0.00495 M Ba(OH)2 is placed in a beaker with 35 mL of H2O. The predicted end point of the titration with 0.00495 M H2SO4 was at a volume of 15.00 mL, but the results of titration consistently gave an end point from 18.20-18.31 mL. A well-reasoned explanation for this result would be:

The concentration of the barium hydroxide was higher than what was reported on the label.

The molecular equation for the reaction in Experiment 4 is: Ba(OH)2 + H2SO4 →BaSO4 + 2 H2O Based on the conductivity properties of these reactants and products that you observed in the lab, which is the correct net ionic equation?

Ba2+(aq) + 2 OH-(aq) + 2 H+(aq) + SO42-(aq) →BaSO4(s) + 2 H2O(l)

In a conductivity titration, 10.15 mL of 4.95 x 10-3 M Ba(OH)2 is titrated by 12.11 mL of a H2SO4 solution. Given the end point volume of titrant, what is the concentration of the H2SO4 solution?

4.15 x 10-3 M

An antacid sample contains 0.0350 moles of magnesium hydroxide. 0.0600 moles of hydrochloric acid are added. Which chemical species would you expect to encounter in excess in solution after mixing?

Reaction Equation: Mg(OH)2 + 2 HCl →2 H2O + MgCl2

Mg2+, HO-, Cl-

If the balanced equation for the reaction of magnesium hydroxide with hydrochloric acid is: 2 HCl (aq) + Mg(OH)2 (s) —►MgCl2 (aq) +2 H2O (l) How many moles of HCl can be neutralized by an antacid containing 6.750x102 mg of magnesium hydroxide? (Mg(OH)2 MW = 58.3197 g/mol)

0.02315 mol

After mixing 0.0400 moles HCl with 0.0350 moles of calcium carbonate in a crushed antacid sample, the pH after mixing was measured initially as 3.4, then increased and stabilized after 2-3 minutes at a value of 4.1. A well-reasoned explanation for this result would be:

Based solely on the number of moles added, there should be excess base, and a pH greater than 7. However, because the solubility of calcium carbonate is low, it is possible that not all moles of base were in solution, and a slowly rising pH could be caused as more base dissolves.

In a calorimetry experiment, 50.0 mL each of two dilute aqueous solutions are combined and produce an extrapolated T = 22.4˚C. If the density of each solution is assumed to be equal to that of water (at 20˚C, d = 0.9982 g/mL), and the specific heat of the solution is also equal to that of water (4.186 J/g˚C), the value of qrxn would be:

-9.36 kJ

In the reaction NaOH(s) + HCl (aq) → NaCl(aq) + H2O(l), the spectator ion(s) would be: (select all that apply.)

Cl-

In calorimetry, the enthalpy change of the reaction, qrxn, is defined to be: (Select all that apply.)

-(qsoln + qcal)

the heat gained or lost by the reaction in units of joules.

To carry out Experiment 6 quantitatively, one major adjustment was made to the procedure followed in Experiment 2. That adjustment was:

The calorimeter was calibrated to account for heat loss from the system.