Chem 310 Lab Oral exam

Why do we use a dewar?

A dewar acts as an insulated container that minimizes heat exchange between the calorimeter and the surroundings. This helps ensure that the measured temperature change is due only to the reaction

Was the reaction exothermic or endothermic? How do you know?

the reaction was exothermic. I know this because the temperatures increased during the reaction, so heat was released by the reaction into the solution.

What was the purpose of the first week of lab?

the purpose of the first week of lab was to understand the expectations for lab, learn how to write lab reports, and understand the layout for this term.

Give one reason why the enthalpy of reaction might be lower than expected.

Heat loss to the surroundings could be a reason. Other possibilities include incomplete reaction, measurement errors in temperature, or inaccuracies in reactant masses.

Why do we need to divide by the mass of solute used to compare to literature values?

Different experiments use different amounts of material. Dividing by the amount of solute normalizes the result so it can be compared directly with literature values .

Describe two differences between the measurements taken on the UV-Vis compared to the fluorimeter.

1. UV-Vis measures absorption of light, while fluorescence measures emitted light after excitation.

2. UV-Vis records wavelengths absorbed by the quantum dots, while the fluorimeter records wavelengths emitted by the excited quantum dots.

How does what you saw under UV light relate to UV-Vis or fluorescence measurements?

The color observed under UV light comes from fluorescence emission. The emission wavelength measured by the fluorimeter corresponds to the color seen by eye. Larger quantum dots emit longer wavelengths (redder colors), while smaller dots emit shorter wavelengths (bluer colors).

How did the quantum dot size change over time? Was it consistent across all times?

The quantum dots generally grew larger with time because the particles continued growing after synthesis began. Growth was not always perfectly consistent; growth rates often slow at longer times as reactants decrease.

Why did we have to take a scan on the UV-Vis rather than measuring at one wavelength?

A scan is needed to locate the absorption peak. Since the peak shifts as quantum dot size changes, measuring only one wavelength could give inaccurate size information.

What does this code accomplish?

The result is a scatter plot of radius versus time.

If the solution was ideal, would you have to worry about not filling your solution to the line initially? Explain.

Not as much. Ideal solutions have no volume change upon mixing because the intermolecular interactions are essentially unchanged.

When measuring vapor pressure, why did you need to withdraw the syringe after injecting the solution?

After injection, the syringe was removed so the flask remained a closed system with a fixed volume. Leaving the syringe attached would alter the vapor space and pressure measurements.

Why did we use mass to determine the number of moles rather than volume?

Mass is conserved and can be accurately converted to moles using molar mass, making composition calculations more reliable. Volume changes when liquids are mixed

What interactions between molecules could cause measured pressure to be higher or lower than expected?

• Stronger A–B interactions than A–A or B–B interactions decrease vapor pressure (negative deviation from Raoult's Law).

• Weaker A–B interactions increase vapor pressure (positive deviation from Raoult's Law).

Explain how you calculated the total and partial molar volumes.

1. Calculate total moles from the masses and molar masses.

2. Calculate mean molar volume:

3. Plot mean molar volume versus mole fraction.

4. Fit the data to a quadratic equation.

5. Take the derivative of the fitted equation.

6. Use equations Va and Vb to determine partial volumes

What quantitative and qualitative observations showed a reaction was happening?

Quantitative: The measured electrode potential changed with time and displayed oscillations.
Qualitative: Possible bubbling, visible changes on the iron electrode surface, and other observable changes noted during the experiment.

How does Euler's method work?

Euler's method solves differential equations by taking small time steps. For each step, the change predicted by the rate equation is added to the previous value to estimate the next value.

What does clicking on the bottom-right corner of a cell in Excel do? Why is this useful?

Double-clicking the fill handle automatically copies the formula down the column. This is useful because it quickly performs calculations for hundreds or thousands of rows.

What was special about how the iron electrode was stored? Why was this necessary?

The iron electrode had to be protected from oxidation before use. Exposure to air and moisture can form an oxide layer (rust), which would affect the reaction surface and alter the electrochemical measurements. This follows directly from the reaction mechanism involving iron oxidation and oxide formation.

What do rate constants tell us about a reaction? Why did only certain rate constants create oscillations in your model?

Rate constants describe how quickly each reaction step occurs. Oscillations only occur when the rates of formation and removal of the different surface species are balanced appropriately. If one step is too fast or too slow relative to the others, the feedback loop required for oscillatory behavior disappears and the system reaches a steady state instead.