Solutions, Separation Techniques, and Spectroscopy

Flashcards covering solution chemistry, concentration calculations, solubility factors, separation methods (distillation and chromatography), electromagnetic radiation, molecular spectroscopy transitions, the photoelectric effect, and Beer-Lambert law spectrophotometry.

In chemistry, how are the solute, solvent, and solution defined?

The solute is the substance being dissolved, the solvent is the substance dissolving the solute, and the solution is the total homogeneous mixture.

Why do ionic compounds such as $CaF_2$ dissolve in water rather than remaining solid?

Water molecules are polar with partial charges; their negative ends attract cations ($Ca^{2+}$) and their positive ends attract anions ($F^-$). These ion-dipole attractions overcome the lattice energy and stabilize the separated ions in solution.

What is a common example of a solid solution mentioned in the notes?

Metal alloys.

What is the formula and definition for molarity ($M$)?

$$\text{Molarity (M)} = \frac{\text{moles of solute}}{\text{Liters of solution}}$$

What is the formula for calculating the mass percent of a solute in a solution?

$$\text{Mass Percent} = \frac{\text{mass of solute}}{\text{mass of solution (solute + solvent)}} \times 100$$

When mixing two liquids, such as $25\,\text{mL}$ of pentane and $45\,\text{mL}$ of hexane, how is the solute identified?

The solute is the substance present in the smaller amount; in this case, pentane ($C_5H_{12}$).

What three simultaneous steps occur during the formation of a liquid solution as shown in this diagram?

Step 1: Expanding the solute (breaking bonds and overcoming intermolecular forces to separate solute particles), Step 2: Expanding the solvent (overcoming intermolecular forces to separate solvent molecules), and Step 3: Interaction of the solute and solvent to form the solution.

Why will a nonpolar solute fail to dissolve in a polar solvent?

The polar solvent molecules experience much stronger intermolecular attractions with each other than with the nonpolar solute. Consequently, it requires more energy to expand the solvent than is released by mixing, so the solvent will not expand to accommodate the solute.

According to the diagram, what occurs to gaseous solute solubility when the piston is pushed down to increase pressure?

Pushing the piston down increases the gas pressure and density above the liquid, raising the rate at which gas molecules strike and enter the solution until a higher-concentration equilibrium is established.

Why does the solubility of a gas in a liquid decrease as temperature increases?

Gas dissolution is generally exothermic, so Le Châtelier's principle shifts the equilibrium toward the gas phase; additionally, dissolved gas particles gain thermal energy to overcome intermolecular attractions and escape into the gas phase.

On what physical property difference does distillation rely to separate components of a mixture?

Differences in volatility (boiling points), which are determined by the strength of intermolecular forces in each component.

What is the general governing principle behind chromatography separations?

Chromatography separates components based on their relative affinities for a stationary phase versus a mobile phase; components more strongly attracted to the mobile phase move the farthest.

In paper chromatography, what substances serve as the stationary and mobile phases?

The paper serves as the stationary phase, and the liquid solvent traveling up the paper serves as the mobile phase.

What constitutes the stationary phase in column chromatography, and which components travel downward the fastest?

Silica gel or silica powder serves as the stationary phase; components with stronger attractions to the mobile phase solvent travel downward the fastest.

In gas chromatography, what are the stationary and mobile phases, and which substances exit first?

The mobile phase is an unreactive carrier gas and the stationary phase is a liquid or solid; gases with the greatest attraction to the carrier gas exit the column first.

What is the formula for calculating the retention factor ($R_f$ value) in chromatography?

$$R_f = \frac{\text{distance traveled by sample components}}{\text{distance traveled by solvent}}$$

What equation relates the speed of light ($c$), wavelength ($\lambda$), and frequency ($\nu$)?

$c = \lambda \nu$, where $c = 3.00 \times 10^8\,\text{m/s}$, $\lambda$ must be expressed in meters, and $\nu$ has units of $1/\text{s}$, $\text{s}^{-1}$, or $\text{Hz}$.

What equation gives the energy ($E$) of a single photon of electromagnetic radiation?

$E = h\nu = \frac{hc}{\lambda}$, where $h = 6.626 \times 10^{-34}\,\text{J}\cdot\text{s}$ (Planck's constant) and $E$ is in Joules per photon ($\text{J/photon}$).

What is the approximate wavelength range of the visible spectrum, and how does energy trend along the electromagnetic spectrum?

Visible light spans from approximately $400\,\text{nm}$ to $700\,\text{nm}$; photon energy increases as wavelength decreases (frequency increases).

What type of electromagnetic radiation induces electronic energy level transitions?

Visible and ultraviolet (UV) radiation.

What region of the electromagnetic spectrum causes the molecular vibrations (bends and stretches) shown in the diagram?

Infrared (IR) radiation causes molecular vibrational transitions; it does not possess enough energy to excite electrons to higher electronic energy levels.

What type of radiation causes molecular rotational transitions about different axes, as depicted in the diagram?

Microwave radiation.

What key principles of the photoelectric effect are demonstrated when red, green, and blue light strike the metal surface?

No electrons are ejected below the threshold frequency (red light). Above the threshold frequency (green light), electrons are emitted; increasing the frequency further (blue light) increases the kinetic energy (speed) of the ejected electrons.

In the photoelectric effect, what is the distinct role of light intensity versus light frequency above the threshold frequency?

Light intensity determines the number of photons striking the surface and controls the quantity of electrons ejected, whereas frequency determines photon energy and controls the kinetic energy (velocity) of the ejected electrons.

Name the primary components of a spectrophotometer in order from light generation to signal readout.

Light source $\rightarrow$ Collimator (lens) $\rightarrow$ Monochromator (prism or grating) $\rightarrow$ Wavelength selector (slit) $\rightarrow$ Sample solution (in cuvette) $\rightarrow$ Detector (photocell) $\rightarrow$ Digital display or meter.

In spectrophotometry, what are incident light ($I_0$) and transmitted light ($I_t$), and under what unique condition are they equal?

$I_0$ is the intensity of light entering the sample, and $I_t$ is the intensity of light exiting; $I_0 = I_t$ only when the sample is pure water (otherwise $I_0 > I_t$).

State the Beer-Lambert Law equation and identify the meaning and units of each term.

$A = abc$ (or $A = \epsilon l c$), where $A$ is absorbance (dimensionless, typically 0 to 1), $a$ (or $\epsilon$) is the molar absorptivity constant in $\text{L}/(\text{mol}\cdot\text{cm})$, $b$ (or $l$) is path length in $\text{cm}$ (usually $1\,\text{cm}$), and $c$ is molar concentration in $\text{M}$.

On a Beer-Lambert plot of absorbance versus concentration, what is represented by the slope of the line?

The slope equals $ab$ (or $\epsilon l$); when the path length $b = 1\,\text{cm}$, the slope directly equals the molar absorptivity constant $a$.

How is the optimal wavelength for spectrophotometric analysis selected from an absorbance spectrum?

Select the wavelength corresponding to the highest peak ($\lambda_{\max}$) on the spectrum (e.g., $640\,\text{nm}$ for green food coloring), avoiding wavelengths where light is strongly reflected or transmitted.