CHEM 2510 / Module 1: Introduction

What is analytical chemistry focus on?

Separation, identification, & determination of the relative amounts of the components of natural and artificial materials.

What are three key classical chemistry methods?

> Volumetric.

> Gravimetric.

> Distillation.

Why are classical methods considered more accurate?

> Instruments can drift, add noise, and require calibration.

> Classical methods measure directly instead of relying on instrument calibration.

What are four key instrumental methods?

> Light-based spectroscopy.

> Mass spectrometry.

> Separation.

> Electrochemical.

What does qualitative analysis tell you?

What components in sample.

What does quantitative analysis tell you?

How much components in sample.

What are the general steps in chemical analysis?

> Make specific questions re. chemical measurement from general questions.

> Search chemical lit. for appropriate procedure / to devise one.

> Select rep material to analyze.

> Convert rep sample into form suitable for analysis.

> Confirm that method reliably produces results answering initial question.

> Measure the [analyte] in identical aliquots. Replicates and uncertainties are to be considered.

> Deliver complete report for intended audience.

> Conclusions may then point to starting over / reformulating question.

> How many # of decimal pts should the uncertainty have?

> How many units past certainty can you estimate?

> The same # of decimal pts as the magnitude.

> One unit past certainty.

What are the four general rules for identifying sig figs?

> All non-zero digits = significant.

> Zeroes appearing btwn non-zeroes = significant.

> Leading zeroes = not significant.

> Trailing zeroes after a decimal point = significant.

> Trailing zeroes w/o decimal point = not significant.

What is the exception to the general rules for sig figs regarding logarithmic measurements?

> # of sig figs in the original number = # of digits after decimal in the log.

> # of digits before decimal in the log = order of magnitude / power of 10.

What is the exception to the general rules for sig figs regarding exact meaurements?

Exact number will never limit the # of sig figs.

This is not super common b/c we always have a degree of uncertainty.

> After performing mathematical operations, how many sig figs can you have?

> When do you round after these operations?

> The least precise measurement sets the limit for how precise the final answer can be and how many sig figs there are.

> You only round your final answer, everything else is not rounded.

What’s the rule of sig figs when it comes to addition and subtraction without a common exponent?

Round off to the # of decimal places in the number w/ fewest decimal places.

What’s the rule of sig figs when it comes to addition and subtraction with a common exponent?

> Convert all to a common exponent.

> Add/sub normally.

> Round to the same # of decimal places as the least precise value.

What’s the rule of sig figs when it comes to multiplication & division?

Round off to the # of sig figs found in value containing lowest # of sig figs.

What condition is reached when no further changes occur in the amounts of reactants and products in a reversible reaction?

Equilibrium. Fwd & rev rxns cont but concentrations = constant.

What is the general equilibrium constant expression for aA + bB ⇌ cC + dD?

K = [C]ᶜ [D]ᵈ / [A]ᵃ [B]ᵇ

What happens if [Reactant] ↑ or [Product] ↑?

↑ Reactant → shift right (more products).

↑ Product → shift left (more reactants).

> How does equilibrium shift if pressure ↑?

> Why does equilibrium shift to the side with fewer moles of gas when pressure increases?

> Why does equilibrium shift to the side with more moles of gas when pressure decreases?

> Shifts to side w/ fewer mol of gas.

> Because fewer gas molecules = smaller V → lowers P, counteracting stress (Le Châtelier).

> Because more gas molecules = larger V → raises P, counteracting drop (Le Châtelier).

How does equilibrium shift if temperature ↑?

> Exothermic → shift left (heat = product).

> Endothermic → shift right (heat = reactant).

> What is K_w?

> How is K_w expressed?

> eq constant for H₂O.

> K_w = [H₃O⁺][OH^-]

> What is ionic strength?

> What is the formula for ionic strength (\mu )?

A measure of the total concentration of ions in a solution.

u=\frac12\Sigma c_{}z^2_{}

Given the following situations, determine (1) what happens ionically & (2) in increase / decrease in ionic strength.

> Precipitation.

> Acid-base neutralization.

> Dilutions.

> Adding inert salt.

> Complex formation.

> Strong acid + strong base

> Precipitation → ions removed → decrease.

> Acid-base neutralization → new salt, new ions made → increase.

> Dilutions → all [ions] go down → decrease.

> Adding inert salt → more ions but no rxn → increase.

> Complex formation → ion binds another, fewer free ions, free ions drop → decrease.

> Strong acid + strong base → ions swapped, none lost → no change.

> What is effective concentration (activity)?

> What’s the formula for activity?

The “real” concentration that drives equilibrium, accounting for ion–ion interactions.

a=\gamma\left\lbrack X\right\rbrack

The coefficient is how we can apply ionic strength to eq.

Why is K more accurate when expressed with activities instead of concentrations?

> In ionic solutions, ions attract/repel → actual “free” conc ≠ measured conc
> So we use activity.

What’s the formula for the activity coefficient?

\log_{y}=\frac{-0.51Z^2\sqrt{I}}{1+\frac{a\sqrt{I}}{305}}

Just get antilog to find y = activity coeff.

What’s the difference between formality (F) and molarity (M)?

> Molarity (M) = [specific chemical species] as it actually exists in solution.

> Formality (F) = initial [species]_tot, assuming it retains its chemical identity as a formula unit, even if it later dissociates or reacts.

> What’s the fraction of dissociation (α)?

> What does it tell you?

> What’s the formula for α?

> Ratio of molecules that dissociate compared to the initial number of molecules.

> How strong dissociation is.

a=\frac{\left\lbrack A^{-}\right\rbrack}{\left\lbrack A^{-}\right\rbrack+\left\lbrack HA\right\rbrack}=\frac{x}{x+\left(F-x\right)}=\frac{x}{F}

> What’s a buffer? What does it do?

> What does it contain?

> Buffer = chemical system resisting ∆pH by neutralizing added acid or added base.

> Buffer contains significant amounts of a weak acid and its conjugate base.

> What does the buffer’s weak acid neutralize?

> What does the buffer’s conjugate base neutralize?

> The weak acid neutralizes the added base, e.g. hydroxide ions.

> The conjugate base neutralizes the added acid, e.g. hydronium ions.

> What does a titration curve show you?

> What does the buffer zone in a titration curve show you?

> In a titration curve, at what pH will the buffer start to “buffer” or does it duty?

> In a titration curve, at what pH will the buffer stop “buffering” or give up?

> What does it mean when pH = pK_a?

> Shows ∆pH of a solution as titrant (acid or base) is added.

> Has a buffer zone that shows you slow change in the pH due to the performance of the WA/CB.

> At pH = pK_a - 1, it starts to buffer or resist pH.

> At pH = pK_a + 1, it stops buffering and gives up resisting to pH changes.

> At pH = pK_a, there is equal amount of [HA] and [A^-], i.e. 50% weak acid and 50% conjugate base.

> If you add a strong acid to a buffer, will the buffer’s weak acid increase or decrease? Will the buffer’s conjugate base increase or decrease?

> If you add base to a buffer, will the buffer’s weak acid increase or decrease? Will the buffer’s conjugate base increase or decrease?

> If you add a strong acid to a buffer, you get more weak acid and less conjugate base.

> If you add a strong base to a buffer, you get less weak acid and more conjugate base.

What is the Henderson-Hasselbalch (HH) equation? What does it calculate for you?

This calculates the pH of the WA-CB pair of our buffer.

What are the three situations you will be placed in, in regard to the HH equation?

> All in one form, wherein the pH is MORE than one unit above the pKa or wherein the pH is LESS than one unit below the pKa.

> Both in equal amounts, wherein pH = pKa.

> pH is WITHIN one unit below or above the pKa and not equalling the pKa.