Laboratory Experiments: Safety, Spectrophotometry, and Kinetics

Lab safety

Wear goggles, closed non slip footwear, lab coats.

Chemical spills

Clean up chemical spills, drips, and bottle rings immediately.

Chemical spills on skin

Should be washed at nearest eyewash, sink, or emergency shower and flush.

Chemical spill in eye

Should be washed at emergency eye wash.

Extra chemical solution disposal

If you put extra chemical solution from the stock carboy share it with other students or discard in fumehood.

Class A fires

Ordinary combustible materials like paper, wood, most plastics.

Class B fires

Flammable/combustible liquids (gasoline, solvents, grease, oil).

Class C fires

Electrical equipment (appliances, wiring, circuit breakers, outlets).

Purpose of experiment 1

Determine amount of ASA in standard over the counter 'aspirin' tablet by converting the acetylsalicylic acid to salicyclic acid and reacting it with ferric chloride to produce a coloured complex.

Salicylate ion reaction

Salicylate ion + Ferric chloride hexahydrate → coloured SA-FeCl3 complex.

Absorbance comparison

The coloured complex absorbance value is then compared to the absorbance values of a series of standard salicylic acid solutions (reacted with ferric chloride) of the known concentration.

Beer's law

Describes the relationship between the absorbance of a solution and its concentration.

Beer's law equation

A=ε⋅b⋅c.

Absorbance (A)

Absorbance measured (unitless).

Molar absorptivity (ε)

Constant for each substance at a given wavelength - L·mol⁻¹·cm⁻¹.

Path length (b)

Path length of the cuvette - cm (usually 1.00cm).

Concentration (c)

Concentration of the solution - mol/L.

Main idea of Beer's law

As concentration of a solution increases, it absorbs more light, so absorbance increases.

Lambda max

525nm is chosen because it is where the absorbance is the highest → best sensitivity.

Standard curve

Absorbance vs concentration → as concentration increased the absorbance value also increased.

Spectrophotometry

Measures how much light is absorbed by a solution to find concentration.

Calorimeter

A device that measures heat changes that occur in physical or chemical processes, no heat enters or leaves allowing the heat flow to be measured by determining the change in temperature.

Heat of reaction (q)

The heat absorbed or released during a complete chemical reaction and is equal to the enthalpy change, delta H.

Specific heat (s)

The heat required to raise the temperature of one gram of a substance.

Heat of solution

The energy change involved in the dissolving process as bonds within a substance are broken and new bonds are formed between the substance and the solvent it is dissolved in.

Calorimeter constant

The heat absorbed in the container itself.

Heat of neutralization

The enthalpy change per mole of reaction.

Exothermic reactions

Solution temperature rises → q is negative and ΔH is negative.

Endothermic reactions

Solution temperature drops → q is positive and ΔH is positive.

Reaction rate

The speed at which a reaction happens, how fast reactants turn into products.

Rate constant

It is proportionality constant between the rate of reaction and the concentration terms.

Reaction order

Describes how the rate of a chemical reaction changes as the concentration of reactants changes.

Ionic strength

A measure of the concentration of ions in a solution.

Buffer

A solution that resists change in pH, made up of weak acid and its conjugate base or vice versa.

Buffer capacity

How much acid/base the buffer can neutralize before pH shifts.

Equivalence point

Moles of acid and moles of base are the same, all weak acid is converted to its conjugate.

Half neutralization point

pH = pKa, equal amounts of acid and base.

Voltaic cells

Produce electrical energy from redox reactions.

Oxidation

Occurs at anode (loss of electrons).

Reduction

Occurs at cathode (gain of electrons).

Electrons flow

From anode → cathode.

Experiment 1 Purpose

Determine amount of ASA in standard over the counter 'aspirin' tablet by converting acetylsalicylic acid to salicyclic acid and reacting it with ferric chloride to produce a coloured complex.

Coloured complex reaction

Salicylate ion + Ferric chloride hexahydrate → coloured SA-FeCl3 complex.

Beer's Law plot

Absorbance (A) vs. concentration (c) — a linear relationship.

λmax in spectrophotometry

It gives the greatest sensitivity (largest absorbance change per concentration).

Blank solution purpose

To zero the spectrophotometer and remove background absorbance.

Finding concentration from absorbance

Use the standard curve equation (e.g., A=mc+b), plug in A and solve for c.

Heat gained/lost formula

q=mcΔT

Enthalpy change per mole formula

ΔH=qn

Exothermic reaction sign of q

Negative (q < 0)

Theoretical ΔH for neutralization

-55.90 kJ/mol

Temperature rise in reaction

Exothermic

Iodide-persulfate reaction overall equation

S2O82−+2I−→I2+2SO42−

Role of thiosulfate (S₂O₃²⁻)

Reacts with I₂ to keep the solution colourless until all thiosulfate is used up.

Rate of reaction calculation

Rate=[I2]t=0.5×[S2O32−]

Ionic strength formula

μ=12∑cizi2

Effect of ionic strength on rate

Rate increases due to stabilization of charged intermediates.

Rate constant formula

k=Rate[I−]m[S2O82−]n

Buffer definition

A solution that resists pH change, made of a weak acid and its conjugate base (or vice versa).

Henderson-Hasselbalch equation

pH=pKa+log⁡([base][acid])

pH at half-equivalence point

pH = pKa

pH at equivalence point of strong acid + strong base

7

Indicator for pH ≈ 4-6 endpoint

Methyl red

Effect of strong acid on buffer pH

The pH changes only slightly.

Oxidation location in voltaic cell

At the anode

Reduction location in voltaic cell

At the cathode

Electron flow direction in voltaic cell

From anode to cathode

E°cell formula

Ecell∘=Ecathode∘−Eanode∘

Purpose of the salt bridge

Maintains charge neutrality by allowing ion flow.

Spontaneity of redox reaction

When Ecell∘>0

Nernst equation usage

To calculate E under non-standard conditions.