Physical Chemistry Practical Module (CHE3722) Vocabulary

Key vocabulary, scientific laws, and laboratory terminology derived from the CHE3722 Third-Year Physical Chemistry Practical Manual.

Arrhenius equation

The exponential relationship between the rate constant and the activation energy for a reaction divided by the temperature, expressed as $k = Ae^{-E_a/RT}$ where $A$ is a constant and $E_a$ is the activation energy.

Pseudo first order reaction

A reaction that effectively follows a first order rate equation, such as the reaction between persulfate and iodide when iodide ion concentration is kept constant.

Conductivity ($\kappa$)

A proportionality constant in the resistance equation with units of $ohm^{-1}\,cm^{-1}$ or $S\,cm^{-1}$.

Molar conductivity ($\Lambda_m$)

The conductivity of a solution divided by its molar concentration, defined by the formula $\Lambda_m = \kappa/c$.

Cell constant ($C$)

A value defined by the geometry of the electrochemical cell ($C = l/A$) used to relate resistance and conductivity via the equation $C = \kappa R$.

Kohlrausch equation

Describes the concentration dependence of molar conductivity in strong electrolytes: $\Lambda_m = \Lambda_m^o - K\sqrt{c}$.

Ostwald's dilution law

A relation derived from the law of mass action used to determine the dissociation constant $K$ for weak electrolytes from conductivity measurements.

Solid solution

A homogeneous solid phase containing two or more components, appearing when A and B have the same type of crystal structure.

Liquidus curve

The solid-liquid equilibrium curve on a phase diagram that describes the temperature at which solid first appears from a cooling liquid solution.

Eutectic point

The point on a phase diagram where the two liquidus curves intersect, representing the composition and temperature ($x_E, T_E$) at which all liquid solidifies at a fixed temperature.

Thermal halt (arrest)

A duration on a cooling curve where the temperature remains constant until solidification is complete, indicating a reduction in the degrees of freedom.

Break

An abrupt change in the slope of a cooling curve occurring when a solid phase first begins to form from a liquid mixture.

Polarimeter

An instrument used to measure the angle of rotation of polarized light passing through a solution containing chiral molecules like sucrose.

Dextrorotatory

A property of a substance, such as sucrose, that rotates the plane of polarized light to the right.

Levorotatory (Laevo-rotatory)

A property of a substance, such as fructose, that rotates the plane of polarized light to the left.

Inversion of sucrose

The formal term for the hydrolysis of sucrose into glucose and fructose, which results in the net rotation of polarized light shifting from right to left.

Specific rotation ($[\alpha]$)

A standardized measure of the rotation of polarized light, calculated as $[\alpha] = 100\alpha/Lc$, where $L$ is the path length in dm and $c$ is concentration in $g/100\,ml$.

Raoult's law

A principle stating that the vapor pressure of a solution component is a good approximation only when its mole fraction is close to unity.

Negative deviation

Occurs when the vapor pressure of a solution is lower than predicted by Raoult's law, often due to strong heterogeneous molecular attractions ($A-B$).

Azeotrope (Constant boiling mixture)

A binary mixture for which the liquid and vapor compositions are identical at a maximum or minimum boiling point, causing it to distill without change in composition.

Refractometer

An instrument used for analyzing the composition of acetone-chloroform specimens by measuring their refractive indices.

Orange book

The specific laboratory notebook provided to UNISA students to record data, measurements, and observations during practical experiments.

Conductance water

Purified water used to prepare standards and rinse cells to ensure accurate conductivity readings without electrolyte contamination.

Newton ($N$)

The SI unit of force, equivalent to $kg\,m\,s^{-2}$.

Joule ($J$)

The SI unit of energy or work, equivalent to $kg\,m^2\,s^{-2}$.