Chemical Reactivity and Acid-Base Chemistry

Comprehensive vocabulary flashcards covering the concepts of chemical reaction drawing conventions, arrow types, reaction profiles, thermodynamics, kinetics, and Brønsted/Lewis acid-base chemistry based on the lecture notes.

Reactant

A substance (also referred to as a substrate or starting material) that is consumed during a chemical reaction.

Reagent

A chemical substance added to a system to bring about a reaction.

Catalyst

A substance that increases the rate of a chemical reaction without modifying the overall standard Gibbs energy change ($\Delta_r G^\circ$).

Thermodynamics

The branch of physical chemistry that provides information regarding the position of equilibrium and how far a chemical reaction will proceed.

Kinetics

The branch of physical chemistry that provides information on the rate and how fast a chemical reaction will proceed.

Gibbs Energy of Reaction ($\Delta_r G$)

The difference in partial molar Gibbs energies between reactants and products at a given composition, which changes over the course of a reaction.

Standard Gibbs Energy of Reaction ($\Delta_r G^\circ$)

The energy change per mole of reaction for pure reactants converting to pure products under standard conditions, which remains constant for a specific reaction.

Exergonic Reaction

A thermodynamically spontaneous reaction where $\Delta_r G^\circ < 0$, resulting in products being favoured at equilibrium.

Endergonic Reaction

A non-spontaneous reaction where $\Delta_r G^\circ > 0$, resulting in reactants being favoured at equilibrium.

Equilibrium Constant ($K$)

A unitless parameter used to quantify the position of equilibrium, expressed as the ratio of activities (or concentrations at low concentration) of products over reactants: $K = \frac{a_C^c \cdot a_D^d}{a_A^a \cdot a_B^b} \approx \frac{[C]^c[D]^d}{[A]^a[B]^b}$.

Activity ($a_i$)

The effective concentration of a species $i$, defined as $a_i = \gamma_i \frac{c_i}{c^\ominus}$, where $\gamma_i$ is the activity coefficient, $c_i$ is the molar concentration, and $c^\ominus = 1\,\text{mol}\cdot\text{dm}^{-3}$ is standard concentration.

Standard Gibbs Energy and Equilibrium Equation

The fundamental thermodynamic equation $\Delta_r G^\circ = -RT \ln(K)$, linking standard Gibbs energy change to temperature $T$, the gas constant $R = 8.314\,\text{J}\cdot\text{K}^{-1}\cdot\text{mol}^{-1}$, and the equilibrium constant $K$.

Kinetic Expression of Equilibrium Constant

The expression defining the equilibrium constant $K$ as the ratio of the forward rate constant ($k_f$) to the backward rate constant ($k_b$): $K = \frac{k_f}{k_b}$.

Le Chatelier’s Principle

A thermodynamic principle stating that if an equilibrium is disturbed by altering conditions, the position of equilibrium shifts to counteract the change in order to re-establish dynamic equilibrium.

Activation Energy ($E_a$)

The kinetic energy barrier that a molecule must overcome for a reaction step to occur, which determines the rate of that reaction step.

Transition State ($\text{TS}^\ddagger$)

A chemical structure located at an energy maximum on an energy profile diagram that contains partial bonds and partial charges, lasting less than a molecular vibration ($\approx 10^{-13}\,\text{s}$) and impossible to isolate.

Hammond Postulate

A postulate stating that a transition state linking two points on a reaction coordinate structurally resembles the nearest stable species (resembling reactants in exergonic reactions and products in endergonic reactions).

Reaction Intermediate

A transient chemical species situated at a localized energy minimum between elementary reaction steps that can, in theory, be isolated and observed spectroscopically.

Rate Equation

A mathematical formula representing the kinetics of a reaction in terms of concentrations and exponents: $\text{rate} = k[A]^x[B]^y$, where $k$ is the rate constant and $x, y$ are reaction orders.

Rate-Determining Step

The slowest individual elementary step in a multistep reaction mechanism, which governs the overall reaction rate.

Brønsted Acid

A chemical species that can donate a proton ($\text{H}^+$).

Brønsted Base

A chemical species that can accept a proton ($\text{H}^+$).

pH

A logarithmic measurement scale of solution acidity defined by the relation $\text{pH} = -\log_{10}([\text{H}^+])$, where smaller values indicate higher proton concentration.

Acidity Constant ($K_a$)

A unitless equilibrium constant that quantifies the extent of proton donation of an acid in water, approximated as $K_a \approx \frac{[\text{A}^-][\text{H}_3\text{O}^+]}{[\text{AH}]}$.

Self-Ionisation Constant of Water ($K_w$)

The equilibrium constant for the self-ionisation of water, $K_w = a_{\text{HO}^-} \cdot a_{\text{H}_3\text{O}^+} \approx [\text{HO}^-][\text{H}_3\text{O}^+]$, which equals $1.00 \times 10^{-14}$ at $298\,\text{K}$.

$$\text{p}K_a$$

The negative base-10 logarithm of the acidity constant ($\text{p}K_a = -\log_{10}(K_a)$), where smaller and more negative values indicate stronger acids.

$$\text{p}K_{aH}$$

A numerical scale used to quantify the strength of a base by measuring the acidity ($\text{p}K_a$) of its conjugate acid (protonated form).

Conjugate Acid-Base Pair Principle

The reciprocal thermodynamic relationship stating that strong acids have weak conjugate bases, and strong bases have weak conjugate acids.

Acid-Base Reaction Equilibrium Constant ($K_c$)

The equilibrium constant for a general proton-transfer reaction between an acid (AH) and a base (B), calculated using the formula $K_c = 10^{(\text{p}K_{aH}(\text{B}) - \text{p}K_a(\text{AH}))}$.

Lewis Acid

A chemical species that can accept an electron pair.

Lewis Base

A chemical species that can donate an electron pair.