Graduate Medical School Admissions Test (GAMSAT) Section III: Chemistry Syllabus

State the postulates of the kinetic theory of ideal gases.

Gas particles have negligible volume, are in constant random motion, undergo perfectly elastic collisions, have no intermolecular forces, and their average kinetic energy is proportional to absolute temperature.

Why do real gases deviate from ideal behaviour, and under what conditions?

Real gases deviate because particles have finite volume and experience intermolecular forces. Deviations are greatest at high pressure and low temperature; ideal behaviour is approached at low pressure and high temperature.

Define enthalpy change ($\Delta H$) and distinguish exothermic from endothermic reactions.

$\Delta H$ is the heat change at constant pressure. Exothermic: $\Delta H < 0$ (releases heat). Endothermic: $\Delta H > 0$ (absorbs heat).

State Hess's law.

Hess's law states that the total enthalpy change of a reaction is independent of the route taken, depending only on the initial and final states. Enthalpy is a state function, so $\Delta H$ values can be added.

How is the enthalpy of reaction calculated from bond enthalpies?

$\Delta H_{\text{rxn}} = \sum (\text{bond enthalpies of bonds broken}) - \sum (\text{bond enthalpies of bonds formed})$. Breaking bonds is endothermic; forming bonds is exothermic.

Define entropy ($S$) and give the sign of $\Delta S$ for melting and for gas formation.

Entropy is a measure of the disorder or number of microstates of a system. Melting (solid to liquid) and forming a gas both increase disorder, so $\Delta S > 0$.

State the Gibbs free energy equation and the condition for spontaneity.

$\Delta G = \Delta H - T\Delta S$. A reaction is spontaneous (feasible) when $\Delta G < 0$, at equilibrium when $\Delta G = 0$, and non-spontaneous when $\Delta G > 0$.

Define the rate of a chemical reaction and give a general rate law expression.

Rate is the change in concentration of a reactant or product per unit time. A general rate law is $\text{rate} = k[A]^m[B]^n$, where $k$ is the rate constant and $m, n$ are the orders with respect to each reactant.

List the factors that affect the rate of a chemical reaction.

Concentration (or pressure for gases), temperature, surface area of solids, presence of a catalyst, and the nature of the reactants (and light for some reactions).

What does the overall order of a reaction mean, and how is the rate constant unit affected?

The overall order is the sum of the exponents in the rate law ($m + n + \ldots$). The units of $k$ depend on the overall order; for an order $p$ reaction, $k$ has units $\text{mol}^{1-p}\,\text{L}^{p-1}\,\text{s}^{-1}$.

State the main postulates of collision theory.

For a reaction to occur, particles must collide with energy equal to or greater than the activation energy and with the correct orientation. Only a fraction of collisions are successful.

Define activation energy ($E_a$).

Activation energy is the minimum energy that colliding particles must possess for a reaction to occur. It is the energy barrier between reactants and products, corresponding to formation of the transition state.

State the Arrhenius equation and explain the effect of temperature on rate.

$k = A e^{-E_a / RT}$, where $A$ is the pre-exponential (frequency) factor. Increasing temperature increases $k$ exponentially, because more molecules have energy $\geq E_a$ (Maxwell-Boltzmann distribution shifts to higher energies).

How does a catalyst increase the rate of a reaction?

A catalyst provides an alternative reaction pathway with a lower activation energy, so a greater fraction of collisions are successful. It is not consumed and does not change $\Delta H$ or the position of equilibrium.

Distinguish homogeneous from heterogeneous catalysis.

Homogeneous catalysis: the catalyst is in the same phase as the reactants (e.g. aqueous catalyst in solution). Heterogeneous catalysis: the catalyst is in a different phase (e.g. a solid catalyst with gaseous reactants, acting at its surface).

What is the role of enzymes as biological catalysts?

Enzymes are protein catalysts that lower the activation energy of biochemical reactions. They are highly specific, binding substrate at an active site (lock-and-key / induced-fit), and are sensitive to temperature and pH.

State Le Chatelier's principle.

If a system at equilibrium is subjected to a change in concentration, pressure, or temperature, the equilibrium position shifts to partially counteract that change.

Write the equilibrium constant expression $K_c$ for $\ce{aA + bB <=> cC + dD}$.

$K_c = \dfrac{[\ce{C}]^{c}[\ce{D}]^{d}}{[\ce{A}]^{a}[\ce{B}]^{b}}$, where concentrations are equilibrium values. Pure solids and liquids are omitted.

How does increasing temperature affect the equilibrium of an exothermic reaction?

For an exothermic forward reaction, increasing temperature shifts the equilibrium to the left (toward reactants) and decreases $K_c$, opposing the added heat. Temperature is the only factor that changes the value of $K$.

Define the Bronsted-Lowry acid and base.

A Bronsted-Lowry acid is a proton ($\ce{H+}$) donor; a Bronsted-Lowry base is a proton acceptor. They react to form conjugate acid-base pairs.

Give the formula for pH and the relationship between $[\ce{H+}]$ and $[\ce{OH-}]$ at $25^{\circ}\text{C}$.

$\text{pH} = -\log_{10}[\ce{H+}]$. At $25^{\circ}\text{C}$, $K_w = [\ce{H+}][\ce{OH-}] = 1.0 \times 10^{-14}$, so $\text{pH} + \text{pOH} = 14$.

Distinguish a strong acid from a weak acid.

A strong acid dissociates completely in water (e.g. $\ce{HCl}$). A weak acid dissociates only partially, establishing an equilibrium, with an acid dissociation constant $K_a = \dfrac{[\ce{H+}][\ce{A-}]}{[\ce{HA}]}$.

Define $K_a$ and $pK_a$, and state what a lower $pK_a$ indicates.

$K_a$ is the acid dissociation constant; $pK_a = -\log_{10} K_a$. A lower $pK_a$ (larger $K_a$) indicates a stronger acid that dissociates more readily.

What is a buffer solution and what is it composed of?

A buffer resists changes in pH on addition of small amounts of acid or base. It is composed of a weak acid and its conjugate base (or a weak base and its conjugate acid) in comparable amounts.

State the Henderson-Hasselbalch equation.

$\text{pH} = pK_a + \log_{10}\dfrac{[\text{A}^-]}{[\text{HA}]}$, relating the pH of a buffer to the ratio of conjugate base to weak acid.

Define the solubility product $K_{sp}$ for a salt $\ce{A_xB_y}$.

For $\ce{A_xB_y(s) <=> x A^{n+} + y B^{m-}}$, $K_{sp} = [\ce{A^{n+}}]^{x}[\ce{B^{m-}}]^{y}$ at saturation. A precipitate forms when the ionic product exceeds $K_{sp}$.

Explain the common ion effect on solubility.

The solubility of a sparingly soluble salt decreases when a soluble salt sharing a common ion is added. By Le Chatelier's principle, the added ion shifts the dissolution equilibrium toward the solid (precipitation).

Define oxidation and reduction in terms of electrons.

Oxidation is loss of electrons (increase in oxidation state); reduction is gain of electrons (decrease in oxidation state). Mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain).

In a galvanic (voltaic) cell, what occurs at the anode and cathode, and how is the cell EMF calculated?

Oxidation occurs at the anode (negative terminal); reduction at the cathode (positive terminal). The standard cell EMF is $E^{\circ}_{\text{cell}} = E^{\circ}_{\text{cathode}} - E^{\circ}_{\text{anode}}$; a positive value indicates a spontaneous reaction.

Distinguish a galvanic cell from an electrolytic cell.

A galvanic (voltaic) cell converts chemical energy into electrical energy via a spontaneous redox reaction ($E^{\circ}_{\text{cell}} > 0$). An electrolytic cell uses an external electrical supply to drive a non-spontaneous reaction ($E^{\circ}_{\text{cell}} < 0$).

Name the functional groups: $\ce{-OH}$, $\ce{-COOH}$, $\ce{-CHO}$, and $\ce{-NH2}$.

$\ce{-OH}$ is hydroxyl (alcohol); $\ce{-COOH}$ is carboxyl (carboxylic acid); $\ce{-CHO}$ is the aldehyde group; $\ce{-NH2}$ is the amino group (amine).

In IUPAC nomenclature, what suffixes denote an alkane, alkene and alkyne?

Alkanes end in -ane (single C-C bonds), alkenes end in -ene (C=C double bond), and alkynes end in -yne (C≡C triple bond).

Distinguish structural (constitutional) isomers from stereoisomers.

Structural isomers have the same molecular formula but different connectivity of atoms. Stereoisomers have the same connectivity but differ in the spatial arrangement of atoms (e.g. geometric and optical isomers).

Distinguish cis-trans (geometric) isomerism from optical isomerism.

Geometric (cis-trans) isomers differ in arrangement about a C=C double bond or ring due to restricted rotation. Optical isomers (enantiomers) are non-superimposable mirror images arising from a chiral centre.

What is a chiral centre, and what are enantiomers?

A chiral centre is a carbon atom bonded to four different groups. Enantiomers are the two non-superimposable mirror-image forms; they rotate plane-polarised light in opposite directions and have identical physical properties otherwise.

Distinguish a nucleophile from an electrophile.

A nucleophile is an electron-pair donor (electron-rich, attracted to positive centres), e.g. $\ce{OH-}$. An electrophile is an electron-pair acceptor (electron-poor, attracted to negative centres), e.g. $\ce{H+}$ or a carbocation.

Compare $S_N1$ and $S_N2$ nucleophilic substitution mechanisms.

$S_N1$ is two-step via a carbocation intermediate, first-order, favoured by tertiary substrates and gives racemisation. $S_N2$ is one-step (concerted), second-order, favoured by primary substrates and proceeds with inversion of configuration.

Describe addition, substitution and elimination reactions.

Addition: two reactants combine into one, breaking a multiple bond (e.g. alkene + $\ce{H2}$). Substitution: one atom/group replaces another. Elimination: a small molecule (e.g. $\ce{H2O}$ or $\ce{HX}$) is removed to form a multiple bond.

What products form when a carboxylic acid reacts with an alcohol?

Esterification produces an ester and water: $\ce{RCOOH + R'OH <=> RCOOR' + H2O}$, typically acid-catalysed and reversible.

Name the four major classes of biomolecules and their monomer building blocks.

Carbohydrates (monosaccharides), proteins (amino acids), lipids (fatty acids + glycerol), and nucleic acids (nucleotides).

Describe the bond and reaction that join amino acids, and the levels of protein structure.

Amino acids join by peptide (amide) bonds formed via condensation (loss of water). Protein structure: primary (sequence), secondary ($\alpha$-helices and $\beta$-sheets via H-bonds), tertiary (3D fold), and quaternary (multiple subunits).