# Chemistry Dictionary

Welcome to this library of definitions for chemistry; from the molecular to the macroscopic. This dictionary should serve only as a quick reference and offers short explicative definitions as refreshers or introductions to core concepts in chemistry. Please do not hesitate to signal any error or suggestion you may have, feel free to use any of the available contact forms on the site with the subject definition and the word you are referencing.

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# A

1. Acid - An acid as per the Brønsted definition is any substance capable of giving off a proton, or Hydrogen particle. As per the Lewis definition an acid is any substance capable of accepting a pair of electrons.

Notice that these two definitions are complementary, in that if: an acid gives off a proton, it is releasing a charged particle H+ into the environment; yet at the same time it must accept those electrons 2e- from the proton in order to release it.

An acid is written as AH and its conjugated base as A-. The couple is written as: AH/A-.

They are linked together by the two half reactions:

$$AH \rightleftharpoons A^- + H^+$$ $$B^- + H^+ \rightleftharpoons BH$$

Thus the two half reactions together form the complete reaction:

$$AH + B^- \rightleftharpoons A^- + BH$$

Notice that the acid is giving up its H to the base, and in exchange the A receives the negative charge - which in fact represents the two electrons given up by the Hydrogen.

If one were to follow the electrons they go from: H to A, and from the B to the H in order to form the bond.

2. Amphoteric - The quality of a substance to be able to act as both an acid and a base. This is an innate quality of water and is seen in autoprotolysis.
3. Antiaromatic - If a molecule has 4n π electrons and has the all other characteristics for aromaticity then it is said to be antiaromatic.
1. The molecule must be cyclic.
2. The molecule must be planar, allows for p-orbitals to be parallel and interact.
3. That the molecule must be formed of a continous ring of conjugated π bonds (p-orbital interaction); if a heteronome participates in conjugation then its doublet must be able to be conjugated with the other π bonds (can be composed of one or more rings).

Example: cyclobutadiene; the number of π delocalised electrons is 4, its ionic form cyclobutadiene(2-) is aromatic (6 e-).

4. Autoprotolysis - A chemical reaction occuring in water between water molecules themselves H2O in which a proton is transfered between two identical molecules, one playing the role of an acid and the other the base (in the Brønsted context).

The chemical reaction is written:

$$2H_2 O \rightleftharpoons H_3 O^+ + HO^-$$

Any solvent which contains acidic hydrogen and lone pairs of electrons with which to accept H+ can demonstrate autoprotolysis, for example ammonia:

$$2NH_3 \rightleftharpoons NH^-_2 + NH^+_4$$
5. Aromaticity (aryl) - This is the property of being cyclic (a closed ring), and planar with a ring of resonance bonds. Aromatic chemical groups are called aryl. Requirements for aromaticity, known as Huckel's rule, are as follows:
1. The molecule must be cyclic; can be composed of one or more rings.
2. The molecule must be planar, allows for p-orbitals to be parallel and interact.
3. That the molecule must be formed of a continous ring of conjugated π bonds (p-orbital interaction); if a heteronome participates in conjugation then its doublet must be able to be conjugated with the other π bonds.
4. The molecule must have 4n + 2 electrons in a conjugated system (n ∈ ℕ); normally on sp2-hybridised atoms, and sometimes sp-hybridised.

To apply the rule, count the number of π eletrons in the molecule, then set the equation equal to this number. Solve for n. If n ∈ ℕ, and the other rules are true then the molecule is indeed aromatic.

Example: benzene, 3 double bonds, 6 π electrons. $$4n + 2 = 6$$ $$4n = 4$$ $$n = 1$$ $$n \in \mathbb{N}$$ In order to know which are the π electrons to be counted, you must look at those which reside in the p-orbitals; the hybrid atoms have one p-orbital each. Thus if each part of the cyclic compound is hybridised sp2 then this means that the molecule is completely conjugated (each atom has one p-orbital) and the electrons in these p orbitals are in fact π electrons.

Aromatic molcules are very stable and do not react easily with other compounds. Most common aromatic compounds are derivatives of benzene (found in petroleum).

Aromaticity does not necessarily indicate an odour. This misnomer comes from the association initially given by August Wilhelm Hofmann in 1855 who studied a class of benzene molecules; which did indeed emit odours.

Chemically aromaticity refers to a conjugated system made of alternating single and double bonds in a ring. This arrangement allows for electrons in the molecule's π system to be delocalised; increasing the molecules stability. Such molecules are represented by a resonance hybrid of different strucutres.

Bond lengths in such a strucutre are the intermediate of the two composing representations. X-ray diffraction has shown that all six carbon-carbon bonds in benzene are of the same length: 1.4 Å. C-C double bonds are typically at 1.35 Å, and single bonds at 1.47 Å.

The ring contains π bonds which are formed from the overlap of atomic p-orbitals; which can interact with each other freely as they are outside the plane of the molecule. Each electron is shared by all six atoms in the ring; there are not enough electrons to form double bonds between all carbon atoms, but the "extra" electrons strengthen the exisiting bonds and give rise to the resonance structure.

6. # B

7. Base - As per the Brønsted definition it is any substance capable of taking a proton, or Hydrogen particle. As per the Lewis definition a base is any substance capable of releasing a pair of electrons.

Notice that these two definitions are complementary, in that if a base takes on a proton, it is accepting a charged particle H+ from the environment; yet at the same time it must release its electrons 2e- and share them with the proton in order to form a bond.

A base is written as B- and its conjugated acid as BH. The couple is written as: B-/BH.

They are linked together by the two half reactions:

$$AH \rightleftharpoons A^- + H^+$$ $$B^- + H^+ \rightleftharpoons BH$$

Thus the two half reactions together form the complete reaction:

$$AH + B^- \rightleftharpoons A^- + BH$$

Notice that the base is accepting an H from the acid, and in turn the A receives the negative charge - which in fact represents the two electrons given up by the Hydrogen.

If one were to follow the electrons they go from hydrogen to the A, then from B to the H in order to form the bond.

8. Benzene (C6H6) - An aromtaic organic molecule of the formula C6H6, molar mass 78.114 g·mol-1. It is cyclic and contains only Carbon and Hydrogen thus it is called a hydrocarbon.

Benzene is naturally found in petroleum and is a foundational chemical in petrochemistry: production of plastics, pharmaceuticals, dyes, detergents, pesticides, rubber, and other important molecules.

It is a liquid, colourless, highly volatile, flammable, and a known human carcinogen.

# D

10. Dissociation Constant (water) - The rate at which water dissociates from its protons. This can be derived from autoprotolysis.

Dissociation constant: $$K_w = 1.0 \cdot 10^{-14}$$

$$K_w = \frac{[H_{3}0^{+}][HO^-]}{[ H_{2}0]^2}$$

Since this is theoretically measured in a pure solution of water and the concentration of pure substances is not taken into account then the formula simplifies to:

$$K_w = [H_{3}0^{+}][HO^-]$$

Moreover, since according to the balanced equation for autoprotolysis [H3O+] = [HO-], and thus we can simplify the formula even more:

$$K_w = X^2$$

Thus:

$$[H_{3}0^{+}] = [HO^-] = \sqrt{1.0 \cdot 10^{-14}} = 1.0 \cdot 10^{-7} mol/L$$

And thus we've demonstrated that water's $$pH = -log(1.0 \cdot 10^{-7}) = 7$$.

# H

12. Hydronium Ion - An oxygen cation attached to three protons, its composition is written as H3O+. This is different from R3O+ known as a oxonium ion and instead of three protons can include any other groups. A hydronium ion is an oxonium ion, but the inverse is not true; as this term refers specifically to ions which have hydrogens.

# O

14. Oxonium Ion - An oxonium ion is any oxygen cation with three bonds, the simplest form is H3O+ but in reality can be R3O+; R meaning any other groups. An oxonium ion is not a hydronium ion, this term refers specifically to ions which have hydrogens.
15. # P

16. pH (Potential Hydrogen) - Loosely speaking a measure of the concentration of free protons in an aqueous solution. A more precise definition is the activity of protons in the solution or a measure of [H3O+]. This allows us to determine the acidity or basicity of a solution. These measurements are all put in the context of water, and relative to water; thus water's pH is equal to 7; acids are less than 7, bases are greater than 7.
In a base: $$[HO^-] > H_3 O^+$$ In an acid: $$[HO^-] < H_3 O^+$$
• Hydroxide ions = [OH-]
• Hydronium ions = [H3O+]

pH is measured on a logarithmic scale, it is measured in units of moles per liter of hydrogen ions often called hydronium ions sometimes less precisely called oxonium ions.

Precisely speaking this measurement of hydrogen ions in a solution can be done experimentally. In fact an experimental measure is designated as p[H]. Moreover, the difference between pH and p[H] is so small that they are sometimes used interchangeably. The way to find this would be by use of the Nernst equation as shown below:

$$E = E^{0} + f\frac{2.303RT}{F}\log\left[ H^{+} \right]$$

The strict definition of pH does not have associated units, and is dimensionless. It is described as the log of the inverse of hydrogen activity in a solution:

$$pH = -log(a_{H^+}) = log\left(\frac{1}{a_{H^+}}\right)$$

But most often used in the following form with [H3O+] being the concentration of hydronium ions in mol/L:

$$pH = -log[H_3 O^+]$$ $$[H_3 O^+] = 10^{-pH}$$
17. pOH (Potential Hydroxide) - The inverse of pH, this measures the potential hydroxide activity of a solution: HO-.
$$pOH = -log[OH^-]$$ $$[OH^-] = 10^{-pOH}$$

Note that the sum of pOH along with pH is equal to the total of the scale.

$$pH + pOH = 14$$

# #

19. π bond (Pi bond) - These are covalent chemical bonds where two lobes of an orbital on one atom overlap with two other lobes of another laterally. The Greek letter π is used to refer to the p-orbitals. The symmettry of the π bond is the same as that of the p-orbital; π bonds occur with p-orbitals and or d-orbitals.

π bonds are weaker than σ bonds, requiring 612 kj of energy to break vs 348 kj respectively, that is 264 kj required to break a π bond; as the σ bond is comprised in the 612 kj initially quoted *(C-C bonds).

Single covalent bonds result in σ bonds, multiple bonds yeild π bonds. These also result in unsaturated molecules, as electrons are used for multiple bond formation rather than fixing H. Note that π bonds do not allow free rotation around the central axis as σ bonds do.