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The Chemistry Of Poisons And Toxins

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All matter is composed of atoms. An atom is a particle that uniquely defines a chemical element (S1.1). Atoms are the smallest component of an element, and an element is a substance which cannot be chemically broken down. Every element on the periodic table has a different atomic mass and is given a unique symbol to distinguish it. For example, chlorine (a toxic gas which attacks the respiratory system, eyes, and skin (Wikipedia, 2019)), has the symbol Cl (S1.5).

A chlorine atom contains 17 protons and 18 neutrons inside the nucleus, and 17 electrons outside the nucleus. Because protons and neutrons both have a relative mass of approximately 1 atomic mass unit (a.m.u), the atomic mass of chlorine equals approximately 35.45 u (S1.2). Chlorine has 3 electron shells, the first consisting of 2 electrons, the second with 8 electrons, and the third with 7 electrons. Because chlorine has 17 electrons, the electron configuration would be: 1s22s22p63s22p5 (Breslyn, 2013). This is because the 1s orbital can only hold two electrons, meaning that the next two would be held by the 2s orbital, then 6 electrons on the 2p orbital, 2 electrons on the 3s orbital, and the final 5 electrons on the 2p orbital (S1.7).

Chlorine-35 and chlorine-37, despite having different atomic mass, are not different elements. These are both isotopes of the element chlorine (Jefferson Lab, 2019) because they will always contain 17 protons. The change in atomic mass is brought upon by the number of neutrons in the nucleus. Chlorine-35 has 18 neutrons, because (using a relative atomic mass of 1 a.m.u) the 17 protons, and 18 neutrons in the nucleus add up to an atomic mass of approximately 35 u. In chlorine-37, because the number of protons does not change, the number of neutrons would be equal to 20, because when the weight of 17 protons and 20 neutrons is combined, the atomic mass of the chlorine atom is approximately equal to 37 u (S1.3, S1.4).

Singular chlorine atoms do not have enough electrons to fill their outer shells, and therefore become unstable (GCSE Science, 2015). This causes the chlorine atoms to seek out other chlorine atoms to share electrons with and fill the empty space on their valence shell. This is called covalent bonding, where two atoms share their electrons (S2.3).

When two or more atoms bond together, they form a molecule of definite shape, size, and arrangement of atoms. For example, hydrogen chloride, which is formed when a hydrogen atom bonds with a chlorine atom (S2.4).

Atoms, or groups of atoms that have been covalently bound, can lose or gain electrons to become ions. When an electron is lost, the atom becomes more positively charged and is called a cation. When an electron is gained, the atom becomes more negatively charged and is called an anion. The anion chloride (Cl-) is formed when the element chlorine gains an electron, or when hydrogen chloride is dissolved in water, or other polar solvents (Polar solvents are solvents that contain a hydrogen atom bonded to an oxygen, nitrogen, or fluorine atom (Wikipedia, 2019)) (S2.5).

An ionic bond is a chemical bond where one or more electrons are transferred from one atom to another. Ionic bonding occurs when the difference in the electronegativity between two atoms is greater than 1.7, and mostly occurs between metal and non-metal atoms. The atom with the higher electronegativity will attract the valence electron away from the atom with the lower electronegativity. When referring to sodium chloride (NaCl), the electronegativity difference between Na (0.93) and Cl (3.16) is 2.1 (Siyavula, 2015), meaning that the chlorine atom will receive electrons from the sodium atom. Sodium has one valence electron, and when it transfers this to chlorine, it will become an Na+ cation. Chlorine, after receiving the electron from the sodium atom, will become a Cl- anion (S2.6).

When two or more atoms bond together, through either covalent or ionic bonding, they form a chemical compound. Compounds can be represented by their chemical formula, which provides all of the atoms that make the compound. For example, sulphur mustard is formed using 4 carbons, 8 hydrogens, 2 chlorines, and 1 sulphur atom; producing the chemical formula:

C4H8Cl2S (S2.7)

Compounds can be further classified based on whether or not they contain carbon-hydrogen bonds. Organic compounds are compounds that have carbon-hydrogen bonds, whereas inorganic compounds do not have any carbon-hydrogen bonds. Sulphur mustard [Bis(2-chloroethyl) sulphide] (PubChem, 2019) is an example of an organic compound because it has 4 carbon-hydrogen bonds. Sodium Hypochlorite (Bleach) is an example of an inorganic compound because it doesn’t have any carbon-hydrogen bonds (S2.11).

Oxidation-Reduction (REDOX) reactions involve the transfer of electrons (LibreTexts, 2019). REDOX reactions are chemical reactions where the oxidation number of an atom, ion, or molecule changes due to the loss or gain of an electron (LibreTexts, 2019). If an electron is lost, then the species is considered to be oxidized and is known as the reducing agent. If an electron is gained, then the species is considered to be reduced, and is known as the oxidizing agent (LibreTexts, 2019). There are several types of REDOX reactions, including: combination, decomposition, single replacement, double replacement, combustion, and disproportionation (LibreTexts, 2019). The following is an example of a decomposition REDOX reaction:

2C4H8Cl2S + 13O2 6H2O + 4HCl + 8CO2 + 2SO

The oxidation numbers for the species can be determined by referring to the oxidation rules:

Therefore, the oxidation number for three of the species in the reaction will be:



Here the Oxidation number will be zero, because Oxygen (O2) is a free element.


The carbon has lost 5 electrons, meaning it has been oxidized, and is therefore the reducing agent. Oxygen has gained 6, meaning that it has been reduced, and is therefore the oxidizing agent. (R1.1)

Acid/base reactions involve the exchange of hydrogen ions (H+) between species (Encyclopaedia Britannica, 2018). Acids and bases can be either weak or strong, and can be used to neutralize each other. Sodium Hypochlorite (Bleach) is a weak base, and if mixed with an acid will produce chlorine gas (McGill, 2017). For example:

NaOCl + 2HCl Cl2 + NaCl + H2O

The hydrogen ions in the hydrochloric acid are exchanged and bond with oxygen to form water (R1.3).

When a chemical reaction takes place, an energy transformation occurs. Energy transformations involving the release of heat are called exothermic, and the absorption of heat are called endothermic (Lumen, 2019). When heat is released or absorbed, the enthalpy can be calculated. Mustard gas is a liquid at standard conditions, and so its enthalpy of formation will be approximately -200.5 kJ/mol (NIST, 2018), and its enthalpy of combustion will be approximately -3163.5 kJ/mol (NIST, 2018).

When completing stoichiometric calculations, the basic quantity used is 1 mole. The mole is the SI unit which measures the number of particles in a specific substance (LibreTexts, 2019). The number of moles in a system can be determined using the atomic mass of an element. The relative atomic mass of an element is the atomic weight of any atom, compared with the atomic weight of one carbon-12 isotope. Carbon-12 is chosen because it has a high abundance, and is solid at room temperature (Wikipedia, 2019).

All chemical reactions can be represented by a balanced equation, such as the REDOX reaction with mustard gas and oxygen. The coefficients of the balanced equation indicate both the number of reacting particles, and the reacting quantities in moles.

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2C4H8Cl2S + 13O2 6H2O + 4HCl + 8CO2 + 2SO

Mustard gas on the reacting side (2C4H8Cl2S) has a coefficient of ‘2’, this means that there are 2 moles of mustard gas present in the balanced equation. (R3.2)

Balanced equations can be used to determine whether a reagent is limiting, or in excess. In the following example, the limiting reagent is found using the stoichiometric ratio:

2Al + 3Cl2 2AlCl3

This equation is already balanced, so the next step is to convert the masses of aluminium (Al), and chlorine (Cl) to moles. The mass of aluminium has been set to 4.80g, and the mass of chlorine has been set to 6.25g. The molar mass of aluminium is approximately equal to 26.98 g/mol (PubChem, 2019), and the molar mass of Cl2 is approximately equal to 70.9 g/mol (PubChem, 2019).

n=mMnAl=4.80g26.98 g/molnAl=1.78×10-1 molnCl2=6.25g70.9g/molnCl2=0.88×10-1 mol

To determine the limiting reagent, the actual ratio can be calculated, then compared with the stoichiometric ratio:

Actual Ratio=moles of Almoles of Cl2 =1.78×10-1 mol0.88×10-1 mol =2.02 mol Al1 mol Cl2 This tells us that for every 2.02 moles of Al for every 1 mole of Cl2. The stoichiometric ratio from the balanced reaction is shown below:

Stoichiometric ratio=2 mol3 mol =0.67 mol Al1 mol Cl2This means there must be at least 0.67 moles of Al for every 1 mole of Cl2. As the actual ratio is greater than the stoichiometric ratio, there is more aluminium than needed to react with each mole of Cl2. This means Cl2 is the limiting reagent, and Al is in excess. (R3.3)

To determine the amount of a reactant, techniques such as volumetric analysis, and gravimetric analysis can be used. Volumetric analysis is any method of quantitative chemical analysis where the amount of a substance is determined by measuring the volume occupied (Britannica, 2014). Gravimetric analysis is a class of techniques used to determine the mass or concentration of a substance by measuring its change in mass (Khan Academy, 2015). Titration is used to determine the unknown concentration of a solution, given the concentration of a different solution. For example:

25.9 mL of a 0.0329 M solution of Ba(OH)2 is needed to neutralize 20.0mL of HCl.

Molarity=molL0.0329 M=x mol0.0259 Lx=0.0329×0.0259 =0.00085211 mol BaOH2Ba(OH)2 + 2HCl 2H2O + BaCl2

Therefore, the number of moles of HCl will be double the number of moles of Ba(OH)2.

nHCl=nBaOH2×2 =0.00085211×2 =0.00170422 molTo calculate the concentration of HCl:

HCl=nHClVHCl =0.001704220.0200 =0.085211 M(R4.1)

The empirical formula can be determined by calculating the mass composition. Mass composition describes the relative quantities of elements in a chemical compound (ThoughtCo, 2019). Example:

A compound is analysed and found to contain 48.04% carbon, 8.08% hydrogen and 43.88% nitrogen. We can calculate the empirical formula by first determining the number of moles of each element present in the compound. For this example, it is assumed that the compound has a mass of 100g.

48.04% of 100g=48.04g of carbon8.08% of 100g=8.08g of hydrogen43.88% of 100g=43.88g of nitrogenThe number of moles can now be calculated:

n=mMnC=48.04 g12.01 g/mol =4.00 molnH=8.08 g1.01 g/mol = 8.00 molnN=43.88 mol14.01 g/mol =3.13 molTherefore, for every 4 carbons, there are 8 hydrogens and 3 Nitrogens. The empirical formula is:


Mustard gas is neutralized when it is mixed with water, and forms thiodiglycol (C4H10O2S) and hydrochloric acid (American Chemical Society, 2005) – as shown in the following reaction:

C4H8Cl2S + 2H2O C4H10O2S + 2HCl

This reaction is non-reversible. A reversible reaction is one where the reactants form products, which react together to give the reactants back (ThoughtCo, 2019). For example:

aA + bB ↔ cC + dD

Here we can see that reactant A and reactant B react to form product C and product D. However, as there is a ↔ in the middle, it is known that product C and product D can reverse the reaction to form reactant A and reactant B.

When a reversible reaction occurs, a dynamic equilibrium exists, meaning that the products and reactants are transitioning at equal rates, causing no net change.


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The Chemistry Of Poisons And Toxins. (2022, February 27). Edubirdie. Retrieved February 3, 2023, from
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