Tutor profile: Jack R.

Using Le Chatelier's Principle, explain why heating some reactions will cause more products to be formed, while heating others will cause less products to be formed.

Le Chatelier's Principle says that if a change is made to an equilibrium reaction, the equilibrium will shift to counter that change. In exothermic reactions, heat is a product of the reaction, so the addition of heat to the reaction will shift the equilibrium toward the reactant side. In endothermic reactions, heat is consumed in the reaction, so the addition of heat will cause more products to be formed.

Why are cysteines so important for protein structure, and why do cysteine mutations often lead to dysfunctional proteins?

The R group of cysteine contains a thiol, which can readily act as a nucleophile. Two cysteines will often form disulfide linkages with one another, in which the two sulfur atoms of the thiols form a covalent bond with one another. These disulfide bonds are very important in determining the tertiary structure of a protein, and mutations in a cysteine that is a part of a disulfide bond will inhibit correct folding of the protein.

Explain why, in general, the electronegativity of an element increases as one moves from left to right across a row on the periodic table.

As one moves from left to right within a row on the periodic table, the atomic number of the element is increased by one. This represents another proton that has been added to the nucleus of the atom. Because protons are positively charged, they attract the negatively-charged electrons. When a nucleus contains more protons, it will more strongly attract the electrons, keeping them closer to the nucleus and making them more resistant to being removed (called ionization).