Читать книгу The Handy Chemistry Answer Book - Justin P. Lomont - Страница 26

What are Lewis acids and bases?

The definition of a species as a Lewis acid or a Lewis base is based on whether a species has a tendency to donate its electrons to another species or whether it tends to accept electrons from another species in a chemical reaction. Lewis acids are electron acceptors, while Lewis bases are electron donors. As you can probably guess, Lewis acids tend to react with Lewis bases, since they each have what the other one is looking for.

What are Bronsted acids and bases?

The Bronsted acid/base definition differs somewhat from the Lewis definition. Rather than focusing on electron acceptors and donors, the Bronsted definition deals with donating or accepting H⁺ atoms (protons) in chemical reactions. A Bronsted acid is a species that tends to donate protons in a chemical reaction, while a Bronsted base is a species that tends to accept those protons. To accept protons, a Bronsted base must typically have at least one pair of nonbonding valence electrons.

What is an amphoteric molecule?

An amphoteric molecule is one that can act as either an acid or a base. Either the Lewis or Bronsted definition of acid/base behavior can be used to define a species as amphoteric.

What is the pK_a of a Bronsted acid?

The pK_a of a Bronsted acid provides a description of the tendency of the acid to donate a proton in an aqueous (water-based) solution. A lower pK_a is associated with a stronger tendency to donate protons, and thus a stronger Bronsted acid. The pK_a is calculated mathematically as the negative logarithm (base 10) of the equilibrium constant for dissociation of a proton from the acid in water.

What is the pH of an aqueous solution?

The pH of an aqueous solution provides a description of the concentration of H₃O+ ions (or water molecules that have accepted an extra proton) in the solution. The pH is calculated as the negative logarithm (base 10) of the activity of H₃O⁺ ions in solution. A pH near 7 indicates that the solution is nearly neutral, in the sense that this would be the pH of pure water without any acid or base added. A pH value lower than 7 indicates that the solution is at least somewhat acidic, while a pH above 7 indicates the solution is basic.

The equation describing the pH of a solution is:

pH = -log [H₃O⁺]

What is a general chemical equation for a combustion reaction?

A combustion reaction involves the burning of a hydrocarbon in the presence of oxygen to form carbon dioxide and water as products. A specific example would be:

CH₄ + 2 O₂ CO₂ + 2 H₂O

In general, the equation for a combustion reaction will look something like:

Hydrocarbon + Oxygen Carbon Dioxide + Water

Why do baking soda and vinegar fizz so much when they react?

The chemical name for baking soda is sodium bicarbonate, and its chemical formula is NaHCO₃. Vinegar consists mainly of acetic acid (CH₃COOH) and water. When baking soda is added to vinegar, the following reaction takes place between the sodium bicarbonate and acetic acid:

NaHCO₃ (aq) + CH₃COOH (aq) CO₂ (g) + H₂O (l) + CH₃COONa (aq)

It’s the released CO₂ that creates the bubbles and foaming appearance you see when the two are mixed.

Why does it get so hot when you pour a strong acid in water?

When a strong acid is poured into water, many proton transfer reactions take place very rapidly, and these are exothermic reactions. The fact that these reactions are exothermic means that they release energy (in the form of heat), increasing the temperature of the solution before the heat has time to dissipate to the surrounding environment.