The Handy Chemistry Answer Book. Justin P. Lomont

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.

CATALYSIS AND INDUSTRIAL CHEMISTRY

      How can we make chemical reactions go faster?

      Two of the most common ways to make a reaction proceed faster are to increase the temperature at which the reaction is carried out or to use a catalyst to lower the energetic barrier to the reaction. With increased temperature, the amount of thermal energy available to overcome the energetic barrier to the reaction is increased. Of course, you’ll also get more products faster if you just increase the concentration(s) of the reactant(s). Light can also serve as the catalyst, breaking a chemical bond to produce a species that is more reactive that the original reactant species.

      Plants like this one combine nitrogen and hydrogen using the Haber-Bosch process to manufacture ammonia.

      What is a catalyst for a reaction?

      A catalyst for a chemical reaction is any chemical species that lowers the amount of free energy necessary to achieve the chemical transformation. This may involve a significant change in the mechanism by which the reaction takes place. Light may also serve as the catalyst.

      What are some industrial processes that make use of catalysts?

      The chemical industry uses catalysts in a staggering array of chemical reactions that make products that we all use every day. The production of ammonia (NH₃) from nitrogen (N₂) and hydrogen (H₂), known as the Haber-Bosch process, uses iron or ruthenium catalysts; this reaction is used to generate about five hundred million tons of fertilizer per year! Most of the world’s plastic is made by one form of catalysis or another, and the refining of crude oil into gasoline and other fuels is also possible because of catalysts. Catalysts are also used in food processing and production—margarine is made by reacting fats with hydrogen gas in the presence of a nickel catalyst.

      What chemicals are produced on the largest scale?

      In terms of volume (not profit), sulfuric acid is the largest chemical produced worldwide. Global consumption is around two hundred million tons per year—a number that is just too impossibly large to imagine. Nitrogen (N₂), ethylene (CH₂CH₂), oxygen (O₂), lime (mainly CaO), and ammonia (NH₃) round out the top five, though sometimes these change.

      What is the difference between heterogeneous catalysis and homogeneous catalysis?

      Homogeneous catalysis takes place when the catalyst and the other reactants are all dissolved in the same solution. Heterogeneous catalysis typically involves the use of a catalyst that is insoluble, or perhaps only weakly soluble, in the solution in which the reaction takes place. Thus, in heterogeneous catalysis, the catalyst and solution may form a suspension, or the catalyst may simply be a solid that is placed in the solution.

OTHER KINDS OF CHEMICAL REACTIONS

      Why does iron rust?

      Rust is iron that has been oxidized. The oxidation of iron, or any iron alloy (See “History of Chemistry”), can occur whenever iron is in the presence of oxygen and water. The chemical reaction involves electrons from the iron being transferred to oxygen atoms, which react with water molecules to eventually form iron oxides. The presence of ions, like salt or H₃O⁺ in acidic solutions, can accelerate the rate of these reactions. Preventing rust usually requires a protective coating that prevents the iron from reacting with oxygen and water.

      Is melting a chemical reaction?

      No, melting is not a chemical reaction. Melting is a change between phases of matter, and it does not involve the breaking or formation of any new chemical bonds. As the temperature increases to cause a substance to melt, the arrangement of the molecules in the solid/liquid changes, but no chemical reaction takes place. The same is true for liquid to gas phase changes as well as for the reverse processes (freezing and condensing).

      Everyone is familiar with rust, which is what happens when iron oxidizes—the result of iron being exposed to water and oxygen.

      What is thermite?

      Thermite is a mixture of a metal powder and a metal oxide that is capable of causing a very strong exothermic reaction.