The Handy Chemistry Answer Book. Justin P. Lomont

a reaction?

      Some reactions can go both in forward and reverse, while others can only go in one direction. For a reaction that can go both ways, the equilibrium constant describes the ratio of products to reactants. For the reaction:

      A B

      The equilibrium constant would be:

      K_eq = [B]/[A]

      For the reaction:

      A + B C

      The equilibrium constant would be:

      K_eq = [C]/[A][B]

      and for the reaction:

      A + B C + D

      The equilibrium constant would be:

      K_eq = [C][D]/[A][B]

      Reactions with a large equilibrium constant (K_eq > 1) favor formation of the products, while reactions with a small equilibrium constant (K_eq < 1) favor formation of the reactants.

      What is Le Chatelier’s principle?

      Le Chatelier’s principle tells us how to predict the effect a change in conditions will have on a chemical equilibrium. It tells us that a system at equilibrium will shift to counteract changes that disturb the equilibrium. These could be changes in concentrations of chemical species, temperature, pressure, or other conditions. The most commonly discussed changes involve changes in concentration of chemical species, so we’ll just focus on those here. For this equilibrium:

      A + B C + D

      If we decrease the concentration of A, some C and D will react to replenish the A that is depleted, so the concentrations of C and D will decrease. As species A is replenished, more B will be created as well. So the net effect is that decreasing the concentration of A will also decrease the concentrations of C and D, and at the same time increase the concentration of B. More generally, decreasing the concentration of a reactant will cause the equilibrium to shift toward the reactants, increasing the concentrations of other reactants and decreasing the concentrations of products. The converse is also true: Decreasing the concentration of a product will cause the equilibrium to shift toward the products, increasing the concentrations of other products and decreasing the concentrations of reactants.

      It is important to keep in mind that Le Chatelier’s principle only applies to reversible chemical processes (chemical equilibria), so everything we have said here does not apply to reactions that can only proceed in the forward direction.

      An example of a free energy diagram.

      What is a free energy diagram for a chemical reaction?

      A free energy diagram is probably easiest to understand by taking a look at one (see diagram) as we explain the key features.

      The y-axis measures the relative free energy of the chemical species we’re dealing with, while the x-axis describes the reaction coordinate (it’s common that going left to right is forward progress in the reaction, but this isn’t necessarily the case 100% of the time). On the left we have our reactants. In general there may be any number of reactants, and here we’ve just denoted two species, A and B. The “hill” in the middle is the energetic barrier to the chemical reaction, and the quantity Ea denotes the height of this energy barrier. The quantity Ea is commonly referred to as the activation energy for the reaction. On the right-hand side of the diagram we have our products. Again, there can be any number of products, and here we’ve denoted them C and D. Finally we have the quantity G, which describes the change in Gibbs free energy associated with the reaction. The fact that the reactants are higher in free energy than the products tells us that this particular example is a spontaneous reaction. If the reactants were lower in free energy than the products, the reaction would not be spontaneous.

      Can chemical reactions involve multiple steps?

      Yes, and many do. While some chemical reactions may only involve a single step, others may involve ten or more elementary steps. Of course, chemists working in different subfields may have different definitions of what constitutes a step of a reaction, depending on what aspects of the reaction they focus on.

      What is an example of a multistep chemical reaction?

      Many reactions in biological chemistry (see also “Biochemistry”) are multistep chemical reactions. Glycolysis, which is the process of breaking down sugar to generate energy, for example, involves ten sequential steps. Each step is carried out by a special type of catalyst, called an enzyme. There are countless multistep processes in biological systems.

      What is meant by “dynamic equilibrium”?

      Equilibrium conditions in a reversible chemical reaction are described as a dynamic equilibrium. This means that even at equilibrium the reaction has not stopped, and the forward and reverse reactions are still taking place. The bulk concentrations of reactants and products don’t change, but this is just because the forward and reverse reactions are happening at equal rates. The reaction never stops, it just reaches equilibrium.

      What is the rate-determining step of a reaction?

      In a reaction with multiple steps, the rate-determining step is the slowest step. It’s the step that limits the rate of formation of the final products, usually because it has the highest activation energy.

       What does it mean when we say a chemical reaction takes one minute?

      When we say a reaction takes one minute, what we’re really saying is that a certain fraction (specifically that fraction is 1-1/e, or about 63%) of the reactant molecules have gone on to form products after one minute has passed (e refers to the irrational number e = 2.7182818…). If we say a reaction takes one year, or any other amount of time, we’re also referring to that same fraction of reactants becoming products. It’s not that 100% of the reactants have become products, but rather just a certain fraction.

      Why are chemical reactions important in biological systems?

      Chemical reactions make everything in your body work! This is true in all living things (plants, animals, insects, bacteria, etc.). Anytime you make even a slight movement, lots of chemical reactions have to take place. Digesting food, building up or breaking down fat, breathing, cell reproduction, or pretty much any other process that happens in your body involves lots of chemical reactions. Chemistry isn’t only important in the lab, it’s essential to everything related to life!

ACIDS AND BASES

      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.