Читать книгу The Handy Chemistry Answer Book - Justin P. Lomont - Страница 24

Fire is one example of a chemical reaction that everyone has seen take place. Fire involves a combustion reaction, which is any reaction where a hydrocarbon reacts with oxygen to form carbon dioxide and water. Another example is when your car accumulates rust. This reaction involves oxidation of the iron in the metal. Lots of complicated chemical reactions are taking place all the time in our bodies too. Every movement you make, for example, involves many chemical reactions taking place in your muscles and nerves.

What is enthalpy?

Enthalpy is a measure of the energy that something contains, and it’s defined as the total heat content of a system. In terms of chemical reactions, we are most often interested in the change in enthalpy (denoted H) associated with a reaction. The H for a reaction is defined as the enthalpy of the products minus the enthalpy of the reactants, and this is typically measured via changes in temperature that take place during the reaction.

What is a calorie?

A calorie is a unit of heat energy defined as the amount of energy it takes to raise one gram of water by one degree Celsius. Calories are also often used to describe the energy content of foods. When used for foods, a “calorie” actually refers to 1,000 calories, or a kilocalorie of energy (which can be rather confusing).

What is a bond enthalpy?

Bond enthalpy refers to the amount of energy it takes to break a chemical bond. This tells us how favorable a chemical bond is relative to the separation of the two fragments on either side of the bond.

What is a heat of formation?

The standard heat of formation for a substance is the change in enthalpy associated with its formation of one mole (see “History of Chemistry”) of a substance from its elements with the constituent elements in their standard states (see “Analytical Chemistry”).

What is Gibbs free energy?

Gibbs free energy is a quantity that describes the amount of useful work (see “Physical and Theoretical Chemistry”) that can be obtained from a system at a constant temperature and pressure. In the context of chemical reactions, changes in Gibbs free energy will typically dictate whether or not a reaction is favorable.

What makes a reaction happen spontaneously?

A spontaneous chemical reaction is one for which the associated change in Gibbs free energy is negative. The fact that a reaction is spontaneous actually doesn’t tell us anything about how quickly the reaction takes place, though. A spontaneous reaction may happen very quickly or take thousands of years!

What is a unimolecular reaction?

A unimolecular reaction involves only a single reactant molecule undergoing a chemical reaction to form products. One possible outcome is that the bonds rearrange within a single molecule to form only one product molecule, while another possibility is that the reactant molecule will fragment, producing multiple product molecules.

What is a bimolecular reaction?

As you might be able to guess if you’ve read the previous question, a bimolecular reaction involves two reactant molecules undergoing a chemical reaction. They may form a single product molecule (if they combine) or multiple product molecules.

What is the equilibrium constant for 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.