The Handy Chemistry Answer Book. Justin P. Lomont
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to describe chemical reactions. It is conventional to list the initial species, or reactants, on the left side of the equation, followed by an arrow, and the final species, or products, on the right side of the equation. The equation below shows the reaction of methane and oxygen to produce water and carbon dioxide.
CH4 + 2 O2
The arrow to the right indicates that the reactant species are converted to the product species during the course of the reaction. In some cases, reactions are reversible, which will be represented by two arrows, one pointing in either direction (
What is the yield for a reaction?
The yield of a chemical reaction is the amount of product that is produced (an example might be two grams). It is often of greater interest to consider the percent yield, which describes the amount of product formed relative to the maximum amount of product one could have expected based on the quantities of reactants used. The percent yield provides a measure of how efficient a process is for producing the target product.
What does selectivity mean for chemical reactions?
Selectivity can have several meanings for chemical reactions, but there are two main categories: either a reaction will occur selectively with a particular chemical species or at a particular location in a molecule to avoid unwanted side-reactions, or a reaction will produce a particular product selectively.
How do modern chemists characterize the products of chemical reactions?
Chemists need to characterize the products of their chemical reactions so that they can be sure of the structure and composition of the molecule(s) they have made. One common way is by measuring the melting point of a solid substance. This doesn’t provide specific information about the arrangement of chemical bonds in a molecule, though, so more advanced techniques are required to completely characterize a molecule. These techniques often involve using electromagnetic radiation to probe the energy levels in the molecules (see “Physical and Theoretical Chemistry” for more on these topics). Knowledge of what energies/wavelengths of light the molecule can absorb can be related directly to structural features of the molecule.
Are chemists still looking for new reactions?
Yes. Chemistry has hundreds of years of knowledge to build on, but it is in no way a complete field. New ways of making existing molecules, and making chemical structures that are completely new to our planet, are absolutely goals of modern chemistry. Developing new chemical reactions, and understanding old ones, are topics of eternal interest to chemists.
What is the law of conservation of matter?
The law of conservation of matter states that matter cannot be created or destroyed. This is relevant to chemical reactions because it tells us that we must have just as many atoms of each element at the beginning of a reaction as we do at the end of a reaction. In the example above, this is reflected by the fact that we use two molecules of oxygen, or four oxygen atoms from our reactants, to produce one molecule of CO2 and two molecules of H2O for a total of four oxygen atoms in our products.
What is the stoichiometry of a reaction?
The stoichiometry of a reaction is closely tied to the idea of the conservation of matter; it tells us the ratio in which the molecules react. Again using the example at the beginning of this chapter, the reaction stoichiometry 1:2:1:2 describes the ratio in which methane and oxygen react to form carbon dioxide and water.
Why do some chemical reactions cause a color change?
The colors we see all have to do with what wavelengths of light something absorbs or reflects. For a chemical reaction to cause a change in the color of something, all that has to happen is that the products of the reaction absorb and reflect different wavelengths of light than the reactants do. We’ll discuss how light interacts with molecules in more detail in “Physical and Theoretical Chemistry.”
What are a couple of examples of familiar chemical reactions?
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.
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