Writing Reactions

• Double displacement reactions—these are also referred to as metathesis reactions; in them, two types of ions are displaced. These usually happen in solution with an insoluble precipitate formed (in precipitation reactions) or water formed (in neutralization reactions). The precipitate will leave the solution, which will drive the solution forward. In a neutralization reaction, there is double displacement between and acid and a base to yield salt plus water.

• Combustion reactions—this is basically the burning of a substance or the reaction of a carbonaceous substance with oxygen. Heat is the main product of this, along with carbon dioxide and water. The combustion of hydrocarbons, such as propane fuel yields CO2 and heat.

• Redox reactions—this is an overarching term that involves the exchange of electrons from one substance to another. We will talk more about redox reactions later.

WRITING REACTIONS

In writing reactions, there are specific rules to follow in order to write an accurate assessment of what goes on in a reaction. The left side of any chemical equation, is where the reactants are listed. These are written in what’s called the stoichiometric coefficients. Basically, this means that the reaction is written so that it becomes clear how many moles of one substance mix with how many moles of another substance. In other words, the reaction proceeds as if molecules react with molecules rather than x grams of a substance mixes with y grams of another substance.

In addition, after the molecular formula for a reactant or end product, you must put the form of the reactant and end product: You write (l) for liquid, (s) for solid, and (g) for gas. Many reactions will say (aq), which means “aqueous” in those reactions that involve a chemical in solution with water. The number of a given substance or element must be the same on the left-hand side of the equation as is the case on the right-hand side of the equation.

An example of how to balance an equation is shown in Figure 29:

Figure 29.

In the figure, you can see that the equations are completely balanced with respect to the numbers of elements or atoms on both sides of the arrow. You need to know how to balance these types of equations as they lie at the root of chemistry and how you understand chemical reactions.

So, in the first case in the figure, you have hydrogen gas and nitrogen gas going to methane. Because of the fact that methane is NH3, you need to have at least three hydrogen atoms on the left side. Because you can’t have half of a hydrogen gas, you must put 3H2 gas molecules on the left, leading to the mixture with N2 gas that must yield 2 molecules of NH3 (or methane).

In real cases of chemical reactions, there will be the possibility of a limiting reactant that will affect the reaction. This will not affect the equation—only the actual number of moles that can be gotten from the reaction. This is also sometimes referred to as the limiting reagent.

In order to have knowledge of the limiting reagent, you need to transfer the number of grams of a substance in an equation and turn it into the number of moles. In the reaction of H2 gas and O2 gas, you can balance the equation that it takes 2 moles of H2 gas and one mole of O2 gas to make 2 moles of H2O. If you have 4 grams of H2 (weighing 2.016 grams per mole, you get 1.98 moles of hydrogen gas). If you do the reaction stoichiometrically-speaking, you need 0.99 moles of oxygen. If you have 20

grams of oxygen, this amounts to 0.625 moles (less than the 0.99 moles necessary). This means that oxygen is the limiting agent.

You also need to understand the idea of theoretical yield. Theoretically, there will be a total reaction in which all of the reactants go to all of the products. This is almost never the case. This leads to the calculation of the percent yield, which is the actual yield divided by the theoretical yield multiplied by 100 percent. It will always be a number less than 100 percent.

When reactions happen in a solution, the reaction is often thought of as the number of moles of a substance divided by the volume of the solution in liters. This refers to as the Molarity, which is the number of moles of the solute divided by the volume of the solution in liters. So, if there is one mole of a solute in a liter of water, the molarity is one.

To do a calculation, imagine putting 100 grams of table salt (NaCl) in 50 milliliters of water. The number of moles (with 22.99 grams per mole plus 35.45 grams per mole of sodium and chloride, respectively), you get a total of 1.7 moles of sodium chloride. Divide this by 50 milliliters of water or 0.05 liters, you get a molarity of 34.2 moles per liter.

There can be a physical change in the properties of a reactant during the chemical reaction. While there will be a physical change, it does not change the actual substance itself—only its form. There may be the transformation of a liquid to a gas or an aqueous solution of something into a precipitated solid. Again, this is a physical change but not a chemical change. When a precipitation reaction occurs, the solid is called a precipitate. The remaining liquid is referred to as the “supernate”. In such a reaction, the equation is written similar to Figure 30 as shown in your manual:

Figure 30.