Reactions in Which a Solid Forms

In this chapter we will study some common types of reactions that take place in water, and we will become familiar with some of the driving forces that make these reactions occur. We will also learn how to predict the products for these reactions and how to write various equations to describe them. 

OBJECTIVE To learn about some of the factors that cause reactions to occur. 

In this text we have already seen many chemical reactions. Now let s consider an important question Why does a chemical reaction occur What causes reactants to want to form products As chemists have studied reactions, they have recognized several tendencies in reactants that drive them to form products. That is, there are several driving forces that pull reactants toward products—changes that tend to make reactions go in the direction of the arrow. The most common of these driving forces are 

OBJECTIVE To learn to identify the solid that forms in a precipitation reaction. 

One driving force for a chemical reaction is the formation of a solid, a process called precipitation. The solid that forms is called a precipitate, and the reaction is known as a precipitation reaction. For example. 

A reaction in which a solid forms and separates from the solution 

What is the equation that describes this chemical change To write the equation, we must decipher the identities of the reactants and products. The reactants have already been described K CrO iaq) and BeL(NO )2(aq). 

Is there some way in which we can predict the identities of the products What is the yellow solid The best way to predict the identity of this solid is to first consider what products are possible. To do this we need to know what chemical species are present in the solution that results when the reactant solutions are mixed. First, let s think about the nature of each reactant in an aqueous solution. 

What Happens When an Ionic Compound Dissolves in Water  

When each imit of a substance that dissolves in water produces separated ions, the substance is called a strong electrolyte. Barium nitrate is a strong electrolyte in water, because each Ba(N03)2 unit produces the separated ions (Ba2 NO3-, NO3 ). 

The idea introduced here is very important when ionic compounds dissolve, the resulting solution contains the separated ions. Therefore, we can 

Electrical conductivity of aqueous solutions, (a) Pure water does not conduct an electric current. The lamp does not light, (b) When an ionic compound is dissolved in water, current flows and the lamp lights. The result of this experiment is strong evidence that ionic compounds dissolved in water exist in the form of separated ions. 

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