CR_Acids and Bases Lesson

Acids and Bases

Another kind of double displacement reaction takes place when an acid and base react with each other. First, you will need to learn the definition of acids and bases. There are several ways to define these terms. The first is called the Arrhenius definition. This is the only definition you need right now. You will learn more in a later unit.

An acid is a substance that produces H+ in solution (reacts with water to produce the hydronium ion, H3O+). • A base is a substance that produces hydroxide ion, OH-, in water.

Acids  and  bases  are common electrolytes, meaning that they conduct electricity (because charged ions flow freely).   They are found in many common substances and also are commonly used in the lab. Some properties of acids and bases can be found in the table below:

Acids taste sour turn blue litmus paper red corrosive to metals release H+ ions in water (Arrhenius definition) react with bases in neutralization relations Bases taste bitter turn red litmus paper blue slippery feel release OH- ions in water (Arrhenius definition) react with acids in neutralization reactions

Vinegar, citrus juices, and cola are all acidic. Some common bases include household ammonia and milk of magnesia.

Acids

In general, acids begin as molecules that when reacted with water, form the hydronium ion. This is an ionization reaction because ions form where none existed before. Since ions are present, the solution conducts electricity.   Substances that conduct electricity are called electrolytes.

The generic ionization equation for an acid is:

HA(g) + H2OH30 (aq) + A- HA(g) acid H2O water H3O+ hydronium ion A- ion

Some specific examples are:

  • LaTeX: HCl_(g)+H_2O \rightarrow H_3O^+_{(aq)}+Cl^-_2HCl(g)+H2O→H3O+(aq)+Cl−2
  • LaTeX: HC_2H_3O_2+H_2O \rightarrow H_3O^++C_2H_3O^-_2HC2H3O2+H2O→H3O++C2H3O−2

image of lemonNotice that in the second example, there are hydrogens in the anion. Those are not the ones that react with water.   So, not every substance that contains hydrogen is an acid.

Because this ionization reaction of an acid is so common, it is often abbreviated to this:

H2O on the arrow HA → H+ + A

This can also be written without the water written over the arrow.

An acid that has only one acidic hydrogen is called a  monoprotic acid. Acids that have more than one acidic hydrogen are referred to as  polyprotic acids. Polyprotic acids are sometimes called the more specific terms of  diprotic  and  triprotic.

Examples of Acids monoprotic acids HCI, HNO3, HC2H3O2 diprotic acids H2SO4, H2CO3 triprotic acids H3PO4

Bases

There are two categories of bases: ionic and molecular bases.  

  • Ionic bases contain OH- or O-2. (NaOH, Ca(OH)2, and CaO are examples)
  • Molecular bases are molecules that when reacted with water form hydroxide.  

B+ H2O → BH+ + OH- B+ base H2O water BH+ cation OH- hydroxide ion

Remember to think of water as HOH (composed of H+ and OH-).   This will help you write these equations.

Some specific examples are:

  • LaTeX: NH_3+H_2O \rightarrow NH^4_{4(aq)}+OH^-_{(aq)}NH3+H2O→NH44(aq)+OH−(aq)
  • LaTeX: CH_3NH_2+H_2O \rightarrow CH_3NH^+_3+OH^-CH3NH2+H2O→CH3NH+3+OH−

Naming Acids and Bases

The naming of ionic bases follows the rules you already know for naming ionic compounds. For example, NaOH is sodium hydroxide. For molecular bases, you will need to memorize the common ones. NH3 is ammonia. CH3NH2 is methylamine.

Acids are named according to a new set of rules.

Binary acids, meaning they contain hydrogen and one other nonmetal, are named with the prefix hydro- and the ending -ic.

Naming Binary Acids hydro _______ic acid

For example, HCl gas is hydrogen chloride, but HCl (aq) is hydrochloric acid. H2S is hydrosulfuric acid.

Acids that contain hydrogen, oxygen, and another element are called oxoacids. These do not use the prefix hydro.   Instead, they are named with the endings, -ic and -ous, followed by the word acid.

  • Polyatomic ions that have the ending - ate change the ending to - ic.
  • (An easy way to remember this is "I ate it, it made me sick!")
  • Polyatomic ions that have the ending - ite change the ending to - ous.
  • (An easy way to remeber this is "A bad dog will bite us.")

For example, to name HNO3(aq), look at the name of NO3-.   NO3- is nitrate. Nitrate changes to nitric. So, HNO3(aq) is nitric acid.

Neutralization Reactions

The H+  ion in the acid reacts with the OH-  ion in the base, causing the formation of water. Generally, the product of this reaction is an ionic salt and water:

 

Example #1: Write the neutralization reaction of hydrobromic acid with sodium hydroxide. Remember that this is just a specific type of double displacement reaction. Also remember that HOH is really H₂O. • Start by writing the reactants. HBr + NaOH → ⚫ Then switch the anions. HBr + NaOH → NaBr + HOH Don't forget to write the products correctly according their charges and balance. This reaction is already balanced! Example #2: Write the neutralization reaction of nitric acid with magnesium hydroxide. • Start by writing the reactants. HNO3 + Mg(OH)2 →→ Then switch the anions. HNO3 + Mg(OH)2 Mg(NO3) + HOH • Write the products correctly according their charges and balance. (And change HOH to H₂O.) 2HNO3 + Mg(OH)2 Mg(NO3)2 + 2H2O

Remember to work on the module practice problems as you complete each section of content.

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