Definition of Acids and Bases and their Properties

How do you identify an acid and a base?  You may say that acid has sour taste. Yes your are right!  There are several examples of acids that you are familiar, like for example ascorbic acid (vitamin c), citric acid, acetic acid in vinegar and lactic acid in milk.  But take note not all acids can be tasted.  You need to use acid-base indicators to identify if a substance is an acid or base. Base, on the other hand is said to be bitter in taste and slippery to touch like detergents.  Below are definitions of acids and bases from different chemists and some of the properties of acid and bases that you can use to identify the difference between the two.

Definition of Acids and Bases

Arrhenius Definition of Acids and Bases

Swedish chemist Svante Arrhenius define acids as substances that produce H+ ions in aqueous solution while bases are substances that produces OH- ions in aqueous solutions.

Let us look at the example below:

The equation above hydrochloric acid when dissolved in water is ionized to form H+ ions and Cl- ions.

The above equation shows that NaOH when dissolved in water is ionized to produce Na+ ions and OH- ions.

Its the H+ ions that makes a substance an acid while  OH- ions make the substance a base. 

Bronsted-Lowry Acids and Bases Definition

Danish chemist Johannes Bronsted  and English chemist Thomas Martin Lowry , define acids as substances that donate protons (hydrogen ions) while bases are substances that accept protons (hydrogen ions).  Their definition is quite broad compared to Arrhenius definition, Arrhenius acids and bases are only limited to aqueous solution, whereas in Bronsted-Lowry definition of acids and bases includes substances capable of donating proton while acids includes any substances capable of accepting proton.

The above equation shows the ionization of hydrochloric acid in water.  But this is not only what happens in the solution the H+ ions or the proton is being attracted to the oxygen of water molecule forming a hydrated proton H3O+, called hydronium ion. Therefore, the ionization of hydrochloric acid in water look like this:

The above equation shows that HCl is the Bronsted-Lowry acid, since it donated proton or H+ ion to H2O, while H2O is the Bronsted-Lowry base since it accepted protons from HCl.  

Let us have an additional example.  Identify if Bronsted-Lowry acid or base:

Given a) HBr and b) NO2-.  

a) HBr is an example of Bronsted-Lowry acid since its capable of donating proton in once dissolve in water.

b) NO2- is an example of Bronsted-Lowry base since it has capability to accepts H+ ion or proton to form HNO2.

General Properties of Acids and Bases

Acids

• Acids have sour taste.
• Acids change blue litmus paper to red.
• Acids react with other metals (higher in position in the activity series) to produce hydrogen gas.
• Acids also react with carbonates and bicarbonates (like NaCO3, NaHCO3)  to produce carbon dioxide gas.
• Aqueous acids are electrical conductor.

Bases

• Bases have bitter taste.
• Bases are slippery to touch.
• Bases change red litmus paper to blue.
• Bases are electrical conductor

Molecular Equation, Ionic Equation, Net Ionic Equation

In my previous post we learn about chemical equation and the types of chemical reactions.  All cited equations are example of molecular equation.  Molecular equations show the reaction of molecules where all species are present which exist as whole unit or molecules, as shown in the example below:

The equation above is an example of molecular equation and a double replacement reaction, It shows the reaction of lead nitrate molecule in reaction with potassium iodide forming lead  iodide precipitate and potassium nitrate molecules.  Although the equation gives all the required information of the reactants and products, it does not show what happens to the substances in solution.

Ionic equation on the other hand is a kind of equation which shows what happens to the substances in solution.  It shows dissolve species as free ions. See the example below:

This shows what happens to the reaction of lead nitrate  and potassium iodide in solution.  If you will notice all reactants and products are dissociated in water except lead iodide.  This is because lead iodide is not soluble in water.  Now in predicting if precipitate will be formed in the reaction, refer to the solubility rules.  Once the solute is soluble it should be written in ionized form as it shown in the example above and if not soluble it should be written as a whole molecule.

Now to determine the net ionic equation, spectator ions are being canceled out or eliminated.  Spectator ions are ions which are not included in the overall reaction  These are ions that are present both in reactants side and in the products side.  After the spectator ions are eliminated, what will be left is the net ionic equation.

After all spectator ions are eliminated the net ionic equation for the above reaction will be:

Net ionic equation shows only the species that take part in the reaction.

In summary, there are several rules to follow in order to come up with a net ionic equation:

1.  A reaction must be written in molecular form specifying the reactants and products if soluble or not in water.  If soluble in water aq is used which means that the substance is dissociated in water, if not soluble, s is written as subscript which means that precipitate is formed.   And the equation must be balanced.

2.  Write the ionic equation.  Substances that are soluble in water must be written in ionized form while the substance which is not soluble in water should be written as a whole molecule.  

3.  Cancel all the spectator ions both in the reactants and products side.  Then write the net ionic equation.

4.  Finalize by checking the charges and number of atoms in the net ionic equation.

Additional Example:

1.  Predict what happens when a potassium phosphate (K3PO4) solution is mixed with calcium nitrate

[Ca(NO3)2] solution. Write the net ionic equation for this reaction.

Step 1.  Write the balanced equation of the reaction.

Step 2.  Write the ionic equation by writing the soluble substances in ionized form.

Step 3.  Cancel all the spectator ions and write the net ionic equation:

Step 4.  Check if the net ionic equation has balance charges and number of atoms

                            Reactants,      +6 and -6 = 0    ,   Products  = 0

TRY THIS:

Write the balanced molecular equation, ionic equation and net ionic equation of the following:

1.  Na2S(aq)   +     ZnCl2(aq)  >>>

2.  K3PO4(aq)   +   Sr(NO3)2(aq)  >>>

3.  Mg(NO3)2 (aq)  +   NaOH(aq)  >>>>

General Properties of Aqueous Solution

Solution is a homogeneous mixture of solute and solvent.  Solute is a substance or component present in smaller quantity or proportion, while solvent is a substance or component present in larger amount or proportion.  There are different types of solution, these are the solid solution, liquid solution and the gaseous solution.  Aqueous solution is a type of liquid solution where the solvent is water. Solutes can either be solid, liquid, or gas.  Example of aqueous solution is sugar dissolved in water, salt dissolved in water, alcohol in water and more.

Aqueous solution can be electrolyte and nonelectrolyte.  Electrolyte is a substance that when dissolve in water it conducts electricity while nonelectrolyte is a substance that does not conduct electricity when dissolve in water.  Electrolytes conduct electricity since the substances is completely breaks up into ions in water while nonelectrolytes, the substances do not dissociate in water.

Electrolytes can be classified into strong electrolytes and weak electrolytes:  Strong electrolytes are those substances that are completely or 100% dissociated in water, while weak electrolytes are substances not completely dissociated in water.  Below are examples of weak and strong electrolytes:

How can we determine if an electrolyte is a strong, weak and nonelectrolyte?

An electrical conductivity apparatus is used to determine if an electrolyte is weak, strong and nonelectrolye.  The two electrodes of electrical conductivity apparatus are placed in the solution, once the bulb light very bright it means that the solution is a strong electrolyte and if light dimly, solution is said to be weak electrolyte.  But if no light produced in the electrical conductivity apparatus then the solution is a nonelectrolyte.

Electrical Conductivity Setup

Difference between strong, weak and nonelectrolyte by means of light produced in an electrical conductivity apparatus.

The  first bulb don't show light which means that the first solution is a nonelectrolyte.  Second bulb lights dimly which indicates that the solution is weak electrolyte and the third bulb lights brightly which indicates that the solution is a strong electrolyte.

What happens when substances dissociate in water?

Water is a polar substance, it can dissolve both ionic and polar substances. Being polar, water has two ends, the positive and negative.  The oxygen is the negative end  and the hydrogen is the positive end.  When an ionic substance such NaCl is dissolved in water, the three-dimensional structure of solid in NaCl is destroyed, and the Na+ and Cl- ions are separated from each other. In solution, each Na+ ion is surrounded by a number of several water molecules orienting their negative end towards the cation.  In the same manner, each Cl- ion is surrounded with several water molecules with their positive ends oriented towards the anion.     The process is called hydration.  A process where ions are surrounded by water molecules arranged in a specific manner.  It helps in the stabilization of the solution which prevents the cations to combine with anions.

Acids and bases are also electrolytes.  Some are 100% ionized in water (strong electrolytes) like HCl (hydrochloric acid)  and HNO3 (nitric acid) while other are just slightly ionized in water (weak electrolytes) like CH3COOH (acetic acid) and HF (hydrofluoric acid).  In water, HCl gas is ionized into H+ ions and Cl- ions completely as shown in the equation below:

Acetic acid (CH3COOH) in water is slightly ionized  in water as shown in the equation below:

The double arrow in the equation above shows that the reaction is reversible, meaning the reaction can be in the forward reaction and reverse reaction.  Initially, acetic acid is ionized to formed into CH3COO- ions and H+ ions,  As time goes on some of the CH3COO- ions recombine with H+ ions forming again CH3COOH molecules.  There comes a time when the rate of forward reaction equalizes the rate of backward reaction, in this point a state of chemical equilibrium is reached.

Note:

Dissociation is used for ionic substances dissolved in water.

Ionized is used for acids and bases dissolved in water.

Percent Yield

Reaction is not always 100 % complete, there are several factors that the actual yield is not equal with theoretical yield.  Theoretical yield is the expected amount of product/s if all limiting reagent has been consumed, while actual yield is the amount of product/s obtained based from actual reaction. For example in making cookies, you are expecting 100 pcs of cookies but only 95 pcs are made.  Therefore the actual yield is 95 pcs and the theoretical is 100 pcs.  There are many factors that can affect why actual yield is not equal with the theoretical yield.

To determine if how efficient is the chemical reaction, percent yield can be calculated.  Percent yield is the proportion of the actual yield and the theoretical yield, as shown in the formula below:

Sample Problem:

Industrially, vanadium metal, which is used in steel alloys, can be obtained by reacting vanadium (V) oxide with calcium at high temperatures

In a process,  154 g of V2O5 reacts with enough  Ca.

a)  Calculate the theoretical yield of V.

b)  Calculate the percent yield if 83 g of V obtained.

Solution:

a.  Calculating theoretical yield 

b.  Percent yield

TRY THIS:

1.  Hydrogen fluoride is used in the manufacture of Freons (which destroy ozone in the stratosphere)  and in the production of  aluminum metal.  It is prepared by the reaction:

In one process, 600 g of CaF2 are treated with an excess of H2SO4.

a)  Calculate the amount of HF produced.

b)  Calculate the percent yield.

2.  When heated, lithium reacts with nitrogen to form lithium nitride. Write the balanced equation of this reaction and what is the theoretical yield of Li3N in g when 12.3 g of Li are heated with 33.6 g of N2?  If the actual yield of Li3N is 5.9 g, what is the percent yield of the reaction?