Acid and Base Word Equations Chemistry Tutorial

Key Concepts

In general an acid reacts with a base to produce a salt and water.

The reaction between an acid and a base can be represented by the general word equation shown below:
acid + base → salt + water

A base is a substance that contains hydroxide ⁽¹⁾
A base is often a compound made up of a metal and hydroxide.

Common bases are: sodium hydroxide, potassium hydroxide, calcium hydroxide.

The reaction between an acid and a metal hydroxide can be represented by the general word equation shown below:
acid + metal hydroxide → salt + water

The name of the salt is determined by the metal part of the hydroxide, and, the acid used:

general word equation:	acid	+	metal hydroxide	→	salt	+	water
hydrochloric acid example :	hydrochloric acid	+	metal hydroxide	→	metal chloride	+	water
nitric acid example :	nitric acid	+	metal hydroxide	→	metal nitrate	+	water
sulfuric acid example :	sulfuric acid	+	metal hydroxide	→	metal sulfate	+	water
phosphoric acid example :	phosphoric acid	+	metal hydroxide	→	metal phosphate	+	water

The chemical reaction between an acid and a base (metal hydroxide) is called a neutralisation reaction (or neutralization reaction).

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Determining the Products of the Chemical Reaction Between an Acid and a Base

If you place some hydrochloric acid in a test tube with a drop of universal indicator, the solution will turn red.
If we then start to add a base, such as sodium hydroxide, the universal indicator will change colour from red-orange to yellow-green, and if you continue to add base you might even make the indicator change colour to blue.
This change in colour tells us that a chemical reaction has taken place.

If we repeat this experiment without using the universal indicator, we could boil off the liquid and find that at about 100°C the temperature doesn't change until all the liquid has boiled off.
The boiling point of water is about 100°C, so we can identify this liquid as water.⁽²⁾
When all the water has boiled off, we would be left with a white solid which is a salt.

You could repeat this chemical reaction using a different acid, sulfuric acid.
In this experiment you add sodium hydroxide to some sulfuric acid in a test tube with a drop of universal indicator.
As the sodium hydroxide is slowly added to the sulfuric acid, the universal indicator changes colour from red-orange to yellow-green (and maybe even to blue).
You could boil off all the liquid and find that it boils at the boiling point of water (about 100°C), and, when all the water has boiled off you would be left with another white solid, another salt.

From these two experiments we could generalise and say that sodium hydroxide will react with an acid to produce a salt and water.
We can write a word equation to represent this chemical reaction:

reactants			→	products
sodium hydroxide	+	acid	→	salt	+	water

What if you repeat these experiments using a different base, potassium hydroxide for example?
If we add potassium hydroxide to hydrochloric acid with a drop of universal indicator, the indicator changes colour from red-orange to yellow-green (and maybe blue) and the liquid can be boiled off at about 100°C leaving a white salt.
If we add potassium hydroxide to sulfuric acid with a drop of universal indicator, the indicator changes colour from red-orange to yellow-green (and maybe blue) and the liquid can be boiled off at about 100°C leaving a white salt.

From these two experiments we could generalise and say that potassium hydroxide will react with an acid to produce a salt and water.
We can write a word equation to represent this chemical reaction:

reactants			→	products
potassium hydroxide	+	acid	→	salt	+	water

From all of the above experiments, we could make an even more general statement about the products of a chemical reaction between an acid and a metal hydroxide (a base):

A metal hydroxide reacts with an acid to produce a salt and water.

and we could write a general word equation to describe this generalisation:

reactants			→	products
metal hydroxide	+	acid	→	salt	+	water
base	+	acid	→	salt	+	water

But what is the "salt" made up of? ⁽³⁾

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Naming the Salt Produced in the Reaction Between an Acid and a Metal Hydroxide (base)

The salt produced when a metal hydroxide reacts with an acid will be made up of 2 parts:

a metal (the metal from the metal hydroxide used in the experiment)

a non-metal (from the acid used in the experiment)

The name of the salt is written as two words:

first word is the name of the metal (from the metal hydroxide)

second word depends on the acid used:
(i) chloride (if hydrochloric acid is used)

(ii) nitrate (if nitric acid is used)

(iii) sulfate (if sulfuric acid is used)

(iv) phosphate (if phosphoric acid is used)

So, in general, the name of the salt produced when a metal hydroxide reacts with an acid will be either:

metal chloride (if hydrochloric acid is used)

metal nitrate (if nitric acid is used)

metal sulfate (if sulfuric acid is used)

metal phosphate (if phosphoric acid is used)

Therefore, we can write some general word equations to describe the chemical reaction between a metal hydroxide and an acid that produces a salt and water:

	reactants			→	products
general word equation:	metal hydroxide	+	acid	→	salt	+	water
Example :	metal hydroxide	+	hydrochloric acid	→	metal chloride	+	water
Example :	metal hydroxide	+	nitric acid	→	metal nitrate	+	water
Example :	metal hydroxide	+	sulfuric acid	→	metal sulfate	+	water
Example :	metal hydroxide	+	phosphoric acid	→	metal phosphate	+	water

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De-coding a Word Equation for the Reaction Between an Acid and a Base (metal hydroxide)

Recall the following facts:

In a word equation the reactants are written on the left hand side of the arrow and each reactant is separated by a plus sign (+)

In a word equation the products are written on the right hand side of the arrow and each product is separated by a plus sign (+)

The name of the base will be two words: metal hydroxide

The name of the acid will be two words, the second word will be acid.

The name of the salt will be two words, the name of the metal is the first word, the second word comes from the name of the acid used.

Water will also be a product when a metal hydroxide reacts with an acid.

For the chemical reaction between an acid and a metal hydroxide (base), the products are a salt and water.
metal hydroxide + acid → salt + water

If we are given the following word equation:

calcium hydroxide + hydrochloric acid → calcium chloride + water

then we can identify the reactants and products of the chemical reaction:

reactants	→	products
calcium hydroxide + hydrochloric acid	→	calcium chloride + water

The reactants are:

calcium hydroxide

hydrochloric acid

The products are:

calcium chloride ⁽⁴⁾

water

We can also identify the base and acid reactants, and the salt and water produced:

reactants			→	products
base	+	acid	→	salt	+	water
calcium hydroxide	+	hydrochloric acid	→	calcium chloride	+	water

base is calcium hydroxide

acid is hydrochloric acid

salt is calcium chloride

liquid is water

And we can infer what the experimenter may have done: some calcium hydroxide was added to some hydrochloric acid.

And we can infer what the experimenter may have observed if an indicator had been added to the acid while the base was added: indicator changed colour.

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Acid + Base Word Equation Problem Solving

The Problem: Chris the Chemist added a drop of phenolphthalein indicator to some nitric acid in a beaker. The solution in the beaker was colourless. Then Chris carefully placed 1 pellet of white sodium hydroxide in the beaker and stirred slowly with a glass stirring rod until the pellet of sodium hydroxide had disappeared completely. Initially, the solution around the pellet turned pink, but turned colourless while Chris stirred. Eventually, the pink colour remained permanently and the pellet of sodium hydroxide had disappeared. Write a word equation to describe the chemical reaction between nitric acid and sodium hydroxide.

The Solution to the Problem

(using the StoPGoPS approach to problem solving)

STOP

STOP! State the Question.

What is the question asking you to do?

Write a word equation for the chemical reaction between sodium hydroxide and nitric aicd.

PAUSE

PAUSE to Prepare a Game Plan

(1) What information (data) have you been given in the question?

(a) Name of the reactants: sodium hydroxide and nitric acid

(b) Evidence of a chemical reaction: indicator changed colour

(2) What is the relationship between what you know and what you need to find out?

General word equation: acid + metal hydroxide → salt + water

The name of the salt will be 2 words: the first word is the name of the metal from the metal hydroxide, the second word comes from the name of the acid:

hydrochloric acid → metal chloride

nitric acid → metal nitrate

sulfuric acid → metal sulfate

phosphoric acid → metal phosphate

GO with the Game Plan

General word equation:	base	+	acid	→	salt	+	water
General word equation:	reactants			→	products
General word equation:	metal hydroxide	+	nitric acid	→	metal nitrate	+	water
Substitute the name of each reactant and each product into the general word equation:
word equation:	sodium hydroxide	+	nitric acid	→	sodium nitrate	+	water

PAUSE

PAUSE to Ponder Plausibility

Have you answered the question?

Yes, we have written a word equation for the reaction between and sodium hydroxide and nitric acid.

Is your answer plausible?

De-code the word equation you wrote to try to re-create Chris's experiment.

The reactants are sodium hydroxide and nitric acid, so Chris added sodium hydroxide to nitric acid.

If Chris added an indicator to the acid then a colour change would be observed as the base (sodium hydroxide) reacted with the acid (nitric acid). Note that Chris would not observe the formation of the salt because it would be soluble, but Chris could evaporate off the liquid in which case a white solid would remain which would be the sodium nitrate salt.

Since we can de-code our word equation to arrive at what Chris actually did and what was observed, we are confident our word equation is correct.

STOP

STOP! State the Solution

sodium hydroxide + nitric acid → sodium nitrate + water

Footnotes:

(1) We will be using the Arrhenius definition of acids and bases for this tutorial.
Acids include: inorganic acids like dilute sulfuric acid, hydrochloric acid and nitric acid, and organic acids like acetic acid found in vinegar.
Bases also include aqueous solutions of ammonia referred to as ammonium hydroxide.

(2) The temperature that water boils at will depend on where you are.
If you happen to be in place where atmospheric pressure is 1 atmosphere, then water will boil at 100°C.
If you are above sea-level, up a hill or a mountain, then the boiling point of water will less than 100°C
If you reduce the air pressure enough, such as in a evacuated bell jar, you can boil water at room temperature!

(3) We could perform some tests on the salt to determine what it is made up of:

(i) Flame Test: identifies the metal by the colour of its flame.

(ii) Solubility Tests: identifies the anion by precipitation of an insoluble solid:

(a) Add barium nitrate: if a precipitate forms the salt solution contain sulfate.

(b) If no precipitate forms in (a) then add silver nitrate solution, if a precipitate forms the salt solution contains chloride.

(4) Some metals, like iron, can form ions with different positive charges. Iron can form ions with a charge of 1+, 2+ or 3+.
The name of the cations of iron are therefore: iron(1+), iron(2+), and iron(3+).
The name of the salt formed by these cations must include the name of the metal AND its charge: iron(1+) chloride, iron(2+) chloride, iron(3+) chloride.
Iron can also form covalent compounds. In this case there is no cation, no positive charge, but we can assign an informal charge based on its oxidation state (which is then referred to as its oxidation state or oxidation number). When we do this, we use Roman numerals to indicate the oxidation state of the metal. For iron the oxidation states are iron(I), iron(II), and iron(III).
The Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005 ("Red Book") for the naming of salts (binary inorganic ionic compounds) recommends the use of the charge number (1+, 2+, 3+) instead of the oxidation state (I, II, III).