Acid + Metal Word Equations Chemistry Tutorial

Key Concepts

In general an acid reacts with a metal to produce a salt and hydrogen gas.⁽¹⁾

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

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

general word equation:	acid	+	metal	→	salt	+	hydrogen gas
hydrochloric acid example :	hydrochloric acid	+	metal	→	metal chloride	+	hydrogen gas
sulfuric acid example :	sulfuric acid	+	metal	→	metal sulfate	+	hydrogen gas

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

If you place a piece of clean magnesium metal in a test tube with hydrochloric acid, bubbles of gas are given off.
You can use the "pop test" to identify this gas as hydrogen gas.
After the reaction finishes (no more bubbles of gas are produced), you could evaporate off all the liquid and you would be left with a white solid which is called a salt.

You could repeat this chemical reaction using a different acid, sulfuric acid.
In this experiment you add a piece of clean magnesium metal to some sulfuric acid in a test tube.
Bubbles of gas are given off which can also be tested using the "pop test" to identify the gas as hydrogen gas.
You could evaporate off the all liquid and be left with another white solid, another salt.

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

reactants			→	products
magnesium	+	acid	→	salt	+	hydrogen gas

What if you repeat these experiments using a different metal, zinc for example?
If we add clean zinc metal to hydrochloric acid, bubbles of hydrogen gas are produced, and after evaporation of the liquid, a white salt is left over.
If we add clean zinc metal to sulfuric acid, bubbles of hydrogen gas are produced, and after evaporation of the liquid, a white salt is left over.

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

reactants			→	products
zinc	+	acid	→	salt	+	hydrogen gas

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:

A metal reacts with an acid to produce a salt and hydrogen gas.

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

reactants			→	products
metal	+	acid	→	salt	+	hydrogen gas

But what is the "salt" made up of? ⁽²⁾

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Naming the Salt Produced in the Reaction Between an Acid and a Metal

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

a metal (the metal 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

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

(ii) sulfate (if sulfuric acid is used)

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

metal chloride (if hydrochloric acid is used)

metal sulfate (if sulfuric acid is used)

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

	reactants			→	products
general word equation:	metal	+	acid	→	salt	+	hydrogen gas
Example :	metal	+	hydrochloric acid	→	metal chloride	+	hydrogen gas
Example :	metal	+	sulfuric acid	→	metal sulfate	+	hydrogen gas

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De-coding a Word Equation for the Reaction Between an Acid and a Metal

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 a metal will be just one word.

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.

Hydrogen, or hydrogen gas, will also be a product when a metal reacts with an acid.

For the chemical reaction between an acid and metal, the products are a salt and hydrogen gas.
metal + acid → salt + hydrogen gas

If we are given the following word equation:

iron + hydrochloric acid → iron(2+) chloride + hydrogen

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

reactants	→	products
iron + hydrochloric acid	→	iron(2+) chloride + hydrogen

The reactants are:

iron

hydrochloric acid

The products are:

iron(2+) chloride (or iron(II) chloride)⁽³⁾

hydrogen

We can also identify the metal and acid reactants, and the salt and gas produced:

reactants			→	products
metal	+	acid	→	salt	+	hydrogen
iron	+	hydrochloric acid	→	iron(2+) chloride	+	hydrogen

metal is iron

acid is hydrochloric acid

salt is iron(2+) chloride

gas is hydrogen

And we can infer what the experimenter may have done: a piece of iron was added to hydrochloric acid.

And we can infer what the experimenter may have observed: bubbles of gas were given off (hydrogen gas).

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Problem Solving

The Problem: In a class demonstration, Chris the Chemist added a small piece of calcium metal to some hydrochloric acid in a large beaker. The reaction proceeded very quickly and produced copious bubbles of hydrogen gas. Write a word equation to describe this chemical reaction.

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 calcium metal and hydrochloric aicd.

PAUSE

PAUSE to Prepare a Game Plan

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

(a) Name of the reactants: calcium metal and hydrochloric acid

(b) Name of the products: hydrogen gas

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

General word equation: metal + acid → salt + hydrogen

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

hydrochloric acid → metal chloride

sulfuric acid → metal sulfate

GO with the Game Plan

General word equation:	metal	+	hydrochloric acid	→	metal chloride	+	hydrogen gas
General word equation:	reactants			→	products
Substitute the name of each reactant and each product into the general word equation:
word equation:	calcium	+	hydrochloric acid	→	calcium chloride	+	hydrogen gas

PAUSE

PAUSE to Ponder Plausibility

Have you answered the question?

Yes, we have written a word equation for the reaction between and calcium metal and hydrochloric acid.

Is your answer plausible?

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

The reactants are calcium and hydrochloric acid, so Chris added calcium to hydrochloric acid.

The products are a salt and hydrogen gas, so Chris would have observed bubbles of gas. 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 calcium chloride 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

calcium + hydrochloric acid → calcium chloride + hydrogen gas

Footnotes:

(1) More accurately: an active metal reacts with a non-oxidising acid to produce a salt and hydrogen gas.
Active metals include: Group 1 metals (alkali metals), Group 2 metals (alkaline earth metals), aluminium, zinc, titanium, manganese, chromium, iron, cadmium, cobalt, nickel, tin, lead (but not copper, silver, gold nor platinum).
Non-oxidising acids include: inorganic acids like dilute sulfuric acid and hydrochloric acid (but not nitric acid which is an oxidisng acid), and organic acids like acetic acid found in vinegar.

(2) 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.

(3) 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).