Question

NOW you should try these steps on the reaction between magnesium metal and hydrochloric acid. 1. Write the balanced molecular equation for the reaction. This reaction is a single replacement (metal + acid), so you should be able to predict what the products of the reaction are. 2. Now look at each specie in your equation and write them as they actually exist. Refer back to parts 1-4 of this exercise as necessary. This will be your total ionic equation: 3. Look at your total ionic equation to see if there are any spectator ions. Cancel out any spectator ions, and check again to see if your equation is balanced with the lowest whole number coefficients. This will be your net ionic equation for the reaction:

Answer 1

Background:

$\rightarrow$ All metals above hydrogen in the activity series react with acid to form salt and hydrogen gas. This type of reaction is called displacement reaction where a metal displaces hydrogen and forms a compound with the negatively charged ion or radical (anion) of an acid (for example chloride (Cl^-), sulfate (SO₄^2-), nitrate(NO^3-), etc.)

$\rightarrow$The formula of a compound depends on the valency of the metal and anion. It can be derived from the criss-cross method as shown below:

A13+ CII- 3+ Al ΑΙ, Cl3 Formula = AICI

$\rightarrow$ If the compound formed is soluble in water, it exists in the form of ions, for example, like all nitrates sodium nitrate is soluble in water and exists as Na⁺_(aq) and NO^3-_(aq) ions {aq represents the state of the substance, it can be solid (s), liquid (l), gaseous (g) or aqueous (aq)}. To know whether a substance is soluble or not, one can refer to solubility charts given in basic chemistry books. It will contain information such as all nitrates and compounds of alkali metals are soluble, chlorides are generally soluble in water (except Ag⁺, Hg₂²⁺, and Pb²⁺) and also shows insoluble compounds such as sulfides, carbonates, and hydroxides with exceptions if any)

Now coming to the parts of the question:

1. Balanced Molecular Equation

In the present case, magnesium (Mg) is a reactive metal and will displace hydrogen from hydrochloric acid to form magnesium chloride. The formula for magnesium chloride can be derived from the criss-cross method as described above. Magnesium has 2+ valency and chloride has 1- valency. So the formula for magnesium chloride will be MgCl₂. Further, it is soluble in water, so the state symbol (aq) will be added in its subscript.

The balanced chemical equation will be:

Mg_(s) + 2 HCl_(aq)$\rightarrow$ MgCl_2(aq) + H_2(g)

Note: Since each atom on either side of the arrow must be balanced i.e. same in number, HCl_(aq) is multiplied by 2 to make H and Cl atoms equal on both sides.

2. Total ionic equation

As mentioned in the background, the soluble substances ( i.e. those having the subscript (aq) in the chemical equation exist as ions) can be written in ionic form. So writing the ionic form of the chemical equation below:

Mg(s) + 2H⁺_(aq) +2Cl^-_(aq)$\rightarrow$ Mg²⁺_(aq) +2Cl^-_(aq) +H_2(g)

Note: The number of atoms must be kept the same while writing the total ionic equation

3. Net ionic equation

Now the ions whose state does not change on either side of the arrow are called spectator ions. They can be canceled just as one does in algebraic equations. Remember to consider the number of atoms while canceling out.

In the present case, 2Cl^-_(aq) ions on both sides of the arrow cancel each other out. The net ionic equation can be written without them as follows:

Mg_(s) + 2H⁺_(aq)$\rightarrow$ Mg²⁺_(aq) +H_2(g).

Note: The net ionic equation should not only have the same number of atoms on both sides of the arrow but must also have equal charges on reactants' and products' side.