Chapter 16, Problem 16.166QP

About half of the hydrochloric acid produced annually in the United States (3.0 billion pounds) is used in metal pickling. This process involves the removal of metal oxide layers from metal surfaces to prepare them for coating, (a) Write the overall and net ionic equations for the reaction between iron(III) oxide, which represents the rust layer over iron, and HCl. Identify the Brønsted acid and base. (b) Hydrochloric acid is also used to remove scale (which is mostly CaCO₃) from water pipes. Hydrochloric acid reacts with calcium carbonate in two stages: the first stage forms the bicarbonate ion, which then reacts further to form carbon dioxide. Write equations for these two stages and for the overall reaction. (c) Hydrochloric acid is used to recover oil from the ground. It dissolves rocks (often CaCO₃) so that the oil can flow more easily. In one process, a 15 percent (by mass) HCl solution is injected into an oil well to dissolve the rocks. If the density of the acid solution is 1.073 g/mL, what is the pH of the solution?

Interpretation Introduction

Interpretation:

The overall and net ionic equations for the reaction between iron(III) oxide, which represents the rust layer over iron, and $\text{HCl}$ has to be written; the Brønsted acid and base has to identified

Concept Information:

A chemical substance that gives hydrogen ions when dissolved in water is called as acid. A chemical substance that gives hydroxide ions in solution is called as base.

Bronsted's definitions of acid and base:

 Bronsted's definition is based on the  chemical reaction that occur when both acids and bases are added with each other. In Bronsted's theory acid donates proton , while base accepts proton from acid.

Example: Consider the following reaction.

$\text{HCl}\text{+}{\text{NH}}_{\text{3}}\to {\text{NH}}_{\text{4}}{}^{\text{+}}\text{+}{\text{Cl}}^{\text{-}}$

Hydrogen chloride donates a proton, and hence it is a Bronsted acid. Ammonia accepts a proton, and hence it is a Bronsted base.

pH definition:

The concentration of hydrogen ion is measured using $\text{pH}$ scale.  The acidity of aqueous solution is expressed by $\text{pH}$ scale.

The $\text{pH}$ of a solution is defined as the negative base-10 logarithm of the hydrogen or hydronium ion concentration.

$\text{pH}\text{=}{\text{-log[H}}_{\text{3}}{\text{O}}^{\text{+}}\text{]}$

To Give: The overall and net ionic equations for the reaction between iron(III) oxide, which represents the rust layer over iron, and $\text{HCl}$; the Brønsted acid and base in the reaction

Answer to Problem 16.166QP

The overall equation is: ${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}{}_{(s)}+{\text{6HCl}}_{(aq)}\to {\text{2FeCl}}_{\text{3}}{}_{(aq)}+{\text{3H}}_{\text{2}}{\text{O}}_{(l)}$

The net ionic equation is: ${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}{}_{(s)}+{\text{6H}}^{\text{+}}{}_{(aq)}\to {\text{2Fe}}^{\text{3+}}{}_{(aq)}+{\text{3H}}_{\text{2}}{\text{O}}_{(l)}$

$\text{HCl}$ is the Bronsted acid and ${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}$ is the Brosnted base

Explanation of Solution

Given data:

About half of the hydrochloric acid produced annually in the United States (3.0 billion pounds) is used in metal pickling. This process involves the removal of metal oxide layers from metal surfaces to prepare them for coating.

Reaction between iron(III) oxide and $HCl$:

The overall equation and net ionic equation for the reaction of iron (III) oxide, representing the rust layer over ion and $\text{HCl}$ can be given as follows,

The overall equation is:

${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}{}_{(s)}+{\text{6HCl}}_{(aq)}\to {\text{2FeCl}}_{\text{3}}{}_{(aq)}+{\text{3H}}_{\text{2}}{\text{O}}_{(l)}$

The net ionic equation is:

${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}{}_{(s)}+{\text{6H}}^{\text{+}}{}_{(aq)}\to {\text{2Fe}}^{\text{3+}}{}_{(aq)}+{\text{3H}}_{\text{2}}{\text{O}}_{(l)}$

Identification of Bronsted acid and base:

Since $\text{HCl}$ donates the ${\text{H}}^{\text{+}}$ ion, it is the Bronsted acid. Each ${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}$ unit accepts six ${\text{H}}^{\text{+}}$ ions; therefore, it is the Bronsted base.

Interpretation Introduction

Interpretation:

Hydrochloric acid reacts with calcium carbonate in two stages. Equations for these two stages and for the overall reaction has to be written

Concept Information:

A chemical substance that gives hydrogen ions when dissolved in water is called as acid. A chemical substance that gives hydroxide ions in solution is called as base.

Bronsted's definitions of acid and base:

 Bronsted's definition is based on the  chemical reaction that occur when both acids and bases are added with each other. In Bronsted's theory acid donates proton , while base accepts proton from acid.

Example: Consider the following reaction.

$\text{HCl}\text{+}{\text{NH}}_{\text{3}}\to {\text{NH}}_{\text{4}}{}^{\text{+}}\text{+}{\text{Cl}}^{\text{-}}$

Hydrogen chloride donates a proton, and hence it is a Bronsted acid. Ammonia accepts a proton, and hence it is a Bronsted base.

pH definition:

The concentration of hydrogen ion is measured using $\text{pH}$ scale.  The acidity of aqueous solution is expressed by $\text{pH}$ scale.

The $\text{pH}$ of a solution is defined as the negative base-10 logarithm of the hydrogen or hydronium ion concentration.

$\text{pH}\text{=}{\text{-log[H}}_{\text{3}}{\text{O}}^{\text{+}}\text{]}$

To Write: Equations for these two stages and for the overall reaction

Answer to Problem 16.166QP

The first stage is: ${\text{CaCO}}_{\text{3}}{}_{(s)}+{\text{HCl}}_{(aq)}\to {\text{Ca}}^{\text{2+}}{}_{(aq)}+{\text{HCO}}_{\text{3}}{{}^{\text{-}}}_{(aq)}+{\text{Cl}}^{\text{-}}{}_{(aq)}$

The second stage is: ${\text{HCl}}_{(aq)}+{\text{HCO}}_{\text{3}}{{}^{\text{-}}}_{(aq)}\to {\text{CO}}_{\text{2}}{}_{(g)}+{\text{Cl}}^{\text{-}}{}_{(aq)}+{\text{H}}_{\text{2}}{\text{O}}_{(l)}$

The overall equation is: ${\text{CaCO}}_{\text{3}}{}_{(s)}+{\text{2HCl}}_{(aq)}\to {\text{CaCl}}_{\text{2}}{}_{(aq)}+{\text{H}}_{\text{2}}{\text{O}}_{(l)}+{\text{CO}}_{\text{2}}{}_{(g)}$

Explanation of Solution

Given data:

Hydrochloric acid is used to remove scale (which is mostly ${\text{CaCO}}_{\text{3}}$) from water pipes. Hydrochloric acid reacts with calcium carbonate in two stages; the first stage forms the bicarbonate ion, which then reacts further to form carbon dioxide.

Hydrochloric acid reacts with calcium carbonate in two stages.

The first stage reaction equation is:

${\text{CaCO}}_{\text{3}}{}_{(s)}+{\text{HCl}}_{(aq)}\to {\text{Ca}}^{\text{2+}}{}_{(aq)}+{\text{HCO}}_{\text{3}}{{}^{\text{-}}}_{(aq)}+{\text{Cl}}^{\text{-}}{}_{(aq)}$

Calcium carbonate reacts with hydrochloric acid and gives bicarbonate ions.

The second stage reaction equation is:

${\text{HCl}}_{(aq)}+{\text{HCO}}_{\text{3}}{{}^{\text{-}}}_{(aq)}\to {\text{CO}}_{\text{2}}{}_{(g)}+{\text{Cl}}^{\text{-}}{}_{(aq)}+{\text{H}}_{\text{2}}{\text{O}}_{(l)}$

Bicarbonate ions further reacts with hydrochloric acid and gives carbon dioxide.

The overall reaction equation is:

The overall reaction equation is given as follows,

${\text{CaCO}}_{\text{3}}{}_{(s)}+{\text{2HCl}}_{(aq)}\to {\text{CaCl}}_{\text{2}}{}_{(aq)}+{\text{H}}_{\text{2}}{\text{O}}_{(l)}+{\text{CO}}_{\text{2}}{}_{(g)}$

The ${\text{CaCl}}_{\text{2}}$ formed is soluble in water.

Interpretation Introduction

Interpretation:

Hydrochloric acid is used to recover oil from the ground. If the density of the acid    solution is $1.073\text{ g/mL}$, the pH of the acid solution has to be calculated

Concept Information:

A chemical substance that gives hydrogen ions when dissolved in water is called as acid. A chemical substance that gives hydroxide ions in solution is called as base.

Bronsted's definitions of acid and base:

 Bronsted's definition is based on the  chemical reaction that occur when both acids and bases are added with each other. In Bronsted's theory acid donates proton , while base accepts proton from acid.

Example: Consider the following reaction.

$\text{HCl}\text{+}{\text{NH}}_{\text{3}}\to {\text{NH}}_{\text{4}}{}^{\text{+}}\text{+}{\text{Cl}}^{\text{-}}$

Hydrogen chloride donates a proton, and hence it is a Bronsted acid. Ammonia accepts a proton, and hence it is a Bronsted base.

pH definition:

The concentration of hydrogen ion is measured using $\text{pH}$ scale.  The acidity of aqueous solution is expressed by $\text{pH}$ scale.

The $\text{pH}$ of a solution is defined as the negative base-10 logarithm of the hydrogen or hydronium ion concentration.

$\text{pH}\text{=}{\text{-log[H}}_{\text{3}}{\text{O}}^{\text{+}}\text{]}$

To calculate: The pH of the given acid solution

Answer to Problem 16.166QP

The pH of the given hydrochloric acid solution is -0.64

Explanation of Solution

Given data: Hydrochloric acid is used to recover oil from the ground. It dissolves rocks (often ${\text{CaCO}}_{\text{3}}$) so that the oil can flow more easily.

In one process, a 15 percent (by mass) HCl solution is injected into an oil well to dissolve the rocks.

The density of the acid solution is 1.073 g/mL

The concentration of the $\text{HCl}$ solution has to be found to determine its pH.

Let’s assume a volume of $1.000\text{ L = 1000 mL}$.

The mass of 1000 mL of solution is:

$\begin{array}{l}1000\text{ mL}\times \frac{1.073\text{ g}}{\text{1 mL}}\\ =1073\text{ g}\end{array}$

The number of moles of $\text{HCl}$ in a 15% solution is:

$\begin{array}{l}\frac{15\%\text{ HCl}}{\text{100\% soln}}\times 1073\text{ g soln}\\ \text{=(1}\text{.6}\times {\text{10}}^{\text{2}}\text{ g HCl)}\times \frac{\text{1 mol HCl}}{36.46\text{ g HCl}}\\ =4.4\text{ mol HCl}\end{array}$

Thus, there are 4.4 moles of $\text{HCl}$ in 1 L solution, and the concentrationis 4.4 M.

The pH of the solution is:

$\begin{array}{l}\text{pH}\text{=}\text{-log(4}\text{.4)}\\ \text{=}\text{-0}\text{.64}\end{array}$

This is a highly acidic solution (note that the pH is negative), which is needed to dissolve large quantities of rocks in the oil recovery process.