Problem 9 - Chemistry: Atoms First

Question

Aqueous hydrogen fluoride (hydrofluoric acid) is used to etch glass and to analyze minerals for their silicon content. Hydrogen fluoride will also react with sand (silicon dioxide). (a) Write an equation for the reaction of solid silicon dioxide with hydrofluoric acid to yield gaseous silicon tetrafluoride and liquid water. (b) The mineral fluorite (calcium fluoride) occurs extensively in Illinois. Solid calcium fluoride can also be prepared by the reaction of aqueous solutions of calcium chloride and sodium fluoride, yielding aqueous sodium chloride as the other product. Write complete and net ionic equations for this reaction.

Step-by-Step Solution

Answer

The balanced equation for reaction (a) is \(\text{SiO}_2(s) + 4\text{HF}(aq) \rightarrow \text{SiF}_4(g) + 2\text{H}_2\text{O}(l)\). The complete ionic equation for reaction (b) is \(\text{Ca}^{2+}(aq) + 2\text{Cl}^{-}(aq) + 2\text{Na}^{+}(aq) + 2\text{F}^{-}(aq) \rightarrow \text{CaF}_2(s) + 2\text{Na}^{+}(aq) + 2\text{Cl}^{-}(aq)\), and the net ionic equation is \(\text{Ca}^{2+}(aq) + 2\text{F}^{-}(aq) \rightarrow \text{CaF}_2(s)\).

Step 1: Write the reaction for silicon dioxide and hydrofluoric acid

When solid silicon dioxide reacts with aqueous hydrogen fluoride, gaseous silicon tetrafluoride and liquid water are the products. To write the balanced chemical equation, start by writing down the reactants and products with their physical states: silicon dioxide (solid) + hydrogen fluoride (aqueous) -> silicon tetrafluoride (gas) + water (liquid). Now, balance the equation so that the number of each type of atom is the same on both sides.

Step 2: Balance the equation

The initial, unbalanced equation is: \(\text{SiO}_2(s) + \text{HF}(aq) \rightarrow \text{SiF}_4(g) + \text{H}_2\text{O}(l)\). Begin by balancing silicon and then oxygen. This requires placing a coefficient of 4 in front of \(\text{HF}\) to balance fluorine and oxygen: \(\text{SiO}_2(s) + 4\text{HF}(aq) \rightarrow \text{SiF}_4(g) + 2\text{H}_2\text{O}(l)\). The balanced equation now shows 1 silicon, 4 fluorines, and 2 oxygens on each side.

Step 3: Write the complete ionic equation for the reaction of solid calcium fluoride and aqueous calcium chloride

To form solid calcium fluoride, \(\text{CaCl}_2\) reacts with \(\text{NaF}\) in aqueous solution. The complete ionic equation includes all the ions present in the reaction: \(\text{Ca}^{2+}(aq) + 2\text{Cl}^{-}(aq) + 2\text{Na}^{+}(aq) + 2\text{F}^{-}(aq) \rightarrow \text{CaF}_2(s) + 2\text{Na}^{+}(aq) + 2\text{Cl}^{-}(aq)\). This shows the ions before and after the reaction occurs.

Step 4: Write the net ionic equation for solid calcium fluoride formation

The net ionic equation includes only the ions that participate in the reaction, omitting the spectator ions. Spectator ions are those that do not change during the reaction. In this case, \(\text{Na}^{+}\) and \(\text{Cl}^{-}\) are spectator ions. Remove them to get the net ionic equation: \(\text{Ca}^{2+}(aq) + 2\text{F}^{-}(aq) \rightarrow \text{CaF}_2(s)\). This shows that calcium ions react with fluoride ions to form solid calcium fluoride.

Key Concepts

Balancing Chemical Equations

To master the science of chemistry, one fundamental skill you need is balancing chemical equations. This process ensures that the Law of Conservation of Mass is followed, which states that matter cannot be created or destroyed in a chemical reaction. Consequently, the number of atoms for each element must remain consistent before and after the reaction.

Starting with a straightforward approach helps build your foundation. Begin by writing down the unbalanced equation with the correct formulas for the reactants and products. Then, methodically adjust the coefficients – the numbers placed in front of compounds – to balance the atoms for each element. For example, in the reaction of silicon dioxide with hydrofluoric acid, we balance silicon and oxygen first, followed by hydrogen and fluorine. This careful step-by-step progression ensures that the number of atoms for each element is balanced on both sides of the equation, illustrating that in a closed system, mass is conserved.

Net Ionic Equations

When delving into reactions that occur in an aqueous solution, you may encounter net ionic equations. These are simplified versions of complete ionic equations that only include the ions and molecules directly involved in the reaction – the ones that undergo a change. Net ionic equations are valuable because they show us the essence of chemical reactions by filtering out the 'spectator ions', which are present but do not participate.

To write a net ionic equation, first, write the complete ionic equation to show all of the ions in the solution. Next, identify and remove the spectator ions – these are ions that appear unchanged on both the reactant and product side of the equation. What remains is the net ionic equation, which distinctly represents the actual chemistry taking place. For example, when calcium ion reacts with fluoride ion to form solid calcium fluoride, the net ionic equation excludes sodium and chloride ions, which are the spectators in the reaction.

Reaction of Silicon Dioxide and Hydrofluoric Acid

The reaction between silicon dioxide (SiO₂) and hydrofluoric acid (HF) is particularly interesting because it has practical applications, such as in the etching of glass and the analysis of mineral silicon content. Silicon dioxide, commonly found as sand, reacts with hydrofluoric acid to produce gaseous silicon tetrafluoride (SiF₄) and liquid water (H₂O).

This reaction is not only a prime example of a chemical equation needing to be balanced but also demonstrates a reaction that changes physical states – from solid and aqueous reactants to gaseous and liquid products. Understanding reactions like this one is essential for chemists, from industrial applications to theoretical studies. The transformation is indicative of a larger group of reactions where acids react with oxides to form salts and water, highlighting the versatility and variety of chemical reactions.