Chapter 8

\(\quad \mathrm{CH}_{4}+\mathrm{O}_{2} \rightarrow \mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O}\) Answer: Because oxygen appears as a pure elemental substance, we save it for last. Starting with carbon (an arbitrary choice), we note that there is one carbon on each side of the equation, meaning \(\mathrm{C}\) is balanced. There are four hydrogen atoms on the left and only two on the right. To fix this, we put a balancing coefficient \(2 \mathrm{in}\) front of the \(\mathrm{H}_{2} \mathrm{O}\) : \(\mathrm{CH}_{4}+\mathrm{O}_{2} \rightarrow \mathrm{CO}_{2}+2 \mathrm{H}_{2} \mathrm{O}\) This balances the hydrogen. Now it's time to balance the oxygen. There are two oxygen atoms on the left and four on the right (two from the \(\mathrm{CO}_{2}\) molecule and two from the two \(\mathrm{H}_{2} \mathrm{O}\) molecules). To balance the equation, we put a \(2 \mathrm{in}\) front of the \(\mathrm{O}_{2}\). \(\mathrm{CH}_{4}+2 \mathrm{O}_{2} \rightarrow \mathrm{CO}_{2}+2 \mathrm{H}_{2} \mathrm{O}\)

\(\mathrm{CaH}_{2}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_{2}+\mathrm{H}_{2}\)

\(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}+\mathrm{O}_{2} \rightarrow \mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O}\)

\(\mathrm{HCl}+\mathrm{Na}_{2} \mathrm{CO}_{3} \rightarrow \mathrm{NaCl}+\mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O}\)

Balance the chemical equation for the combustion of hexane, \(\mathrm{C}_{6} \mathrm{H}_{14}(l)\). The unbalanced equation is: \(\mathrm{C}_{6} \mathrm{H}_{14}(l)+\mathrm{O}_{2}(g) \rightarrow \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g)\) Answer: Balance C: \(\mathrm{C}_{6} \mathrm{H}_{14}(l)+\mathrm{O}_{2}(g) \rightarrow 6 \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g)\) Balance H: \(\mathrm{C}_{6} \mathrm{H}_{14}(l)+\mathrm{O}_{2}(g) \rightarrow 6 \mathrm{CO}_{2}(g)+7 \mathrm{H}_{2} \mathrm{O}(g)\) Balance O: \(\mathrm{C}_{6} \mathrm{H}_{14}(l)+\frac{19}{2} \mathrm{O}_{2}(g) \rightarrow 6 \mathrm{CO}_{2}(g)+7 \mathrm{H}_{2} \mathrm{O}(g)\) Multiply through by 2 to get rid of the fractional coefficient: \(2 \mathrm{C}_{6} \mathrm{H}_{14}(l)+19 \mathrm{O}_{2}(g) \rightarrow 12 \mathrm{CO}_{2}(g)+14 \mathrm{H}_{2} \mathrm{O}(g)\)

Balance the chemical equation: \(\mathrm{C}_{6} \mathrm{H}_{12}(l)+\mathrm{O}_{2}(g) \rightarrow \mathrm{H}_{2} \mathrm{C}_{6} \mathrm{H}_{8} \mathrm{O}_{4}(l)+\mathrm{H}_{2} \mathrm{O}(I)\)

Balance the chemical equation: \(\mathrm{CH}_{3} \mathrm{NH}_{2}(g)+\mathrm{O}_{2}(g) \rightarrow \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g)+\mathrm{N}_{2}(g)\)

\(\quad \mathrm{KBr}(a q)+\mathrm{Cl}_{2}(a q) \rightarrow \mathrm{KCl}(a q)+\mathrm{Br}_{2}(a q)\)

\(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{~s}) \stackrel{\text { Heat }}{\longrightarrow} \mathrm{PbO}(\mathrm{s})+\mathrm{NO}(\mathrm{g})+\mathrm{O}_{2}(g)\)

A solution of sodium sulfate is combined with a solution of barium chloride. Does a precipitation reaction occur? If so, which salt precipitates?

Aqueous solutions of lead nitrate and sodium sulfate combine to produce a precipitate. (a) What are the spectator ions? (b) Write the net ionic equation for this precipitation reaction.

Aqueous solutions of nickel(II) nitrate and ammonium phosphate combine to produce a precipitate. (a) What are the spectator ions? (b) Write the net ionic equation for this precipitation reaction.

(a) Write the intact-compound equation and the net ionic equation for the neutralization reaction that occurs between aqueous solutions of sulfuric acid and potassium hydroxide. (b) What salt would be isolated if you evaporated the water from this solution after neutralization?

It takes one molecule of \(\mathrm{NaOH}\) to neutralize one molecule of \(\mathrm{HCl}\), but it takes two molecules of \(\mathrm{NaOH}\) to neutralize one molecule of \(\mathrm{H}_{2} \mathrm{SO}_{4}\). Explain why this is true.

Write the intact-compound equation and the net ionic equation for the neutralization of aqueous hydrofluoric acid by aqueous calcium hydroxide. What is the name of the salt that forms?

Tap water contains dissolved oxygen gas, \(\mathrm{O}_{2}(a q)\). Adding heat to tap water causes the dissolved oxygen to leave. Which of the two equations below represents a chemical reaction, and which does not? Defend your answer. (a) \(2 \mathrm{H}_{2} \mathrm{O}_{2}(l) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{O}_{2}(g)\) (b) \(\mathrm{H}_{2} \mathrm{O}(l)+\mathrm{O}_{2}(a q)\) Heat \(\mathrm{H}_{2} \mathrm{O}(l)+\mathrm{O}_{2}(g)\)

Someone claims that a substance has undergone a chemical reaction. What would they have to demonstrate in order to prove this?

For most gas-phase chemical reactions, where does the energy come from to break covalent bonds in reactant molecules?

Consider a gas-phase reaction. (a) Cooling the mixture of reactants can slow and even stop the chemical reaction. Explain why this is so. (b) Increasing the number of reactant molecules in the flask makes the reaction go faster. Explain why this is so.

Describe how you would explain to someone how a chemical reaction changes one set of compounds into another.

Why must a chemical equation be balanced?

Balance this chemical equation by inspection: \(\mathrm{C}_{2} \mathrm{H}_{4}+\mathrm{O}_{2} \rightarrow \mathrm{CO}_{2}+\mathrm{H}_{2} \mathrm{O}\)

Translate this balanced chemical equation into words: \(\mathrm{CH}_{4}+2 \mathrm{O}_{2} \rightarrow \mathrm{CO}_{2}+2 \mathrm{H}_{2} \mathrm{O}\) Methane

Balance this chemical equation by inspection: \(\mathrm{Fe}_{2} \mathrm{O}_{3}+\mathrm{C} \rightarrow \mathrm{Fe}+\mathrm{CO}_{2}\)

Balance this chemical equation by inspection: \(\mathrm{Al}_{2} \mathrm{Cl}_{6}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Al}(\mathrm{OH})_{3}+\mathrm{HCl}\)

Balance this chemical equation by inspection: \(\mathrm{KClO}_{3} \rightarrow \mathrm{KCl}+\mathrm{O}_{2}\)

Consider the following chemical equation: \(\mathrm{C}_{2} \mathrm{H}_{2}+\mathrm{O}_{2} \rightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{CO}_{2}\) (a) Balance this chemical equation by inspection. (b) Translate the balanced equation into words. The compound \(\mathrm{C}_{2} \mathrm{H}_{2}\) is called acetylene and is used as a fuel for torches that cut through steel.

John writes the chemical equation \(\mathrm{O}_{2}+\mathrm{O}_{2} \rightarrow 1 \frac{1}{3} \mathrm{O}_{3}\). Sue claims that is not a chemical reaction because there is no chemical change, saying "Oxygen is on both sides of the arrow!" (Hint: \(\mathrm{O}_{2}\) is oxygen that you breathe; \(\mathrm{O}_{3}\) is ozone, which is quite toxic.) Sue also says it's not balanced. Which of Sue's complaints is correct? Explain.

Balance this chemical equation and assign it a reaction type: \(\mathrm{N}_{2} \mathrm{O}_{5}(g) \rightarrow \mathrm{NO}_{2}(g)+\mathrm{O}_{2}(g)\)

Balance this chemical equation and assign it a reaction type: \(\mathrm{CO}(g)+\mathrm{NO}(g) \rightarrow \mathrm{CO}_{2}(g)+\mathrm{N}_{2}(g)\)

Balance this chemical equation and assign it a reaction type: \(\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{3}(a q)+\mathrm{Na}_{2} \mathrm{~S}(a q) \rightarrow \mathrm{Fe}_{2} \mathrm{~S}_{3}(s)+\mathrm{NaNO}_{3}(a q)\)

Balance this chemical equation and assign it a reaction type: \(\mathrm{SO}_{2}(g)+\mathrm{O}_{2}(g) \rightarrow \mathrm{SO}_{3}(g)\)

Balance this chemical equation and assign it a reaction type: \(\mathrm{Li}(\mathrm{s})+\mathrm{N}_{2}(g) \rightarrow \mathrm{Li}_{3} \mathrm{~N}(s)\)

Consider the following unbalanced chemical equation: \(\mathrm{C}_{5} \mathrm{H}_{10} \mathrm{O}_{2}(1)+\mathrm{O}_{2}(g) \rightarrow \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g)\) (a) Balance this equation. (b) Classify the reaction type and explain. (c) Because one of the reactants is oxygen, what can you call this type of reaction?

Under the proper conditions, hydrogen peroxide, \(\mathrm{H}_{2} \mathrm{O}_{2}(l)\), can be converted to water and oxygen gas. Write a balanced equation for this reaction and assign it a reaction type.

Compare and contrast combination reactions and decomposition reactions. Give an example of each.

"Hard" water is hard because it contains \(\mathrm{Ca}^{2+}(a q)\) and \(\mathrm{Mg}^{2+}(a q)\) ions. It can be softened by adding borax powder, which is sodium tetraborate, \(\mathrm{Na}_{2} \mathrm{~B}_{4} \mathrm{O}_{7} .\) Adding borax removes the \(\mathrm{Ca}^{2+}(a q)\) and \(\mathrm{Mg}^{2+}(a q)\) from solution. Write a net ionic equation for each of these two reactions. What ion replaces the \(\mathrm{Ca}^{2+}(a q)\) and \(\mathrm{Mg}^{2+}(a q)\) ions in the water?

Fill in this table: $$ \begin{array}{lll} & & \text { Soluble or } \\ \text { Name } & \text { Formula } & \text { insoluble } \\ \hline \text { Sodium phosphate } & & \\ & \mathrm{Ba}\left(\mathrm{CH}_{3} \mathrm{CO}_{2}\right)_{2} & \\ & \left(\mathrm{NH}_{4}\right)_{2} \mathrm{~S} & \\ \text { Iron(II) carbonate } & & \\ \text { Mercury(II) chloride } & & \\ & \mathrm{Co}(\mathrm{OH})_{2} & \\ & \mathrm{HgCl} & \\ \hline \end{array} $$

Write a net ionic equation for the precipitation reaction, if any, that occurs when aqueous solutions of the following ionic compounds are mixed: (a) \(\mathrm{Pb}\left(\mathrm{NO}_{3}\right)_{2}\) and \(\mathrm{NaCl}\) (b) \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}\) and \(\mathrm{NiSO}_{4}\) (c) \(\mathrm{NaCl}\) and \(\mathrm{KNO}_{3}\)

Write a net ionic equation for the precipitation reaction, if any, that occurs when aqueous solutions of the following ionic compounds are mixed: (a) \(\mathrm{Bi}\left(\mathrm{NO}_{3}\right)_{3}\) and \(\mathrm{NaOH}\) (b) \(\mathrm{K}_{2} \mathrm{SO}_{4}\) and \(\mathrm{SrI}_{2}\) (c) \(\mathrm{Cu}\left(\mathrm{CH}_{3} \mathrm{COO}\right)_{2}\) and \(\mathrm{Na}_{3} \mathrm{PO}_{4}\)

Aqueous solutions of iron(III) sulfate and barium hydroxide are combined. Does a precipitate form? If yes, write a net ionic equation for the precipitation reaction.

Aqueous solutions of calcium chloride and potassium carbonate are combined. Does a precipitate form? If yes, write a net ionic equation for the precipitation reaction.

You need calcium sulfate, but there is none in the lab. However, there are lots of other ionic compounds on the shelves. (a) Propose a synthesis for calcium sulfate. (b) Write a complete ionic equation for the synthesis. (c) Write a net ionic equation for the synthesis. (d) Write the equation you wrote for (b) as an English sentence.

You need nickel(II) hydroxide, but there is none in the lab. However, there are lots of other ionic compounds on the shelves. Propose a synthesis for nickel(II) hydroxide.

You dissolve some silver nitrate in your tap water from home, and the water turns cloudy. What chemical species might be in your tap water?

When an aqueous solution of ammonium sulfate is added to an aqueous solution of calcium nitrate, a precipitate forms. (a) Write a net ionic equation for the precipitation. (b) Write the precipitation reaction in a way that emphasizes that a double- replacement reaction has taken place.

An aqueous solution of hydrosulfuric acid, \(\mathrm{H}_{2} \mathrm{~S}(a q)\), reacts with an aqueous solution of \(\mathrm{NaOH}(a q)\). (a) Write the complete ionic equation for the acid-base neutralization that occurs. (Hint: \(\mathrm{S}^{2-}(a q)\) ion is present in the neutralized solution.) (b) What salt would remain behind if you evaporated the water from the neutralized solution?

When an aqueous solution of \(\mathrm{H}_{2} \mathrm{~S}(a q)\) is mixed with an aqueous solution of copper(II) sulfate, a black precipitate of CuS forms. (a) Write and balance the full reaction equation for the reaction of \(\mathrm{H}_{2} \mathrm{~S}(a q)\) with \(\operatorname{CuSO}_{4}(a q)\). (b) Explain how your answer to (a) demonstrates that \(\mathrm{H}_{2} \mathrm{~S}(a q)\) is an acid. (c) Write the net ionic equation for the formation of \(\operatorname{CuS}(s)\)

Suppose you had oxalic acid, \(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\), and you wanted to make sodium oxalate, \(\mathrm{Na}_{2} \mathrm{C}_{2} \mathrm{O}_{4} .\) How might you do this?

Write (a) a reaction with all species intact and using \((a q),(s)\), or \((l)\) for the reaction between aqueous solutions of sulfuric acid and calcium hydroxide. (b) What salt is formed as a result of neutralization?