Question

Write a molecular equation for the precipitation reaction that occurs (if any) when each pair of solutions is mixed. If no reaction occurs, write NO REACTION. (a) sodium carbonate and lead(II) nitrate (b) potassium sulfate and lead(II) acetate (c) copper(II) nitrate and barium sulfide (d) calcium nitrate and sodium iodide

Short answer

(a) Na2CO3(aq) + Pb(NO3)2(aq) -> PbCO3(s) + 2NaNO3(aq); (b) K2SO4(aq) + Pb(C2H3O2)2(aq) -> PbSO4(s) + 2KC2H3O2(aq); (c) Cu(NO3)2(aq) + BaS(aq) -> Ba(NO3)2(aq) + CuS(s); (d) NO REACTION.

Step-by-step solution

- Write down the reactants for each pair

Determine the chemical formulas for each reactant in the pairs given. (a) Sodium carbonate: Na2CO3; Lead(II) nitrate: Pb(NO3)2. (b) Potassium sulfate: K2SO4; Lead(II) acetate: Pb(C2H3O2)2. (c) Copper(II) nitrate: Cu(NO3)2; Barium sulfide: BaS. (d) Calcium nitrate: Ca(NO3)2; Sodium iodide: NaI.

- Predict the products using double displacement

Exchange the positive ions (metal cations) between the reactants to form new combinations, which would be the expected products of a double displacement reaction. Check solubility rules to determine which products, if any, will precipitate out of solution as a solid. Insoluble compounds will form a precipitate.

- Write the balanced molecular equations

(a) Na2CO3(aq) + Pb(NO3)2(aq) -> PbCO3(s) + 2NaNO3(aq). (b) K2SO4(aq) + Pb(C2H3O2)2(aq) -> PbSO4(s) + 2KC2H3O2(aq). (c) Cu(NO3)2(aq) + BaS(aq) -> Ba(NO3)2(aq) + CuS(s). (d) Ca(NO3)2(aq) + NaI(aq) -> CaI2(aq) + 2NaNO3(aq).

- Include physical states and check for NO REACTION cases

Add physical states to each compound (solid, liquid, aqueous, or gas) and make sure that they align with the expected solubility. If no insoluble products are formed, write NO REACTION.

- Finalize the equations

For each reaction, if a precipitate forms, the final equation is already stated in step 3. If all products are soluble, write NO REACTION.

Key concepts

Molecular Equation

In chemistry, a molecular equation provides a concise way of expressing a chemical reaction using the chemical formulas of the reactants and products involved. It shows all the reactants and products as if they were intact, undissociated compounds. This equation represents the overall reaction without indicating the ionic nature of the substances when they are in aqueous solutions.

For instance, when writing a molecular equation for the reaction between sodium carbonate ((Na2CO3)) and lead(II) nitrate ((Pb(NO3)2)), the complete chemical formulas for all reactants and products are used, showing the exchange of partners in a double displacement reaction. A molecular equation for this process would look like this: Na2CO3(aq) + Pb(NO3)2(aq) -> PbCO3(s) + 2NaNO3(aq). Here, the physical states of the compounds are also included to show that lead(II) carbonate is insoluble and precipitates out of the solution, while the sodium nitrate remains in solution.

Double Displacement Reaction

A double displacement reaction, also known as a metathesis reaction, is a type of chemical reaction where two compounds exchange ions to form two new compounds. These reactions generally involve two ionic compounds in aqueous solution and can often result in the formation of a precipitate, a gas, or a weak electrolyte.

During the process, the cations of one compound switch places with the cations of the other compound. For example, when potassium sulfate ((K2SO4)) is mixed with lead(II) acetate ((Pb(C2H3O2)2)), they undergo a double displacement reaction to form lead(II) sulfate ((PbSO4)) and potassium acetate ((KC2H3O2)). Thus, the ions 'K+' and 'Pb2+' displace each other, resulting in new combinations which are expressed in the resulting formula: K2SO4(aq) + Pb(C2H3O2)2(aq) -> PbSO4(s) + 2KC2H3O2(aq). The solid lead(II) sulfate formed is the precipitate indicating a chemical reaction has occurred.

Solubility Rules

Solubility rules are a set of empirically derived guidelines that determine the solubility of various ionic compounds in water. They help predict whether a compound will remain in solution or form a precipitate when mixed in an aqueous solution. These rules are essential when writing molecular equations for double displacement reactions, as they allow us to identify the products that will precipitate out of the solution.

For example, common rules include that most nitrates ((NO3)), acetates ((C2H3O2)), and alkali metal salts are soluble in water. In contrast, most carbonates ((CO3)), sulfides ((S)), and lead salts are insoluble, unless paired with a soluble cation like those of alkali metals. Using solubility rules, one can predict that when solutions of copper(II) nitrate ((Cu(NO3)2)) and barium sulfide ((BaS)) are combined, copper(II) sulfide ((CuS)) will precipitate because sulfides are generally insoluble except for those with alkali metals and a few other exceptions: Cu(NO3)2(aq) + BaS(aq) -> Ba(NO3)2(aq) + CuS(s).

Chemical Formulas

Chemical formulas represent the composition of molecules and compounds using symbols for the elements and numeric subscripts to show the number of each type of atom present. The formula provides essential information that is used to predict the behavior of chemical substances during reactions.

Understanding how to interpret and write chemical formulas is crucial for balancing equations and understanding reaction mechanisms. In the context of precipitation reactions, correctly identifying the formulas for the reactants is the first step in predicting the products that will form through a double displacement reaction. For instance, when the formulas for calcium nitrate ((Ca(NO3)2)) and sodium iodide ((NaI)) are listed, one can immediately recognize their composition, positively charged ions ((Ca2+) and (Na+)), and negatively charged ions ((NO3) and (I)). This knowledge is then used to predict the corresponding products: Ca(NO3)2(aq) + NaI(aq) -> CaI2(aq) + 2NaNO3(aq). The correct chemical formulas ensure the proper stoichiometry of the final balanced molecular equation.