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Ch.4 - Reactions in Aqueous Solution
Chapter 4, Problem 41a,b

Write balanced molecular and net ionic equations for the following reactions, and identify the gas formed in each: (a) solid cadmium sulfide reacts with an aqueous solution of sulfuric acid (b) solid magnesium carbonate reacts with an aqueous solution of perchloric acid.

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Write the formula for solid cadmium sulfide and sulfuric acid. Cadmium sulfide is CdS and sulfuric acid is H2SO4.
Combine the reactants to form the products. Since sulfuric acid is a strong acid, it will react with cadmium sulfide to form cadmium sulfate (CdSO4) and hydrogen sulfide gas (H2S).
Write the balanced molecular equation: CdS(s) + H2SO4(aq) → CdSO4(aq) + H2S(g).
Identify the spectator ions in the reaction. In this case, cadmium (Cd) and sulfate (SO4) ions are spectators.
Write the net ionic equation by removing the spectator ions: CdS(s) + 2H+(aq) → Cd2+(aq) + H2S(g). The gas formed in this reaction is hydrogen sulfide (H2S).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Balancing Chemical Equations

Balancing chemical equations involves ensuring that the number of atoms for each element is the same on both the reactant and product sides. This is based on the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction. A balanced equation provides a clear representation of the reactants and products involved in a reaction.
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Molecular and Net Ionic Equations

A molecular equation shows the complete chemical formulas of reactants and products, while a net ionic equation focuses on the species that actually participate in the reaction, omitting spectator ions. To derive the net ionic equation, one must first write the balanced molecular equation, then dissociate the soluble ionic compounds into their respective ions, and finally eliminate the spectator ions to reveal the core reaction.
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Gas Formation in Reactions

Gas formation in chemical reactions often indicates a change in state and can be identified by observing the production of bubbles or effervescence. In the context of acid-base reactions, gases such as hydrogen sulfide or carbon dioxide may be produced. Recognizing the gas formed is crucial for understanding the reaction's dynamics and the products involved.
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