Problem 94 - Chemistry The Molecular Nature of Matter

Question

Write the balanced molecular and net ionic equation for the reaction between sulfuric acid and potassium cyanide.

Step-by-Step Solution

Answer

Molecular equation: H2SO4 + 2KCN -> K2SO4 + 2HCN. Net ionic equation: H2SO4 + 2CN- -> SO4^2- + 2HCN.

Step 1: Write the Unbalanced Molecular Equation

Write out the chemical formulas for sulfuric acid and potassium cyanide, and their reaction products. Sulfuric acid (H2SO4) reacts with potassium cyanide (KCN) to produce potassium sulfate (K2SO4) and hydrogen cyanide (HCN). The unbalanced molecular equation is: H2SO4 + KCN -> K2SO4 + HCN

Step 2: Balance the Molecular Equation

Balance the numbers of atoms for each element on both sides of the equation. In this case, we balance the potassium (K), sulfur (S), and cyanide (CN) group.The balanced molecular equation is: H2SO4 + 2KCN -> K2SO4 + 2HCN

Step 3: Split the Soluble Electrolytes

Rewrite the balanced molecular equation and split the soluble electrolytes into their constituent ions. This is done because these compounds dissociate into ions when dissolved in water. The equation now looks like this:H2SO4 + 2K+ + 2CN- -> 2K+ + SO4^2- + 2HCN

Step 4: Cancel Spectator Ions

Identify and cancel the spectator ions, which are ions that do not change during the reaction. In this case, the potassium ions (K+) are spectator ions. Removing them, we get the net ionic equation:H2SO4 + 2CN- -> SO4^2- + 2HCN

Step 5: Confirm Conservation of Charge

Check that the charges are balanced on both sides of the net ionic equation. On the left-hand side, you have H2SO4 (which is neutral) and 2 moles of CN-, for a total charge of -2. On the right-hand side, you have SO4^2- and 2 moles of HCN (which is neutral), so the charge is also -2. The net ionic equation is therefore balanced.

Key Concepts

Chemical Reaction Balancing

Understanding the concept of chemical reaction balancing is crucial for students studying chemistry. It's like solving a puzzle where each piece is an atom, and our goal is to have the same number of each type of atom on both sides of the equation.

To balance the equation, start by listing all the elements involved in the reaction on both sides. For our sulfuric acid (H₂SO₄) and potassium cyanide (KCN) reaction, we need to balance potassium (K), sulfur (S), hydrogen (H), oxygen (O), carbon (C), and nitrogen (N). For every element, adjust the coefficients – which are the numbers in front of the chemical formulas – until the number of atoms for each element is equal for the reactants and the products.

For instance, since there are two potassium atoms in potassium sulfate (K₂SO₄), we need two KCN molecules to balance the K atoms. Consequently, this also results in two HCN molecules, which correspondingly balances the cyanide groups.

Molecular Equation

A molecular equation is a way to represent the reactants and products of a chemical reaction using their full chemical formulas without indicating the ionic character of the compounds. This form of equation displays the chemical formulas of the substances as if they were molecules, even though they might actually exist as ions in solution.

Using the provided sulfuric acid and potassium cyanide example, the molecular equation illustrates how one molecule of H₂SO₄ reacts with two molecules of KCN to form one molecule of K₂SO₄ and two molecules of HCN. It does not reflect the reality that these substances may dissociate into their constituent ions when in aqueous solution. So, writing and balancing the molecular equation is the first step towards understanding the underlying stoichiometry of the reaction.

Spectator Ions

Spectator ions are essentially the 'bystanders' of a chemical reaction. These are ions that do not participate in the actual chemical change but simply 'watch' as the other ions undergo the transformation. Their presence is due to the dissociation of the soluble salts in the solution.

In our example, the potassium ions (K+) do not participate in the formation of the end products—they are present on both sides of the reaction equation. Therefore, when we write the net ionic equation, we exclude them. Identifying and eliminating spectator ions simplifies the equation and lets us focus on the truly reactive species, speaking more accurately to the chemistry that happens on a molecular level.

Conservation of Charge

The conservation of charge is a fundamental principle in chemistry that states the total charge must be the same on both sides of a chemical reaction. In other words, the number of positive and negative charges must balance out to zero if the reactants and products are neutral, or to an identical net charge in each.

In the balanced net ionic equation for the reaction between H₂SO₄ and KCN, we see that the charges on both sides of the arrow are equal; summing up to -2 due to the two cyanide ions (CN^-) on the reactant side and the sulfate ion (SO₄^2-) on the product side. This satisfies the law of conservation of charge, confirming that our balancing of the equation is not only mathematically correct but also physically plausible.