Question

quiz 8.6be- counting atoms, chemical equations and the law of conservation of mass

the photo shows the reaction between zinc and hydrochloric acid that forms zinc chloride and hydrogen gas.

analyze the chemical equations. which of these formulas correctly shows how matter is conserved?

○ zn + 2hcl → zncl₂ + h₂

○ zn + 4hcl → zncl₂ + 2h₂

○ zn + 2hcl → zncl₂ + 2h₂

○ zn + hcl → zncl₂ + h₂

Explanation:

Step1: Recall conservation of mass (balance atoms)

For a chemical equation to show conservation of mass, the number of each type of atom must be equal on both sides (reactants and products). Let's analyze each option by counting atoms of Zn, H, and Cl.

Option 1: $\boldsymbol{\ce{Zn + 2HCl -> ZnCl_{2} + H_{2}}}$

• Zn atoms: Left = 1, Right = 1 (balanced).
• Cl atoms: Left (from 2HCl) = \(2 \times 1 = 2\), Right (from \(\ce{ZnCl_{2}}\)) = 2 (balanced).
• H atoms: Left (from 2HCl) = \(2 \times 1 = 2\), Right (from \(\ce{H_{2}}\)) = \(2 \times 1 = 2\) (balanced).

Option 2: $\boldsymbol{\ce{Zn + 4HCl -> ZnCl_{2} + 2H_{2}}}$

• Cl atoms: Left (4HCl) = \(4 \times 1 = 4\), Right (\(\ce{ZnCl_{2}}\)) = 2 (unbalanced).

Option 3: $\boldsymbol{\ce{Zn + 2HCl -> ZnCl_{2} + 2H_{2}}}$

• H atoms: Left (2HCl) = \(2 \times 1 = 2\), Right (\(2\ce{H_{2}}\)) = \(2 \times 2 = 4\) (unbalanced).

Option 4: $\boldsymbol{\ce{Zn + HCl -> ZnCl_{2} + H_{2}}}$

• Cl atoms: Left (HCl) = 1, Right (\(\ce{ZnCl_{2}}\)) = 2 (unbalanced).
• H atoms: Left (HCl) = 1, Right (\(\ce{H_{2}}\)) = 2 (unbalanced).

Answer:

$\boldsymbol{\ce{Zn + 2HCl -> ZnCl_{2} + H_{2}}}$ (the first option: Zn + 2HCl → ZnCl₂ + H₂)