Question

demonstrate understanding

1. explain why it is important that a chemical equation be balanced in terms of the fact that atoms are conserved.
2. list three types of physical evidence that indicate a chemical reaction has occurred.
3. compare and contrast a skeleton equation and a chemical equation.
4. explain why it is important to reduce coefficients in a balanced equation to the lowest-possible whole-number ratio.
5. analyze when balancing a chemical equation, can you adjust the subscript in a formula? explain.
6. assess is the following equation balanced? if not, correct the coefficients to balance the equation. $2\ce{k2cro4 (aq)} + \ce{pb(no3)2 (aq)} \

ightarrow 2\ce{kno3 (aq)} + \ce{pbcro4 (s)}$

1. evaluate aqueous phosphoric acid and aqueous calcium hydroxide react to form solid calcium phosphate and water. write a balanced chemical equation for this reaction.

Explanation:

1. Balancing reflects the law of conservation of mass (atoms can’t be created/destroyed), so reactant and product atom counts match.
2. Common physical evidences include color change, gas production (bubbles), and precipitate formation.
3. Skeleton equations show reactants/products but no balanced coefficients; chemical equations are balanced (equal atom counts).
4. Lowest ratios give the simplest, standard representation of the reaction stoichiometry.
5. No—subscripts define compound identity; changing them creates different substances, not balancing.
6. Count atoms: Left has 4 K, 1 Cr, 4 O (from $K_2CrO_4$), 1 Pb, 2 N, 6 O (from $Pb(NO_3)_2$); Right has 2 K, 1 N, 3 O (from $KNO_3$), 1 Pb, 1 Cr, 4 O (from $PbCrO_4$). Correct by changing $KNO_3$ coefficient to 4 and $K_2CrO_4$ to 1: $K_2CrO_4(aq) + Pb(NO_3)_2(aq) → 2KNO_3(aq) + PbCrO_4(s)$.
7. Write formulas: $H_3PO_4(aq) + Ca(OH)_2(aq) → Ca_3(PO_4)_2(s) + H_2O(l)$. Balance Ca (3 on right → 3 $Ca(OH)_2$), P (2 on right → 2 $H_3PO_4$), then H and O: $2H_3PO_4(aq) + 3Ca(OH)_2(aq) → Ca_3(PO_4)_2(s) + 6H_2O(l)$.

Answer:

1. Balancing follows conservation of mass (atoms conserved).
2. Color change, gas production, precipitate formation.
3. Skeleton: unbalanced; chemical: balanced (equal atoms).
4. Simplest stoichiometric representation.
5. No—subscripts define compound identity.
6. No; balanced equation: $K_2CrO_4(aq) + Pb(NO_3)_2(aq) → 2KNO_3(aq) + PbCrO_4(s)$.
7. $2H_3PO_4(aq) + 3Ca(OH)_2(aq) → Ca_3(PO_4)_2(s) + 6H_2O(l)$.