Question

1. choose the best answer.

a reaction in which heat is required is called ______.
electrolysis
catalytic reaction
thermal decomposition
dissociation

1. choose the best answer.

what is this reaction classified as?
$2\ce{na3po4} + 3\ce{cacl2} \
ightarrow 6\ce{nacl} + \ce{ca3(po4)2}$
combustion
oxidation - reduction
double replacement
single replacement

Explanation:

Question 18

• Electrolysis: Uses electricity, not just heat.
• Catalytic reaction: Involves a catalyst, not defined by heat requirement.
• Thermal decomposition: Reactions where heat is needed to break down a compound (e.g., \( \text{CaCO}_3 \overset{\Delta}{

ightarrow} \text{CaO} + \text{CO}_2 \)), so heat is required.

• Dissociation: Can occur without heat (e.g., ionic compounds in water), not solely heat - dependent.

• Combustion: Involves a substance reacting with \( \text{O}_2 \) to produce heat/light, not seen here.
• Oxidation - reduction: Requires electron transfer (change in oxidation states). In \( 2\text{Na}_3\text{PO}_4 + 3\text{CaCl}_2

ightarrow 6\text{NaCl} + \text{Ca}_3(\text{PO}_4)_2 \), Na is +1, P is +5, O is -2, Ca is +2, Cl is -1 throughout; no oxidation state change.

• Double replacement: Ions swap partners. \( \text{Na}^+ \) swaps with \( \text{Ca}^{2+} \), \( \text{PO}_4^{3 - } \) swaps with \( \text{Cl}^- \), forming new compounds (NaCl and \( \text{Ca}_3(\text{PO}_4)_2 \)), matching the double - replacement pattern (\( \text{AB}+\text{CD}

ightarrow\text{AD}+\text{CB} \)).

• Single replacement: One element replaces another in a compound, not the case here (no single element replacing another).

Answer:

C. thermal decomposition

Question 19