Question

1. choose the best answer. which of these is not a double replacement reaction? \\( 2\text{na}_3\text{po}_4 + 3\text{cacl}_2 \

ightarrow 6\text{nacl} + \text{ca}_3(\text{po}_4)_2 \\) \\( 2\text{hcl}+\text{caco}_3 \
ightarrow \text{cacl}_2 + \text{h}_2\text{o} + \text{co}_2 \\) \\( \text{h}_3\text{po}_4 + 3\text{koh} \
ightarrow \text{k}_3\text{po}_4 + 3\text{h}_2\text{o} \\) \\( \text{h}_2\text{so}_4 + 2\text{nano}_2 \
ightarrow 2\text{hno}_2 + \text{na}_2\text{so}_4 \\)

Explanation:

Step1: Recall double replacement reaction

A double replacement reaction is a type of chemical reaction where two compounds exchange ions to form two new compounds, generally in the form \(AB + CD ightarrow AD + CB\), and often involves the formation of a precipitate, water, or a gas (but for gas, sometimes it's a result of a decomposition of an intermediate).

Step2: Analyze each reaction

- **First reaction (\(2\ce{Na3PO4}+3\ce{CaCl2} ightarrow6\ce{NaCl}+\ce{Ca3(PO4)2}\))**:
Sodium and calcium ions exchange places (phosphate and chloride also exchange). So, it is a double replacement (ions from \(\ce{Na3PO4}\) and \(\ce{CaCl2}\) swap to form \(\ce{NaCl}\) and \(\ce{Ca3(PO4)2}\)).
- **Second reaction (\(2\ce{HCl}+\ce{CaCO3} ightarrow\ce{CaCl2}+\ce{H2O}+\ce{CO2}\))**:
Let's break it down. First, \(2\ce{HCl}+\ce{CaCO3}\) can be seen as a double replacement forming \(\ce{CaCl2}\) and \(\ce{H2CO3}\) (since \(H^+\) from \(\ce{HCl}\) and \(Ca^{2+}\) from \(\ce{CaCO3}\) swap, \(Cl^-\) and \(CO_3^{2 - }\) swap). But \(\ce{H2CO3}\) decomposes into \(\ce{H2O}\) and \(\ce{CO2}\). However, the initial step is a double replacement? Wait, no—actually, this is an acid - carbonate reaction. But let's check the other reactions too.
- **Third reaction (\(\ce{H3PO4}+3\ce{KOH} ightarrow\ce{K3PO4}+3\ce{H2O}\))**:
This is an acid - base neutralization reaction, which is a type of double replacement reaction. The \(H^+\) from \(\ce{H3PO4}\) and \(K^+\) from \(\ce{KOH}\) swap, and \(OH^-\) and \(PO_4^{3 - }\) swap to form \(\ce{K3PO4}\) and \(\ce{H2O}\) (water is formed). So it is a double replacement.
- **Fourth reaction (\(\ce{H2SO4}+2\ce{NaNO2} ightarrow2\ce{HNO2}+\ce{Na2SO4}\))**:
Here, \(H^+\) from \(\ce{H2SO4}\) and \(Na^+\) from \(\ce{NaNO2}\) swap, \(SO_4^{2 - }\) and \(NO_2^-\) swap to form \(\ce{HNO2}\) and \(\ce{Na2SO4}\). So it is a double replacement.

Now, re - examining the second reaction: \(2\ce{HCl}+\ce{CaCO3} ightarrow\ce{CaCl2}+\ce{H2O}+\ce{CO2}\). The reaction of an acid with a carbonate produces a salt, water, and carbon dioxide. The key here is that the formation of \(\ce{CO2}\) comes from the decomposition of \(\ce{H2CO3}\) (which would be the product of the double replacement of \(2\ce{HCl}+\ce{CaCO3}\) to form \(\ce{CaCl2}+\ce{H2CO3}\)). But actually, this reaction is a combination of a double replacement (to form \(\ce{H2CO3}\)) and a decomposition (of \(\ce{H2CO3}\) into \(\ce{H2O}\) and \(\ce{CO2}\)). However, the other reactions are straightforward double replacements, while this one has an additional decomposition step. But wait, no—let's think again. A double replacement reaction typically forms two compounds. In this case, the products are three substances (\(\ce{CaCl2}\), \(\ce{H2O}\), \(\ce{CO2}\)), while the other reactions form two compounds. Wait, no—the first reaction forms two compounds, the third forms two (a salt and water), the fourth forms two. The second reaction forms three. But actually, the correct way: a double replacement reaction is \(AB + CD ightarrow AD + CB\). Let's write the second reaction as \(2\ce{HCl}+\ce{CaCO3}\). If we consider \(\ce{CaCO3}\) as \(CaO\cdot CO_2\), but better to use ion exchange. \(\ce{HCl}\) is \(H^+ + Cl^-\), \(\ce{CaCO3}\) is \(Ca^{2+}+CO_3^{2 - }\). So swapping \(H^+\) with \(Ca^{2+}\) and \(Cl^-\) with \(CO_3^{2 - }\) would give \(CaCl_2\) and \(H_2CO_3\). Then \(H_2CO_3\) decomposes to \(H_2O + CO_2\). So the overall reaction is a double replacement followed by a decomposition. But the other reactions are pure double replacements (forming two compounds). Wait, no—the third reaction is acid - base, which is double replacement (forms a salt and water, two compounds). The fourth forms two compounds. The first forms two compounds. The second forms three substances. But actually, the reaction \(2\ce{HCl}+\ce{CaCO3} ightarrow\ce{CaCl2}+\ce{H2O}+\ce{CO2}\) is not a double replacement reaction in the strict sense because a double replacement should produce two compounds, and here we have three products due to the decomposition of the intermediate carbonic acid. Wait, no—carbonic acid decomposes, but the initial step is double replacement. Wait, maybe I made a mistake. Let's check the definition again. A double - replacement reaction is a chemical reaction where the cations and anions of two ionic compounds exchange places to form two new compounds. In the reaction \(2\ce{HCl}+\ce{CaCO3}\), \(\ce{HCl}\) is a strong acid (ionizes into \(H^+\) and \(Cl^-\)), \(\ce{CaCO3}\) is a salt (ionizes into \(Ca^{2+}\) and \(CO_3^{2 - }\) in solution, though it's sparingly soluble). So the ion exchange would be \(H^+\) (from \(\ce{HCl}\)) and \(Ca^{2+}\) (from \(\ce{CaCO3}\)) swap, \(Cl^-\) (from \(\ce{HCl}\)) and \(CO_3^{2 - }\) (from \(\ce{CaCO3}\)) swap. So the initial products should be \(CaCl_2\) and \(H_2CO_3\) (which is a compound). But \(H_2CO_3\) is unstable and decomposes into \(H_2O\) and \(CO_2\). So the overall reaction has three products, while the other reactions have two products (even if one is water, like in the acid - base reaction). Wait, the acid - base reaction (\(\ce{H3PO4}+3\ce{KOH}\)) forms \(\ce{K3PO4}\) (a salt) and \(H_2O\) (two compounds). The reaction \(2\ce{HCl}+\ce{CaCO3}\) forms \(\ce{CaCl2}\) (a salt), \(H_2O\) (a compound), and \(CO_2\) (a gas, not a compound from the double replacement). So the key is that in a double replacement, we should have two new compounds. In this case, the second reaction has three products, so it's not a double replacement (or it's a double replacement plus a decomposition, but the question is which is NOT a double replacement). So the reaction \(2\ce{HCl}+\ce{CaCO3} ightarrow\ce{CaCl2}+\ce{H2O}+\ce{CO2}\) is not a double replacement reaction (it's an acid - carbonate reaction which is a type of reaction that includes a double replacement and a decomposition, but the other reactions are pure double replacements).

Answer:

The reaction \(2\ce{HCl}+\ce{CaCO3}
ightarrow\ce{CaCl2}+\ce{H2O}+\ce{CO2}\) (the second option) is NOT a double replacement reaction.