Let's analyze the given chemical equations to identify which one shows a reversible reaction.
1. Equation 1:
[tex]\[ 2 Na + \bar{F}_2 \rightarrow 2 NaF \][/tex]
This equation represents a single direction reaction where sodium (Na) reacts with fluorine ([tex]\(\bar{F}_2\)[/tex]) to form sodium fluoride (NaF). The arrow ([tex]\(\rightarrow\)[/tex]) indicates that the reaction proceeds only in the forward direction.
2. Equation 2:
[tex]\[ CaCO_3 \rightarrow CaO + CO_2 \][/tex]
This equation shows the decomposition of calcium carbonate (CaCO_3) into calcium oxide (CaO) and carbon dioxide (CO_2). The single arrow ([tex]\(\rightarrow\)[/tex]) indicates that the reaction proceeds only in the forward direction.
3. Equation 3:
[tex]\[ NH_4Cl (s) \leftrightarrow NH_3(g) + HCl(g) \][/tex]
This equation clearly shows a reversible reaction. The double arrow ([tex]\(\leftrightarrow\)[/tex]) indicates that the reaction can proceed in both the forward and the reverse directions. Ammonium chloride (NH_4Cl) can decompose into ammonia (NH_3) and hydrogen chloride (HCl), and these products can also react to form ammonium chloride again.
4. Equation 4:
[tex]\[ 2 H_2O_2(aq) \xrightarrow{Bt} 2 H_2O(l) + O_2(g) \][/tex]
This equation shows the decomposition of hydrogen peroxide (H_2O_2) into water (H_2O) and oxygen (O_2), facilitated by a catalyst (Bt). The single arrow ([tex]\(\rightarrow\)[/tex]) indicates that the reaction proceeds only in the forward direction.
After analyzing all the equations, the correct answer is the third equation:
[tex]\[ NH_4Cl (s) \leftrightarrow NH_3(g) + HCl(g) \][/tex]
Thus, the equation that shows a reversible reaction is:
[tex]\[ \boxed{3} \][/tex]
