Question

balance the following equation: _ $p_4$ + _ $o_2$ → ___ $p_2o_5$ *
$p_4 + 5o_2 → 2p_2o_5$
$p_4 + 3o_2 → 2p_2o_5$
$4p_4 + 5o_2 → p_2o_5$
$2p_4 + 5o_2 → 2p_2o_5$
which of the following below is a single replacement reaction? *
$cl_2 + 2nabr → 2nacl + br_2$
$agno_3 + nacl → agcl + nano_3$
$caco_3 → cao + co_2$
$cao + h_2o → ca(oh)_2$

Explanation:

First Sub - Question (Balancing the Chemical Equation)

Step1: Analyze P atoms

On the left, we have \( P_4 \), so the number of P atoms is 4. On the right, in \( P_2O_5 \), each \( P_2O_5 \) has 2 P atoms. Let the coefficient of \( P_2O_5 \) be \( x \) and the coefficient of \( P_4 \) be \( y \). We have \( 4y=2x \), or \( 2y = x \).

Step2: Analyze O atoms

On the left, \( O_2 \) has 2 O atoms. Let the coefficient of \( O_2 \) be \( z \). On the right, \( P_2O_5 \) has 5 O atoms, so the number of O atoms on the right is \( 5x \). The number of O atoms on the left is \( 2z \), so \( 2z = 5x \).

Step3: Test the options

• Option 1: \( P_4+5O_2

ightarrow2P_2O_5 \)

• For P: Left - 4, Right - \( 2\times2 = 4 \) (balanced).
• For O: Left - \( 5\times2=10 \), Right - \( 2\times5 = 10 \) (balanced).
• Option 2: \( P_4 + 3O_2

ightarrow2P_2O_5 \)

• O: Left - \( 3\times2 = 6 \), Right - \( 2\times5=10 \) (not balanced).
• Option 3: \( 4P_4+5O_2

ightarrow P_2O_5 \)

• P: Left - \( 4\times4 = 16 \), Right - 2 (not balanced).
• Option 4: \( 2P_4+5O_2

ightarrow2P_2O_5 \)

• P: Left - \( 2\times4 = 8 \), Right - \( 2\times2=4 \) (not balanced).

• A single - replacement reaction has the form \( A + BC

ightarrow AC + B \) (where A and B are elements, and BC is a compound).

• Option 1: \( Cl_2+2NaBr

ightarrow2NaCl + Br_2 \)

• Here, \( Cl_2 \) (element) replaces \( Br \) in \( NaBr \) (compound) to form \( NaCl \) and \( Br_2 \) (element). This follows the single - replacement pattern.
• Option 2: \( AgNO_3+NaCl

ightarrow AgCl + NaNO_3 \)

• This is a double - replacement reaction (ions swap: \( Ag^+ \) and \( Na^+ \) swap anions \( NO_3^- \) and \( Cl^- \)).
• Option 3: \( CaCO_3

ightarrow CaO + CO_2 \)

• This is a decomposition reaction (a compound breaks into simpler substances).
• Option 4: \( CaO + H_2O

ightarrow Ca(OH)_2 \)

• This is a combination reaction (substances combine to form a single compound).

Answer:

\( \boldsymbol{P_4 + 5O_2
ightarrow2P_2O_5} \) (the first option)

Second Sub - Question (Identifying Single - Replacement Reaction)