Question

coaching time

1. can you do it?

$\boldsymbol{2}$ h$_2$ + $\boldsymbol{1}$ o$_2$ $
ightarrow$ $\boldsymbol{2}$ h$_2$o

$\boldsymbol{2}$ na + $\boldsymbol{1}$ cl$_2$ $
ightarrow$ $\boldsymbol{2}$ nacl

1. a harder one!

_co$_2$ + _h$_2$o $
ightarrow$ ___c$_6$h$_{12}$o$_6$ + ___o$_2$

1. error analysis (whats wrong? fix it)

2 h$_2$ + o$_2$ $
ightarrow$ 2 h$_2$o$_2$

1. reaction types? (s, d, c, sr, dr)

$square$ zn + hcl $
ightarrow$ zncl$_2$ +h$_2$
$square$ agno$_3$ + nacl $
ightarrow$ agcl + nano$_3$
$square$ ch$_4$ + o$_2$ $
ightarrow$ co$_2$ + h$_2$o
$square$ caco$_3$ $
ightarrow$ cao + co$_2$

Explanation:

Step1: Balance photosynthesis reaction

Balance carbon first:

There are 6 C in $\text{C}_6\text{H}_{12}\text{O}_6$, so put 6 before $\text{CO}_2$:
$6\text{CO}_2 + \text{H}_2\text{O}
ightarrow \text{C}_6\text{H}_{12}\text{O}_6 + \text{O}_2$

Balance hydrogen next:

There are 12 H in $\text{C}_6\text{H}_{12}\text{O}_6$, so put 6 before $\text{H}_2\text{O}$:
$6\text{CO}_2 + 6\text{H}_2\text{O}
ightarrow \text{C}_6\text{H}_{12}\text{O}_6 + \text{O}_2$

Balance oxygen last:

Left side: $6\times2 + 6\times1 = 18$ O. Right side: 6 O in $\text{C}_6\text{H}_{12}\text{O}_6$, so need 12 more O from $\text{O}_2$, put 6 before $\text{O}_2$:
$6\text{CO}_2 + 6\text{H}_2\text{O}
ightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Step2: Fix incorrect reaction

The reaction $2\text{H}_2 + \text{O}_2
ightarrow 2\text{H}_2\text{O}_2$ is wrong because hydrogen and oxygen react to form water ($\text{H}_2\text{O}$), not hydrogen peroxide. Balance the correct reaction:
$2\text{H}_2 + \text{O}_2
ightarrow 2\text{H}_2\text{O}$

Step3: Classify reaction types

Reaction 1: $\text{Zn} + \text{HCl}

ightarrow \text{ZnCl}_2 + \text{H}_2$
A single element replaces another in a compound: Single Replacement (SR). First balance it:
$\text{Zn} + 2\text{HCl}
ightarrow \text{ZnCl}_2 + \text{H}_2$

Reaction 2: $\text{AgNO}_3 + \text{NaCl}

ightarrow \text{AgCl} + \text{NaNO}_3$
Ions swap between two compounds: Double Replacement (DR).

Reaction 3: $\text{CH}_4 + \text{O}_2

ightarrow \text{CO}_2 + \text{H}_2\text{O}$
A fuel reacts with oxygen to form $\text{CO}_2$ and $\text{H}_2\text{O}$: Combustion (C). Balance it:
$\text{CH}_4 + 2\text{O}_2
ightarrow \text{CO}_2 + 2\text{H}_2\text{O}$

Reaction 4: $\text{CaCO}_3

ightarrow \text{CaO} + \text{CO}_2$
One compound breaks into two: Decomposition (D).

Answer:

1. $6\text{CO}_2 + 6\text{H}_2\text{O}

ightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

1. The product is incorrect; the correct balanced reaction is $2\text{H}_2 + \text{O}_2

ightarrow 2\text{H}_2\text{O}$
4.

• $\text{Zn} + 2\text{HCl}

ightarrow \text{ZnCl}_2 + \text{H}_2$: SR (Single Replacement)

• $\text{AgNO}_3 + \text{NaCl}

ightarrow \text{AgCl} + \text{NaNO}_3$: DR (Double Replacement)

• $\text{CH}_4 + 2\text{O}_2

ightarrow \text{CO}_2 + 2\text{H}_2\text{O}$: C (Combustion)

• $\text{CaCO}_3

ightarrow \text{CaO} + \text{CO}_2$: D (Decomposition)