Question #08090

1 Answer
Jul 13, 2014

The reason is that the positive part of one compound must combine with the negative part of the other compound.

This is an example of a double replacement reaction.

The more positive part of a compound comes first in the formula.

We could write the reaction between AB +CD → ? as

A⁺B⁻ + C⁺D⁻ → ?

When the positive ions change partners, A⁺ combines with D⁻, and C⁺ combines with B⁻.

The reaction is then

A⁺B⁻ + C⁺D⁻ → A⁺D⁻ + C⁺B⁻ or AB + CD → AD + CB

The formula BC would say incorrectly that B is the positive part of the compound.

Thus, in your reaction,

2Na⁺,2Cl⁻(aq) + Hg₂²⁺,2NO₃⁻ (aq) → 2Na⁺, 2NO₃⁻(aq) + Hg₂²⁺,2Cl⁻(s)