Title: 4. REACTION CLASSIFICATION. 4.1. Single-replacement
Course: Chemist€ry

Now, we already know that “CHAMELEON video”, from previous scenario, Chemist€ry.I, is not a magical trick. Let’s help our ancestors to classify chemical reactions solving a final che-mistery question regarding “elixir of life” or the “Legendary potion” (at point 1 of Chemist€ry.I).

1. REACTION CLASSIFICATION

4.1 Single-replacement reaction.

When one element is substituted for another element in a compound, generating a new element and a new compound as products. For instance,

2HCl(aq) + Mg(s) → MgCl_2(aq) + H_2(g)

2NaBr(aq) + Cl_2(g) → 2NaCl(s) + Br_2(g)

At the single-replacement reaction there is one element as a reactant and another element as a product.

But not all single-replacement reactions will occur, only elements on top of the column are able to replace the elements below them on the periodic table but not the other way around. For example, the elements called halogens (F, Cl, Br, I) follow a pattern, which is:

Fluorine is able to replace Chlorine, Bromine and Iodine.

Chlorine is able to replace Bromine and Iodine but not Fluorine.

Bromine is able to replace Iodine but not Fluorine neither Chlorine.

Thus, the reaction represented by

MgBr_2(s) + Cl_2(g) → MgCl_2(s) + Br_2(ℓ) will occur, but the reaction MgBr_2(s) + I_2(s) → MgI_2(s) + Br_2(ℓ) will not.

When replacing anions (ions negatively charged), reactivity trends are easy to predict using their relative positions on the periodic table.

However, replacing cations (ions positively charged), chemical reactivity trends are more complicated because there are much more elements that can form cations. A list of activity series for single-replacement reactions is shown below.

Activity Series for Cation Replacement in Single-Replacement Reactions^5

- Li
- K
- Ba
- Sr
- Ca
- Na
- Mg
- Al
- Mn
- Zn
- Cr
- Fe
- Ni
- Sn
- Pb
- H_2
- Cu
- Hg
- Ag
- Pd
- Pt
- Au

An element on top will replace an element below it in compounds undergoing a single-replacement reaction. Elements will not replace elements above them in compounds.

Example

Use the activity series to predict the products, if any, of each equation. Fill the gaps, if need it.

- SnCl_2 + Fe →  _________? + __________?
- SnCl_2 + Au →  _________? + __________?

Solution

- Because iron is above tin, it will replace tin in the compound. The products of this single-replacement reaction are FeCl_2 and Sn.
- Gold is below tin in the activity series. No reaction is predicted.

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