Do you have a redox equation you don't know how to balance? Besides simply balancing the equation in question, these programs will also give you a detailed overview of the entire balancing process with your chosen method.

- Ion-electron method (also called the half-reaction method)
- Oxidation number change method
- Aggregate redox species method (or ARS method) - New on periodni.com [1]

by ARS method

The aggregate redox species method, or the ARS method in short, is an improved oxidation number change method that successfully solves even reactions that cannot be 'cleanly' separated into partial reactions of oxidation and reduction.

* Step 1. Write the unbalanced equation* ('skeleton equation') containing all of the reactants and products of the chemical reaction. Redox equations that need to be balanced can often be written without water molecules, H

Ca_{3}(PO_{4})_{2}(s) + H_{2}SO_{4}(aq)
→
H_{3}PO_{4}(aq) + CaSO_{4}(s)

* Step 2. Identify the redox couples in the reaction*.

* a) Assign the oxidation numbers* for each atom in the equation (see: Rules for assigning oxidation numbers). The use of the oxidation numbers greatly simplifies identifying which element in a reaction is oxidized and which element is reduced.

Ca+2_{3}(P+5O-2_{4})_{2} + H+1_{2}S+6O-2_{4}
→
H+1_{3}P+5O-2_{4} + Ca+2S+6O-2_{4}

* b) Identify and write out all redox couples* of atoms that have been oxidized (their oxidation number has increased) or reduced (their oxidation number has decreased). Determine the transfer of electrons for each redox couple, but make sure that the number of atoms that have been oxidized (or reduced) is equal on both sides of the equation. If necessary, write down the stoichiometric coefficients in front of the species.

Missing oxidation reaction

( )

Missing reduction reaction

( )

* Step 3. Aggregate the redox species* into one equation (ARS equation). Species with redox atoms are combined in such a way that all redox atoms are balanced in the ARS equation.

This is not a redox reaction. The ARS equation doesn't have any members.

* Step 4. Balance the charges and other atoms.* The non-redox species are added to the ARS equation and the atoms and charges are balanced. Adding species without redox atoms into the 'empty' ARS equation will result in the skeleton equation.

Ca_{3}(PO_{4})_{2} + 3H_{2}SO_{4}
→
2H_{3}PO_{4} + 3CaSO_{4}

* Step 5. Simplify the equation.* The same species on opposite sides of the arrow can be canceled. If necessary, the whole equation can be divided with the largest common divisor in order to make the coefficients as small as possible.

Ca_{3}(PO_{4})_{2} + 3H_{2}SO_{4}
→
2H_{3}PO_{4} + 3CaSO_{4}

* Step 6. Check the balance of charges and elements.* Like any chemical reaction, a redox reaction must be balanced by mass and charge. Check if the sum of each type of atom on one side of the equation is equal to the sum of the same atoms on the other side. Check if the sum of electrical charges on the left side of the equation is equal to those on the right side. It doesn't matter what the sum of the charges is as long as it's equal on both sides.

ELEMENT | LEFT | RIGHT | DIFFERENCE |
---|---|---|---|

Ca | 1*3 | 3*1 | 0 |

P | 1*2 | 2*1 | 0 |

O | 1*8 + 3*4 | 2*4 + 3*4 | 0 |

H | 3*2 | 2*3 | 0 |

S | 3*1 | 3*1 | 0 |

Charge | 0 | 0 | 0 |

Since the sum of individual atoms on the left side of the equation matches the sum of the same atoms on the right side, and since the charges on both sides are equal, we can write a balanced equation.

Ca_{3}(PO_{4})_{2} + 3H_{2}SO_{4} → 2H_{3}PO_{4} + 3CaSO_{4}

- E. Generalić, N. Vladislavić, Aggregate Redox Species Method - An Improved Oxidation Number Change Method for Balancing Redox Equations,
*Chemistry Journal*, Vol.**4**, No. 3, 43-49 (2018)

Citing this page:

Generalic, Eni. "Balancing redox reactions by aggregate redox species method." *EniG. Periodic Table of the Elements*. KTF-Split, 18 Jan. 2024. Web. {Date of access}. <https://www.periodni.com/ars_method.php>.

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