### This is an **oxidation-reduction** (redox) reaction:

* 6 HI + 6 e- → 6 H0 (reduction)*

*6 HI + 6 e- → 6 H0 (reduction)*

* 3 Fe0 - 6 e- → 3 FeII (oxidation)*

*3 Fe0 - 6 e- → 3 FeII (oxidation)*

*H3PO4* is an **oxidizing** agent, *Fe* is a **reducing** agent.

*H3PO4*

*Fe*

### Input interpretation

* Fe + H3PO4 → Fe3(PO4)2 + H2*

**Iron phosphoric acid hydrogen**

### Balanced equation

Balance the chemical equation algebraically:

* Fe + H3PO4 → Fe3(PO4)2 + H2*

Add stoichiometric coefficients, c, to the reactants and products:

* c1Fe + c2H3PO4 → c3 Fe3(PO4)2 + c4H2*

Set the number of atoms in the reactants equal to the number of atoms in the products for **Fe, H, O and P:**

*Fe: C1 = 3c3*

*H: 3c2=2c4*

* O: 4c2=8c3*

* P: c2 = 2c3*

Since the coefficients are relative quantities and underdetermined, choose a coefficient to set arbitrarily. To keep the coefficients small, the arbitrary value is ordinarily one. For instance, set **c3 = 1 **and solve the system of equations for the remaining coefficients:

* C1 = 3 C2=2*

* C3 = 1 C4=3*

Substitute the coefficients into the chemical reaction to obtain the balanced

equation:

**Answer:**

* Fe + H3PO4 → Fe3(PO4)2 + H2*