# Cus + 10 hno3 → cu(no3)2 + h2so4 + 8 no2 + 4 h2o

In the oxidation number change method the underlying principle is that the gain in the oxidation number (number of electrons) in one reactant must be equal lớn the loss in the oxidation number of the other reactant.

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Step 1. Write down the unbalanced equation ("skeleton equation") of the chemical reaction. All reactants & products must be known. For a better result write the reaction in ionic size.

Step 2. Separate the process into lớn half reactions. A redox reaction is nothing but both oxidation and reduction reactions taking place simultaneously.

a) Assign oxidation numbers for each atom in the equation. Oxidation number (also called oxidation state) is a measure of the degree of oxidation of an atom in a substance (see: Rules for assigning oxidation numbers).

b) Identify & write out all redox couples in reaction. Identify which reactants are being oxidized (the oxidation number increases when it reacts) and which are being reduced (the oxidation number goes down). Write down the transfer of electrons. Carefully, insert coefficients, if necessary, to lớn make the numbers of oxidized and reduced atoms equal on the two sides of each redox couples.

c) Combine these redox couples inlớn two half-reactions: one for the oxidation, và one for the reduction (see: Divide the redox reaction inlớn two half-reactions).

Step 3. Balance the atoms in each half reaction. A chemical equation must have the same number of atoms of each element on both sides of the equation. Add appropriate coefficients (stoichiometric coefficients) in front of the chemical formulas to lớn balance the number of atoms. Never change any formulas.

a) Balance all other atoms except hydrogene & oxyren. We can use any of the species that appear in the skeleton equations for this purpose. Keep in mind that reactants should be added only lớn the left side of the equation và products khổng lồ the right.

b) Balance the charge. For reactions in an acidic solution, balance the charge so that both sides have sầu the same total charge by adding a H+ ion to lớn the side deficient in positive sầu charge.

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c) Balance the oxygene atoms. Cheông xã if there are the same numbers of oxygene atoms on the left and right side, if they aren"t equilibrate these atoms by adding water molecules.

Balanced half-reactions are well tabulated in handbooks & on the web in a "Tables of standard electrode potentials". These tables, by convention, contain the half-cell potentials for reduction. To make the oxidation reaction, simply reverse the reduction reaction & change the sign on the E50% value.

Step 4. Make electron gain equivalent khổng lồ electron lost. The electrons lost in the oxidation half-reaction must be equal the electrons gained in the reduction half-reaction. To make the two equal, multiply the coefficients of all species by integers producing the lowest common multiple between the half-reactions.

Step 5. Add the half-reactions together. The two half-reactions can be combined just like two algebraic equations, with the arrow serving as the equals sign. Recombine the two half-reactions by adding all the reactants together on one side and all of the products together on the other side.

Step 6. Simplify the equation. The same species on opposite sides of the arrow can be canceled. Write the equation so that the coefficients are the smallest phối of integers possible.

Finally, always check lớn see that the equation is balanced. First, verify that the equation contains the same type and number of atoms on both sides of the equation.

Second, verify that the sum of the charges on one side of the equation is equal khổng lồ the sum of the charges on the other side. It doesn"t matter what the charge is as long as it is the same on both sides.

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