Question

What is the balanced chemical equation for developing photographic films?
(A) $2\,AgBr\, + \,2N{a_2}{S_2}{O_3}\, \to \,N{a_3}[A{g_2}{({S_2}{O_3})_2}]\, + \,2NaBr$
(B) $AgBr\, + \,N{a_2}{S_2}{O_3}\, \to \,Na[Ag{({S_2}{O_3})_2}]\, + \,NaBr$
(C) $AgBr\, + \,2N{a_2}{S_2}{O_3}\, \to \,N{a_3}[A{g_2}{({S_2}{O_3})_2}]\, + \,NaBr$
(D) $2AgBr\, + \,2N{a_2}{S_2}{O_3}\, \to \,N{a_2}[A{g_2}{({S_2}{O_3})_2}]\, + \,2NaBr$

Answer 1

A chemical equation is a written representation of any chemical reaction having reactants written on the left side and products on the right side. A given chemical equation must be balanced that means the no of atoms of a given element is equal on both the reactant and product side.

Complete Step-by-step Solution:
A chemical equation needs to be balanced because of the law of conservation of matter and there is the only rearrangement of atoms in a chemical reaction neither new atoms are created nor destroyed so the number of atoms of any element taking part in the reaction needs to be equal on the reactant and product side. The most widely used method for balancing an equation is the trial and error method as there is no specific formula-based method for balancing equations. There are a few key steps in balancing an equation. They are as:
Identify the most complex substance, begin with that substance, and choose one element that appears in only one reactant and product. Adjust the stoichiometric coefficients of those molecules to obtain the same number of atoms on both sides and do the same with the rest of the elements.
In the given reaction silver bromide is reacting with sodium thiosulphate to give sodium argento thiosulphate and sodium bromide. The formula of sodium argento thiosulphate is $N{a_3}[Ag{({S_2}{O_3})_2}]\,$ . We first used $2$ as the coefficient on both the reactant and calculated that we only need to change the coefficient of sodium thiosulphate as $2$ and we get the whole reaction balanced.
Hence, the correct option is (C).

NOTE:
A Photographic film is a strip or sheet of transparent film base which is coated on one side with an emulsion of gelatine that contains very small crystals of silver halide that are very light sensitive. The sensitivity, contrast, and resolution of the film depend on the size of the film.