Some photographic film is coated with crystals of AgBr suspended in gelatin. Some of the silver

step 1 The equilibrium constant for this reaction can be determined by first looking up the equilibrium

Some photographic film is coated with crystals of AgBr...

Some photographic film is coated with crystals of $\mathrm{AgBr}$ suspended in gelatin. Some of the silver ions are reduced to silver metal upon exposure to light. Unexposed $\mathrm{AgBr}$ is then dissolved with sodium thiosulfate in the fixing step. $$ \begin{aligned} & \mathrm{AgBr}(\mathrm{s})+2 \mathrm{~S}_2 \mathrm{O}_3{ }^{2-}(\mathrm{aq}) \rightleftharpoons \\ & {\left[\mathrm{Ag}\left(\mathrm{S}_2 \mathrm{O}_3\right)_2\right]^{3-}(\mathrm{aq})+\mathrm{Br}^{-}(\mathrm{aq})} \\ & \end{aligned} $$ (a) What is the equilibrium constant for this reaction? (b) What mass of $\mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3$ must be added to dissolve $1.00 \mathrm{~g}$ of $\mathrm{AgBr}$ suspended in $1.00 \mathrm{~L}$ of water?

Some photographic film is coated with crystals of $\mathrm{AgBr}$ suspended in gelatin. Some of the silver ions are reduced to silver metal upon exposure to light. Unexposed $\mathrm{AgBr}$ is then dissolved with sodium thiosulfate in the fixing step.
$$
\begin{aligned}
& \mathrm{AgBr}(\mathrm{s})+2 \mathrm{~S}_2 \mathrm{O}_3{ }^{2-}(\mathrm{aq}) \rightleftharpoons \\
& {\left[\mathrm{Ag}\left(\mathrm{S}_2 \mathrm{O}_3\right)_2\right]^{3-}(\mathrm{aq})+\mathrm{Br}^{-}(\mathrm{aq})} \\
&
\end{aligned}
$$
(a) What is the equilibrium constant for this reaction?
(b) What mass of $\mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3$ must be added to dissolve $1.00 \mathrm{~g}$ of $\mathrm{AgBr}$ suspended in $1.00 \mathrm{~L}$ of water?

Key Concepts

Chemical Equilibrium

Chemical equilibrium describes a state in which the forward and reverse reactions occur at the same rate, leading to constant concentrations of reactants and products over time. It is a fundamental concept in chemistry that helps predict how a system will respond to changes in concentration, pressure, or temperature.

Equilibrium Constant Expression for Heterogeneous Reactions

For reactions involving more than one phase, such as solids and aqueous species, the equilibrium constant (K) is defined by including only the concentrations of species in the solution or gas phase. Solids and pure liquids are omitted because their activities are constant. This simplification is essential when writing the equilibrium expressions for reactions like dissolution or precipitation.

Stoichiometry in Reaction Calculations

Stoichiometry involves the quantitative relationships between reactants and products in a chemical reaction. It uses the mole concept to convert between mass and moles and apply the ratios derived from the balanced chemical equation. This concept is crucial for calculating necessary reactant amounts or the expected yield of a product.

Complex Ion Formation

Complex ion formation refers to the process where central metal ions bond with surrounding ligands to form a complex. The formation of complex ions can alter solubility and shift equilibria, making it a valuable tool in chemical processes, such as selectively dissolving a sparingly soluble salt by forming a soluble complex.

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