In the reaction \( \text{H}_2\text{Se} + \text{H}_2\text{O} \rightarrow \text{SeH}^- + \text{H}_3\text{O}^+ \), hydrogen selenide (\( \text{H}_2\text{Se} \)) acts as the acid because it donates a proton (H\(^+\)) to water. Water (\( \text{H}_2\text{O} \)) acts as the base because it accepts the proton from \( \text{H}_2\text{Se} \). The conjugate acid formed is hydronium (\( \text{H}_3\text{O}^+ \)), which is created when water accepts the proton, and the conjugate base is selenide (\( \text{SeH}^- \)), which is the result of \( \text{H}_2\text{Se} \) losing a proton.
In this reaction, \( \text{H}_2\text{Se} \) is a stronger acid because selenium is less electronegative than oxygen, making it more willing to donate a proton compared to water. This tendency conforms to the general trend of acid strength, where elements situated further left in their respective periods (like selenium compared to oxygen) are usually more likely to donate protons. Thus, the different electronegativities help explain the behavior of the acids and bases in this reaction.