Why does this equation account for the fact that the gas volume is unchanged?
H2Se + Sn ==> SnSe + H2
H2Se and H2 are gases. Sn and SnSe are solids. 1 mol gas and 1 mol of solid on the left. 1 mol gas and 1 mol solid on the right. Volume doesn't change.
The equation you provided,
H2Se + Sn ==> SnSe + H2,
accounts for the fact that the gas volume is unchanged because the number of moles of gaseous reactants and gaseous products remains the same.
In this equation, you have one mole of gas, H2, as a reactant on the left side, and one mole of gas, H2, as a product on the right side. This means that the total number of moles of gas remains constant throughout the reaction.
Similarly, on the left side, you have one mole of H2Se, a gas, and on the right side, you have SnSe, a solid. Since the reaction involves the conversion of one mole of gaseous H2Se to one mole of solid SnSe, the number of moles of gaseous reactant decreases while the number of moles of solid product increases by the same amount.
Therefore, even though there is a change in the state of matter from gas to solid, the overall volume of the system remains unchanged because the decrease in gas volume is compensated by the increase in solid volume.
To summarize, the equation accounts for the fact that the gas volume is unchanged because the number of moles of gaseous reactants and products is balanced, resulting in no net change in the overall volume of the system.