Excess Water in the Reaction of Calcium Phosphide

When one mole of calcium phosphide (Ca3P2) reacts with an excess of water (H2O), it undergoes a chemical reaction to produce phosphine gas (PH3) and calcium hydroxide (Ca(OH)2).

The balanced chemical equation for this reaction is as follows:
Ca3P2 + 6H2O -> 2PH3 + 3Ca(OH)2

Now, let's discuss the concept of excess water and why it is represented as 6H2O in the equation.

Excess Water
In a chemical reaction, reactants are often written with stoichiometric coefficients representing the ratio in which they react. However, when one of the reactants is in excess, it means that there is more than enough of it to react completely with the other reactant.

In the given equation, excess water implies that a large excess of water is used in the reaction. This means that there is an abundant amount of water compared to the amount required for the reaction to go to completion.

Stoichiometric Coefficients
The stoichiometric coefficients in a balanced chemical equation represent the molar ratios between the reactants and products. These coefficients indicate the number of moles of each substance involved in the reaction.

In the case of excess water, the coefficient of 6H2O indicates that six moles of water are used for each mole of calcium phosphide. This ratio is chosen based on the stoichiometry of the reaction and is necessary to ensure that all of the calcium phosphide is consumed.

Alternative Stoichiometric Coefficients
In some cases, the stoichiometric coefficients can be different depending on the specific reaction conditions or the desired outcome. However, in this particular reaction, the coefficient of 6H2O is appropriate to ensure complete reaction with calcium phosphide.

It is important to note that stoichiometric coefficients should always be in their simplest whole number ratio. In this case, any other number for the coefficient of water would not result in a balanced equation or complete reaction.

Conclusion
In summary, excess water is represented as 6H2O in the reaction between calcium phosphide and water. This indicates that six moles of water are used for each mole of calcium phosphide to ensure complete reaction. The stoichiometric coefficients in a balanced chemical equation are chosen based on the molar ratios between the reactants and products, and they should be in their simplest whole number ratio.