In the reaction, H2+I2—>2HI, why is I2 used in large excess?
In the reaction H2 + I2 → 2HI, I2 is used in large excess to ensure that the reaction goes to completion and to increase the yield of the desired product, HI. This concept is based on the principle of Le Chatelier's principle and the reaction's equilibrium.
To understand this, let's first consider the balanced chemical equation:
H2 + I2 → 2HI
This equation represents a reaction between hydrogen gas (H2) and iodine gas (I2) to produce hydrogen iodide (HI). In theory, for every 1 mole of H2, 1 mole of I2 would be needed to form 2 moles of HI. However, in practice, it is usually beneficial to add an excess of one of the reactants, which in this case is I2.
By adding I2 in large excess, several advantages are gained:
1. Favoring the forward reaction: Since the reaction is reversible, meaning HI can also decompose back into H2 and I2, adding excess I2 shifts the equilibrium towards the right, favoring the formation of HI. According to Le Chatelier's principle, if one of the reactants is present in excess, the equilibrium will shift to utilize more of that reactant and produce more of the product. In this case, the excess I2 prevents the reverse reaction from occurring and helps maximize the amount of HI formed.
2. Higher reaction rate: Excess I2 ensures that there is always a sufficient amount of I2 available for the reaction to occur. This leads to a higher reaction rate because the likelihood of successful collisions between the reactant molecules (H2 and I2) is increased. Without excess I2, the reaction rate could be slower due to limited availability of I2 molecules.
3. Increased product yield: By adding excess I2, the production of HI is maximized. This means that the reaction will result in a higher yield of HI, resulting in more product being produced. Without the excess I2, the amount of HI formed would be limited by the amount of I2 available, which could reduce the overall yield.
In summary, adding I2 in large excess ensures that the reaction goes to completion, shifts the equilibrium towards the formation of HI, increases the reaction rate, and ultimately leads to a higher yield of the desired product.