Final answer:
The presence of a magnesium oxide layer on magnesium metal would cause a negative effect on the calculated experimental hydrogen generation (rexp) since it acts as a protective shield that hinders the reaction between magnesium and hydrochloric acid.
Step-by-step explanation:
If magnesium has a coating of magnesium oxide on its surface, the reactivity of the magnesium with hydrochloric acid (HCl) to produce hydrogen gas could be less than expected. The presence of magnesium oxide (MgO) acts as a protective layer, which inhibits the reaction between the pure magnesium (Mg) and HCl, hence the calculated experimental hydrogen generation (rexp) would likely be lower than the theoretical yield. Therefore, the effect would be negative (-), because the magnesium oxide prevents the direct contact between magnesium and HCl, thus slowing or preventing the reaction from occurring efficiently.
In a typical reaction setup without the oxide coating, magnesium reacts with hydrochloric acid to form magnesium chloride (MgCl₂) and hydrogen gas (H₂) as shown:
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
Without the oxide coating, magnesium would react more readily, producing hydrogen gas as observed by the disappearance of the solid magnesium and the evolution of gas bubbles. However, the presence of magnesium oxide changes this behavior, leading to a decreased rate of reaction and a decreased volume of hydrogen gas produced. It's worth noting that the reaction can still proceed, but the initial rate of reaction would be lower until enough MgO is removed to expose the pure Mg to the acid.