What are the products when calcium hydride, CaH₂, reacts with water?
ACaO and H₂O
BCa(OH)₂ and H₂
CCa and H₂O₂
DCaH₂ does not react with water
The H⁻ ion is a strong base and a reducing agent. It takes a proton from water to form H₂: CaH₂ + 2H₂O → Ca(OH)₂ + 2H₂. This is why saline hydrides are used as drying agents and as sources of hydrogen.
Question 2 True / False
In sodium hydride, NaH, hydrogen has an oxidation state of +1.
TTrue
FFalse
Answer: False
Sodium is far more electropositive than hydrogen, so it gives up its electron and hydrogen becomes the hydride ion H⁻, with oxidation state −1.
Question 3 Multiple Choice
Which of these is best classified as a metallic (interstitial) hydride?
ALiH
BPdHₓ
CH₂S
DB₂H₆
Palladium absorbs large amounts of hydrogen into holes in its metal lattice, giving a nonstoichiometric solid that keeps metallic character. LiH is saline, and H₂S and B₂H₆ are molecular.
Question 4 Short Answer
Water has a much higher boiling point than hydrogen sulfide even though H₂O has the lower molar mass. Explain why.
Think about your answer, then reveal below.
Model answer: Oxygen is small and highly electronegative, so water molecules form strong hydrogen bonds with one another. Sulfur is larger and less electronegative, so H₂S molecules interact mainly through weaker dipole and London forces.
The same effect makes NH₃ and HF boil far above the trend set by the heavier hydrides of their groups. Without hydrogen bonding, water would be expected to boil well below 0 °C.