How many electrons does a single oxygen gain or lose in the following reaction?
h 2 + o 2 → h 2 o

it gains 2 electrons

Explanation:

atoms want to be more stable, for oxygen it's number is 8, 2 less than the stable ring of 10. and hydrogen has a single electron, 1 less than the stable ring of 2. so oxygen takes 2 electrons 2 make it stable and hydrogen becomes attatched to the oxygen atom to form a covalent bond

answer: enthalpy

explanation:

for a exothermic reaction, the energy is released i.e. the energy of the products is lower than the energy of the reactants.

= enthaply

= energy of products

= energy of reactants

as , thus comes out to be negative. negative value of enthalpy shows that the reaction is exothermic.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.