Pls help ASAP ! Identify the following reaction: H2 + O2 → H2O Group of answer choices

synthesis reaction

decomposition reaction

single replacement reaction

double replacement reaction

Synthesis Reaction

Explanation:

Here we see H2 and O2 are being combined H2O. Synthesis reactions are where we see a combination to make a single product.

So here it'd be A + B =AB

2Na(s)+Cl2(g)→2NaCl(s)

Decomposition would be the break down of a compound into 2, the opposite of synthesis.

AB = A + B

2HgO(s)→2Hg(l)+O2(g)

Single replacement is where  1 element replaces another in a reaction, or switches place with 1 element.

AY + B = A + BY

2 HCl(aq) + Zn(s) → ZnCl2(aq) + H2(g)

Double replacement is similar but 2 elements are switch/replaced.

AY + BX = AX + BY

CuCl2(aq) + 2 AgNO3(aq) → Cu(NO3)2(aq) + 2 AgCl(s)

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