Which of the following is a synthesis reaction?
c2h4 + 3 o2 yields 2 co2 + 2 h2o
2hbr yields h2 + br2
co2 + h2o yields h2co3
f2 + 2 hcl yields cl2 + 2 hf

Synthesis reaction is a reaction where two or more substances combine to form a new compound. It is a reaction which releases energy in the form of heat and light. Therefore, it is an exothermic reaction. From its the definition the correct answer is the third option where carbon dioxide and water combine to form carbonic acid. 

The correct answer is CO2 + H2O yields H2CO3

Explanation:

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Synthesis reactions are those that two compounds or elements come together to form another.

Let's analyze each of the reactions:

C2H4 + 3 O2 yields 2 CO2 + 2 H2O in this reaction there is a recombination of elements

2HBr yields H2 + Br2 in this reaction the hydrogen bromide is broken and two simpler molecules are formed

CO2 + H2O yields H2CO3 in this reaction two compounds come together and form one

F2 + 2 HCl yields Cl2 + 2 HF in this reaction there is a rearrangement of elements

After the analysis, we conclude that the correct answer is CO2 + H2O yields H2CO3