Answer :
The answer is (1). The electrons transfer form one reactant to another means that there is the change of valence. For other choices, the valence of reactant does not change.
Answer : The correct reaction is, (1) [tex]2Ca(s)+O_2(g)\rightarrow 2CaO(s)[/tex]
Explanation :
(1) [tex]2Ca(s)+O_2(g)\rightarrow 2CaO(s)[/tex]
In this reaction, the oxidation state of calcium (Ca) changes from (0) to (+2) and the oxidation state of oxygen (O) changes from (0) to (-2). In this reaction the electrons are transferred from one reactant (calcium) to another reactant (oxygen). This is a synthesis reaction.
(2) [tex]AgNO_3(aq)+KCl(aq)\rightarrow AgCl(s)+KNO_3(aq)[/tex]
In this reaction, there is no changes in the oxidation state of the elements. So, in this reaction, no electrons transferred from one reactant to another reactant. This is a double-displacement reaction.
(3) [tex]HCl(aq)+NaOH(aq)\rightarrow NaCl(aq)+H_2O(l)[/tex]
In this reaction, there is no changes in the oxidation state of the elements. So, in this reaction, no electrons transferred from one reactant to another reactant. This is a neutralization reaction.
(4) [tex]H_3O^+(aq)+OH^-(aq)\rightarrow 2H_2O(l)[/tex]
In this reaction, there is no changes in the oxidation state of the elements. So, in this reaction, no electrons transferred from one reactant to another reactant. This is a combination reaction.
Hence, the correct reaction is, (1) [tex]2Ca(s)+O_2(g)\rightarrow 2CaO(s)[/tex] in which the electrons transferred from one reactant to another reactant.
