Answer:
  First of all, redox potential, which is the parameter determining the reducing power, is discussed. Then we will talk about the silver mirror reaction.

About the redox potential of glucose and fructose
  We do not know whether the redox potentials of glucose and fructose have already been investigated or not. However, these values would not be useful for this experiment even if they were already known, because these reactions are nonreversible, that is, there are no equilibrium states. Vitamin C (ascorbic acid) is well known as the typical example. The oxidation process of vitamin C is very complex. If oxidized ascorbic acid is stable and it can be reduced to the original ascorbic acid, the redox potential could be determined. However, if the backward reaction does not happen, the value of the redox potential would be meaningless. Probably the case of glucose and fructose would be identical to this example case for ascorbic acid.

About the reaction of glucose and fructose
  Suppose that the redox potentials of glucose and fructose are known. Even so, it is very difficult to estimate the rate of the reactions.
  
  We will examine the following half-reactions.

  Ag⁺ + e^- ® Ag     E₀ = 0.799 V
  H⁺ + e^- ® (1/2)H₂     E₀ = 0.000 V

Thus, if hydrogen gas is blown into the water solution containing silver ion, silver ion should be reduced to silver metal. Actually, however, this reaction will not occur at all. It is very difficult to estimate from redox potentials whether the reaction will occur easily or not, because it depends not only on redox potential but also on the activation energy of the reaction.
  
About the reduction of formaldehyde and glucose in the silver-mirror reaction
  It is true that glucose has reducing power. For example, it is reported that the test agent for diabetes, which is not silver (Tollens’ reagent) but copper (Benedict’s reagent), reacts with 0.1% glucose in urine. But a teacher told us that formaldehyde seems more reactive in the silver-mirror reaction than glucose. The result of this questioner’s experiment may support what this teacher told us.

  Now we ask this questioner "if your friends also did the same experiment, what results did they get?" These results would be good parameters in estimating whether your result is right or wrong (left: the photo of a silver mirror reaction). To increase your confidence in your results we recommend that you repeat your experiment, if you have time. 
 
About the silver-mirror reaction
  When you did this experiment, we assume that you added ammonia water to the silver nitrate solution, causing the solution to become basic. The following is an explanation for this procedure.

1. Why is ammonia water added?
     The purpose of adding the ammonia water is to weaken the reduction of silver ion, which makes more beautiful silver.
     
     Ag⁺ + e^- ® Ag 　　　E₀ = 0.799 V
     Ag(NH₃)₂⁺ + e^- ® Ag + 2NH₃ 　　　E₀ = 0.373 V
     
   The half equations above indicate that ammonia forms a complex with silver ion, which is more difficult to reduce than silver ion itself. This is not surprising because silver ion forms a more stable complex with ammonia than with water.
     
     If silver ion is employed without ammonia, the silver ion is reduced so quickly that colloidal silver metal would appear. The solution would become a black and cloudy liquid.
     

2. Why are basic conditions necessary?
     Because formaldehyde and glucose are oxidized more easily under basic conditions.
     
     HCHO + H₂O ® HCOOH + 2H⁺ + 2e^-
     
   This half-reaction indicates that the reaction proceeds more easily to the right side under basic conditions. 

   
Acknowledgement
  We would like to thank an anonymous professor for his careful teaching and kind support for this answer.
  
 

This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp.  Please let us know if you find any errors.