confused

[abi ayan]

Though there is hydrogen bond between the molecules of glucose then how it forms hydrogen bond with water when dissolving ?

 

[hypervalent_iodine]

The exact same way. I'm unsure of where your confusion is.

[abi ayan]

My confusion is that the H-bond between the glucose is break down and new H-bond is formed between the molecules of glucose and water.And now my doubt is that the both H-bonds stated above has equal strength(formed between o-H and H).so why the existing H-bond between the molecules of the glucose breaks down??

[Enthalpy]

Hydrogen bonds can have varied strength. Not only because of proximity effects in the molecule (compare an carboxylic acid with an alcohol), but also because in a solid, not all bonds may find the optimum angle to the neighbour molecules - or even, some bonds may be frustrated because of geometry.

 

There is also a more general reason.

 

Dissolution, or mixing, (or chemical reactions) also happens when it is energetically neutral or slightly defavourable. For instance glycerine or erythritol mix or dissolve well with water but the mix is very cold to the hand.

 

Imagine one single sugar piece dropped in the Ocean. The sucrose molecules dissolve, drift away, and have no chance to deposit again on the sugar piece. The sugar piece dissolves completely, even if the reaction was defavourable. To get an equilibrium, the amount of water must be smaller, so that the concentration of dissolved sugar reaches an equilibrium, where as many molecules per second dissolve and crystallize. This "equilibrium concentration" (or "saturation concentration" for a dissolution) depends on the temperature and, for a gas, on the pressure.

 

A statistical computation of dissolved molecules gluing or not to the solid would be extremely complicated, but fortunately, thermodynamics can relate the equilibrium concentration to simple values like the heat of dissolution.

 

In thermodynamics, what drives reactions and equilibria is not the heat of reaction but the entropy, which also depends on concentrations (though a big heat of reaction may overwhelm other contributions in entropy and decide alone).

 

By the way, the conservation of energy already tells that heat alone can't decide what direction a reaction will take, because if chemical energy has decreased, energy has increased somewhere else. The view of thermodynamics is instead that energy tends to dilute naturally, and entropy is a measure of how equally distributed heat is.

 

Take a meteoroid as an example. Drop it from 100,000km height: it and Earth lose gravitational energy which heats the meteoroid, the air and Earth. No energy has been lost, but it was previously concentrated in one dimension (the height of the object) and is diluted into heat after the impact, where each atom has gained kinetic energy, so heat is energy spread over >10²³ dimensions. Entropy tells the falling rock story happens, while a hot rock won't spontaneously climb - energy alone would not tell it.

 

Because heat is very dilute energy, conversions into heat occur easily, while the conversion of heat into work (engines), electricity, chemical energy... is difficult and demands engineering and machines.

[abi ayan]

thanks for explaining

[Bromo_DragonFly]

if all hydrogen bonds were unbreakable, then we wouldn't have any Silica Gel Chromatography. I suggest you to read about column chromatography to see how conditions can force a hydrogen bond to establish or break.