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Consider the reaction of $\ce{AgNO3}$ and $\ce{HCl}$. I read that silver chloride would be formed. But $\ce{H} > \ce{Ag}$ in reactivity, then how could $\ce{Ag}$ displace $\ce{H}$ from $\ce{HCl}$ ?

If there was a reaction, $\ce{AgCl}$ and $\ce{HNO3}$ were to form. But then these would react again to give us the original compounds back. The actual question was to write down the reaction between $\ce{AgNO3}$ and $\ce{HCl}$. I (wrongly) realized that the compounds wouldn't react because of the reasons stated.

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    $\begingroup$ Why should they react back again to the original compounds? There must be some driving force for the reaction in the first place. $\endgroup$ – Jannis Andreska Aug 19 '14 at 19:51
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One of the most difficult parts of chemistry is learning to recognize a type of reaction based solely on its reactants. This has to be done before you can apply a reaction pattern to the problem, and so it is critical to get this step right in the beginning.

In this case, you are looking at the reaction:

$$ \ce{AgNO3 + HCl -> AgCl + HNO3} $$

You already know the products, but are questioning how these products can be formed since the activity of H is greater than Ag, implying that Ag is easier to oxidize than H. This is true - but take a look at the oxidation numbers of each species in this reaction:

$\ce{Ag: +1 -> +1}$

$\ce{NO3: -1 -> -1}$

$\ce{H: +1 -> +1}$

$\ce{Cl: -1 -> -1}$

Since there is no overall transfer of electrons, this can't be a redox reaction, and that means activities won't matter here.

That means there must be another driving force for this reaction - another reaction pattern that fits better.

I don't want to give you the answer directly, but I will give you some advice that should help:

Write the full equation - including the phases.

$$ \ce{AgNO3(aq) + HCl(aq) -> AgCl(s) + HNO3(aq)} $$

See if you can find another reaction pattern that fits this equation better. Later, I'll update this answer to show how you can identify the correct pattern for aqueous reactions using only the reactants.

Update

As I mentioned, one of the hardest parts of chemistry is learning to identify reaction patterns based only on the reactants. This is a double-displacement (or metathesis, or precipitation) reaction. It is easy to see now, given that the reactants are aqueous and at least one of the products is solid (the precipitate). Formation of the solid is the driving force for this reaction - the quick explanation is that the forces attracting silver and chloride ions together are stronger than the solvation forces between those ions and water, as well as the forces holding them to sodium and nitrate ions.

This is good, but how do we predict that this is a precipitation reaction ahead of time?

The key is to have a good understanding of the three common types of salt/acid/base reactions that occur in aqueous solution, and to learn to recognize clues in the reactants.

Briefly, the three types that are most commonly seen are:

  1. Precipitation
  2. Acid/base (Arrhenius definition)
  3. Redox (single displacement)

You can recognize each of these by the reactants if you know what to look for.

Precipitation - Two soluble salts (ionic compounds), or a salt with an acid or base.

Acid/base - An Arrhenius acid and base (compound containing hydroxide)

Redox - a salt or acid and an elemental metal

There are many other types of reactions that occur in aqueous solution, and many variations of the acid/base and redox category, but these three cover the cases most commonly seen in a classroom.

Once you have identified the likely pattern that the reaction will follow, the next step is to predict the products using that pattern to see if they make sense. If they do, then you have likely chosen correctly.

You can use this algorithm for more advanced chemistry as well - in organic chemistry, for example, one of the major goals is to learn to predict reactions based on functional groups. Once you can identify functional groups and have memorized reaction patterns for them, it becomes possible to predict a huge range of reactions.

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  • $\begingroup$ I don't know, I'm not good at chemistry. But is that a double-displacement reaction ? Just guessing from the phases. $\endgroup$ – Mriganka Basu Roy Chowdhury Aug 20 '14 at 7:59
  • $\begingroup$ @Mriganka, yes - this is a double displacement (a.k.a. precipitation, a.k.a. metathesis) reaction. The giveaway in this case is the insoluble product $\ce{AgCl}$. However, the real question is - how do we figure this out from just the reactants? See my edit for more info. $\endgroup$ – thomij Aug 20 '14 at 15:56
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One thing of paramount importance in Chemistry is to visualize concepts. According to the solubility table, nitrates are always soluble, so the strong ionic bond between silver ions and nitrate ions are broken by water molecules because of ion-dipole attraction. By definition, acids ionize in water to give mobile ions, so hydrogen chloride in aqueous solution gives out hydrogen ions (and form hydronium ions) and chloride ions.

Hence, you can see that there are five types of particles in the solution now: $\ce{H2O}$ molecules, $\ce{Ag+}$ ions, $\ce{NO3-}$ ions, $\ce{H+}$ ions, and $\ce{Cl-}$ ions. (Slight ionization of water is neglected in this case.) They freely bump into each other as they are mobile. However, this freely moving condition is inhibited by the interaction between $\ce{Ag+}$ ions and $\ce{Cl-}$ ions. According to the solubility table, $\ce{AgCl}$ is insoluble in water. When $\ce{Ag+}$ ions and $\ce{Cl-}$ ions bump into each other, they strongly attract each other, in which the strong ionic force cannot be separated by the ion-dipole force between them and $\ce{H2O}$ molecules. As a result, $\ce{AgCl}$ evolves as a white solid.

Although some may speak this of an example of double displacement reaction, this has no direct relationship with reactivity of hydrogen and silver because there is no transfer of electrons (which occurs in redox reactions).

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But H>Ag in reactivity, then how could Ag displace H from HCl ?

What do you mean that hydrogen is more reactive than silver? In what context?

If there was a reaction, AgCl and HNO3 were to form. But then these would react again to give us the original compounds back.

How do you know? Why would these form? Why not silver nitrate? Why not hydrochloric acid?

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  • $\begingroup$ How about $\ce{AgNO3 + HCl <=> AgCl + HNO3}$? $\endgroup$ – ashu Aug 20 '14 at 5:32
  • $\begingroup$ What about that equation? $\endgroup$ – Dissenter Aug 20 '14 at 5:33
  • $\begingroup$ 'How do you know? Why would these form? Why not silver nitrate? Why not hydrochloric acid?' is creating too much confusion in your answer. If by this, you meant that they (AgNO3 + HCl & AgCl + HNO3) will be in equilibrium (did I get it right?), consider writing a simple amswer with an equation. $\endgroup$ – ashu Aug 20 '14 at 5:45
  • $\begingroup$ The OP didn't really put effort into the problem the second time around, and another poster has already provided the answers, so these are just thought questions. $\endgroup$ – Dissenter Aug 20 '14 at 5:46
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    $\begingroup$ I am going to leave this for the time being, but if you put a little bit of explanation after your rhetorical questions that adds to what thomij has already contributed, it should be fine. Otherwise, it should probably be a comment. $\endgroup$ – jonsca Aug 20 '14 at 7:30
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The silver displaces the hydrogen because this is a double replacement equation, they just switch their anion. When the silver would not be able to displace the hydrogen is when It would have been a single replacement equation like Ag + HCl

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