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Lab 4: metathesis: to exchange or not

Objectives

  • To give practice writing equations for metathesis reactions, including net ionic equations
  • To illustrate the concept of solubility and the effect of temperature and crystallization

 Grading

You will be determined according to the following:

  • Pre-lab (10%)
    • Must attach graph
  • Lab Report Form (80%)
    • Must include detailed observations for each reaction
  • TA Evaluation of lab procedure (10%)

Before coming to lab…

  • Complete the pre-lab exercise, including the plot (due at the beginning of lab)
  • Read the instructions for the lab and refresh your memory on anything that isn’t clear by reading your textbook

Introduction

In molecular equations for many aqueous reactions, cations and anions appear to exchange partners. These reactions conform to the following general equation:

Equation 1: AX + BY AY + BX size 12{"AX"+"BY" rightarrow "AY"+"BX"} {}

These reactions are known as metathesis reactions. For a metathesis reaction to lead to a net change in solution, ions must be removed from the solution. In general, three chemical processes can lead to the removal of ions from solution, comcomitantly serving as a driving force for metathesis to occur:

1. The formation of a precipitate2. The formation of a weak electrolyte or nonelectrolyte3. The formation of a gas that escapes from solution

The reaction of barium chloride with silver nitrate is a typical example:

Equation 2: BaCl 2 ( aq ) + 2AgNO 3 ( aq ) Ba ( NO 3 ) 2 ( aq ) + 2AgCl ( s ) size 12{"BaCl" rSub { size 8{2} } \( "aq" \) +"2AgNO" rSub { size 8{3} } \( "aq" \) rightarrow "Ba" \( "NO" rSub { size 8{3} } \) rSub { size 8{2} } \( "aq" \) +"2AgCl" \( s \) } {}

This form of the equation for this reaction is referred to as the molecular equations. Since we know that the salts BaCl 2 size 12{"BaCl" rSub { size 8{2} } } {} , AgNO 3 size 12{"AgNO" rSub { size 8{3} } } {} , and Ba ( NO 3 ) 2 size 12{"Ba" \( "NO" rSub { size 8{3} } \) rSub { size 8{2} } } {} are strong electrolytes and are completely dissociated in solution, we can more realistically write the equation as follows:

Equation 3: Ba 2 + ( aq ) + 2Cl ( aq ) + 2Ag + ( aq ) + 2NO 3 ( aq ) Ba 2 + ( aq ) + 2NO 3 ( aq ) + 2AgCl ( s ) size 12{"Ba" rSup { size 8{2+{}} } \( "aq" \) +"2Cl" rSup { size 8{ - {}} } \( "aq" \) +"2Ag" rSup { size 8{+{}} } \( "aq" \) +"2NO" rSub { size 8{3} rSup { size 8{-{}} } } \( "aq" \) rightarrow "Ba" rSup { size 8{2+{}} } \( "aq" \) +"2NO" rSub { size 8{3} rSup { size 8{-{}} } } \( "aq" \) +"2AgCl" \( s \) } {}

This form, in which all ions are shown, is known as the complete ionic equation. Reaction occurs because the insoluble substance AgCl precipitates out of solution. The other product, barium nitrate, is soluble in water and remains in solution. We see that Ba 2 + size 12{"Ba" rSup { size 8{2+{}} } } {} and NO 3 size 12{"NO" rSub { size 8{3} rSup { size 8{ - {}} } } } {} ions appear on both sides of the equation and thus do not enter into the reaction. Such ions are called spectator ions. If we eliminate or omit them from both sides, we obtain the net ionic equation:

Equation 4: Ag + ( aq ) + Cl ( aq ) AgCl ( s ) size 12{"Ag" rSup { size 8{+{}} } \( "aq" \) +"Cl" rSup { size 8{ - {}} } \( "aq" \) rightarrow "AgCl" \( s \) } {}

This equation focuses our attention on the salient feature of the reaction: the formation of the precipitate AgCl. It tells us that solutions of any soluble Ag + salt size 12{"Ag" rSup { size 8{+{}} } "salt"} {} and any soluble Cl salt size 12{"Cl" rSup { size 8{ - {}} } "salt"} {} , when mixed, will form insoluble AgCl. When writing net ionic equations, remember that only strong electrolytes are written in the ionic form. Solids, gases, nonelectrolytes, and weak electrolytes are written in the molecular form. Frequently the symbol (aq) is omitted from ionic equations. The symbols (g) for gas and (s) for solid should not be omitted. Thus, Equation 4 can be written as

Equation 5: Ag + + Cl AgCl ( s ) size 12{"Ag" rSup { size 8{+{}} } +"Cl" rSup { size 8{ - {}} } rightarrow "AgCl" \( s \) } {}

Consider mixing solutions of KCl and NaNO 3 size 12{"NaNO" rSub { size 8{3} } } {} . The ionic equation for the reaction is

Equation 6: K + ( aq ) + Cl ( aq ) + Na + ( aq ) + NO 3 ( aq ) K + ( aq ) + NO 3 ( aq ) + Na + ( aq ) + Cl ( aq ) size 12{K rSup { size 8{+{}} } \( "aq" \) +"Cl" rSup { size 8{ - {}} } \( "aq" \) +"Na" rSup { size 8{+{}} } \( "aq" \) +"NO" rSub { size 8{3} rSup { size 8{-{}} } } \( "aq" \) rightarrow K rSup { size 8{+{}} } \( "aq" \) +"NO" rSub { size 8{3} rSup { size 8{-{}} } } \( "aq" \) +"Na" rSup { size 8{+{}} } \( "aq" \) +"Cl" rSup { size 8{ - {}} } \( "aq" \) } {}

Questions & Answers

how to know photocatalytic properties of tio2 nanoparticles...what to do now
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s. Reply
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Devang Reply
are you nano engineer ?
s.
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Mostly, they use nano carbon for electronics and for materials to be strengthened.
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is Bucky paper clear?
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s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
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of graphene you mean?
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or in general
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in general
s.
Graphene has a hexagonal structure
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I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
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Porter
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AMJAD
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Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
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AMJAD
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In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
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Source:  OpenStax, Gen chem lab. OpenStax CNX. Oct 12, 2009 Download for free at http://cnx.org/content/col10452/1.51
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