2 int qualitative

8/13/2019 2 Int Qualitative

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Author: J R Reid

Qualitative

AnalysisQualitative definitions ofsolubilitySolubility rulesIdentifying ionsBalancing ionic equationsComplex ionsBalancing complex ion equations


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Qualitative?

We can analyse chemicals in two mainways:

Quantitative an analysis using quantities likevolume, mass or amount

Qualitative an analysis using qualities suchas colour change, smell or precipitation(cloudiness)

The purpose of this form of qualitativeanalysis is to identify the presence ofcertain ions in a solution


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Solubility

When you add a chemical to water it coulddo two things:

It could dissolve and the water will turn seethrough (transparent) this is a soluble

substanceIt might just sit there and not dissolve it isinsoluble

If an insoluble substance is made in water

the liquid will turn cloudy, this is calledprecipitation. A precipitate will settle overtime and the solid will form a layer on thebottom of the test-tube


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Solubility Rules - Intro

Ions come in two forms:Anions negatively charged ions

Cations positively charged ions

Every ionic substance is made from a

cation and an anionCertain ions easily dissolve, others tendto be insoluble. They all follow certainrules

We can use these rules in order to predictwhat ions are present in an unknownsolution


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Solubility Rules - Details

These are the general rules:All Group 1 and ammonium (NH4

+) ions aresoluble

All nitrates are soluble

All chlorides are soluble (except for Ag and Pb)

All sulphates are soluble (except for Ba and Pb)

All carbonates are insoluble (except for Group1 and NH

4

+)

All hydroxides are insoluble (except for Group1, NH4

+ and Ba)


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Identifying Ions

We could use these rules to work out ifhydroxides or chlorides are present in asolution. (What is one difference betweenthe two ions?)

To make life easier we could use a flowchart that shows the order that we need todo tests in order to eliminate possibleionsThese two flow charts divide all ions into

two groups anions and cations. Once wehave identified one of the ions in asolution we need to identify the other


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Identifying Anions: CO32,Cl,I,SO42,NO3,OH

litmus remains redSO4

2, Cl, I, NO3

add Ba(NO3)2solutionwhite precipitate

SO42

add AgNO3solution

no precipitate

precipitateCl, I

add dilute NH3solution

precipitate disappears

Cl

precipitate remainsI

no precipitateNO3

new sample

add red litmus litmus goes blueOH, CO32

add dilute HClsolution

bubbles of gasCO3

2

no bubblesOH


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new sample

precipitate disappearsAl3+, Zn2+, Pb2+

precipitate remainsMg2+, Ba2+

add 2 drops, thenexcess NH3solution

white precipitate formsAl3+, Zn2+, Pb2+, Mg2+,

Ba2+

add excessNaOH solution

add 2 drops of diluteNaOH solution.

no precipitateNH4

+, Na+

add NaOH solution,heat, test gas with red

litmus.

litmus staysred Na+

litmus goesblue NH4

+

blue precipitate thendeep blue solution

Cu2+

blue precipitate formsCu2+

brown precipitate formsAg+

brown precipitatethen colourlesssolution Ag+

add 2 drops, then excessNH3solution

add 2 drops, then excessNH3solution

orange precipitate formsFe3+

green precipitate formsFe2+

white precipitate formsAl3+, Pb2+

white precipitate forms and

disappears, Zn2+

add dilute H2SO4solution

colourless solutionAl3+ white precipitatePb2+

add dilute H2SO4solution

colourless solutionMg2+

white precipitateBa2+

add 2 dropsKSCN solution

dark red solutionconfirms Fe3+

Identifying Cations: NH4, Na, Mg2 , Ag, Fe2 , Fe3 , Cu2 , Al3 , Pb2 , Zn2 , Ba2

new sample

newsample

newsample

newsample

new sample


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Identifying Anions I

Use the anion flowchart to identify theanion used below:

Add Ba(NO3)2 Add AgNO3 Add NH3Add redlitmus


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Identifying Anions II

Use the anion flowchart to identify theanion used below:

Add HClAdd redlitmus


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Identifying Cations I

Use the cation flowchart to identify thecation used below:

Add NaOH Add KSCN


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Identifying Cations II

Use the cation flowchart to identify thecation used below:

Add NaOH Add NH3


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Balancing Equations

In the assessment you will have to showbalanced equations for the precipitationreactions. A precipitation reaction is one thatproduces a cloudy liquid because of the presenceof a solid in the liquid.In other words two ions will join together to forma solid. For example:

Pb2+ + 2OH- Pb(OH)2All of the charges of the ion can be found on

your flow chart but you have to put themtogether and balance the equation properly.How do I fix this?

Al3+ + NO3- AlNO3


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Complex Ions

You may have noticed that in some occasions we had aprecipitate (a solid) then we added excess (more) chemicalsto it. Occasionally the precipitate ended up disappearing this indicates that the solid has turned back into an ion, acomplexion.

A complex ion is a metal ion that has got extra negativechemicals (ligands) stuck to it. Instead of balancing out thecharge it ends up retaining its charge and turns into an ionagain. Here are some examples:

Cu2+ + 4NH3 Cu(NH3)42+

Al3+ + 3OH- Al(OH)3 + OH- Al(OH)4

-

The hardest part about complex ions is that you have toremember how many ligands are stuck to it


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Balancing Complex Ion Equations

With previous ionic equations all we had to do was tomake sure that the ions charges matched.Unfortunately with complex ions we work backwards:

1. Memorise how many ligands match up with the metal ione.g. Al3+has 4 OH-ions

2. Write that down as your product: Al(OH)4-3. Now write down the reactants: Al3++ 4OH-

You may be thinking, Oh no! I have to remember awhole heap of numbers and ligands. Well, yes you doBUT it is not as bad as it seems.

On the next page we will investigate trends with thecomplex ions that you will need to memorise


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Complex Ion Trends

Cu(H2O)62+ Zn(OH)42-

Zn(H2O)62+ Zn(NH3)4

2+

Al(H2O)63+ Cu(NH3)42+

Al(OH)4- Ag(NH3)2

+

Pb(OH)42- FeSCN2+

Cu(H2O)62+ Zn(OH)42-

Zn(H2O)62+ Zn(NH3)4

2+

Al(H2O)63+ Cu(NH3)42+

Al(OH)4- Ag(NH3)2

+

Pb(OH)42- FeSCN2+

Trends:1. All complex ions containing water have SIX ligands2. The rest have FOUR ligands (except Silver (2) and Iron (1))

B l i E i


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Balancing Exercises -Precipitation

Precipitates:

Ba2+(aq) + SO42-

(aq) (s)

(aq) + (aq)

AgCl(s)

Ag+(aq) + I-(aq)

+

Al(OH)3(s)

Pb2+(aq) + OH-(aq)

+ Ba(OH)2(s)

B l i E i


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Balancing Exercises Complex Ions

Complex Ions:

(aq) + (aq) Cu(NH3)42+

(aq)

Al3+(aq)

+ OH-(aq)

(aq)

+ Zn(NH3)42+

(aq)

Ag+(aq) + NH3

(aq)

+ Pb(OH)42-

(aq)

Fe3+(aq) + SCN-(aq)

B l i E i


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Balancing Exercises Acid Reactions

Acids + Carbonates:

Na2CO3(s)+ HCl(aq)

K2CO3(s)+ HCl(aq)

CaCO3(s)+ HCl(aq)

B l i E i


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Balancing Exercises Extras

Create balanced equations for theformation of:

+ Mg(OH)2(s)

+ Zn(OH)42-

(aq)

+ AgOH(s)

+ FeSO4(s)

2 int qualitative

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