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- Strong vs. Weak Acids- Indicators

Mr. Shields Regents Chemistry U15 L04

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Acid strength

We all know there must be a difference betweenDifferent acids.

For instance Vinegar is acetic acid and we allPut vinegar on our food from time to time.

People used to chew willow bark to help reduce fever.In doing so they were swallowing salicylic acid (an OrganicAcid).

But would anyone try putting sulfuric acid(car battery acid) instead of vinegar on their foodand eat it???

I HOPE NOT !!!

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Strong vs. Weak acids/basesObviously some acids are stronger than others.But what makes one acid strong and the otherWeak?

A Strong acid is one that is mostly dissociated(separated into ions) when dissolved in Water.

The same can be said for bases. Strong bases areAlso highly dissociated in water.

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-Do not confuse strong acid vs. weak acids & bases with concentration.

- 6M Acetic acid (CH3COOH) is just as concentrated as 6M H2SO4 or 6M HCl

But it is a much weaker acid

NOTE:

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Strong & Weak Acids

Example of a Strong Acid:HCl H+ + Cl-

Lots of H+ & Cl-. Very little undissociated HCl in Sol’n.

Example of a Strong Base:KOH K K++ + OH + OH--

Lots of K+ & OH-. Very little undissociated KOH in Sol’n.

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Strong vs. Weak acids/bases

So if strong acids and bases are highly dissociated In solution what defines a weak acid or base?

Weak acids are NOT highly dissociated into theirrespective ions in solution

The same can be said for weak bases. They areAlso NOT highly dissociated in solution.

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Weak Acids and bases

• Weak acids/bases don’t ionize very much in solution. Instead Weak acids/bases don’t ionize very much in solution. Instead they tend to reach they tend to reach an equilbrium that is mostly the an equilbrium that is mostly the undissociated acid or baseundissociated acid or base::

C2H4O2 H+ + C2H3O2- (4% dissociated)

Lots of C2H4O2. Very little H+, C2H3O2-.

(CH3COOH) (CH3COO-)

NH4OH NH4+ + OH- (in equilibrium)

Lot’s of NH4OH and very little NH4+, OH-

“Vinegar”

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The greater the dissociation of the acid (or base)The Stronger the acid (or base)

Strength of acid/base

1% 5% 40% 90% 100%Degree ofdissociation

Metals react more vigorously with strong acidsthan weak acids

Mg + HCl MgCl2 + H2 Strong acid/ Fast reaction

Mg + CH3COOH CH3COOMg + H2 Weak Acid/ Slow reaction

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Mg &Weakeracid

Mg &Strongacid

Like H2SO4 LikeAcetic Acid

H2

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Strong Acids• HCl• HBr

• H2SO4 Sulfate

• HNO3Nitrate

Strong Bases• NaOH• KOH

• Ca(OH)2

• Mg(OH)2

Weak Acids

•H2C2H3O2 (acetic)

•HCN

•HF

•HNO2 (Nitrite) or H2SO3 (Sulfite)

•H2CO3

Weak Bases

•NH3

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IndicatorsRemember that pH is a measure of how much H+

is in solution. Let’s look at how we can measure pH.

There are two ways we might do this:

1) Measure pH directly using electronic instruments2) Use INDICATORS

Indicators are substances that change colorBased on the pH of the solution.

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IndicatorsLook at Table M in your Reference Table Booklet. The tablelists the color changes of specific indicators at specific pH’s

It also listsThe useful pHRange of eachIndicator.

Using theseIndicators we Can determineThe pH of anunknown.

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Color range of a few indicators

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Solving Problems w/ Indicators

Problem: An Unknown sol’n turns methyl orangeYellow & turns bromocresol green…green. Whatis the pH of the solution?

2) Methy orange if pH > 4.4 it’s yellow

1) Check Methyl orange in table M:

3) Check Bromocresol green <3.8 yellow >5.4 blue

4) pH must be >4.4 but < 5.4 and a green color must be the midpoint of 3.8 and 5.4 or about 4.6

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Solving Problems w/ Indicators

Problem: An Unknown sol’n causes Bromothymol blue to turn blue and phenolphthaleinTo turn colorless. What is the approximate pH ofThe solution?

2) Bromthymol Blue if pH is >7.6 it’s blue

1) Check Bromothymol blue pH range in table M:

3) Check Phenolphthalein <8.2 colorless

4) pH is between 7.6 and 8.2