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Mr. Shields Regents Chemistry U12 L05

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Rate of Sol’nWe talked about HOW ionic solids go into solutionWe talked about HOW ionic solids go into solutionBut what factors affect how But what factors affect how QUICKLYQUICKLY they go they go Into solution.Into solution.

In other words, what can we say about whatIn other words, what can we say about whatThings affect the Things affect the RATE RATE OF SOLUTIONOF SOLUTION??

It turns our there are It turns our there are 3 factors3 factors that can affect that can affectThe rate of sol’n:The rate of sol’n:

1) Surface Area1) Surface Area2) Temperature2) Temperature3) Mechanical Stirring3) Mechanical Stirring

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Surface area

Let’s look at each of these 3 factors that affect therate of solution.

The first factor is SURFACE AREA

From experience we know that 5g of granulatedSugar will dissolve faster than a 5g lump of sugar

But why?

What exactly is going on when sugar goesinto Solution?

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Surface areaSince both water and sugar are polar molecules …

1. Polar water aligns it’s positive chg with sugars negative charge & vice versa

2. The dipole-dipole forces of attraction in sugar are overcome by the forces of attraction of the water

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Surface area3. Molecules leave the clumps of sugar

- water surrounds the molecule in the solvation (in this case hydration) process

Water Molecules

Sucrose

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Surface area So what’s the physical difference between Granular Sugar and a single lump of sugar?

- Granular sugar consists of many small particles

- And because we have these small particles

their surface area is much larger than the surface area of the single lump

OK. So why does the increased surfacearea increase the rate of solution?

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Surface area

With greater surface area many more particlesOf sugar are exposed to water molecules per Unit time

Therefore, more solvent comes into contactWith sugar molecules more quickly

Since more waterCan come intoContact with moreSolute, the soluteGoes into sol’nMore quickly

Surface area: 108

“Lump of sugar”

9 x 2

92

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SA= 24 x 6 = 144

“granular sugar”

32 2

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Temperature

Now let’s see how temperature can affect the rate of Solution…

What happens to KE when temp. increases?

Recall KE = ½ mv2

As Temp inc the KE increases, thereforeMolecular velocity increases.

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Temperature

If solvent molecules are Moving faster at HIGHERTemperature what’s happening at the surface of the soluteIn a given period of time?

1. More molecules will reach the surface of the solute during a given time

AND

2. Solvated ions can leave the area faster allowing new solvent molecules to arrive at the remaining unsolvated solute surface

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Temperature

3. Lastly remember solvation (hydration) requires:

a)Breaking the Force of attraction between solute ions (endothermic)b) Breaking the Force of attraction between solvent Molecules (also endothermic).

Higher Velocitysolvent moleculescan provide thisnecessary energy .

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Stirring

Stirring or agitation is the last thing we willDiscuss that affects the rate of dissolving.

This is a process we do almost subconsciously.For instance, when we make a cup of instantCoffee or when we add sugar to ice tea.

But why does stirring increaseThe rate of dissolving?

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StirringAs solvent molecules remove solute ions from the Crystal lattice surface several things happen:

1)The concentration of solvatedsolute in the Immediate vicinity of the crystal surface increases

2) This increased concentrationof solvated solute Impedes newsolvent from reaching the surfaceOf the solute.

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StirringStirring (or agitation) “sweeps away” the increasedConcentration of solute from the vicinity ofThe surface of the undissolved solute

And Stirring also makes “fresh” solvent availableat the Solute surface

Solvated solute is concentratedOver solute surface

Solutesurface

Solvated solute

Fresh solvent sweepsAway solvated soluteExposing fresh solute

Fresh solvent