1 mr. shieldsregents chemistry u12 l05 2 rate of sol’n we talked about how ionic solids go into...
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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 .
0
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0min
1min
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gra
ms
dis
solv
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Temp 0 deg
Temp 50 deg
RATE
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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
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