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USC / Math / MATH 142 / What causes a solute to be dissolved?

# What causes a solute to be dissolved? Description

##### Description: Notes Cover: Principles of Solubility The Solution Process Spontaneity Gas as a Solute
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CHEM 112 Notes (1-13-16)

## What causes a solute to be dissolved?

Clicker Question: Calculate the molarity of a 12.0% sulfuric acid solution (H2SO4; 98.08 g/mol)  having a density of 1.080 g/mL.

�� =������

��

12% H2SO4

MM=98.08 g/mol

d=1.080 g/mol

12g H2SO4 (per 100g of solution)

12�� ��2����4

98.08 ��/������= 0.122 ���������� ��2����4

100�� ���������������� ×1��

1.080 ����= 92.59���� = 0.09259��

0.122������ ��2����4

0.09259��= 1.32��

Remember: Theory guides, but experiment decides

## Why is water a universal solvent?

-Chemistry is an experimental science. Actually doing the experiment will yield more  accurate solutions than theory

Principles of Solubility:

∙ Dynamic—able to change it

o Temperature can change the dynamic process

∙ Why this happens:

o Enthalpy (ΔH):

▪ Heat given off or absorbed (endothermic/exothermic)

▪ Usually a spontaneous reaction is exothermic and gives off heat, but some  spontaneous solutions are endothermic

∙ Types of Interactions:

o Solvent-solvent

o Solute-solute

o Solute-solvent

## Will iodine dissolve better in hexane or water?

∙ The Solution Process:

o First need to undo solvent and solute (requires energy)

o Separation of solute and of solvent is endothermic

o Bringing the two together is exothermic

o See “Enthalpy of Solution” figure in lab manual

ΔHsolution = ΔH1st step + ΔH2nd step + ΔH3rd step

o 1st step:

▪ Absorbs energy to separate solute We also discuss several other topics like How did the atomic bomb affect society?

o 2nd step:

▪ Absorbs energy to separate solvent

o 3rd step:

▪ Releases energy to combine both

∙ Spontaneity:

o Solid < Liquid < Gas (least spontaneity to most spontaneity)

∙ Spontaneity and Solubility:

o What causes a solute to be dissolved?

▪ Like dissolves like

∙ Intermolecular Forces:

o Dipole-dipole

▪ Polar molecules

o London Dispersion Forces

▪ Everything has it, but it’s the only force nonpolar

molecules have, so it’s typically associated with

nonpolar molecules

o Hydrogen Bonding

▪ Molecule must be polar and have H-F, H-O, or H-N

o Strengths:

▪ Strongest = covalent bonds

∙ All intermolecular forces are significantly

weaker than covalent bonds If you want to learn more check out Who is henry brackenridge?

▪ Weak = hydrogen bonds

▪ Very weak = dipole-dipole

▪ VERY very weak = London dispersion forces

∙ Relative Solubility:

o Ex: hexane and water

C

C C H H

C

C C

:O:

(Hexane) (Water)

▪ Won’t dissolve—not alike

∙ London dispersion forces holding hexane won’t break for water

∙ Water has hydrogen bonds

∙ Water is polar, and hexane is nonpolar

o Will iodine dissolve better in hexane or water?

I I If you want to learn more check out What is the first american constitution?

▪ Iodine has London dispersion forces, but no hydrogen bonds We also discuss several other topics like What is a major cell nutrient produced during photosynthesis, a raw material for other molecules?

▪ Answer: Hexane We also discuss several other topics like Why do companies go into advertising their products?

o NOTE: If it’s not an ionic salt or doesn’t have hydrogen bonds, it won’t dissolve  in water

∙ Why is water a universal solvent?

o For an ionic compound to dissolve, ions have to separate and solvent (water) has  to separate

o Next step: hydration

▪ Water molecules separate ions

Ionic Compound Water

r

e

r

d

e

r

d

o

r

s

i

o

s

d

i

e

d

s

e

a

Ions Separate Water Separates

s

e

a

r

e

c

r

n

c

I

n

I

Hydration

∙ Gas can be a solute

o Pressure will have a big impact in this case

o Henry’s Law:

▪ C = KD (directly proportional)

▪ C—concentration (could be molarity-M, molality-m, or partial pressure PA)

▪ K-value gets smaller as temperature increases

▪ As temperature increases, solubility increases (trend)

o Example of Henry’s Law worked out:

▪ Given:

∙ H2O at 20°C

∙ Partial pressure = 8.00 torr

∙ K = 3.91 x 10-2 molal/atm

▪ Conversions:

∙ 1atm = 760torr

o ���� =8.0�������� ×1������

760��������= 0.01052������

▪ Work:

∙ �� = 3.91 × 10−2 ����������

������× 0.01053������

∙ �� = 4.12 × 10−4����������

o A + B C + heat (exothermic—releases heat)

o A + B + heat C (endothermic—absorbs heat

▪ If you increase the temperature of an exothermic reaction (-ΔH), you’re  adding more product, so it will go in the other direction to reach  equilibrium

o ↑T of exothermic reaction (-ΔH)—equilibrium shifts to reactants (decreases  solubility because you are shoving solute out of the solution)

o ↑T of endothermic reaction (ΔH)—equilibrium shifts to products (increase  solubility because you are helping the solute dissolve)

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