Chemistry 5,6, 7 & (8) Solutions, Equilibria in aqueous solution, chemical kinetics & catalysis, (Redox)

Solution (definition)

A homogeneous mixture composed of: solvent & solute

Solvent

Substance present in greater amount

→ dissolves the solute

→ water in water-salt solution

Solute

The substance being dissolved

→ salt (NaCl) in water-salt solution

Properties of water: why is water extremely important in biology & chemistry?

Chemistry:

→ cause it’s a universal solvent

→ surrounds ions to stabilize solutions

→ provides the physical space for reactions

Biology:

→ transport for nutrients & waste in blood & sap

→ temp. Stability: absorbs body heat without big temp. changes

→ acts as reactant in hydrolysis reactions

Another chemistry fact about water?

It’s polar molecules form strong hydrogen bonds

Water molecule

H₂O

The oxygen atom is more electronegative than hydrogen

→ oxygen has partial negative charge (δ⁻)

→ hydrogen has partial positive charge (δ⁺)

= water polar molecule!

Hydrogen bonds in water: water molecules attract each other through hydrogen bonds (explains many properties of water)

Name important properties of water

1. high boiling point: hydrogen bonds require energy to break

2. Cohesion: water molecules stick together → important for transport in plants

3. Adhesion: water sticks to other surfaces

4. Excellent solvent: water dissolves many ionic & polar subs → example: NaCl dissolves because water separates Na⁺ and Cl⁻ ions

IMAT Tip for dissolving of substances

→ like dissolves likes

A molecule or solution of mixture likes to dissolve the solute that’s alike

Example: polar substances dissolve in polar solvents!

Solubility (definition)

The maximum amount of solute that can dissolve in a solvent at a given temperature

Factors that affecting solubility

1. Temperature:

   for many solids: Temperature high → solubility high

   for gases: Temperature high → solubility low

2. Pressure:

   important for gases: Pressure high → solubility high

3. Nature of substances: polar subs dissolve better in polar solvents

Concentration of solutions (important formulas for calculation)

Describes how much solute is present in a solution, with calculation of:

1. Molarity (M)

M = n/V

M: Molarity (mol/L)

n: Moles of solute (mol)

V: Volume of solution (L)

Alternative formulas to find moles & volume of solution:

find moles: n = M x V

Find volume: V = n/M

Example: how much M in 1 mole NaCl in 1 L Solution:

M = 1 mol / 1 = 1 M NaCl

2. Mass concentration

p = m/V

p or D: density/pressure g/L

m: Mass of substance (in g or kg)

V: Volume occupied by the substance in mL usw

Alternative formulas:

find mass: m = p x V

find volume: V = m/p

3. Percentage concentration:

% mass/volume → w/V (weight by volume)

% (w/v) = (g/mL) x 100

g= mass of solute in grams

mL= total volume of solution in millilitres

% mass/mass → w/w (weight by weight)

% (w/w) = (g1/g2) x 100

g1: mass of solute in grams

g2: mass of total solution in grams

% volume/volume → v/v (volume by volume)

% (v/v) = (mL1/mL2) x 100

mL1: volume of solute in mL

mL2: total volume of solution mL

What unit do you use to calculate Molarity?

MOLES not grams!

Chemical equlilibrium

a state where forward reaction state = reverse reaction rate

→ means reaction continues, but concentrations remain constant

Example: A ⇌ B

Dynamic equilibrium: reaction is not stopped → both directions continue

Le Chatelier’s Principle: If a system at equilibrium is disturbed, it shifts to oppose the change.

Factors:

1. Concentration

Adding reactant:

→ equilibrium shifts toward products

⸻

2. Temperature

Increasing temperature favours the direction that absorbs heat.

⸻

3. Pressure

Important for gases.

Higher pressure favours fewer gas molecules.

Equilibrium Constant (Basic Idea)

The equilibrium constant (K) describes the position of equilibrium.

Large K:

→ products favoured

Small k:

→ reactants favoured

⸻

6.3 Equilibrium in Water

Water can self-ionise:

H₂O ⇌ H⁺ + OH⁻

⸻

In pure water:

H⁺ concentration = OH⁻ concentration

Neutral solution.

Reaction rate

how fast reactants are converted into products

Factors affecting reaction rate:

1. Temperature:

   Temp high → particles move faster, more collisions → reaction faster

2. Concentration:

   Concentration high → more collisions → faster reaction

3. Surface are:

   smaller particles → larger surface are → faster reaction

Catalysts

Increase reaction rate without being consumed

Activation energy

The min. energy needed for a reaction to occur

Catalysts work by → reducing activation energy

Biological catalysts: enzymes bzw. proteins that speed up biochemical reactions

Enzymes: are specific, are not consumed, work through active site

REMEMBER: catalysts don’t change final equilibrium position → but make it reached faster

Oxidation & Reduction

Oxidation → loss of e-

Reduction → gain of e-

Oxidation numbers: a number representing the apparent charge of an atom in a compound

Rule: a free element has a oxidation number of 0, since it’s not reacting with another element, so there is no oxidation or reduction happening (O2 = 0, Fe= 0 usw.)

usually when an element is reacting to another the oxidation number changes, for example has O2 normally -2, since it’s taking e- & is than more negative, since it takes up e- (is being reduced)

Typical oxidation number of Hydrogen?

Usually +1, since it’s giving 1 e- away → is being oxidated

Simple ions & it’s oxidation number

Oxidation number = charge of ion

Example: Na⁺ = +1

→ being oxidated

Balancing redox reactions (basic)

Ensure: numbers of atoms equal, total charge equal, electron transfer must be conserved

Redox in biology: which cellular processes involve redox reactions?

Cellular respiration → glucose is oxidised, oxygen is reduced