The Chemistry of Life Water: Acids, Basis, & pH copyright cmassengale

The Chemistry of Life

Water: Acids, Basis, & pH

copyright cmassengale

Acids and BasesAcids and Bases

• One of the most important aspects of a living system is the degree of acidity or alkalinity

– A water molecule is neutral, but can react to form hydrogen and hydroxide ions.

– Water disassociates (ionizes) in itself.– Breaking apart of the water molecule into

two ions of opposite charge (due to strong attraction of oxygen atom of one molecule for H atom of another water molecule)

– Usually does this for 1/10,000,000 or 1E-7

H2O H+ (hydrogen ion) + OH- (hydroxide ion)

H+ + H2O H3O (hydronium ion)

Water DissociationWater Dissociation

Water DissociationWater Dissociation• 1 water molecule in 550 million 1 water molecule in 550 million

naturally dissociates into a naturally dissociates into a Hydrogen IonHydrogen Ion and a and a Hydroxide Hydroxide IonIon

Hydrogen Ion Hydroxide IonHydrogen Ion Hydroxide Ion Acid Acid BaseBase

H2O H+ + OH-


5

“Dissociation” of water

The pH ScaleThe pH Scale

• Water dissociation is the basis for this scale• pH is a measure of this dissociation.

According to the following formula: pH = -Log [H+]

• logarithmic scale for comparing the relative concentrations of hydronium ions and hydroxide ions in a solution

• Each pH unit represents a factor of 10X change in concentration


How much stronger is a How much stronger is a pH3 than a pH of 5?pH3 than a pH of 5?

The pH ScaleThe pH Scale• Indicates the concentration of H+ ions• Ranges from 0 – 14• pH of 7 is neutral• pH 0 up to 7 higher acidity … H+ >

OH-

• pH above 7 to 14 higher alkalinity… OH- > H+


Concentration

The pH ScaleThe pH Scale

• At a pH of 7, the concentration of H+ ions and OH- ions is equal.


AcidsAcids•Strong

Acid = pH 1-3

•High in H+

ions•Lower

number of OH- ions


AcidsAcids• An acid is any compound that forms H+

ions in solution (Simple definition)• Number of hydronium ions in solutions

is greater than the number of hydroxide ions

•HCl H+ + Cl-


Acids Facts

• Sour taste• Affect the color of indicators: turn

litmus paper from blue to red• pH 1-6.9• common acids: sulfuric, hydrochloric,

nitric, and acetic acids

BasesBases•Strong Base

= pH 11 – 14•High in OH-

ions•Lower in

number of H+ ions


BasesBases• A base is a compound that produces

hydroxide ions (OH- ions) in solution.• Number of hydroxide ions in solution is

greater than the number of hydronium ions•NaOH Na+ + OH-


Bases Facts

• Bases are slippery• Bitter taste • Turn litmus paper from red to blue• pH 7.1-14• potassium hydroxide, calcium hydroxide,

sodium hydroxide, and ammonium hydroxide

• the lower the pH the stronger the acid

• the higher the pH the stronger the base

• pH 7.0 is neutral

REMEMBER!!REMEMBER!!

Salt

•Salt is a compound formed when an acid is mixed with a base (positive ion from a base and a negative ion from an acid)

Neutralization

•A neutralization reaction occurs when an acid and a base combine to form salt and water

Precipitate

•A precipitate is an insoluble substance crystallizes out of solution (ex: salt from a neutralization reaction)

Precipitation reaction

•The process of forming a precipitate is precipitation

Buffers• The pH of the fluids within most cells in the

human body must generally be kept between 6.5 and 7.5.

Controlling pH is important for maintaining homeostasis.

Remember one example we already learned about of something in biology that needs specific pH to work?

BuffersBuffers


• Control of pH is very important

• Most enzymes function only within a very narrow pH

• Control is accomplished with buffers made by the body

• Buffers keep a neutral pH (pH 7)

BuffersBuffers• Weak acids or basesWeak acids or bases that that

react with strong acids or react with strong acids or basesbases

• Made by the bodyMade by the body• Prevent sharp, sudden Prevent sharp, sudden

changes in pH changes in pH (keep pH (keep pH neutral)neutral)

Weak AcidWeak Acid Weak BaseWeak Basecopyright cmassengale

BuffersBuffers


• Buffers neutralize small amounts of either an acid or base added to a solution

• Complex buffering systems maintain the pH values of your body’s many fluids at normal and safe levels

BuffersBuffers


•Buffers resist changes to the pH of a solution when H+ or OH- is added to the solution.

•Buffers accept hydrogen ions from the solution when they are in excess and donate hydrogen ions when they have been depleted.

Some Key Points for the Test•Many properties of water are emergent properties due to hydrogen bonding.

•The cohesion of water molecules to each other is exploited by plants and animals.

•Water resists temperature changes by absorbing lots of heat.

•Lower density of ice causes it to float & insulate the water below.

•The polarity of water allows it to dissolve other polar molecules.

•Non-polar compounds are hydrophobic and not easily dissolved in water.

•Adding or removing hydrogen ions changes the pH of a solution.

•Buffers resist pH changes by accepting or donating H ions when [H+] changes.


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The Chemistry of Life Water: Acids, Basis, & pH copyright cmassengale