Biology - acids and bases

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  • Created by: Ivana
  • Created on: 28-11-13 12:15

Concept

> Bronsted: acids and base reactions involve the transfer of an H+ ion (proton)
Acid = proton donor (has a conjugate base)
Base = proton acceptor (has a conjugate acid)
>An acids conjugate base is formed by removing the acids proton. 
Acid and conjugate base are opposite in strength HCl (Strong acid) CL- weak base. Cl- anion is HCl's conjugate base.  

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Strong and weak acids

-Dissociation equillibrium = a measure of the strength of an acid
OR Ka - the dissociation constant.

HA (acid) + H20 (base)  <-> H3o+ (acid)  +   A- (base)

ka= [H3o+] [A+]       [product]
    [HA]            [reactant]

>A strong acid (proton donor): 
Readily dissociates at all pH values - willing to donate and hydrogen ion
Has a high Ka value
Eg HCl has a Ka = 1 x 10'3 and completely dissociates at pH 0-14 

>A weak acid
Dissociation is pH dependent - less willing to donate a hydrogen ion
Small Ka value 
Eg Acetic acid has a Ka = 1.8 x 10 '-5 and starts to dissociate at pH 3 - full dissociated above pH7 

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Strong and weak acids #2

Weak acid = Low ph -> molecular form
High pH -> ionic form
Strong acid = dissociates at lower pH

AS YOU INCREASE THE pH THERE IS A HIGHER LEVEL OF DISSOCIATION OF H+ IONS, GENERALLY 

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Ion product (kw)

The ion product of water is always constant (1.0 x 10'-14) 
each ion = square root of 1.0 x 10'-14

This is worked out by rearranging the formula for the equilibrium constant (keq) to isolate the products (kw) 

So if the concentration of H+ increases above 1 x 10'-7 then the concentration of OH- must decrease below 1x10'-7 

keq = product/reactant 

kw= keq x reactant = product

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Water

-Contains acidic ion (H3O+) called an hydronium ion
-Contains a basic ion (Oh-) called a hydroxide ion

H3O+ will give up its proton to OH- from 2H20 -> neutral. 
this reaction happens continuously in water
overall the sample is neutral because there are equal amounts of H3O+ and OH-

For most reactions the acids and bases are not present in equal amounts and this imabalnce is what allows a chemical reaction to occur 

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Properties of acids and bases

  • ACIDS
    Sour taste
    String the skin
    Can corrode metals and skin
    Can be sued a s a reactant during electrolysis due to the presence of mobile ions 
    Turn litmus paper red
    Turn red/orange on universal indicator
    pH level lower than pH7
  • BASE
    Slimy/soapy feel due to saponficiation of lipids in human skin
    Concentrated or strong bases are caustic on organic matter and react violenty with acidic substances
    Turn litmus paper blue
    pH level is higher than 7
    Bitter in taste 
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pOH and pH

H+ INCREASES
pH INCREASES
OH- DECREASES
pOH DECREASES 

The expression of pOH is sometimes used to describe the basicity or OH- concentration of a solution

pOH -> pOH = -log[OH-]

pH+pOH = 14 ALWAYS.

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Titration -buffering region

(http://static.newworldencyclopedia.org/thumb/1/15/Buffer_titration.svg/250px-Buffer_titration.svg.png)BUFFERING REGION: pH doesnt increase as much as in the buffering region when OH- is added

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Buffers

  • Proteins have pH optima for their activity and for their ideal storage conditions
  • Outside these pH values a protein may become denatured and inactivated 
  • Maintaining the pH of a solution is vitally important in biology
  • A BUFFER -> composed of a weak acid and one of its salts (conjugate base). OR a weak base and one of its salts (conjugate acid) 
  • A buffer can maintain the pH of a solution by soaking up (on addition of acids or releasing protons (on addition of base). 
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Henderson Hasselbach equation

-used to determine the quantitative aspects of buffers

-pKA > acid dissociation constant

-pH = pKA + log10 [A-] PROTON ACCEPTOR -acetate

               [HA] PROTON DONOR acid

SAMPLE QUESTION: Calculate the pH of a sodium acetate buffer solution containing 0.25M of acetic acid and 0.5M sodum acetate. The pKA of acetic acid is 4.76.

pH = 4.76 + log10 [0.5]
            [0.25] 
  log10  [0.5]      =0.30
[0.25] 
4.76 + 0.30 = 5.06 

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Acid titration curve

(http://static.newworldencyclopedia.org/thumb/1/15/Buffer_titration.svg/250px-Buffer_titration.svg.png)-Titration curve = serves to profile the unknown solution Eg the pH of a solution. 
-Calculate the unkown amount of acid in the recieving flask by measuring the amount of base/titrant it takes to neutralise the acid.
-From the henderson hasslebach equation we know that pH = the pka of the indicator at the end point of the indicator.
-We know the pH of the solution and the vol. of titrant we can deduce how much base was needed to neutralise the sample.

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FREE ENERGY

Gibbs Free energy equation:
 (http://www.chem.tamu.edu/class/majors/tutorialnotefiles/gfe1.gif)
Free energy= sum of its enthalpy (H) - the product of the temperature (kelvin) and the entropy of the system (s)

H = total heat content of a system
S= the unavailability of a systems thermal energy for conversion into mechanical work. 
H-TS = entropy change 

molecules prefer to be unrestricted. This happens when the entropy of the system is increase. 
Solid -> (entropy increased) -> liquid -> (entropy increased) -> gas

-ve(http://www.chem.tamu.edu/class/majors/tutorialnotefiles/delta.gif)G  = reaction is said to be favourable and the energy released can be utilised to do work
+ve(http://www.chem.tamu.edu/class/majors/tutorialnotefiles/delta.gif) = the reaction is said to be unfavourable
(http://www.chem.tamu.edu/class/majors/tutorialnotefiles/delta.gif) = 0 the reaction is at equilibrium

 

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