Arrhenius Theory

1
Acids and Bases

• Lesson 1
• Arrhenius Theory
• Bronsted-Lowry Theory

2
History of Acids and Bases
In the early days of chemistry chemists were
organizing physical and chemical properties of
substances. They discovered that many substances
could be placed in two different property
categories

• Substance B
• Bitter taste
• Reacts with fats to make soaps
• Do not react with metals
• Turns red litmus blue
• Reacts with A substances make salt and water

• Substance A
• Sour taste
• Reacts with carbonates to make CO2
• Reacts with metals to produce H2
• Turns blue litmus pink
• Reacts with B substances to make salt water

Arrhenius was the first person to suggest a
reason why substances are in A or B due to their
ionization in water.
3
Arrhenius Theory
The Swedish chemist Svante Arrhenius proposed the
first definition of acids and bases. (Substances
A and B became known as acids and
bases) According to the Arrhenius model
acids are substances that dissociate in water to
produce H ions and bases are substances that
dissociate in water to produce OH- ions NaOH
(aq) ? Na (aq) OH- (aq) Base HCl (aq)
? H (aq) Cl- (aq) Acid
4
Acids and Bases

• Before
• HCl(g) ? H(aq) Cl-(aq)
• Acids dissociate in water
• H and Cl- ions floating around in water
• Not really the case.

5
Acids and Bases

• Hydrogen
• 1 proton, 1 electron
• When it forms an ion it loses an electron
• H
• Because it loses its electron we call H ions a
  proton
• Reactions where H ions are transferred from one
  thing to another are called proton transfers

6
Acids and Bases

• Charge on H ions is very small, and very
  concentrated
• Because of this it is attracted to anything
  remotely negative
• Recall Lewis Dot structures.
• Oxygen 6 valence electrons
• Hydrogen 1 valence electron

7
Acids and Bases

• When they form water they shared electrons
• Oxygen has more electrons and because of the
  shared electrons are closer to oxygen then
  Hydrogen
• This makes water a polar covalent molecule
• more electrons close to oxygen end of the water
  molecule, that end has a partial negative charge.
  
• Less electrons around the hydrogen end that
  ends a has a partial positive charge

8
Acids and Bases
9
Acids and Bases

• Partial negative charge on oxygen very attractive
  to our H ion
• H will sit on one of electrons pairs of oxygen
  atoms
• Brings a positive charge
• H3O hydronium ion
• This ion will always form when an acids is
  dissolved in water
• H ion from acid will always go to nearest
  molecule to from H3O

10
Acids and Bases

• For this reason, a hydronium ion a hydrated
  proton.
• Adding water to something is called hydration.  
• So if you were a proton, you would have a water
  molecule added to you.
• All acid solutions contain hydronium (H3O) ions.
  It is the hydronium ion which gives all acids
  their properties (like sour taste, indicator
  colours, reactivity with metals etc. )
• HCl(g) ? H(aq) Cl-(aq) NO
• HCl(g) H2O(l) ? H3O(aq) Cl-(aq) YES!!
  

11
Acids and Bases

• The proton (H) has been transferred from the HCl
  molecule to a water molecule, to form a hydronium
  (H3O) ion and a Cl- ion.
•  This type of reaction is called ionization
  (because ions are being formed)
• NOT an H atom that is moving. The H atom leaves
  its electron behind with the Cl, so it is H (a
  proton) that moves to the water molecule. The
  Cl-, now having the electron that H left behind,
  gains a negative charge.

12
Acids and Bases

• All acids behave similarly in water they donate
  (or give) a proton (H) to the water, forming
  hydronium ion (H3O) and the negative ion of the
  acid.
• Another example might be the ionization of nitric
  acid (HNO3)
• HNO3 (l) H2O (l) ? H3O(aq)
  NO3-(aq)

13
Acids and Bases

• NOT A WORKSHEETbut more practice problems!
• Complete equations for the following acids
  ionizing in water
• HClO (g)
• H2SO4 (l) (assume only 1 H is removed.)
• CH3COOH (l) (assume the H on the right end comes
  off.)
• HSO4- (aq)

14
Bronsted-Lowry Definition of Acids and Bases

• You might recall that the definition of an acid
  according to Arrhenius was a substance that
  released H ions (protons) in water.
• Bronsted and Lowry came up with a theory which is
  more useful when dealing with equilibrium and
  covers a wider range of substances.
• Bronsted-Lowry
• An acid is any substance which donates (gives) a
  proton (H) to another substance.
• A base is any substance which accepts (takes) a
  proton from another substance
• -OR-
• A Bronsted Acid is a proton donor
• A Bronsted Base is a proton acceptor

15
Bronsted-Lowry Definition of Acids and Bases

• HCl H2O ? H3O Cl-
  
•  
• We see that the HCl is donating the proton and
  the water is accepting the proton.
• Therefore HCl is the Bronsted acid and H2O is the
  Bronsted base.
• HCl H2O ? H3O Cl-
  
• acid base

16
Bronsted-Lowry Definition of Acids and Bases

• NH3 H2O ? NH4
  OH-
• Now, the NH3 on the left has changed into NH4 on
  the right, that means it must have accepted
  (taken) a proton.
• Since it has accepted a proton its called a
  base.
• The H2O, this time has donated (lost) a proton as
  it changed into OH-.
• Since it has donated a proton its called an
  acid.
• So now we can label these
• NH3 H2O ? NH4
  OH-
• base acid

17
Bronsted-Lowry Definition of Acids and Bases

• Water is an acid and base?
• Sometimes water acts like a base (takes a proton)
  and sometimes it acts like an acid (donates a
  proton).
• This is just like you. If you buy something
  (donate money) you are a buyer. If you sell
  something (accept money), you are a seller. Im
  sure you have been both at various times.
• Animals that can live either in the water or on
  land are called amphibians. (Yes, this is still
  Chemistry just in case youre wondering!)
• For things that can be either / or , we can use
  the prefix amphi
• A substance that can act as either an acid or a
  base is called amphiprotic.

18
Bronsted-Lowry Definition of Acids and Bases

• Water (H2O) is an example of an amphiprotic
  substance. When it was with HCl, it acted like a
  base, but when it was with NH3, it acted like an
  acid.
• Not only molecules can lose or gain protons. Ions
  can too
• When something loses a proton (acts as an acid),
  it turns into something with one less H and one
  less () charge
•  When something accepts a proton (acts as a
  base), it turns into something with one more H
  and one more () charge
• So what you have to do is look at the right side
  of the equation, and see whether the substance
  gained or lost a proton.

19
Bronsted-Lowry Definition of Acids and Bases

• HCO3- HSO4- ? H2CO3
  SO42-
• HCO3- must have accepted a proton to form H2CO3
• So it is the base.
•  HSO4- must have donated a proton to form
  SO42-
• so it must be the acid.
• so the answer is
• HCO3- HSO4- ? H2CO3
  SO42-
• base acid