Electrochemistry
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Electrochemistry. Determine the oxidation numbers for atoms in compounds and ions Identify reactions as redox or non-redox Include: oxidizing agent, reducing agent. Additional KEY Terms. both atoms and the electrons must be balanced in redox reactions. x 2 atoms = 2e -.

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Electrochemistry

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Electrochemistry

Electrochemistry


Electrochemistry

  • Determine the oxidation numbersfor atoms in compounds and ions

  • Identify reactions as redox or non-redox

  • Include: oxidizing agent, reducing agent

Additional KEY Terms


Electrochemistry

  • both atomsand the electronsmust be balancedin redoxreactions

x 2 atoms = 2e-

gains 1e-

0

+1

-1

0

oxidation numbers:

2

Cu + Cl2 → Cu+ + Cl-

2

2

x 2 = 2e-

loses 1e-

  • done by assigningoxidation numbers to allelements


Electrochemistry

**There is a written difference between ion charge and oxidation number**

Mg (s) + Cl2 (g) → MgCl2 (s)

Cl + 1e- → Cl1-

Mg → Mg2+ + 2e-

Magnesium ion

ion charge: 2+

oxidation #:+2

Chlorine ion

ion charge: 1-

oxidation #:-1

Sometimes these numbers are the same (like above) sometimes they are very different – which is why we write them differently


Electrochemistry

Oxidation number represents the chargethe atom would haveifevery bond were ionic.

O#s are based on the electronegativity of the atoms

1. Assign known numbers first, calculate others.

  • All uncombinedelements (& diatomics) – zero

O2

0

Cu

0

  • Monatomic ion -equals ion charge

Cl-

-1

Fe3+

+3


Electrochemistry

  • Alkalimetals (IA)– always +1

  • Earth metals (IIA)– always +2

  • Halogens(VIA) – usually-1

  • Hydrogen– usually+1

  • Oxygen – usually-2

Except metal hydrides (NaH)

Except peroxides (H2O2)

  • 2. Neutral compound:

  • Sumof O#s must equal zero

  • 3. Charged compound:

  • Sum of O#s must equalcharge of compound


Electrochemistry

  • 2. Neutral compound:

  • Sum of O# must equal zero

Assign O#s for each atom in K2Cr2O7

Step 1: Start with atoms which are known.

O: –2
K: +1

Step 2: Solve for other atoms.

+6

+1

Multiple the O#s by the subscripts for each atom to get the sum

-2

2

K2CrO7

+2

??

+12

= 0

-14

K = +1 Cr = +6 O = –2.


Electrochemistry

  • 3. Charged compound:

  • Sum of O# must equalcharge of compound

Assign O#s for each atom in NO3-

Step 1: Start with atoms which are known.

O: –2

Step 2: Solve for other atoms.

-2

+5

NO3-

+5

= -1

-6

??

N = +5 O = –2.


Electrochemistry

Use ox.numbers to determine if reaction is a redox reaction.

-2

+1

-2

+4

+4

+1

-2

SO2 + H2O → H2SO3

+4

-2

+2

-4

+2

-6

+4

Remember: Redox involves the transfer of electrons – no change in O#s during the reaction means NO redox

NOT a redox reaction


Electrochemistry

Is the following reaction a redox reaction?

-2

+5

0

+1

-2

+1

+5

0

Cu(s) + 2 AgNO3(aq) → CuNO3(aq) + 2 Ag(s)

0

+5

+1

-6

+5

+1

0

-6

Oxidation cannot occur without reduction

Redox reaction

• Cu – oxidized(loss of electrons)


• Ag – reduced(gain of electrons)


Electrochemistry

Oxidizing agent - causes the oxidation of another substance.

AgNO3 is the oxidizing agent

Reducing agent - causes the reduction of another substance.

Cu is the reducing agent

Think opposites: if the compound is the “oxidizing agent” of the reaction, then it is reduced in the process and vice versa

+1

0

Cu(s) + 2 AgNO3(aq) → CuNO3(aq) + 2 Ag(s)

+1

0


Electrochemistry

Identify the substance oxidized, the substance reduced, the oxidizing agent and the reducing agent.

+1

0

-2

+2

-2

-2

+1

+1

+5

-2

2 HNO3(aq) + 3 H2S(g) → 2 NO(g) + 3 S(s) + 4 H2O(l)

+2

0

+5

+1

-2

+2

-2

+2

-6

-2

S – oxidized

N – reduced

H2S – reducing agent

HNO3 – oxidizing agent

Substance - means element

Agent – means compound


Electrochemistry

How many electrons are transferred in the reaction below:

gains 3e-

+1

0

-2

+2

-2

-2

+1

+1

+5

-2

3

HNO3(aq) + H2S(g) → 2 NO(g) + 3 S(s) + 4 H2O(l)

2

loses 2e-

Stoichiometry (coefficients and subscripts) used to determined

total electronstransferred

S: (3 atoms) x (2e- lost) = 6 electrons lost

N: (2 atoms) x (3e- gained) = 6 electrons gained


Electrochemistry

  • CAN YOU / HAVE YOU?

  • Determine the oxidation numbersfor atoms in compounds and ions

  • Identify reactions as redox or non-redox

  • Include: oxidizing agent, reducing agent

Additional KEY Terms


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