Electrochemistry

Chapter 6 – Electrochemistry What are electrolytes? _ substances

 conduct electricity

 molten state or aqueous solution

 undergo chemical changes Why electrolytes can conduct

electricity?  Presence of free moving ions.

What are free moving ions?  Free moving cations or anions or both. What compounds have free

moving ions?

 _{Ionic compounds}

 _{in molten state or aqueous solution} What is the meaning of

“chemical changes”?

 _{changes of charge of ions or}  _{any particles}

What are particles?  _atoms  _molecules  _ions Hydrochloric acid (HCl) is an

electrolyte or not? Why?

 _{It is an electrolyte}

 _{Although HCl is a covalent compound, it dissolves in water}  _{and produce hydroxonium ions (H}

3O+) and chloride ions (Cl-).  _{HCl (aq) + H}

2O(l) → H3O+(aq) + Cl-(aq) [refer textbook pg. 117] Ammonia (NH3) solution is an

electrolyte or not? Why?

 _{It is an electrolyte}  _{Although NH}

3 is a covalent compound, it dissolves in water  _{and produce ammonium ions (NH}

4+) and hydroxide ions (OH-).

 _NH

3 (aq) + H2O(l) NH4+(aq) + OH-(aq) [refer textbook pg. 118] Why molten lead is an

electrolyte?

 _{In molten state, there are free moving lead ions (Pb}2+_). What is an ion?  _{It must have charge (positive or negative).}

 _{Cation or Anion} Why glucose solution is not

an electrolyte?

 _{Glucose are covalent compounds}

 _{Although Glucose dissolve in water, but}

 _{it exist as molecules that are neutral (without charge)}  _{It does not have any free moving ions}

 _{Cannot conduct electricity} Molecules have charge or not?

Why?

 _{Molecules do not have charge, they are neutral.}

 _{A molecule is formed by sharing of electrons of two or more unstable} non-metal atoms.

 _{The number of electrons in a molecule is equal to the total proton} number of the atoms.

 _{The total charge is neutral.} Why cation has positive

charge?

 _{Number of electrons in the cation is less than the proton number.} Why anion has negative

charge?

 _{Number of electrons in the anion is more than the proton number.} What is electrolysis?  _{a process of decomposition (分解)of}

 _{a compound in molten or aqueous state (solution)}  _{by using a direct electric current.}

Electrolytic cell consists of?  _{a battery}  _{an electrolyte}  _{two electrodes}

What is anode?  _{electrode connected to positive terminal of an electric source} What is cathode?  _{electrode connected to negative terminal of an electric source} What are the anions and

cations present in: (short-form:

mt = molten sl = solution)

1. Write out the chemical formula. For example AxBy 2. Determine the cations and anions.

Molten AxBy:

Cation : Ay+_{(l) Anion: B}x-_(l) AxBy Solution:

Cations : Ay+_{(aq), H}+_{(aq) Anion: B}x-_{(aq), OH}-_(aq) *** For solution, it must also have H+ _{and OH}-_.

*** cations and anions in molten state must be in liquid (l) state. *** cations and anions in solution must be in aqueous (aq) state.

 mt sodium chloride Cation: Na+_{(l) Anion: Cl}-_(l)

 mt zinc bromide Cation: Zn2+_{(l) Anion:Br}-_(l)

 mt aluminium oxide Cation: Al3+_{(l) Anion: O}2-_(l)

 mt sodium oxide Cation: Na+_{(l) Anion: O}2-_(l)

 mt potassium iodide Cation: K+_{(l) Anion: I}-_(l)

 mt lead(II) bromide Cation: Pb2+_{(l) Anion: Br}-_(l)

 sodium chloride sl Cation: Na+_{(aq), H}+_{(aq) Anion: Cl}-_{(aq) , OH}-_(aq)

 mt magnesium chloride Cation: Mg2+_{(l) Anion: Cl}-_(l)

 mt sodium iodide Cation: Na+_{(l) Anion: I}-_(l)

 mt potassium bromide Cation: K+_{(l) Anion: Br}-_(l)

 mt zinc iodie Cation: Zn2+_{(l) Anion: I}-_(l)

 copper(II) sulphate sl Cation: Cu2+_{(aq), H}+_{(aq) Anion: SO}

42-(aq), OH-(aq)

 dilute sulphuric acid Cation: H+_{(aq) Anion: SO}

42-(aq), OH-(aq)

 potassium iodide sl Cation: K+_{(aq), H}+_{(aq) Anion: I}-_{(aq), OH}-_(aq)

 dilute hydrochloric acid Cation: H+_{(aq) Anion: Cl}-_{(aq), OH}-_(aq)

 copper(II) nitrate sl Cation: Cu2+_{(aq), H}+_{(aq) Anion: NO}

3-(aq), OH-(aq)

 silver nitrate sl Cation: Ag+_{(aq), H}+_{(aq) Anion: NO}

3-(aq), OH-(aq) Cations and anions will be

attracted to which electrodes? _ Cations will be attracted to Cathode_{Anions will be attracted to Anode} Write the half equation for the

discharge of GE = gain electronsCation discharge its electrons by gaining electrons.LE = lose electrons Anion discharge its electrons by losing electrons.

 lead(II) ion. Pb2+_{(aq) + 2e}- _{→ Pb(s) or Pb}2+_{(l) + 2e}- _{→ Pb(s) GE}

 chloride ion. 2Cl-_{(aq) → Cl}

2(g) + 2e-or 2Cl-(l) → Cl2(g) + 2e- LE

 copper(II) ion. Cu2+_{(aq) + 2e}-_{→ Cu(s) or Cu}2+_{(l) + 2e}- _{→ Cu (s) GE}

 hydroxide ion. 4OH-_{(aq) → 2H}

2O(l) + O2(g) + 4e- LE

 sodium ion. Na+_{(aq) + e}-_{→ Na(s) or Na}+_{(l) + e}-_{→ Na(s) GE}

 bromide ion. 2Br-_{(aq) → Br}

2(g) + 2e- or 2Br-(l) → Br2(g) + 2e- LE

 fluoride ion. 2F-_{(aq) → F}

2(g) + 2e- or 2F-(l) → F2(g) + 2e- LE

 magnesium ion. Mg2+_{(aq) + 2e}-_{→ Mg(s) or Mg}2+_{(l) + 2e}-_{→ Mg(s) GE}

 oxide ion. 2O2-_{(l) → O}

2(g) + 4e- LE

 aluminium ion. Al3+_{(aq) + 3e}-_{→ Al(s) or Al}3+_{(l) + 3e}- _{→ Al(s) GE}

 zinc ion. Zn2+_{(aq) + 2e}-_{→ Zn(s) or Zn}2+_{(l) + 2e}- _{→ Zn(s) GE}

 potassium ion. K+_{(aq) + e}-_{→ K(s) or K}+_{(l) + e}- _{→ K(s) GE}

 iron(II) ion. Fe2+_{(aq) + 2e}- _{→ Fe(s) or Fe}2+_{(l) + 2e}- _{→ Fe(s) GE}

 hydrogen ion. 2H+_{(aq) + 2e}-_{→ H}

2(g) or 2H+(l) + 2e- → H2(g) GE

 copper(I) ion Cu+_{(aq) + e}-_{→ Cu(s) or Cu}+_{(l) + e}- _{→ Cu(s) GE}

What precaution should we take during electrolysis of molten compound which contain chloride and bromide ions?

 Carry out in a well-ventilated place

 Chlorine and bromine vapour are poisonous and pungent.

What are the observations, conclusions and products in anode and cathode in electrolysis of :

 Observation = what you see or feel.

 Conclusion = conclusion according to the observation

 Product = the substance at anode or cathode

 mt sodium chloride Anode : a gray substance is deposited Cathode : a greenish-yellow gas is given off Conclusion :

 mt sodium chloride conducts electricity.

 Chlorine gas is given off at the anode.

 Sodium is deposited at the cathode Product:

Anode : chlorine Cathode : Sodium

 mt lead(II) bromide Anode : a gray substance is deposited Cathode : a greenish-yellow gas is given off Conclusion :

 mt lead(II) bromide conducts electricity.

 Bromine gas is given off at the anode.

 Lead is deposited at the cathode Product:

Anode : Lead Cathode : Bromine

How to predict the products of

electrolysis? P1. Identify cations and anions present in compound._{P2. Identify cations attracted to cathode.}

P3. Identify anions attracted to anode.

P4. Determine number of electrons involved in discharge

(The number of electrons is equal to the charge on the ion, for example, Cu2+ _{will discharge 2 electrons)}

P5. Write half equation of cation gaining electrons P6. Write half equation of anion losing electrons.

P7. Write balance chemical equation by adding the two half-equations. *** Product of electrolysis is at the right hand side of chemical equation. What happens during an

electrolysis of molten lead(II) bromide, PbBr2(l)?

 _{Molten lead(II) bromide, PbBr}

2(l) is an electrolyte

 _{It has free moving lead ion, Pb}2+_{(l) and P1}

 _{free moving bromide ions, Br}-_{(l). P1}

 _{The positive ions (cations), lead ions, Pb}2+_{(l) move to cathode. P2}  _{The negative ions (anions), bromide ions, Br}-_{(l) move to anode. P3}  _{At cathode, a lead ion, Pb}2+_{(l) discharge by accepting 2 electrons P4}

to form a lead atom.

 _Pb2+_{(l) + 2e}- _{→ Pb(s) P5}

 _{Lead metal is formed at cathode.}

 _{At anode, a bromide ion, Br}- _{discharge by donating 1 electron to P4} form a bromine atom.

 _{2 bromine atoms combine to form a bromine gas, Br}

2 molecule.  _2Br-_{(l) → Br}

2(g) + 2e- P6

 _{Bromine gas is released at anode.}

 _{The overall equation of the electrolysis of molten lead(II) bromide,} PbBr2 is : Pb2+(l) + 2Br-(l) → Pb(s) + Br2(g) P7 What happens during an

electrolysis of molten sodium chloride, NaCl(l)?

 _{Molten sodium chloride, NaCl(l) is an electrolyte}

 _{It has free moving sodium ion, Na}+ _{and P1}

 _{free moving chloride ions, Cl}-_{. P1}

 _{The positive ions (cations), sodium ions, Na}+ _{move to cathode. P2}  _{The negative ions (anions), chlorides ions, Cl}- _{move to anode. P3}  _{At cathode, a sodium ion, Na}+ _{discharge by accepting 1 electron P4}

to form a sodium atom.

 _Na+_{(l) + e- → Na(s) P5}

 _{Sodium metal is formed at cathode.}

 _{At anode, a chloride ion, Cl}- _{discharge by donating 1 electron to form}

a chlorine atom. P4

 _{2 chlorine atoms combine to form a chlorine gas, Cl}

2 molecule.  _2Cl-_{(l) → Cl}

2(g) + 2e- P6

 _{Chlorine gas is released at anode.}

 _{The overall equation of the electrolysis of molten Sodium Chloride,} NaCl is : Na+_{(l) + 2Cl}-_{(l) → Na(s) + Cl}

2(g) P7

What happens during an electrolysis of molten lead(II) Oxide, PbO(l)?

 _{Molten lead(II) oxide, PbO(l) is an electrolyte}

 _{It has free moving lead ion, Pb}2+ _{and P1}

 _{free moving oxide ions, O}2-_{. P1}

 _{The positive ions (cations), lead ions, Pb}2+ _{move to cathode. P2}  _{The negative ions (anions), oxide ions, O}2- _{move to anode. P3}  _{At cathode, a lead ion, Pb}2+ _{discharge by accepting 2 electrons to form}

a lead atom. P4

 _Pb2+_{(l) + 2e- → Pb(s) P5}

 _{Lead metal is formed at cathode.}

 _{At anode, an oxide ion, O}2- _{discharge by donating 2 electrons to form}

an oxygen atom. P4

 _{2 oxygen atoms combine to form an oxygen gas, O}

2 molecule.  _2O2-_{(l) → O}

2(g) + 4e- P6

 _{Oxygen gas is released at anode.}

 _{The overall equation of the electrolysis of molten lead(II) oxide, PbO} is : Pb2+_{(l) + 2O}2-_{(l) → Pb(s) + O}

2(g) P7

What happens during an electrolysis of Copper(II) Sulphate solution, CuSO4(aq)?

 _{Copper(II) Sulphate solution, CuSO}

4(aq) is an electrolyte  _{It has free moving copper(II) ion, Cu}2+_{, sulphate ions, SO}

42- P1

 _{free moving hydrogen ions, H}+ _{and hydroxide ions, OH}-_{. P1}  _{The positive ions (cations), copper(II) ions, Cu}2+ _{and hydrogen ions,}

H+ _{move to cathode. P2}

 _{The negative ions (anions), sulphate ions, SO}

42- and hydroxide ions,

OH- _{move to anode. P3}

 _{At cathode, a copper(II) ion, Cu}2+ _{is selectively discharged by}

accepting 2 electrons to form a copper atom. P4

 _{Copper metal is formed at cathode.}

 _{At anode, a hydroxide ion, OH}- _{is selectively discharged by donating}

electrons to form water and oxygen gas. P4

 _4OH-_{(aq) → 2H}

2O(l) + O2(g) + 4e- P6

 _{Oxygen gas is released at anode.}

 _{The overall equation of the electrolysis of Copper(II) Sulphate} solution, CuSO4(aq) is :

2Cu2+_{(aq) + 4OH}-_{(aq) → 2Cu(s) + 2H}

2O(l) + O2(g) P7

Draw a labelled diagram to show the set-up of apparatus for an electrolysis of molten lead(II) Oxide, PbO(l).

 _{Refer figure 6.3 in Practical Book, page 60}  _{Change Solid lead(II) bromide, PbBr}

2 to Solid lead(II) Oxide, PbO.

What happens during an electrolysis of dilute sulphuric acid, H2SO4(aq)?

 _{Dilute sulphuric acid, H}

2SO4(aq) is an electrolyte  _{It has free moving sulphate ions, SO}

42-, P1

 _{free moving hydrogen ions, H}+ _{and P1}

 _{free moving hydroxide ions, OH}-_{. P1}

 _{The positive ions (cations), hydrogen ions, H}+ _{move to cathode. P2}  _{The negative ions (anions), sulphate ions, SO}

42- and hydroxide ions,

OH- _{move to anode. P3}

 _{At cathode, a hydrogen ion, H}+ _{discharge by accepting 1 electrons to}

form a hydrogen atom. P4

 _{2 hydrogen atoms combine to from a hydrogen gas, H}

2 molecule.  _2H+_{(aq) + 2e- → H}

2(g) P5

 _{Hydrogen gas is released at cathode.}

 _{At anode, a hydroxide ion, OH}- _{is selectively discharged by donating}

electrons to form water and oxygen gas. P4

 _4OH-_{(aq) → 2H}

2O(l) + O2(g) + 4e- P6

 _{Oxygen gas is released at anode.}

 _{The overall equation of the electrolysis of Dilute sulphuric acid,} H2SO4(aq) is :

4H+_{(aq) + 4OH}-_{(aq) → 2H}

2(g) + 2H2O(l) + O2(g) P7 The electrochemical series  _{Figure 6.3 in textbook page 101}

Draw a labelled diagram to show the set-up of apparatus for an electrolysis of dilute sulphuric acid, H2SO4(aq).

 _{Refer figure 6.5 in Practical Book page 62.}  _{Change electrolyte to dilute sulphuric acid, H}

2SO4(aq)

Unit of concentration  mol dm-3 _{or g dm}-3 Factors that affect the

electrolysis of an aqueous solution.

 _{Position of ions in the electrochemical series}  _{Concentration of ions in the electrolytes}  _{Types of electrodes used in the electrolysis} What happens during an

electrolysis of 1.0 mol dm-3

hydrochloric acid, HCl(aq)?

 _{1.0 mol dm}-3 _{hydrochloric acid, HCl(aq) is an electrolyte.}

 _{It has free moving chloride ions, Cl}-_{, P1}

 _{free moving hydrogen ions, H}+ _{and hydroxide ions, OH}-_{. P1}  _{The positive ions (cations), hydrogen ions, H}+ _{move to cathode. P2}  _{The negative ions (anions), Cl}- _{and OH}- _{move to anode. P3}  _{At cathode, a hydrogen ion, H}+ _{discharge by accepting 1 electrons to}

form a hydrogen atom. P4

 _{2 hydrogen atoms combine to from a hydrogen gas, H}

 _2H+_{(aq) + 2e- → H}

2(g) P5

 _{Hydrogen gas is released at cathode.}

 _{At anode, a chloride ion, Cl}- _{is selectively discharged by donating 1}

electron to form a chlorine atom. P4

 _{2 chlorine atoms combine to form a chlorine gas, Cl}

2 molecule.  _2Cl-_{(aq) → Cl}

2(g) + 2e- P6

 _{Chlorine gas is released at anode.}

 _{The overall equation of the electrolysis of 1.0 mol dm}-3 _hydrochloric acid, HCl(aq) is :

2H+_{(aq) + 2Cl}-_{(aq) → H}

2(g) + Cl2(g) P7

What happens during an electrolysis of 0.001 mol dm-3

hydrochloric acid, HCl(aq)?

 _{0.001 mol dm}-3 _{hydrochloric acid, HCl(aq) is an electrolyte.}

 _{It has free moving chloride ions, Cl}-_{, P1}

 _{free moving hydrogen ions, H}+ _{and P1}

 _{free moving hydroxide ions, OH}-_{. P1}

 _{The positive ions (cations), hydrogen ions, H}+ _{move to cathode. P2}  _{The negative ions (anions), chloride ions, Cl}- _{and hydroxide ions, OH}

-move to anode. P3

 _{At cathode, a hydrogen ion, H}+ _{discharge by accepting 1 electrons to}

form a hydrogen atom. P4

 _{2 hydrogen atoms combine to from a hydrogen gas, H}

2 molecule.  _2H+_{(aq) + 2e- → H}

2(g) P5

 _{Hydrogen gas is released at cathode.}

 _{At anode, a hydroxide ion, OH}- _{is selectively discharged by donating}

electrons to form water and oxygen gas. P4

 _4OH-_{(aq) → 2H}

2O(l) + O2(g) + 4e- P6

 _{Oxygen gas is released at anode.}

 _{The overall equation of the electrolysis of 1.0 mol dm}-3 _hydrochloric acid, HCl(aq) is :

4H+_{(aq) + 4OH}-_{(aq) → 2H}

2(g) + 2H2O(l) + O2(g) P7 Draw a labelled diagram to

show the set-up of apparatus for an electrolysis of

Copper(II) Sulphate solution, CuSO4(aq) with Copper electrode.

 _{Refer practical book 68, figure 6.9}

What happens during an electrolysis of Copper(II) Sulphate solution, CuSO4(aq) with Copper electrode?

 _{Copper(II) Sulphate solution, CuSO}

4(aq) is an electrolyte

 _{It has free moving copper(II) ion, Cu}2+_{, P1}  _{free moving sulphate ions, SO}

42-, P1

 _{free moving hydrogen ions, H}+ _{and hydroxide ions, OH}-_{. P1}  _{The positive ions (cations), copper(II) ions, Cu}2+ _{and hydrogen ions,}

H+ _{move to cathode. P2}

 _{The negative ions (anions), sulphate ions, SO}

42- and hydroxide ions,

OH- _{move to anode. P3}

 _{At cathode, a copper(II) ion, Cu}2+ _{is selectively discharged by}

accepting 2 electrons to form a copper atom. P4

 _Cu2+_{(aq) + 2e- → Cu(s) P5}

 _{Copper metal is formed at cathode.}

 _{At anode, the copper anode corrodes and dissolves in the} electrolyte.

 _{A copper atom donates 1 electron to form a copper(II) ion, P4} Cu2+_.

 _{Cu(s) → Cu}2+_{(aq) + 2e}- _P6

 _{The concentration of the copper(II) ions in the solution remains} unchanged.

 _{This is because of the rate of formation of copper(II) ions, Cu}2+ _at

the anode is the same as the rate of discharge of copper(II) ions, Cu2+ _{at the cathode.}

 _{The blue colour of the solution remain unchanged.}

 _{The overall equation of the electrolysis of Copper(II) Sulphate} solution, CuSO4(aq) is :

Cu2+_{(aq) + Cu(s) → Cu(s) + Cu}2+_{(aq) P7}

How do you test the present of

hydrogen and oxygen? _ Hydrogen gives a “pop” sound with a lighted wooden splinter_{Oxygen rekindles a glowing wooden splinter.} What are the observations and

products in the electrolysis of dilute sulphuric acid?

What is the overall result?

Observations:

Cathode : A gas is given off that gives a “pop” sound with a lighted wooden splinter

Anode : A gas is given off that rekindles a glowing wooden splinter Ratio of gas at cathode to volume at anode is 2:1

Products:

Cathode : Hydrogen Anode: Oxygen Overall result:

1. Electrolysis of dilute sulphuric acid is the decomposition of water 2. Concentration of the sulphuric acid inc as the electrolysis progresses. What are the observations and

products in the electrolysis of 0.005 mol dm-3 _potassium iodide solution and

2 mol dm-3 _{potassium iodide} solution?

0.005 mol dm-3 _{potassium iodide solution} Observations:

Cathode : A gas is given off that gives a “pop” sound with a lighted wooden splinter

Anode : A gas is given off that rekindles a glowing wooden splinter Products:

Cathode : Hydrogen Anode: Oxygen 2 mol dm-3 _{potassium iodide solution}

Observations:

Cathode : A gas is given off that gives a “pop” sound with a lighted wooden splinter

Anode : A yellow solution that gives a dark blue colour with iodine solution.

Products:

Cathode : Hydrogen Anode: Iodine What are the observations and

products in the electrolysis of 0.001 mol dm-3 _hydrochloric acid and 1 mol dm-3

hydrochloric acid?

0.001 mol dm-3 _{hydrochloric acid} Observations:

Cathode : A gas is given off that gives a “pop” sound with a lighted wooden splinter

Anode : A gas is given off that rekindles a glowing wooden splinter Products:

Cathode : Hydrogen Anode: Oxygen 1 mol dm-3 _{hydrochloric acid}

Observations:

Cathode : A gas is given off that gives a “pop” sound with a lighted wooden splinter

Anode : a greenish-yellow gas is given off that bleach moist litmus paper.

Products:

Cathode : Hydrogen Anode: Chlorine What are the reactive metals

can be extracted from their ores by electrolysis?

 _Aluminium  _magnesium In the electrolysis of the

reactive metal, metals are formed through the reaction at

 _{the cathode}

 _{metal ions are cations}

 _{cations move to cathode and discharge there.} Why cryolite, Na3AlF6 is

added to aluminium oxide?

 _{lower its melting points.}

 _{It serves as Catalyst (refer Form 5 – Chapter 1 – Rate of Reaction)} How do you purify an impure

copper through electrolysis?

 _{anode : Impure copper}  _{cathode : Pure copper}

 _{The electrolyte must have Copper(II) ions}  _{refer figure 6.5 in textbook page 102.}

 _{The impure copper will corrodes and dissolve in the electrolyte.}  _{Cu(s) → Cu}2+_{(aq) + 2e}

- _{Pure copper will form at cathode.}  _Cu2+_{(aq) + 2e- → Cu(s)}

 _{The overall equation of the electrolysis is :} Cu2+_{(aq) + Cu(s) → Cu(s) + Cu}2+_(aq) How do you electroplate a cup

with copper through electrolysis?

 _{anode : Copper electrode}  _{cathode : Cup}

 _{refer figure 6.6 in textbook page 103.}  _{refer the answer of the above question.} What is voltaic cell (vc)?  2 diff. metals

 being immersed into an electrolyte and

 connected by wire. Give an example of vc.  A cell with 2 diff metals

 inserted into a lemon and

What is Daniell cell? _ A voltaic cell

 Zinc and copper are used as electrodes.

 Each of the metals is immersed into a solution of its ions such as

 Zinc Sulphate solution, ZnSO4(aq) and

 Copper(II) Sulphate solution, CuSO4(aq)

 The 2 solutions are connected through a salt bridge or a porous pot.

 Refer Photograph 6.2 and 6.3 in textbook page 105. A solution of zinc ion _ Zinc Sulphate solution, ZnSO4(aq)

 Zinc Nitrate solution, Zn(NO3)2(aq)

 Zinc Chloride solution, ZnCl2(aq)

 Further details please refer chapter 8 - Salts A solutions of copper ion  Copper(II) Sulphate solution, CuSO4(aq)

 Copper(II) Nitrate solution, Cu(NO3)2(aq)

 Copper(II) Chloride solution, CuCl2(aq)

 Further details please refer o chapter 8 – Salts

o Form 5 chapter 3 – Oxidation and Reduction What is the function of the

salt bridge in Daniell Cell (see photograph 6.2 in textbook page 105)?

 To complete the electric circuit by allowing ions to flow from one half-cell to another

What is the function of the porous pot in Daniell Cell (see photograph 6.3 in textbook page 105)?

 To complete the electric circuit by allowing ions to flow from one half-cell to another

In vc, how do we determine its negative (-ve) terminal and positive (+ve) terminal?

-ve terminal

 Metal with higher position in electrochemical series (e/s). +ve terminal

 Metal with lower position in e/s. 1. Determine the –ve

terminal and +ve terminal in vc with following metals immersed in Sodium Chloride solution. 2. Write the 2 half equations.

3. Write the overall eq.

1. -ve terminal (-t)

o Metal with higher position in electrochemical series (e/s). +ve terminal (+t)

o Metal with lower position in e/s. 2. –t releases its electrons:

M(s) → Mn+_{(aq) + ne}- _{, M = -t}

Cation in the electrolyte with lower position in e/s receives the electrons released by –t. For example, cations in Sodium Chloride solution are Na+_{(aq) and H}+_{(aq). H}+ _{has lower position in e/s compared} to Na+_{. Thus, H}+_{(aq) will receives the electrons.}

2H+_{(aq) + 2e- → H} 2(g)

3. Write balance chemical equation by adding the two half-equations.

 Potassium and Sodium 1. -t: Potassium +t: Sodium 2. K(s) → K+_{(aq) + e}

-2H+_{(aq) + 2e- → H} 2(g)

3. 2K(s) + 2H+_{(aq) → 2K}+_{(aq) + H} 2(g)

 Calcium and Potassium 1. -t: Potassium +t: Calcium 2. K(s) → K+_{(aq) + e} -2H+_{(aq) + 2e- → H} 2(g) 3. 2K(s) + 2H+_{(aq) → 2K}+_{(aq) + H} 2(g)

 Zinc and Sodium 1. -t: Sodium +t: Zinc 2. Na(s) → Na+_{(aq) + e}

-2H+_{(aq) + 2e- → H} 2(g)

3. 2Na(s) + 2H+_{(aq) → 2Na}+_{(aq) + H} 2(g)

 Iron and Silver 1. -t: Iron +t: Silver

2. Fe(s) → Fe2+_{(aq) + 2e} -2H+_{(aq) + 2e- → H}

2(g)

3. Fe(s) + 2H+_{(aq) → Fe}2+_{(aq) + H} 2(g)

 Magnesium and Copper 1. -t: Magnesium +t: Copper 2. Mg(s) → Mg2+_{(aq) + e}

-2H+_{(aq) + 2e- → H} 2(g)

3. Mg(s) + 2H+_{(aq) → Mg}2+_{(aq) + H} 2(g)

 Aluminium and Iron 1. -t: Aluminium +t: Iron 2. Al(s) → Al3+_{(aq) + e}

-2H+_{(aq) + 2e- → H} 2(g)

3. 2Al(s) + 6H+_{(aq) → 2Al}3+_{(aq) + 3H} 2(g)

 Copper and Lead 1. -t: Lead +t: Copper

2. Pb(s) → Pb2+_{(aq) + e} -2H+_{(aq) + 2e- → H}

2(g)

3. Pb(s) + 2H+_{(aq) → Pb}2+_{(aq) + H} 2(g)

 Lead and Iron 1. -t: Iron +t: Lead

2. Fe(s) → Fe2+_{(aq) + 2e} -2H+_{(aq) + 2e- → H}

2(g)

3. Fe(s) + 2H+_{(aq) → Fe}2+_{(aq) + H} 2(g)

 Aluminium and Sodium 1. -t: Sodium +t: Aluminium 2. Na(s) → Na+_{(aq) + e}

-2H+_{(aq) + 2e- → H} 2(g)

3. 2Na(s) + 2H+_{(aq) → 2Na}+_{(aq) + H} 2(g)

 Lead and Silver 1. -t: Lead +t: Silver

2. Pb(s) → Pb2+_{(aq) + e} -2H+_{(aq) + 2e- → H}

2(g)

3. Pb(s) + 2H+_{(aq) → Pb}2+_{(aq) + H} 2(g)

 Silver and Tin 1. -t: Tin +t: Silver

2. Sn(s) → Sn2+_{(aq) + e} -2H+_{(aq) + 2e- → H}

2(g)

3. Sn(s) + 2H+_{(aq) → Sn}2+_{(aq) + H} 2(g)

 Tin and Zinc 1. -t: Zinc +t: Tin Zinc

2. Zn(s) → Zn2+_{(aq) + e} -2H+_{(aq) + 2e- → H}

2(g)

3. Zn(s) + 2H+_{(aq) → Zn}2+_{(aq) + H} 2(g)

 Copper and Silver 1. -t: Copper +t: Silver 2. Cu(s) → Cu2+_{(aq) + e}

-2H+_{(aq) + 2e- → H} 2(g)

3. Cu(s) + 2H+_{(aq) → Cu}2+_{(aq) + H} 2(g) In Daniel Cell (DC), what are

the metals used as –t and +t? -ve terminal+ve terminal : Copper: Zinc

In DC, -t is immersed in _____________ solution. Zinc Sulphate In DC, +t is immersed in _____________ solution. Copper(II) Sulphate The 2 half cells in DC are

connected by ___________. _ Salt bridge or_{Porous pot / device} Half equation in –t of DC Zn(s) → Zn2+_{(aq) + e} -Half equation in +t of DC Cu2+_{(aq) + 2e- → Cu(s)}

Overall equation in DC Zn(s) + Cu2+_{(aq) → Zn}2+_{(aq) + Cu(s)} As the DC reaction

progresses, what happen to the ions in 2 half cells?

 Zinc ions in the Zn(s)/Zn2+_{(aq) half-cell increase.}

 Copper(II) ions in the Cu(s)/Cu2+_{(aq) half-cell decrease.} The differences between

primary cells (p/c) and secondary cells (s/c)

 Primary cells are non-rechargeable cells

 Secondary cells are rechargeable cells

Examples of p/c _ Dry cell/zinc-carbon cell, alkaline-manganese cell, silver oxide cell and lithium cell

Examples of s/c  Nickel-cadmium cells, lithium ion cell, nickel metal hydride (NiMH batteries) and lead-acid accumulator (car battery)

Compare an electrolytic cell

and a voltaic cell: Electrolytic Cell Voltaic Cell

 Function Uses direct electric current to produce a non-spontaneous chemical change

Uses a spontaneous chemical change to produce a direct electric current

 Structure 2 electrodes immersed in an electrolyte. The two electrodes are connected to a battery.

2 different metal electrodes immersed in an electrolyte. Two 2 electrodes are connected to a device that uses electricity such as bulb. Could also be made up of 2 half-cells.

 Polarity of electrodes -ve elelectrode = cathode

+ve electrode = anode -ve terminal+ve terminal = < reactive metal= > reactive metal

 Process occurring at negatively charged electrode

Electrons are gained by cations. The atoms of the –t lose electrons to form ions.

 Process occurring at positively charged electrode

Electrons are released by anions. The ions from the electrolyte gain electrons to form atoms.

 Flow of electrons Fr. anode to +t of battery and then fr. –t of battery to cathode

From (fr.) –t to +t

 Products  Electrolyte = molten ionic compound (i.c.)

- Constituent elements of i.c.

 Electrolyte = aq solution

- depend on factors affecting the selective discharge of ions.

Electricity

Find the position in electrochemical series (e.s.) for the following pair of metals:

A and C A x A x volt C B and C B y y < x A B x volt y volt C A and D A z z > x A B x volt y volt z volt C D Do the question in form 4

chemistry textbook page 109

(Work This Out 6.6) No.

Negative terminal

Positive

terminal Cell voltage

(a) (i) (ii) (iii) (iv) Magnesium Magnesium Zinc Zinc Silver Zinc Lead Silver 3.0V + 0.5V = 3.5V 3.0V – 1.1V = 1.9V 1.1V – 0.4V = 0.7V 1.1V + 0.5V = 1.6V (b) Metal Q Silver 2.5V

Mg-Ag produces 3.5V whereas Zn-Ag produces 1.6V, so metal Q must be in between Mg and Zn as 2.5 V is in between 3.5V and 1.6V.

Do the Review Questions in form 4 chemistry textbook pg 113, 114 and 115

1. A is not electrolyte as it has no free moving electrons. It has free moving molecules only.

B is electrolyte as it has free moving electrons.

C is not electrolyte as it has no free moving electrons. It has free moving molecules only.

D is not electrolyte as it has no free moving electrons. 2. Molten Aluminium Oxide has Al3+_{(l) and O}2-_{(l) ions.}

Al3+_{(l) discharges its ions at cathode:} Al3+_{(l) + 3e}- _{→ Al(s)}

O2-_{(l) discharges its ions at anode:} 2O2-_{(l) → O}

2(g) +

4e-3. Hydrochloric acid has H+_{(aq), Cl}-_{(aq) and OH}-_{(aq) ions.}

Test tube X cover electrode positive (anode), thus OH-_{(aq) will be} selectively discharged at anode and produce oxygen gas which can be tested by a glowing splinter.

Lime water → CO2, A lighted splinter → H2, A moist litmus paper → acid or alkali

4. cathode: iron key and anode: silver → B 5. Refer to Electrochemical series (e.s.)

(K, Na, Ca, Mg, Al, Zn, Fe, Sn, Pb, Cu, Ag)

Copper(Cu) and Magnesium(Mg) has the longest distance – D 6. Negative terminal = Magnesium

(as it has higher position than Zn in e.s.) Mg(s) → Mg2+_{(aq) + 2e}

-7. A it will increase the voltage as the distance of 2 metals in e.s. become bigger.

B it will lower the voltage as the distance of 2 metals in e.s. become smaller.

C it has no effect of changing electrolyte. D it has no effect in change the container.

8. Q>P (2.5V) , Q>R (3.0V) , R>S (0.2V) → Q>P>R>S 9. Q > Pb, Q < Fe → Metal Q = Sn

Metal which is higher than Sn in e.s. = Mg, Zn, Al 10. I and III

Subjective Questions

(a) _{X = Silver, Y = impure silver}

(in electroplating, anode: metal; purification, anode: impure metal) (b) _Ag+_{(aq) → Ag(s) + e}

-(c) _{Y dissolve in silver nitrate solution which decrease its mass and anion} Y in silver nitrate solution discharge its electrons and deposits at Z which increase its mass.

(d) _{Unchange as the rate of X dissolve in silver nitrate solution is same as} the rate of X discharge its electrons and deposit at Iron ring.