Electrolysis Flashcards
In an electrolytic cell, how can you tell the nature of the electrolyte through the bulb?
When the switch is on, the electrolyte starts dissociating. Current remaining the same, the glow on the bulb indicates whether the electrolyte is a strong electrolyte - by bright glow of bulb, weak electrolyte - dim glow of bulb, non-electrolyte - when bulb doesn’t glow
Define electrolysis
Electrolysis is the decomposition of a chemical compound (electrolyte) in the fused or aqueous state by the passage of a direct electric current resulting in discharge of ions - as neutral atoms - at the respective electrodes
Which is the negative and positive electrode?
Cathode is the negative electrode
Anode is the positive electrode
Which reaction takes place at the cathode?
Cations gravitate to the cathode as it is the negative electrode, where they gain an electron to become a neutral atom - resulting in a reduction reaction
Which reaction takes place at the anode?
Anions gravitate to the anode as it is the positive electrode, where they lose an electron to become a neutral atom - resulting in an oxidation reaction
What type of reaction is electrolysis?
It involves a chemical change and is a redox reaction
Define electrolytes
Chemical compounds which conduct electricity in the fused or aqueous solution state & - undergo chemical decomposition due to the flow of current through them
What type of compounds are electrolytes?
Ionic compounds
What are the particles in electrolytes?
Ions only or Ions and molecules only
Give examples of electrolytes
Acids - dil. HCl, HNO3, H2SO4
Alkalis - KOH, NaOH solutions
Ionic salts - PbBr2 (molten), CuSO4 [aq.]
Define non-electrolytes
Chemical compounds which do not conduct electricity in the fused or aqueous soln. state & - do not undergo chemical decomposition due to the flow of current through them
What type of compounds are non-electrolytes?
Covalent compounds
What are the particles in non-electrolytes?
Molecules only
Give examples of non-electrolytes
Pure or distilled water, Alcohol, Kerosene, glucose, sucrose, sugar solution, Carbon disulphide (CS₂), liquid carbon tetrachloride (CCl₄)
Define strong electrolytes
They are electrolytes which allow a large amount of electricity to flow through them and hence are good conductors of electricity
How are strong electrolytes dissociated in fused or aq. soln. state?
They are almost completely dissociated in fused or aqueous solution state
Which particles are found in strong electrolytes?
Mainly ions only
What is the general prototype for examples of strong electrolytes?
Generally all strong acids and bases and most salts of strong acids
Give examples of strong electrolytes
Acids - dil. HCl, H2SO4, HNO3, HBr, HI,
Bases - NaOH, KOH, LiOH solns.
Salts - NaCl, Na2SO4, NaNO3, CuCl2 (Copper (II) Chloride), PbSO4 (Lead(II) sulfate), Pb(NO3)2 (Lead(II) nitrate), PbBr2 (Lead(II) bromide), AgI (Silver iodide) aq. solns.
Define weak electrolytes
They are electrolytes which allow small amounts of electricity to flow through them and hence are poor conductors of electricity
How are weak electrolytes dissociated in fused or aq. soln. state?
They are partially dissociated in fused or aqueous solution state
Which particles are found in weak electrolytes?
Ions and unionised molecules
What is the general prototype for examples of weak electrolytes?
Generally all weak acids and bases and most salts of weak acids
Give examples of weak electrolytes
Acids - Carbonic, acetic, oxalic, formic
Bases - NH4OH, Ca(OH)2, Mg(OH)2
Salts - Sodium - carbonate, bicarbonate, oxalate and formate aq. solns.
Define an electrolytic cell
The device in which electrolysis is carried out is called the electrolytic cell or voltameter which contains electrodes [cathode & anode] and the electrolytic solution
What are electrodes made of?
Metal or carbon
When are graphite (carbon) electrodes used?
When the products formed during electrolysis react with the metallic electrode
What is the anode?
It is the electrode connected to the positive terminal of the battery
Which ions migrate to the anode? Why?
Due to being connected to the positive terminal of the battery, anodes acquire a positive charge during electrolysis and hence, anions migrate to the anode
How do anions become neutral atoms at the anode?
The anions donate excess electrons to the anode and are oxidised to neutral atoms
What is the nature of anode?
It is the oxidising electrode
What is the cathode?
It is the electrode connected to the negative terminal of the battery
Which ions migrate to the cathode? Why?
Due to being connected to the negative terminal of the battery, the cathode acquires a negative charge during electrolysis. Hence, cations migrate to the cathode
How do cations become neutral atoms at the cathode?
The cations gain excess electrons and are reduced to neutral atoms
Which is the reducing electrode?
The cathode
By which electrode do electrons enter the electrolyte?
Cathode
By which electrode do electrons leave the electrolyte?
Anode
Define ions
They are atoms or groups of atoms which carry a positive or negative charge and become free and mobile when an electric current is passed through an aq. soln. of a chemical compound
Differentiate between anions and cations in terms of loss or gain of electrons
Anions donate or lose electrons to the anode (oxidation process) and get oxidised to neutral atoms, whereas cations gain or accept electrons from the cathode (reduction process) and get reduced to neutral atoms.
Write the formula which depicts oxidation
Cl^1- - 1e- -> Cl
Write the formula which depicts reduction
Na^1+ + 1e- -> Na
Who first explained the process of electrolysis
Swedish chemist Avante Arrhenius in 1887
Give the postulates of Arrhenius’ theory
- An electrolyte on dissolving in water dissociates into free cations and anions and allows the flow of electric current through it
- The degree of dissociation is the extent to which an electrolyte dissociates into ions
- All ions carry an electric charge and are responsible for the flow of current through the solution
- The number of positive charges on the ions equals the number of negative charges and thus the solution is in electrolytic equilibrium
What is the degree of dissociation?
It is the extent to which an electrolyte dissociates into ions
What does the amount of electricity conducted by the electrolyte depend upon?
The concentration of ions in the solution
What is electrolytic equilibrium?
The no. of positive charges on the ions equals the no. of negative charges and thus the solution is in electrolytic equilibrium
Where is an equilibrium also established?
Between the ions produced and unionized molecules
Which are the metallic ions?
Cations
Which are the non-metallic ions?
Anions
What does the preferential discharge of the ions depend on?
Its position in the electrochemical series
What is the number of electrons gained by the anode equal to?
The no. of electrons donated by the cathode
Where are the products of electrolysis formed?
At the anode and cathode itself
Where does the exchange of electrons take place in electrolysis?
At the surface of the electrodes
What is liberated at the cathode?
Hydrogen gas and metals - electropositive elements
What is liberated at the anode?
Non-metals - electronegative elements
Define electrolytic dissociation
The process due to which an ionic compound in the fused state or i aq. soln. state dissociates into ions by passage of electric current through it
Differentiate between electrolytic dissociation and ionisation in terms of where they take place
Electrolytic dissociation takes place at in electrovalent compounds, whereas ionisation takes place in covalent compounds
What does electrolytic dissociation involve?
It involves separation of ions of the ions which are already present in an ionic compound
Give example of a compound in which electrolytic dissociation takes place
PbBr2 ⇌ Pb^2+ + 2Br^1-
What does ionisation involve?
It involves formation of charged ions from the molecules which are not in the ionic state
Give example of a compound in which ionisation takes place
HCl [aq.] ⇌ H^1+ + Cl^1-
How does ionisation involve atoms changing into ions?
Mg -> Mg^2+ + 2e-
When will sodium chloride conduct electricity?
Only in the fused or aqueous solution state
Name a compound that is a non-electrolyte in the solid state, but conducts electricity in the molten or aqueous state
Sodium chloride
What is essential for conducting electricity?
Free ions
Why is an ionic compound in the solid state a bad conductor of electricity?
Ionic compounds contain positively charged metallic ions & negatively charged non-metallic ions. These ions are not free but held together by strong electrostatic force of attraction.
How to make ionic compounds good conductors of electricity?
When an ionic compound is heated strongly, the ions gain KE and break lose and move freely. The molten soln. becomes a good conductor of electricity
What does NaCl dissociate into, on being dissolved in water?
Sodium ion [Na+] and chloride ions [Cl-] which move freely in soln.
Why does NaCl dissociate into ions on being dissolved in water?
The slightly negatively charged oxygen atoms of water exerts a pull on the positively charged sodium ion. A similar pull is exerted by the slightly positively charged hydrogen atoms of the water on the negatively charged chloride ions. Thus, Na and Cl ions become free in soln.
Name the compounds which are non-electrolytes in the gaseous state but ionise in aq. soln. state
Polar covalent compounds - HCl, NH3
In which states does HCl not conduct an electric current?
In the gaseous state and in the pure liquid state
How is HCl polar covalent in nature?
It shows charge distribution in its molecules such that the hydrogen atom has a slight positive charge & chlorine atom a slight negative charge
What happens when HCl is added to water?
Water being a polar solvent, the slightly negatively charged oxygen atom of the water exerts an electrostatic pull on positively charged hydrogen ion of HCl. Thus H+ ions combine with water forming hydronium ions [H3O+] and the residual chloride ions remain in solution
Differentiate between metallic and electrolytic conduction in terms of transfer of matter
In electrolytic conduction, there is a transfer of ion. The flow of electricity decomposes the electrolyte and new products are formed. Whereas in metallic conduction, there is no transfer of matter. The flow of electricity only produces heat energy and no new products are formed
Why is copper metal a non-electrolyte?
It is a good conductor of electricity, but it is a non-electrolyte because it does not undergo chemical decomposition due to the flow of electric current through it
On what basis are acids, bases and salts classified into strong or weak electrolytes?
On the degree of dissociation
Which acids ionise on dissolution in water?
Mineral acids
What do mineral acids furnish in soln.?
H+ ions
What do bases furnish in soln.?
Furnish OH- ions in soln.
What do ionic salts furnish in soln.?
Furnish positive ions other than H+ ions and negative ions other than OH- ions
Name some strong alkalis
Hydrochloric acid, Nitric acid, Sulphuric acid
Name some strong bases
Potassium hydroxide, Sodium hydroxide, Lithium Hydroxide
Name some strong salts
Lead Bromide, Copper Chloride, Silver Nitrate
Name some weak salts
Sodium carbonate, Potassium bicarbonate, Lead acetate
Give another name for the electrochemical series
Metal activity series
Define the electrochemical series
Depending on the ease with which the metals lose their electrons and form ions, they are arranged in a series known as electrochemical series
Which metals ionize most readily?
The ones that are discharged with great difficulty
Where are the metals which ionize most readily placed in the electrochemical series?
At the top of the series
