Reactions with Acids and the Reactivity Series

Reaction of Metals with Acids

When metals react with dilute acids, they produce salt and hydrogen gas.

General Reaction

$$\text{Metal} + \text{Dilute Acid} \rightarrow \text{Salt} + H_2\uparrow$$

(Refer to Chapter 2 — Acids, Bases and Salts.)

Major Examples

With Sodium (very dangerous — explosive): $$2Na + 2HCl \rightarrow 2NaCl + H_2\uparrow$$

With Magnesium (common classroom demonstration): $$Mg + 2HCl \rightarrow MgCl_2 + H_2\uparrow$$

With Zinc (very frequently asked): $$Zn + 2HCl \rightarrow ZnCl_2 + H_2\uparrow$$

$$Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2\uparrow$$

With Iron: $$Fe + 2HCl \rightarrow FeCl_2 + H_2\uparrow$$

$$Fe + H_2SO_4 \rightarrow FeSO_4 + H_2\uparrow$$

With Aluminium: $$2Al + 6HCl \rightarrow 2AlCl_3 + 3H_2\uparrow$$

Intensity of Reaction

More reactive metal = faster reaction.

Order of intensity (visually observed):

• Na/K — explosive (never in classroom!)
• Mg — vigorous bubbling, lots of heat
• Zn — moderate bubbling
• Fe — slow reaction
• Cu, Au — no reaction at all!

Metals That Don't React with Acids

Behaviour of Cu, Ag, Au

Cu, Ag, Au — no reaction with dilute $HCl$ or $H_2SO_4$.

Why? — These are below hydrogen in the reactivity series.

The Main Rule

Only metals above hydrogen (in the reactivity series) can release $H_2$ from acids.

Some Exceptions with Concentrated Acids

Although Cu doesn't react with dilute acid, it does react with concentrated $H_2SO_4$ or concentrated $HNO_3$.

Cu + Concentrated $H_2SO_4$: $$Cu + 2H_2SO_4 \xrightarrow{\Delta} CuSO_4 + 2H_2O + SO_2\uparrow$$

Cu + Concentrated $HNO_3$: $$Cu + 4HNO_3 \rightarrow Cu(NO_3)_2 + 2H_2O + 2NO_2\uparrow$$

(This is not displacement — it is redox. Here Cu is oxidised, $H_2$ is not produced.)

With Gold — Aqua Regia

Gold — unaffected by all acids.

Exception — Aqua Regia (Royal Water):

Concentrated $HNO_3$ + Concentrated $HCl$ (in 1:3 ratio)

Only this can dissolve gold.

Meaning of the name:

• Aqua = water
• Regia = royal
• 'Royal Water' — because it can dissolve the 'king of metals' (gold).

Why doesn't metal give $H_2$ with $HNO_3$?

$HNO_3$ is a strong oxidising agent. It immediately converts any $H_2$ released into $H_2O$. That is why metals do not give $H_2$ with $HNO_3$ — they give $NO_2$ or $NO$ instead.

Exception: Mg and Mn — give $H_2$ with very dilute $HNO_3$.

$$Mg + 2HNO_3(\text{very dilute}) \rightarrow Mg(NO_3)_2 + H_2\uparrow$$

Displacement Reactions — From Metal Salt Solutions

Principle

A more reactive metal displaces a less reactive metal from its salt solution.

General Form

$$A + BC \rightarrow AC + B$$

(Where A and B are metals, and A is more reactive.)

Famous Example

Zn + CuSO₄ (blue) → ZnSO₄ (colourless) + Cu (red):

$$Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)$$

Visual Observations:

• Blue colour of $CuSO_4$ fades gradually.
• A red/brown layer of Cu is deposited on the Zn strip.
• Zn slowly dissolves.

Here Zn displaced Cu — because Zn is more reactive.

Other Examples

1. Fe + CuSO₄: $$Fe + CuSO_4 \rightarrow FeSO_4 + Cu$$ (Light green FeSO₄ formed; copper coating on iron)

2. Cu + AgNO₃: $$Cu + 2AgNO_3 \rightarrow Cu(NO_3)_2 + 2Ag$$ (Colourless AgNO₃ becomes blue Cu(NO₃)₂; silver coating on copper)

3. Mg + CuSO₄: $$Mg + CuSO_4 \rightarrow MgSO_4 + Cu$$ (Colourless MgSO₄; Cu layer on Mg)

When Displacement Doesn't Happen

Less reactive metal → no reaction with salt of more reactive metal.

Examples (no reaction):

Cu + ZnSO₄ → no reaction

(Because Cu is less reactive than Zn.)

Ag + CuSO₄ → no reaction

(Because Ag is less reactive than Cu.)

An Experiment — Confirming the Activity Series

4 test tubes:

• A: $FeSO_4$ + Zn piece → Fe deposits
• B: $ZnSO_4$ + Cu piece → no reaction
• C: $CuSO_4$ + Ag piece → no reaction
• D: $AgNO_3$ + Cu piece → Ag deposits

Conclusion: $Zn > Fe > Cu > Ag$

The Reactivity Series

What is it?

An ordered list of metals from most reactive to least reactive.

The Complete Reactivity Series

(Top = more reactive, Bottom = less reactive)

K     Potassium    ↑ Highly reactive
Na    Sodium
Ca    Calcium
Mg    Magnesium
Al    Aluminium    Moderately reactive
Zn    Zinc
Fe    Iron
Pb    Lead
H     Hydrogen   ← ↓ Reference line
Cu    Copper
Hg    Mercury
Ag    Silver
Au    Gold         ↓ Least reactive

Mnemonic to Remember

In English: "Please Stop Calling Me A Zebra Instead Try Learning How Copper Saves Gold"

P - Potassium (K) S - Sodium (Na) C - Calcium (Ca) M - Magnesium (Mg) A - Aluminium (Al) Z - Zinc (Zn) I - Iron (Fe) T - Tin/Lead (Pb) [adjusted] L - Lead (Pb) H - Hydrogen (H) C - Copper (Cu) S - Silver/Mercury (Hg/Ag) G - Gold (Au)

(Adjusted to fit standard metals)

Division

1. Highly Reactive (K to Mg):

• React with cold/hot water.
• Vigorous reaction with all acids.
• Extracted by electrolysis.

2. Moderately Reactive (Al to Pb):

• React with steam.
• React with dilute acids.
• Extracted by reduction with carbon.

3. Least Reactive (Cu to Au):

• No reaction with water/acid.
• Found in free state or extracted by heating.

Role of Hydrogen

H — a reference point (although it's a non-metal).

Only metals above H release $H_2$ from acids.

Metals below H (Cu, Hg, Ag, Au):

• No reaction with dilute acids.
• Do not give $H_2$.

Applications of the Reactivity Series

1. Storage

Metal	Storage
Na, K	In kerosene (to protect from air)
P (phosphorus — non-metal)	In water
Mg, Al etc.	Open (protective layer)
Au, Ag	As-is (inert)

2. Displacement — Practical Uses

(a) Thermite Reaction:

Reduction of $Fe_2O_3$ by Al — used for joining railway tracks.

$$2Al + Fe_2O_3 \rightarrow Al_2O_3 + 2Fe + \text{lots of heat}$$

Very fast reaction, lots of heat is released. Molten Fe drips down.

(b) Silver Mirror Manufacturing:

Cu vessel coated with $AgNO_3$ solution — silvery shiny coating forms.

3. Method of Extraction

The reactivity of a metal determines the method of its extraction.

Series	Metals	Method
Top	Na, K, Ca, Al	Electrolysis (Section 7)
Middle	Zn, Fe, Pb	Reduction by carbon
Bottom	Cu, Hg, Ag, Au	Often found in free state

(Detailed in Section 7)

4. Electrochemical Cells

More reactive metal — negative end of cell (anode). Less reactive — positive end (cathode).

Example: Daniell Cell

• Zn (more reactive) — anode
• Cu (less reactive) — cathode

5. Most Reactive (Top Five) — At a Glance

Remember: "K Na Ca Mg Al" These are the 'Top 5' of the reactivity series.

6. Which Metal Can Displace Which Salt?

Examples:

• Zn → salts of Cu, Hg, Ag, Au (yes)
• Cu → salts of Ag, Au (yes)
• Cu → salts of Zn, Fe (no — less reactive)
• Au → salt of any metal (no — least reactive)

7. An Interesting Confirmation — Reactivity Visible Through Heat Too!

Oxides of more reactive metals do not decompose even on strong heating. Oxides of less reactive metals — easily decomposed.

Examples:

• $2HgO \xrightarrow{\Delta} 2Hg + O_2$ (easily)
• $2Ag_2O \xrightarrow{\Delta} 4Ag + O_2$ (easily)
• $Na_2O$ — does not decompose on heating! (very stable)

[Board Important] The reactivity series is asked every year in some form.

🧠 Memory Capsule

A quick glance just before the board exam.

1. Metal + Acid

$$\text{Metal} + \text{Dilute Acid} \rightarrow \text{Salt} + H_2\uparrow$$

Essential:

• $Mg + 2HCl \rightarrow MgCl_2 + H_2$
• $Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2$
• $Fe + 2HCl \rightarrow FeCl_2 + H_2$
• $2Al + 6HCl \rightarrow 2AlCl_3 + 3H_2$

2. Who Doesn't Give $H_2$?

• Cu, Hg, Ag, Au — below H.
• HNO₃ — no metal gives $H_2$ (it's an oxidiser).
• Exception: very dilute $HNO_3$ + Mg/Mn → $H_2$.

3. Aqua Regia

Conc. $HNO_3$ + Conc. $HCl$ (1:3) — dissolves gold.

4. Displacement Reaction

A + BC → AC + B (A more reactive)

Essential:

• $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$ (blue → colourless)
• $Fe + CuSO_4 \rightarrow FeSO_4 + Cu$
• $Cu + 2AgNO_3 \rightarrow Cu(NO_3)_2 + 2Ag$

5. Reactivity Series

Top to bottom — Most reactive to least:

K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au

(H — reference line)

6. Practical Uses of the Series

Series	Property	Examples
Top	Vigorous with water/acid	Na, K, Ca
Middle	Slow with steam, acid	Zn, Fe, Al
Bottom	Only with strong acid	Cu, Au

7. Thermite

$$2Al + Fe_2O_3 \rightarrow Al_2O_3 + 2Fe + \text{heat}$$ Use: joining railway tracks.

8. Board's 'Golden' Questions

1. What is the reactivity series? — full list.
2. Zn + CuSO₄ — with colour change.
3. Why doesn't Cu react with dilute HCl?
4. What is aqua regia?
5. Use of thermite reaction.

Final Formula: More reactive metal = higher in series = faster reaction with acid/salts = loses electrons easily.

Solved Examples

Example 1: NCERT — Zn + Dilute $H_2SO_4$

What happens when zinc metal is added to dilute $H_2SO_4$?

Solution:

Reaction:

$$Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)\uparrow$$

Observations:

1. Vigorous bubbling is observed — hydrogen gas.
2. Zn slowly dissolves.
3. Solution remains colourless.
4. Test tube becomes slightly warm (exothermic).

Confirmation of Hydrogen: Bring a burning matchstick (splint) near the mouth of the tube — it burns with a 'pop' sound.

Products formed:

• $ZnSO_4$ = zinc sulphate (colourless)
• $H_2$ = hydrogen gas

What kind of reaction is this?

• Displacement (Zn displaced H)
• Redox

[NCERT textbook question — frequently asked]

Example 2: NCERT — Cu and Dilute $HCl$

What happens when a Cu strip is dipped in dilute $HCl$?

Solution:

Answer: No reaction will occur.

Reason:

Look at the reactivity series:

$...Pb > **H** > Cu > Hg > Ag > Au$

Cu is below H.

Rule: Only metals above hydrogen can release $H_2$ from acids.

Which metals react with dilute acids?

• K, Na, Ca → yes (explosive)
• Mg, Al, Zn, Fe → yes (normal reaction)
• Pb → yes (slow)
• Cu, Hg, Ag, Au → no

Exception:

• Cu reacts with concentrated $H_2SO_4$ or $HNO_3$ — but this is not displacement, it is redox. $$Cu + 2H_2SO_4 \xrightarrow{\Delta} CuSO_4 + 2H_2O + SO_2\uparrow$$

(Here $H_2$ is not released — instead $SO_2$ is.)

Practical:

• Acidic foods can be stored in copper vessels.
• Copper pipes last in acidic water.

[NCERT — important explanation]

Example 3: NCERT — Zn + $CuSO_4$ (Colour Change)

What happens when a Zn strip is dipped in blue $CuSO_4$ solution?

Solution:

Reaction:

$$Zn(s) + CuSO_4(aq) \rightarrow ZnSO_4(aq) + Cu(s)$$

Observations:

1. The blue colour of $CuSO_4$ solution fades gradually.
2. A red/brown coating of Cu is deposited on the Zn strip.
3. After some time, the solution becomes colourless.
4. The Zn strip becomes lighter (loses mass).

Analysis:

What happened?

• Reactivity series: $Zn > Cu$.
• Zn displaced Cu from $CuSO_4$.
• $Zn$ → $Zn^{2+}$ (went into solution).
• $Cu^{2+}$ → $Cu$ (deposited on the strip).

This is a displacement reaction.

Reason for blue colour:

• $CuSO_4$ solution — blue (due to Cu²⁺ ions).
• $ZnSO_4$ solution — colourless (Zn²⁺ ions are colourless).

Why is this possible? Zn is more reactive — it loses electrons easily. Cu²⁺ accepts electrons to become Cu.

Reverse experiment: Dip a Cu strip in $ZnSO_4$ solution — no reaction.

(Because Cu is less reactive than Zn.)

[NCERT — appears every year in board]

Example 4: NCERT — Activity Series Confirmation Experiment

4 test tubes were taken — A, B, C, D. Each contained Fe, Cu, Zn, Ag salt solutions. A metal was added to each in turn. Determine the order of reactivity from these results.

Tube	Solution	Metal Added	Result
A	$FeSO_4$	Cu	No reaction
B	$CuSO_4$	Zn	Brown coating, colour disappears
C	$ZnSO_4$	Ag	No reaction
D	$AgNO_3$	Cu	Shiny coating, blue solution

Solution:

Analysis:

(B): $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$ This means Zn > Cu (Zn is more reactive)

(D): $Cu + 2AgNO_3 \rightarrow Cu(NO_3)_2 + 2Ag$ This means Cu > Ag (Cu is more reactive)

(A): Cu + FeSO₄ → No reaction This means Fe > Cu (Fe is more reactive)

(C): Ag + ZnSO₄ → No reaction This means Zn > Ag (Zn is more reactive; confirmation)

Combining all:

$$\boxed{Zn > Fe > Cu > Ag}$$

This is the same order as the reactivity series!

Key Insight: The reactivity series was determined by such salt-solution experiments.

[NCERT textbook experiment — repeatedly asked]

Example 5: NCERT — Aqua Regia

What is aqua regia? What does it do?

Solution:

Definition: Aqua Regia = Concentrated $HNO_3$ + Concentrated $HCl$, in the ratio 1:3.

Meaning of the name:

• 'Aqua' = water (Latin)
• 'Regia' = royal
• 'Royal Water' — because it can dissolve the 'king of metals' gold.

Reaction with Gold:

$$Au + 3HCl + HNO_3 \rightarrow AuCl_3 + NO\uparrow + 2H_2O$$

(Simplified form)

Why are all acids ineffective, but aqua regia effective?

• Concentrated $HNO_3$ alone — does not dissolve Au.
• Concentrated $HCl$ alone — does not dissolve Au.
• Both together — dissolve Au! (synergy effect)

$HNO_3$ liberates $Cl_2$ from $HCl$ — which reacts with Au to form $AuCl_3$.

Properties:

• Highly corrosive liquid.
• Yellow-orange colour.
• Sharp, pungent smell.
• Stored in glass containers.

Uses:

1. Refining of gold.
2. Dissolving platinum.
3. Laboratory testing.

Aqua Regia for Gold and Platinum — Which Metals are Unaffected?

Only a few — like Rhodium (Rh), Tantalum (Ta) — are unaffected even by aqua regia.

[NCERT — interesting fact]

Example 6: NCERT — Thermite Reaction

What is the thermite reaction? Describe its uses.

Solution:

Definition: A very vigorous reduction of $Fe_2O_3$ by aluminium powder — releases so much heat that the iron formed melts.

Reaction:

$$2Al(s) + Fe_2O_3(s) \xrightarrow{\text{heat}} 2Fe(l) + Al_2O_3(s) + \text{lots of heat}$$

Properties:

1. Highly exothermic reaction.
2. Temperature reaches up to 3000°C.
3. Iron formed is molten.
4. Flame and sparks are visible from a distance.

Why is this possible?

• Al is more reactive than Fe.
• Al displaced Fe from $Fe_2O_3$.
• Very fast reaction = lots of heat.

Practical Uses

1. Welding Railway Tracks:

• For joining broken tracks.
• Molten Fe flows into the gap and solidifies, joining them.
• Called 'thermite welding'.

2. Joining Machine Parts:

• Filling cracks in heavy machinery.

3. Emergency Fuses:

• In military applications.

Reduction of Other Metals

The thermite principle is also used in extracting Mn, Cr etc.:

$$8Al + 3Mn_3O_4 \rightarrow 4Al_2O_3 + 9Mn$$

$$2Al + Cr_2O_3 \rightarrow Al_2O_3 + 2Cr$$

Precautions:

• Wear safety goggles.
• Have fire extinguishers ready.
• Perform in open spaces.
• Start from a distance.

[Board: 5-mark question]

Example 7: NCERT — Three Questions from Activity Series

(a) Can Cu displace Fe from $FeSO_4$ solution? (b) Can Zn displace Mg from $MgSO_4$ solution? (c) Can Fe displace Cu from $CuSO_4$ solution?

Solution:

Rule: Only a more reactive metal can displace another.

Reactivity Series: $Mg > Zn > Fe > Cu$

(a) Cu + FeSO₄ → ?

Compare: Cu vs Fe

• Fe is more reactive (Fe > Cu).
• Cu is less reactive than Fe.

Answer: No. Cu cannot displace Fe.

Reaction: Cu + FeSO₄ → No reaction.

(b) Zn + MgSO₄ → ?

Compare: Zn vs Mg

• Mg is more reactive (Mg > Zn).
• Zn is less reactive than Mg.

Answer: No. Zn cannot displace Mg.

Reaction: Zn + MgSO₄ → No reaction.

(c) Fe + CuSO₄ → ?

Compare: Fe vs Cu

• Fe is more reactive (Fe > Cu).
• Cu is less reactive than Fe.

Answer: Yes! Fe can displace Cu.

Reaction: $$Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$$

Observations:

• Blue solution turns light green (FeSO₄ is light green).
• Red-brown Cu coating on Fe.

Key Insight: In any metal-salt-solution experiment — first check the series, then write the reaction.

[NCERT textbook question]

Example 8: NCERT — Hg and Concentrated Acids

Mercury (Hg) does not react with dilute $HCl$. Yet, how is mercury used commercially?

Solution:

Behaviour of Hg:

Reactivity series: $...Cu > Hg > Ag > Au$

Hg is below H.

Dilute $HCl/H_2SO_4$: No reaction.

Concentrated $HNO_3$: $$Hg + 4HNO_3 \rightarrow Hg(NO_3)_2 + 2H_2O + 2NO_2\uparrow$$

With Air (on heating): $$2Hg + O_2 \xrightarrow{\Delta} 2HgO$$

Decomposition of HgO gives back Hg: $$2HgO \xrightarrow{\Delta} 2Hg + O_2$$

(This is a 'circular' reaction.)

Commercial Uses

Hg's low reactivity is beneficial:

1. Thermometers:

• Hg remains unaffected — by glass, air, normal temperature.
• Linear thermal expansion.

2. Barometers:

• Accurate pressure measurement.
• Negligible Hg vapour.

3. Electrical Switches:

• Hg flows — accurate contact.

4. Fluorescent Lamps:

• Hg vapour gives ultraviolet light.

5. Alloys (Amalgams):

• Hg + other metal = amalgam (dentistry, gold extraction).

Hazards

Hg is highly toxic (biologically):

• Vapour is dangerous if inhaled.
• Accumulates in body — nerve damage.
• 'Minamata Disease' (Japan, 1956) — from Hg pollution.

Reduced use in modern times:

• Hg-thermometers — replaced by digital.
• LEDs — better than Hg-lamps.

[Board: 3-mark]

Example 9: Numerical — Zn + HCl

How many grams of $H_2$ gas is released when 13 g of Zn is reacted with dilute $HCl$? (Zn=65, H=1)

Solution:

Reaction:

$$Zn + 2HCl \rightarrow ZnCl_2 + H_2\uparrow$$

Molecular Mass:

• 1 mol Zn = 65 g
• 1 mol H₂ = 2 g

Ratio: 65 g Zn → 2 g H₂

From 13 g Zn:

$$\text{H}_2 = \frac{2}{65} \times 13 = \frac{26}{65} = 0.4 \text{ g}$$

Answer: 0.4 g of $H_2$ released.

Volume too (at NTP):

• 1 mol = 22.4 L
• 0.4 g H₂ = 0.2 mol
• Volume = 0.2 × 22.4 = 4.48 L

[Board: 3-mark numerical]

Example 10: An Interesting Question — Cu Strip and AgNO₃

What happens when a Cu strip is dipped in $AgNO_3$ solution?

Solution:

Reaction:

$$Cu(s) + 2AgNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + 2Ag(s)$$

Observations:

1. A shiny silver coating forms on the Cu strip (or fluffy Ag crystals).
2. The solution changes from colourless to blue.
3. $Cu^{2+}$ ions in solution — blue.
4. $Ag$ metal — silvery surface.

Analysis:

Reactivity: Cu > Ag.

Cu displaced Ag:

• $Cu \rightarrow Cu^{2+} + 2e^-$
• $2Ag^+ + 2e^- \rightarrow 2Ag$

This displacement — also a redox reaction.

Reverse Experiment: Ag strip in $CuSO_4$ solution → no reaction.

(Because Ag is less reactive than Cu.)

Decorative Use — Silver Mirror

Based on this principle:

• Light coating with $AgNO_3$ solution on Cu vessels.
• Beautiful 'silver mirror' production.
• Used in serving platters.

Electroplating:

• More controlled coating with electric current.
• (Detailed in Section 8.)

[Board + Practical]

Example 11: NCERT — A Mixed Reaction

Which of the following will react? Write the possible reactions.

(a) Mg + Zn(NO₃)₂ (b) Pb + CuSO₄ (c) Ag + Pb(NO₃)₂ (d) Fe + ZnSO₄

Solution:

Reactivity Series (for this question): $$Mg > Zn > Fe > Pb > Cu > Ag$$

(a) Mg + Zn(NO₃)₂

• Mg > Zn? Yes! (Mg is more reactive)

Reaction occurs: $$Mg(s) + Zn(NO_3)_2(aq) \rightarrow Mg(NO_3)_2(aq) + Zn(s)$$

(b) Pb + CuSO₄

• Pb > Cu? Yes! (Pb is more reactive)

Reaction occurs: $$Pb(s) + CuSO_4(aq) \rightarrow PbSO_4(s) + Cu(s)$$

(PbSO₄ is sparingly soluble — white precipitate)

(c) Ag + Pb(NO₃)₂

• Ag > Pb? No! (Ag is less reactive than Pb)

No reaction.

(d) Fe + ZnSO₄

• Fe > Zn? No! (Fe is less reactive than Zn)

No reaction.

Summary Table

Pair	More Reactive	Reaction
Mg + Zn(NO₃)₂	Mg	Yes
Pb + CuSO₄	Pb	Yes
Ag + Pb(NO₃)₂	Pb (in salt)	No
Fe + ZnSO₄	Zn (in salt)	No

Key Insight: For displacement — the metal strip must be more reactive than the metal in the salt.

[NCERT textbook question]

Example 12: An Interesting Demonstration — Iron Nails and CuSO₄

Take 5 iron nails. Dip them in $CuSO_4$ solution. What will be observed after a day?

Solution:

Reaction:

$$Fe(s) + CuSO_4(aq) \rightarrow FeSO_4(aq) + Cu(s)$$

Observations (with time):

Immediately (0 min):

• Blue solution.
• Shiny Fe nails.

5 min:

• Light red-brown coating starts on nails.
• Solution colour slightly faded.

1 hour later:

• Thick Cu coating on nails.
• Solution greenish-blue.

24 hours later (whole day):

• Nails completely covered with red-brown Cu.
• Solution light green (colour of FeSO₄).
• Blue colour of solution disappeared.

Analysis

What happened?

1. Fe → Fe²⁺ + 2e⁻ (electrons lost).
2. Cu²⁺ + 2e⁻ → Cu (electrons gained).
3. Fe displaced Cu.

Why is this possible?

• Fe is more reactive than Cu.
• Fe loses electrons easily.

Practical Uses

This principle — used in many industrial processes:

1. Extraction of Cu:

• From dilute $CuSO_4$ solution by Fe.

1. Metal Coating:

• In some experiments.

Caution

Fe + CuSO₄ — Not Reversible!

That is, once Cu is deposited on Fe — the blue colour does not come back.

[Board + Lab]

Example 13: A Challenge — Order of Five Metals

From the results of the following experiments, arrange metals A, B, C, D, E in increasing order of reactivity:

• A displaces B from $B_2SO_4$ solution.
• B displaces D from $D_2SO_4$ solution.
• C displaces A from $A_2SO_4$ solution.
• E does not react with cold water; but D displaces E from $E_2SO_4$ solution.

Solution:

Analysis:

(1) A > B (A displaced B)

(2) B > D (B displaced D)

(3) C > A (C displaced A)

(4) D > E (D displaced E)

Combining all:

From (3): C > A From (1): A > B From (2): B > D From (4): D > E

Final Order (most reactive to least):

$$C > A > B > D > E$$

In Increasing Order of Reactivity:

$$\boxed{E < D < B < A < C}$$

Explanation

Why is it like this?

• E is the least reactive — does not react with cold water.
• C is the most reactive — can displace all others.

Application

The actual reactivity series was determined by such experiments.

Can you identify?

If C = Mg, A = Zn, B = Fe, D = Cu, E = Au — then the series matches!

[Board: 5-mark logical question]

Example 14: NCERT — Concentrated HNO₃ and Metals

When Mg, Cu, Zn are added to concentrated $HNO_3$, why is $H_2$ not released?

Solution:

Basic Rule: Metal + dilute acid → salt + $H_2$.

But — this rule fails with concentrated $HNO_3$.

Why?

$HNO_3$ is a strong oxidising agent.

That is — even if some $H_2$ is released, $HNO_3$ immediately oxidises it back to $H_2O$.

$2H_2 + O \rightarrow 2H_2O$ (rapidly)

Instead — N in $HNO_3$ comes to a lower oxidation state.

Actual Reactions

Cu + Concentrated $HNO_3$:

$$Cu + 4HNO_3 \rightarrow Cu(NO_3)_2 + 2H_2O + 2NO_2\uparrow$$

(Brown-red $NO_2$ gas; not $H_2$!)

Cu + Dilute $HNO_3$:

$$3Cu + 8HNO_3 \rightarrow 3Cu(NO_3)_2 + 4H_2O + 2NO\uparrow$$

(Colourless $NO$ gas — turns to $NO_2$ in air.)

Zn + Concentrated $HNO_3$:

$$Zn + 4HNO_3 \rightarrow Zn(NO_3)_2 + 2H_2O + 2NO_2\uparrow$$

Exception — Mg and Mn

Very Dilute $HNO_3$ + Mg:

$$Mg + 2HNO_3(\text{very dilute}) \rightarrow Mg(NO_3)_2 + H_2\uparrow$$

Here Mg is highly reactive — and $HNO_3$ is so dilute that oxidation is difficult.

Mn behaves similarly.

Summary

Situation	Product
Mg/Mn + very dilute $HNO_3$	$H_2$
Metal + dilute $HNO_3$	$NO$ + salt
Metal + concentrated $HNO_3$	$NO_2$ + salt
Cu + dilute/conc. $H_2SO_4$	No $H_2$ (Cu below H)
Cu + conc. $H_2SO_4$ + heat	$SO_2$ + salt

Key Insight: Oxidising acids — break the general rule.

[NCERT important explanation]

Example 15: A Mixed — Al, Zn, Fe — Which is More Reactive?

Discuss the relationship between Al, Zn, Fe. Write reactions with dilute acid.

Solution:

Reactivity Series (this part):

$$Al > Zn > Fe$$

Reactions — with Dilute $HCl$

Al + Dilute HCl: $$2Al + 6HCl \rightarrow 2AlCl_3 + 3H_2\uparrow$$

(Very vigorous reaction, lots of bubbles)

Zn + Dilute HCl: $$Zn + 2HCl \rightarrow ZnCl_2 + H_2\uparrow$$

(Moderately vigorous, good bubbling)

Fe + Dilute HCl: $$Fe + 2HCl \rightarrow FeCl_2 + H_2\uparrow$$

(Slow, fewer bubbles — especially at room temperature)

Order of Intensity

Intensity of reaction: Al > Zn > Fe

Reasons:

• Al — 3 outer electrons, more reactive.
• Zn — 2 outer electrons.
• Fe — d-block, moderate.

Some Surprising Facts

Al is not 'instant' in classroom — why?

• Al has a thin $Al_2O_3$ layer.
• First the acid dissolves this layer.
• Then the actual reaction begins.
• That is — first delay, then rapid.

What slows Fe?

• Fe has a thin oxide layer too.
• Dilute acid dissolves this easily.
• Yet the reaction is moderate.

Practical Consequences

1. Which metal vessels are safe?

• Al: not for acidic foods (but $Al_2O_3$ protects).
• Fe: rust is a risk.
• Stainless steel: safest (Cr-coating).

2. Pipes/Faucets:

• Galvanised Fe — Zn coating.
• Cu pipes — most durable (but expensive).

[Board: 3-5 marks]

Example 16: A Concluding Question — Detailed

(a) Write the complete reactivity series. (b) Which metals — Cu, Zn, Mg — can displace from AgNO₃ solution? (c) What is released by dilute acid? How to confirm? (d) Formula and use of aqua regia.

Solution:

(a) Reactivity Series

Most to least reactive:

$$\text{K} > \text{Na} > \text{Ca} > \text{Mg} > \text{Al} > \text{Zn} > \text{Fe} > \text{Pb} > \text{(H)} > \text{Cu} > \text{Hg} > \text{Ag} > \text{Au}$$

Mnemonic: "Please Stop Calling Me A Zebra Instead Try Learning How Copper Saves Gold"

(b) Which can displace Ag?

Look at series: Mg, Zn, Cu — all more reactive than Ag!

All three can displace Ag from $AgNO_3$.

Reactions:

$$Cu + 2AgNO_3 \rightarrow Cu(NO_3)_2 + 2Ag$$ $$Zn + 2AgNO_3 \rightarrow Zn(NO_3)_2 + 2Ag$$ $$Mg + 2AgNO_3 \rightarrow Mg(NO_3)_2 + 2Ag$$

(c) Dilute acid + metal → ?

Product: Salt + $H_2$ gas.

Confirmation of $H_2$:

'Pop' Test:

• Collect gas in test tube.
• Bring a burning matchstick near it.
• If $H_2$ — burns with a loud 'pop' sound.

Reaction: $$2H_2 + O_2 \rightarrow 2H_2O$$

(Cold water vapour formed.)

(d) Aqua Regia

Formula: Concentrated $HNO_3$ + Concentrated $HCl$ (1 : 3)

Meaning: 'Royal Water' — dissolves gold.

Main Uses:

1. Refining of gold.
2. Dissolving platinum.
3. Laboratory testing.

Special: 'Magic' — neither acid alone dissolves Au, but together they form $Cl_2$ — which dissolves Au.

[Board: 5-mark mixed question]