17.3-17.4Free
Your guide: Miss SanaCounts atoms the way you count rupees: carefully.
Limestone is CaCO3. Heat it in a kiln and it breaks down.
Why does BaCO3 need more heat than MgCO3? Let us find out.
Compare, then decompose.
Both groups have highly reactive metals. The difference comes from ionization energy.
Group 1 metals lose one electron. Group 2 metals lose two, which needs more energy.
So Group 2 metals are reactive, but less than Group 1 metals in the same period.
Both groups get more reactive going down. Atoms get bigger and ionization energy falls.
Table 17.2: Group 1 has ns1, forms M+, is very soft, has low density and low melting points.
Group 2 has ns2, forms M2+, is harder, with higher density and higher melting and boiling points.
Oxides: Group 1 forms M2O, for example Na2O. Group 2 forms MO, for example CaO. Group 2 oxides are less basic.
Hydroxides: Group 1 is highly soluble and strongly basic. Group 2 is less soluble and less basic.
Thermal decomposition: one compound breaks into two or more simpler substances when heated. It usually needs energy, so it is endothermic.
A Group 2 carbonate gives a metal oxide and carbon dioxide.
MgCO3(s) → MgO(s) + CO2(g)
CaCO3(s) → CaO(s) + CO2(g)
A Group 2 nitrate gives a metal oxide, brown nitrogen dioxide and oxygen.
2Ca(NO3)2(s) → 2CaO(s) + 4NO2(g) + O2(g)
Thermal stability of carbonates and nitrates increases down the group.
BeCO3 decomposes at the lowest temperature. BaCO3 needs the highest.
Table 17.3, decomposition temperature in °C: BeCO3 250, MgCO3 540, CaCO3 900, SrCO3 1289, BaCO3 1360.
The reason is polarizing power. A small, highly charged cation such as Be2+ distorts the large carbonate ion.
This weakens the bonds in the anion, so heating breaks it down more easily.
Down the group the cation gets larger. Its polarizing power falls. So the compound is more stable.
The same argument holds for nitrates. Be(NO3)2 is the least stable and Ba(NO3)2 the most stable.
Step 1 / 7
Limestone is CaCO3. Heat it in a kiln and it breaks down.
Why does BaCO3 need more heat than MgCO3? Let us find out.
Compare, then decompose.
Both groups have highly reactive metals. The difference comes from ionization energy.
Group 1 metals lose one electron. Group 2 metals lose two, which needs more energy.
So Group 2 metals are reactive, but less than Group 1 metals in the same period.
Both groups get more reactive going down. Atoms get bigger and ionization energy falls.
Table 17.2: Group 1 has ns1, forms M+, is very soft, has low density and low melting points.
Group 2 has ns2, forms M2+, is harder, with higher density and higher melting and boiling points.
Oxides: Group 1 forms M2O, for example Na2O. Group 2 forms MO, for example CaO. Group 2 oxides are less basic.
Hydroxides: Group 1 is highly soluble and strongly basic. Group 2 is less soluble and less basic.
Thermal decomposition: one compound breaks into two or more simpler substances when heated. It usually needs energy, so it is endothermic.
A Group 2 carbonate gives a metal oxide and carbon dioxide.
MgCO3(s) → MgO(s) + CO2(g)
CaCO3(s) → CaO(s) + CO2(g)
A Group 2 nitrate gives a metal oxide, brown nitrogen dioxide and oxygen.
2Ca(NO3)2(s) → 2CaO(s) + 4NO2(g) + O2(g)
Thermal stability of carbonates and nitrates increases down the group.
BeCO3 decomposes at the lowest temperature. BaCO3 needs the highest.
Table 17.3, decomposition temperature in °C: BeCO3 250, MgCO3 540, CaCO3 900, SrCO3 1289, BaCO3 1360.
The reason is polarizing power. A small, highly charged cation such as Be2+ distorts the large carbonate ion.
This weakens the bonds in the anion, so heating breaks it down more easily.
Down the group the cation gets larger. Its polarizing power falls. So the compound is more stable.
The same argument holds for nitrates. Be(NO3)2 is the least stable and Ba(NO3)2 the most stable.
Q1. Quick Check 17.2 (a): state two physical differences between Group 1 and Group 2 metals.
Group 1 metals are very soft and can be cut with a knife. Group 2 metals are harder. Group 1 metals have lower melting points and lower densities than Group 2 metals. (Table 17.2.)
Q2. Quick Check 17.2 (b): general formula of a Group 1 oxide and a Group 2 oxide.
Group 1: M2O. Group 2: MO.
Q3. Quick Check 17.2 (c): balanced equation for the thermal decomposition of SrCO3.
SrCO3(s) → SrO(s) + CO2(g) (Same pattern as the book's MgCO3 and CaCO3.)
Q4. Quick Check 17.2 (d): why is magnesium carbonate less thermally stable than calcium carbonate?
Mg2+ is smaller than Ca2+. It has more polarizing power. It distorts the carbonate ion more and weakens its bonds. So MgCO3 decomposes at a lower temperature (540 °C against 900 °C).
Q5. Quick Check 17.2 (e): write the decomposition equations for magnesium nitrate and barium nitrate. Which decomposes at a lower temperature?
2Mg(NO3)2(s) → 2MgO(s) + 4NO2(g) + O2(g) 2Ba(NO3)2(s) → 2BaO(s) + 4NO2(g) + O2(g) Magnesium nitrate decomposes at the lower temperature, because stability increases down the group.
✗ “Carbonates get less stable down Group 2.”
✓ They get more stable. BaCO3 needs the highest temperature.
✗ “Nitrates give CO2 on heating.”
✓ Nitrates give NO2 and O2. Carbonates give CO2.
✗ “Group 2 metals are more reactive than Group 1 in the same period.”
✓ Group 1 is more reactive. It has only one electron to lose.
1. Which carbonate needs the highest temperature to decompose?
(a) Table 17.3 gives 1360 °C for BaCO3.
2. Products of heating a Group 2 nitrate:
(a) This matches the book's Ca(NO3)2 equation.
3. The reason nitrate stability rises down the group:
(a) Bigger cations distort the anion less.
4. Which is a difference between Group 1 and Group 2?
(a) Table 17.2: Group 2 is harder than Group 1.
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