Group 2

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Trends in physical properties

ATOMIC RADIUS INCREASES DOWN GROUP 2

  • The more layers of electrons, the more space they will take up, electrons repel each other, electrons get larger down group 2

FIRST IONISATION ENERGY DECREASES DOWN GROUP 2

  • The increase in nuclear charge is offset by the shielding of inner electrons, distance between the outer electrons and nucleus increases so they become easier to remove and ionisation energe falls
  • Atoms are held together by attraction of nuclei to delocalised electrons, as atoms get bigger the nuclei get further away from the delocalised electrons so attractions decrease.
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Reactions with water

GROUP 2 METALS BECOME MORE REACTIVE WITH WATER AS YOU GO DOWN GROUP 2

Beryllium has no reaction with water or steam

 Mg burns in steam and reacts with cold water

(http://www.chemguide.co.uk/inorganic/group2/mgh2oeqn.gif)

(http://www.chemguide.co.uk/inorganic/group2/mgh2oeqn2.gif)

Calcium, strontium and barium all react with cold water with increasing vigour

(http://www.chemguide.co.uk/inorganic/group2/xh2oeqn.gif)

The reactions become easier as the energy needed to form positive ions falls. This is mainly due to the decrease in ionisation energy down the group = lower activation energies = faster reactions.

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Solubility of hydroxides

HYDROXIDES BECOME MORE SOLUBLE AS YOU GO DOWN GROUP 2

Magnesium Hydroxide = insoluble in water

Calcium hydroxide = used as limewater, 1l of pure water will dissolve 1g of Ca(OH)2

Barium hydroxide = soluble enough to produce a solution of 0.1moldm-3 at room temp

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Magnesium hydroxide (milk of magnesia) is used to neutralise excess stomach acid

Calcium hydroxide is a stonger base than magnesium hydroxide and is used in agriculture to adjust the pH of acid soils and lakes

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Solubility of sulfates

SULFATES BECOME LESS SOLUBLE AS YOU GO DOWN GROUP 2

Two common examples illustrate this trend:

  1. A familiar reaction is that between magnesium and dilute sulfuric acid, producing hydrogen gas and a colorless solution of magnesium sulfate. Notice that a solution, and not a precipitate, is formed, implying that magnesium sulfate is soluble.
  2. Barium sulfate exists as a white precipitate in solution. The ready formation of a precipitate indicates that barium sulfate is quite insoluble. In fact, 1 liter of water dissolves about 2 mg of barium sulfate at room temperature.

Barium chloride  (Ba(Cl2)) is used as a test for sulfate ions because Ba2+ reacts with SO42- ions to produce barium sulfate which is insoluble so a precipitate will be formed

Barium sulfate (BaSO4) is also used as a 'barium meal' in x-rays which passes through the body when ingested and absorbs x-rays so can be seen and shows any structural abnormalities.

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