Chapter 8: Reactivity trends (8.3) Flashcards
Test for Carbonate ions (CO3 2-)
dissolve in distilled water. Add nitric acid (HNO3). Bubble gas through limewater to test for CO2. If limewater turns cloudy then test is positive
Test for sulphate ions (SO4 2-)
dissolve in distilled water. Add barium chloride. If white precipitate (barium sulphate) forms then test is positive
Test for halide ions (Cl-, Br-, I-)
Dissolve in distilled water. Add nitric acid then silver nitrate. Since silver halides are insoluble a coloured precipitate shows a positive test. Cl- = white. Br- = cream. I- = yellow
Test for ammonium ions (NH4+)
add aqueous sodium hydroxide. Gently heat mixture. Use damp red litmus paper to test the gas evolved. If litmus paper turns blue then ammonia gas is present and the test is positive
Test for hydroxide ions (OH-)
add universal indicator and the solution will turn blue
Why do you do the carbonate test first?
You do the carbonate test first as it does not interfere with the other tests as none of the other tests produce bubbles with dilute acids
Why do you do the sulphate test second?
Silver carbonate and Barium Carbonate also form white precipitates therefore you have to remove carbonate impurities before hand
Why do you do the halide test last?
You are adding solutions containing Ag+ (aq) ions but silver carbonate and sulphate are both insoluble in water and will form as precipitates so this test must be carried out last.
What happens when magnesium is burnt in oxygen?
It burns with a bright white light and forms magnesium oxide
Water happens when a group 2 metal is reacted with water?
It creates an alkaline hydroxide and hydrogen gas
What happens when you react an acid with a metal?
You get a salt and hydrogen gas.
Why does reactivity increase down group 2?
As you move down the group the elements lose their 2 outer electrons more easily (lower ionisation energy) this is because they have a weaker attraction to the nucleus and due to electron shielding.
What happens to the alkalinity or the group 2 hydroxides as you move down the group?
Solubility increases - so the solution contains more OH- ions and therefore the PH increases and the alkalinity also increases.
How do you test the PH of all the group 2 hydroxides ?
Add a spatula of each group 2 hydroxides to water and shake - you will get a saturated solution with some white solid undissolved at the bottom. Measure the PH and the alkalinity will increase down the group.
How are group 2 compounds used in agriculture?
It is added to field as line by farmers to increase the PH of acidic soils, neutralises acid forming neutral water.