4.1 [Group 2] Flashcards
what is the definition of first ionisation energy?
First ionisation energy of an element is the energy required to remove an electron from each atom in one mole of atoms in the gaseous state.
What is the definition of second ionisation energy?
Second ionisation energy of an element is the energy required to remove an electron from each singly charged positive ion in one mole of positive ions in the gaseous state.
What are the trends for ionisation energy in the group 2 elements?
First and second ionisation energy decreases down the group, atomic radius increases down the group as well making it easier to remove electrons.
What are the factors to consider when explaining trends in ionisation energy?
Nuclear charge (number of protons)
The orbital in which the electron exists
The shielding effect the repulsion between filler inner shells and electrons being removed.
Why does Ionisation energy decrease down group 2?
As nuclear charge increases, so do the forces of attraction for the electron being removed also increases means an increase in ionisation down the group.
As each quantum shell is added, energy of the outermost electron increases.
As the number of filled inner shells increases, the forces of repulsion on the electron being removed increases meaning a decrease in ionisation energy down the group.
One of the factor causes an increase two causes a decrease the combined effect of the last 2 factors outweighs the effect of the first factor explaining a decrease down the group.
What are the basic trends in reactivity for group 2 elements?
In all of these reactions element changes into an M2+ ion, there is a general increase in reactivity down the group. This can be explained by the decrease in energy needed to remove the 2 electrons from each atom of the element.
What happens when Magnesium burns in oxygen?
Burns with a bright white flame
What are the reactions with Group 2 elements in the air?
When calcium, strontium and barium are heated in air, the reactions are more vigerous. This may be hard to see if the reactions are not fresh samples.
How do Group 2 elements react with chlorine?
Group 2 elements combine with chlorine when heated in the gas. Just like reactions with oxygen the reactions with chlorine become more vigorous down the group, although this is harder to see.
General equation for the reactions
M + Cl2 > MCl2
Products are M2+ and Cl-
How do Group 2 elements react with water?
Reaction between Magnesium and water is very slow and does not proceed completely. Calcium, Strontium and barium react with increasing vigour, which can be seen by increase in effervescence.
General equation for this reaction is:
M + 2H20 > M(OH)2 + H2
Products are hydrogen gas and hydroxides containing M2+ and OH- ions
How does Calcium react with water?
Calcium hydroxide is only slightly soluble in water so the liquid in this experiment goes cloudy as a precipitate of calcium hydroxide forms.
Ca + 2H20 > Ca(OH)2 + H2
How does Magnesium react with steam?
Magnesium reacts differently when heated in stem - It rapidly forms magnesium oxide (white solid) hydrogen gas in a vigerous reaction. Equation for this reaction
Mg + H2O > MgO + H2
Hydrogen formed burned it leaves the tube. This is for safety reasons prevent the escape of highly flammable gas into the laboratory.
What are the reactions of Group 2 oxides with water?
The Group 2 oxides are classes as basic oxides. Which means they can react with water to form alkalis. These reactions occur when oxides are added to water. Only observation is that the solid react to form a colourless solution.
General equation:
MO + H20 > m(OH)2
Hydroxide ions are formed which shows the solution as alkaline.
What are Basic Oxides?
Basic Oxides are oxides of metals that react with water to form metal hydroxides, and with acids to form salts and water.
What are the trends in solubility of the hydroxides?
The pH value of the alkaline solution formed depends partly on the relative amounts of oxide and water, but is also affected by the differences in solubility of the hydroxides.
When MgO reacts with water the Magnesium hydroxide formed has a low solubility in water. The solubility of the Group 2 hydroxides increase down the group. Therefore, the maximum alkalinity of the solutions formed increase down the group.
What is the test for carbon dioxide?
Limewater is needed for the test of CO2. The limewater goes cloudy as a white precipitate forms. Limewater is the name used for a saturate aqueous solution of Calcium Hydroxide. Carbon dioxide reacts to form Calcium carbonate which is insoluble in water and is the white precipitate.
CO2 + Ca(OH)2 > CaCO3 + H2O
What is Milk of Magnesia?
Indigestion remedy called Milk of Magnesia. Contains a standard solution of Magnesium Hydroxide mixed with extra solid Magnesium hydroxide acts as a anti-acid.
Human stomach contains HCl to digest food, sometimes if there is too much can cause indigestion. Taking milk of Magnesia neutralises some of the HCl and relieves the symptoms.
Mg(OH)2 + 2HCl > MgCl2 + 2H20
Although Hydroxide ions attack human tissue, very low solubility of Magnesium hydroxide means the concentration of OH ions in the medicine is very low cannot affect health.
How do the Group 2 Oxides and Hydroxides react with acids?
All of the G2 oxides and Hydroxides react with acids to form salts and water. Described as neutralisations reactions.
During the reactions the only observations made are that a white solid reacts to form a colourless solution. The reactions are endothermic so may use to measure enthalpy change.
Mgo + H2SO3 > MgSO4 + H20
How have farmers used Calcium hydroxides in agriculture?
For centuries, farmers have used lime to control soil acidity so that a greater yield of crops can be obtained.
Lime is mostly calcium hydroxide obtained from limestone, and neutralises excess acidity in the soil. Using nitric acid to represent the acid in the soil
Ca(OH)2 + 2HNO3 > Ca(NO3)2 + 2H2O
What are the trends in solubility of the Group 2 sulfates and hydroxides?
All G2 nitrates and chlorides are soluble, but the solubility of G2 sulfates decreases down the group
Magnesium sulfate is classed as soluble
Calcium sulfate is slightly soluble
Strontium sulfate and barium sulfate are insoluble.
Solubility increases down the group meaning Magnesium hydroxide is the least soluble and Barium being the most. Magnesium can be used as medicine as it can neutralise acids.
What is the test for Sulfate ions?
The presence of sulfate ions in aqueous solution can be shown by adding a solution containing barium ions ( barium nitrate or chloride). Any sulfate ions will react with added barium ions to form a white precipitate of barium sulfate.
Ba2+ + SO4 2- > BaSO4
There are other anions that form a white precipitate with Barium ions, like carbonate ions. So in the test there must be H+ ions present to prevent Barium carbonate forming as a white precipitate. Dilute Nitric acid or Hydrochloric acid is therefore added as part of the test
How would you test for the presence of Sulfate ions in a solution of sodium sulfate?
Add dilute nitric acid and barium nitrate solution, a white precipitate will form.
Ba(NO)3 + Na2SO4 > BaSO4 + 2NaNO3
What are Barium meals?
Barium is poisonous to humans however barium sulfate is used in hospitals this is because it is insoluble although containing barium ions these ions are not free to move. Allows soft tissue to be seen more easily on X-rays
What is the definition of thermal stability?
Thermal Stability is a term that indicates how stable a compound is when it is heated. Does it decompose at all (very thermally stable) or does it decompose as much as possible ( not at all thermally stable).
How do Group 2 nitrates and carbonates thermally decompose?
Group 2 carbonates and nitrates are very different compared to Group 1 when heated the reasons for this are:
1.The charge on a Group 2 cation is double of that on a Group 1 cation.
2.The ionic radius of a Group2 ion is smaller than that of a Group 1 cation in the same period.
3.The nitrate and carbonate anions are more complex than the CL- ion.
What are the explanations for why Group 2 carbonates and nitrates thermally decompose?
The larger more complex nitrate ion can change into the smaller more stable nitrite ion, or oxide ion by decomposing and releasing oxygen gas or nitrogen dioxide gas.
The larger more complex carbonate ion can change into the smaller more stable oxide ion by decomposing and releasing Carbon dioxide gas.
The stabilities of the nitrate and Carbonate anions are influenced by the change and size of the cations present smaller and more highly charged cations can affect these anions more.
What is the thermal stability like of nitrates?
All of the nitrates of the G1 and G2 elements are white solids. When they are heated, they all decompose to nitrates or oxides. Give of nitrogen dioxide (brown fumes) and or oxygen. If the nitrate contains water of crystallisation then steam will also be observed.
If no brown fumes are observed this indicates a lesser decomposition that can be represented by
Metal nitrate > metal nitrate + oxygen
If brown fumes are observed this indicates a greater decomposition can be represented by
Metal nitrate > Metal oxide + nitrogen dioxide + Oxygen.
Nitrates and Nitrites are sometimes differentiated by oxidation numbers - Nitrate (V) for nitrate, and nitrate for nitrite.
What are the typical observations obtained by heating samples of nitrates in test tubes over a Bunsen burner flame?
Lithium nitrate -Brown fumes Greater decomposition
Sodium nitrate - No brown fumes Lesser decomposition
Potassium nitrate - No brown fumes Lesser decomposition
Rubidium nitrate - No brown fumes Lesser decomposition
Caesium nitrate - No brown fumes Lesser decomposition
Beryllium nitrate - Brown fumes Greater decomposition
Magnesium nitrate - Brown fumes Greater decomposition
Calcium nitrate - Brown fumes Greater decomposition
Strontium nitrate - Brown fumes Greater decomposition
Barium nitrate Brown fumes Greater decomposition
What are the trends for the decomposition of nitrates in G1 and G2?
Greater decomposition occurs when:
The cation has a 2+ charge
The cation has a 1+ charge and is the smallest Group 1 cation
What is the thermal stability like of carbonates?
All of the carbonates of the G1 and G2 elements are white solids. When they are heated, they either do not decompose or decompose to oxides and give of carbon dioxides.
As the gas given off is colourless and the carbonate and oxide are both white solids, there are no observations that can be made.
What are the comparisons between different G1 and G2 carbonates when heated in a test tube over a Bunsen flame?
Lithium carbonate - Decomposition
Sodium carbonate - No decomposition
Potassium carbonate - No decomposition
Rubidium carbonate - No decomposition
Caesium carbonate - No decomposition
Beryllium carbonate - Decomposition
Magnesium carbonate - Decomposition
Calcium carbonate - Decomposition
Strontium carbonate - Decomposition
Barium carbonate - Decomposition
What is the trend for the thermal decomposition of Carbonates?
Pattern is similar to that of the nitrates. Lithium compound and all of the G2 compounds behave differently to the other G1 compounds.
Lithium carbonate decomposes at lower temperatures than the other G1 Carbonates. Which don’t unless at very high temperatures.
All G2 carbonates decompose in the same way but with increasing difficulty down the Group.
Decomposition occurs when the cation has a 2+ charge or is the smallest 1+ Group cation.
What is the exact definition of thermal stability?
Thermal stability is a measure to the extent to which a compound decomposes when heated.
What is a flame test?
A flame test is a simple chemical test to identify the presence of cations in a compound. However, it is important to know how to do one carefully and interpret the results.
A flame test can indicate the presence of some metals in G1 and G2 of the periodic cable.
How can a flame test be carried out?
A small quantity of a solid is available in a glass dish.
Wear safety glasses and a lab coat. Within a fume cupboard and light a Bunsen burner.
Use a dropper to add a few drops of concentrated hydrochloric acid to the solid and mix together so that the metal compound begins to dissolve. HCl can convert any metal compound to a chloride which are more volatile than other salts so give better results.
Dip a clean metal wire (platinum or nichrome) into the mixture to obtain a sample of the compound.
Hold the end of the wire or rod in the flame and observe the colour.
What are the problems with the flame test?
Many compounds contain small amounts of sodium compounds as impurities, so the intense colour of sodium can mask other colours.
Describing colours with words is subjective, people have different levels of colour vision, word description of colours may be different as well.
What are the colour descriptions for the Group 1 cations?
Lithium : red
Sodium : yellow/orange
Potassium : lilac
Rubidium : red/purple
Caesium : blue/violet
What are the colour descriptions for the Group 2 cations?
Beryllium : no colour
Magnesium : no colour
Calcium : Brick red
Strontium : Crimson red
Barium : Green
What causes the colour in flame tests?
Electron transitions, Electrons occupy orbitals in specific energy levels in an atom these are often represented using electron configurations.
Electrons can absorb energy and move to higher energy levels. The term ground state is used to describe an atom with all its electrons in their lowest possible energy levels. if an electron moves to a higher energy level, new situation described as an ‘excited state’. Movement of an electron to an excited state occurs during a flame test.
This movement is immediately followed by the return of the electron to its ground state, which releases energy. If this energy corresponds to radiation in the visible light spectrum then a characteristic colour appears.
For example the visible light spectrum covers the part of electromagnetic spectrum in the wavelength. the electron transition in sodium corresponds to a wave length of about 590nm which is yellow-orange part of the spectrum. So this is the colour of sodium compound in a flame test. The electron transition in Magnesium corresponds to a wavelength outside the visible light spectrum to no flame colour.
What is the test for ammonia ions?
Cation does not give a colour in a flame test in ammonium ion. The usual test for ammonia ions in a solid solution is to add sodium hydroxide solution and warm the mixture. The addition of sodium hydroxide causes this reaction
NH4 + OH- > NH3 + H20
The warming releases ammonia gas. Ammonia can be recognised by its smell a simple chemical test is use damp red litmus paper, which turns blue (ammonia common alkaline gas) hydrogen chlorine gas reacts with ammonia to form white fumes of ammonium chloride.
