chapter 8- reactivity trends
Describe and explain the trend in atomic radii of the elements Li to F across Period 2 of the Periodic Table.
(As you go across the period) The atomic radii decreases The nuclear charge increases OR protons increase electrons are added to the same shell OR shielding remains the same greater (nuclear) attraction on (outer) electrons / (outer) shell(s)
Some scientists believe that chlorine compounds should not be present in drinking water. Suggest one reason why scientists may be worried by the presence of these compounds.
(Chlorine compounds are) carcinogenic OR (Chlorine compounds are) toxic OR poisonous
Under different conditions, chlorine reacts differently with aqueous sodium hydroxide. A disproportionation reaction takes place as shown below. 3Cl 2(g) + 6NaOH(aq) 5NaCl (aq) + NaCl O3(aq) + 3H2O(l) State what is meant by disproportionation and show that disproportionation has taken place in this reaction. Use oxidation numbers in your answer.
(Disproportionation) is the (simultaneous) oxidation and reduction of the same element (in the same redox reaction. Cl in Cl2 is 0 AND Cl in NaCl is -1 AND Cl in NaClO3 is +5 Chlorine has been oxidised from 0 to +5 AND Chlorine has been reduced from 0 to -1 3Cl2 + 6NaOH 5NaCl + NaClO3 + 3H2O
A teacher heats potassium chlorate(V), KClO3. The equation is given below. 2KClO3(s) 2KCl(s) + 3O2(g) (i) This is an example of a redox reaction. What other type of reaction takes place?
(Thermal) decomposition
The teacher heats 0.824 g of KClO3. Calculate the volume of oxygen produced, in cm3, measured at room temperature and pressure. Give your answer to the nearest whole number
(amount of KClO3) = 0.824/122.6 OR = 0.00672 (mol) (amount O2) = (mol of KClO3) 0.00672 × 3/2 OR = 0.0101 (mol) (volume of O2) = 0.0101 × 24 000 = 242 (cm3)
What volume of CO2, in dm3, would have been given off when measured at room temperature and pressure? The molar mass of MgCO3 = 84.3 g mol-1
1.47 = 0.0174 mol of MgCO3 84.3 0.0174 24.0 = 0.418 dm3 OR (Calculator value 24.0) = 0.419 dm3
Fluorine reacts with boron, B, to form the fluoride BF3. (i) Suggest an equation for this reaction.
2B + 3F2 ----> 2BF3
Chlorine is formed when KMnO4 reacts with hydrochloric acid. The ionic equation for this redox reaction is 16H+ + 2MnO4− + 10Cl ----->2Mn2+ + 8H2O + 5Cl2 Deduce the half-equation for the oxidation of chloride ions to chlorine.
2Cl− ---->Cl2 + 2e−
Magnesium and strontium are in Group 2 of the Periodic Table. (i) When reacted with oxygen, magnesium forms a white powder called magnesium oxide. Write the equation for the reaction of magnesium with oxygen.
2Mg+O2---->2MgO
Hydrogen chloride can be made from sodium chloride and concentrated sulphuric acid. Write a balanced chemical equation to represent this reaction
2NaCl + H2SO4 ----> Na2SO4 +2HCL
When sodium chlorate(I), NaClO, is heated, sodium chlorate(V) and sodium chloride are formed Write the ionic equation for this reaction.
3OCl- -----> ClO3- + 2Cl-
The electron configuration of bromine contains outermost electrons in the 4th shell. Using your knowledge of Group 7 elements, complete the electron configuration of bromine. 1s22s22p63s23p6 ..
3d10 4s2 4p5
A student is given a sample of an unknown Group 2 chloride. • The student dissolves 2.86 g of the chloride in water. • The student adds excess aqueous silver nitrate. • 8.604 g of solid silver chloride, AgCl, forms. (i) Calculate the amount, in moles, of AgCl that forms. The molar mass of AgCl = 143.4 g mol-1.
8.604/143.4 = 0.06(00) (mol)
Ammonium chloride, NH4Cl, is a salt which has covalent bonding, dative covalent (coordinate) bonding and ionic bonding. (i) What is a dative covalent (coordinate) bond?
A shared pair of electrons AND both electrons are donated by one atom
Explain the term oxidising agent in terms of electron transfer.
A substance which gains/removes/accepts electrons
(iii) Write an ionic equation, including state symbols, for the reaction that took place. ...................................................................................................................................... [1]
Ag+ (aq) + I-(aq) ---> AgI(s)
Write the ionic equation for this reaction. Include state symbols.
Ag+(aq) + Cl-(aq) AgCl(s)
A student was provided with an aqueous solution of calcium iodide. The student carried out a chemical test to show that the solution contained iodide ions. In this test, a precipitation reaction took place. (i) State the reagent that the student would need to add to the solution of calcium iodide.
AgNO3(aq) OR silver nitrate OR AgNO3
Draw a 'dot-and-cross' diagram to show the bonding in NH4Cl. Show the outer electrons only.
Ammonium ion with three covalent 'dot-and-cross' bonds AND one dative covalent bon
Write the formula of the Group 2 carbonate which decomposes at the highest temperature. .....................................................................................................................................
BaCO3 OR RaCO3
Predict a value for the boiling point of hydrogen fluoride. Explain your reason for choosing this value. Predicted value
Boiling point between 200 and 300 (K) )±73 to +27 if value quoted in °CHydrogen bonding is present. Stronger forces between molecules /stronger intermolecularforces (than in other hydrogen halides.
showequation- bromine oxidises iodide ions to iodine
Br2 + 2I ̄ ——> I2 + 2Br ̄
This question is about Group 2 elements and compounds. (a) Group 2 carbonates undergo thermal decomposition. (i) Write the equation for the thermal decomposition of calcium carbonate. Include state symbols. ...................................................................................................................................... [1]
CaCO3(s) --->CaO(s) + CO2(g)
State which of the elements chlorine or bromine is the stronger oxidising agent and explain the importance of this in the extraction of bromine from seawater, as represented in the equation above.
Chlorine (is stronger oxidising agent) So it can accept electrons from Br±/so it can displace Br±/ so can oxidise Br±
Explain why chlorine has a lower boiling point than bromine.
Chlorine is smaller than bromine OR has fewer electrons / electron shells OR It is smaller / It has a smaller atomic radius / it is a smaller molecule / atom. The forces between chlorine / Cl2 molecules are weaker (than the forces between bromine / Br2 molecules)(or converse for bromine)OR chlorine / Cl2 has weaker / fewer / less (VdW) intermolecular forces / forces between molecules (or converse for bromine)
show equation- chlorine oxidises bromide ions to bromine
Cl2 + 2Br ̄ ——> Br2 + 2Cl ̄
show equation-chlorine oxidises iodide ions to iodine
Cl2 + 2I ̄ ——> I2 + 2Cl ̄
Write equations for these two reactions of chlorine with sodium hydroxide: equation for reaction with dilute sodium hydroxide, ........................................................................................................................................... equation for reaction with hot concentrated sodium hydroxide.
Cl2 + 2NaOH →NaClO + NaCl + H2O 3 3Cl2 + 6NaOH → NaClO3 + 5NaCl + 3H2O Cl2 and NaOH as reactants AND NaClO3 and NaCl as products
Chlorine and its compounds have wide uses in chemistry. (a) In drinking water, HClO kills bacteria. (i) Write an equation to show how HClO can form in drinking water. ..................................................................................................................................... [1]
Cl2 + H2O HClO + HCl
Sulfur exists as S8 molecules and chlorine as Cl2 molecules. Use this information to explain the difference in their melting points.
Cl2 OR S8 has intermolecular OR van der Waals' forces 2 S8 has stronger intermolecular forces OR van der Waals' forces than Cl2 OR S8 has more electrons.nuclear charge increases/ protons increase
Chlorine gas reacts with water as shown below. Cl 2(g) + H2O(l)-----> HCl O(aq) + HCl (aq Using oxidation numbers, explain why this reaction is an example of disproportionation.
Cl2 is 0 AND HCl is -1 AND HClO is (+)1 Chlorine has been both oxidised and reduced OR Chlorine's oxidation state has increased and decreased Chlorine has been oxidised (from 0) to +1 AND chlorine has been reduced (from 0) to -1 (These two points together subsume the second marking point)
This question is about elements in Period 2 of the Periodic Table. (a) Lithium has a giant metallic structure and a boiling point of 1342 °C. Describe, with the aid of a labelled diagram, the structure and bonding in lithium and explain why lithium has a high boiling point.
Diagram showing a regular arrangement of labelled 'Li+' or '+ ions' with some attempt to show electrons Scattering of labelled electrons between other species OR a statement anywhere of delocalised electrons (can be in text or in diagram) The attraction between + ions and e- is strong OR metallic bonding is strong
Fluorine is a gas at room temperature and has a very low boiling point of -188 °C. (i) Draw a 'dot-and-cross ' diagram to show the bonding in a fluorine molecule. Show the outer electrons only.
Dot and cross bond + 6 matching electrons on each F atom
The Group 7 element chlorine reacts with sodium hydroxide, NaOH, under different conditions to give different products. Chlorine reacts with aqueous sodium hydroxide to form bleach. Write the equation and state the conditions for this reaction. equation ............................................................................................................................ conditions ................................................................................................................
Equation 2NaOH + Cl2 NaCl + NaClO + H2O Conditions cold AND dilute (sodium hydroxide
Nitrogen can also form a fluoride, NF3, which has a permanent dipole. Explain why NF3 has a permanent dipole
F is more electronegative than N OR -F-N+ Dipoles do not cancel OR NF3 is pyramidal (in words) / asymmetrica
Explain why fluorine has a low boiling point.
F2 has induced dipoles OR temporary dipoles OR van der Waals' forces (between the molecules) which are weak
Magnesium reacts with dilute acids.Describe what you would expect to see when magnesium ribbon is added to an excess of dilute hydrochloric acid
Fizzes OR bubble. Mg dissolves OR Mg disappear
(ii) Predict the structure and bonding of Ba3N2. ...................................................................................................................................... [1] (iii)
Giant ionic (lattice)
The boiling points of three hydrogen halides are shown below. Hydrogen chloride 188 Hydrogen bromide 206 Hydrogen iodide 238 Explain the trend in boiling point of the three hydrogen halides.
Going down groupNumber of electrons increases (1) Accept reverse argument going up group Reject arguments based on quantum shells. So Van der Waals force (between molecules) increases
Deduce the amount, in moles, of the Group 2 chloride that the student dissolves. Hence deduce the relative atomic mass and the identity of the Group 2 metal. Give the relative atomic mass to one decimal place. You must show your working
If a Group 2 chloride is used amount of Group 2 chloride = ½ × 0.0600 OR = 0.0300 mol Mass of 1 mol of Group 2 chloride = 2.86 = 95.3(3) 0.0300 [Relative atomic mass of M = 95.3(3) - 71.0) = 24.3 (g mol-1)] AND metal = Mg
Explain why lithium fluoride conducts electricity when molten but not when solid
Ions cannot move in a solid Ions can move OR are mobile when molten
State one benefit for public health, of the reaction between chlorine gas and water
Kills bacteria OR 'kills germs' kills micro-organisms OR makes water safe to drink OR sterilises water OR 'disinfect
Fluorine reacts with lithium at room temperature to form a white crystalline solid, lithium fluoride. Lithium fluoride is a good conductor of electricity when molten but not when solid. (i) Draw a 'dot-and-cross ' diagram to show the bonding in lithium fluoride. Show the outer electrons only
Li shown with either 2 or 0 electrons and F shown with 8 electrons with 7 crosses and one dot (or vice versa) correct charges on both ions
Write an equation, including the state symbols, for the reaction between solid lithium chloride and concentrated sulphuric acid.
LiCl(s) + H2SO4(l) ----->LiHSO4(s) + HCl(g)
Seawater contains aqueous bromide ions. During the manufacture ofbromine, seawater is treated with chlorine gas and the following reaction occurs: 2Br- + Cl2 -----> Br2 + 2Cl- Explain the term oxidation in terms of electron transfer.
Loss of electron
Solids exist as lattice structures. (a) Giant metallic lattices conduct electricity. Giant ionic lattices do not. If a giant ionic lattice is melted, the molten ionic compound will conduct electricity. Explain these observations in terms of bonding, structure and particles present
Metallic lattice has delocalised OR mobile electrons OR metallic bonding has delocalised OR mobile electrons Ionic lattice has no mobile ions OR ionic solid has no mobile ions molten ionic (compounds) have mobile ion
The third period of the Periodic Table features the elements magnesium and chlorine. The table below shows the melting points of these elements. magnesium;650 chlorine;-101 Describe the structure and bonding shown by these elements. Use your answer to explain the difference in melting points.
Mg has a giant structure 6 Additional Guidance Mg has metallic bonding OR description of metallic bonding as positive ions and delocalised electrons. (There is electrostatic attraction between) positive ions and electrons) Cl has a simple molecular OR simple covalent (lattice) Cl has van der Waals' forces (between molecules). van der Waals' forces are weak and metallic bonds are strong
Hydrated strontium chloride, SrCl 2•6H2O, has a molar mass of 266.6 g mol-1. A student heats 5.332 g of SrCl O. 2•6H2 The SrCl 2•6H2O loses some of its water of crystallisation forming 3.892 g of a solid product. Use the information above to determine the formula of the solid product.
Mol of SrCl2•6H2O = (5.332 / 266.6) = 0.02 mol Correctly calculates Mol of water given off [(5.332 - 3.892)/18] = 0.08 mol Correctly calculates 0.08/0.02 = 4 mol of water lost from one mol of SrCl2•6H2O Therefore Answer = SrCl2•2H2O
Give the formulae of the ions present in NH4Cl.
NH4+ AND Cl-
Chlorine forms another chlorate called sodium chlorate(VII), used in the manufacture of matches. Suggest the formula of sodium chlorate(VII
NaClO4
The element strontium forms a nitrate, Sr(NO3)2, which decomposes on heating as shown below. 2Sr(NO3)2(s)---> 2SrO(s) + 4NO2(g) + O2(g Using oxidation numbers, explain why the reaction involves both oxidation and reduction
O goes from -2 to 0 N goes from +5 to +4 N is reduced AND O is oxidised
Chlorine reacts directly with Group 2 elements to form chlorides that are very soluble in water. Aqueous chloride ions can be detected by adding aqueous silver nitrate. The appearance of solid silver chloride, AgCl, confirms the presence of chloride ions. (i) State the type of reaction that has taken place
Precipitation
Strontium reacts in a similar way to magnesium.Describe one difference you might observe if strontium, instead of magnesium, was reacted with dilute hydrochloric acid.
Quicker OR more vigorous OR gets hotter
A chemist carries out reactions of barium and barium nitride, Ba3N2. Reaction 1 Barium is reacted with water. Reaction 2 Barium nitride is reacted with water, forming an alkaline solution and an alkaline gas. Reaction 3 Barium is reacted with an excess of oxygen at 500°C, forming barium peroxide, BaO2. (i) Write equations for Reaction 1 and Reaction 2. Reaction 1: ........................................................................................................................ Reaction 2: ........................................................................................................................
Reaction 1: Ba + 2H2O Ba(OH)2 + H2 Reaction 2: Ba3N2 + 6H2O 3Ba(OH)2 + 2NH3
Group 2 elements react with halogens. (a) Describe and explain the trend in reactivity of Group 2 elements with chlorine as the group is descended
Reactivity increases (down the group) Increasing size mark Atomic radius increases OR There are more shells Increased shielding mark There is more shielding Nuclear attraction (to electron) mark Nuclear attraction (to electron) decreases OR (outermost) electrons experience less attraction (to nucleus) OR Increased nuclear charge is outweighed by increased shielding/distance Ease of electron loss mark Easier to remove (outer) electron(s) OR Ionisation energy decreases Quality of written communication electron(s) OR ionisation OR ionization OR oxidise OR oxidize spelled correctly at least once for last marking poin
Name the shape of, and state the bond angles in, a BF3 molecule. Explain why BF3 has this shape.
Shape: trigonal planar Bond angle: 120° Explanation: Pairs of electrons repel (one another equally) Boron has 3 bonded pairs (and 0 lone pairs)
) Solid SiO2 melts at 2230 °C. Solid SiCl 4 melts at -70 °C. Neither of the liquids formed conducts electricity. Suggest the type of lattice structure in solid SiO2 and in solid SiCl 4 and explain the difference in melting points in terms of bonding and structure.
SiO2 is giant covalent (lattice) SiCl4 is simple molecular (lattice) van der Waals' forces in SiCl4 Covalent bonds broken in SiO2 Forces OR bonds are stronger in SiO2 (than in SiCl4) OR more energy is needed to break forces OR bonds in SiO2 (than in SiCl4)
The student repeated the experiment a further three times, using the same number of moles of CaCO3, SrCO3 and BaCO3. What trend in the behaviour of the Group 2 carbonates would be observed by the student?
The ease of (thermal) decomposition decreases (down the group)
Group 2 elements and compounds show periodic trends. One trend is shown by the effect of heat upon Group 2 carbonates. A student carried out an experiment to find out the volume of carbon dioxide obtained by heating a weighed sample of magnesium carbonate. The student placed a 1.47 g sample of MgCO3 into a test-tube and heated it until there was no further change in mass. The following reaction took place. MgCO3(s)---> MgO(s) + CO2(g) What type of reaction is this?
Thermal decomposition
The solid lattice structure of ammonia, NH3, contains hydrogen bonds. (i) Draw a diagram to show hydrogen bonding between two molecules of NH3 in a solid lattice.
Two (or more) ammonia molecules with at least one H+ and at least one N- (can be on the same or different molecules) H-bond between H in one ammonia and lone pair of N in another ammonia molecule
What observation would show that the solution contained iodide ions? ......................................................................................................................................
Yellow AND precipitate
Deduce the oxidation number of iodine in the following species. I2O7 IO-4
[+4] [-1] [+2] [-1] [0]
For each pair of compounds, give a reagent (or combination of reagents) that, when added separately to each compound, could be used to distinguish between them. State what is observed in each case. Butan−2−ol and 2−methylpropan−2−ol Reagent . Observation with butan−2−ol ........................................................................................................................ ........................................................................................................................ Observation with 2−methylpropan−2−ol
acidified potassium dichromate or K2Cr2O7 / H2SO4 (orange to) green solution OR goes green. colourless solution OR goes colourless M3 (solution) remains purple or no reaction or no (observed) change
How would you confirm that a solution said to be HCl(aq) contained chloride ions?
add silver nitrate (solution) / correct formula AgN03 (aq) white ppt /solid (1)soluble in dilute ammonia /ammonia solution (1)oradd lead nitrate/ethanoate (solution) (1)white ppt. (1)soluble in hot water (
The student is provided with an aqueous solution of calcium bromide that is contaminated with calcium iodide. The student carries out the same chemical test but this time needs to add a second reagent to show that iodide ions are present. State the second reagent that the student would need to add. ..............................................................
concentrated (aqueous) NH3
Give a chemical test for chlorine, stating what you would do and what you would see.
damp litmus paper (1) bleached (1) or damp starch-iodide paper (1) goes blue (1)Displacement acceptable
Chlorine reacts differently with dilute and concentrated aqueous solutions of sodium hydroxide. When chlorine reacts with dilute sodium hydroxide, one of the products is sodium chlorate (I). This is the reaction that is used to manufacture bleach. When chlorine is reacted with hot concentrated sodium hydroxide, a different reaction takes place. One of the products is NaCl O3, used as a weedkiller.In each reaction, chlorine has been both oxidised and reduced. (i) What term is used to describe a redox reaction in which an element is both oxidised and reduced?
disproportionation
what type of reactions is this
disproportionation
Hydrogen chloride is soluble in water. Explain why the solution is acidic.
dissociates /reacts/lionises/changes into ions (as it dissolves) (1)forming H+ ions / H3O+ ions/ donates a proton to water.This makes the solution an acid
Group 2 elements also react with dilute hydrochloric acid. Describe and explain the trend in reactivity of the Group 2 elements with dilute hydrochloric acid as the group is descended. [5]
reactivity increases (down the group) Increasing size mark atomic radii increases OR there are more shells Increased shielding mark there is more shielding Nuclear attraction mark The nuclear attraction decreases OR (outermost) electrons experience less attraction (to nucleus) OR Increased shielding / distance outweighs the increased nuclear charge easier to remove (outer) electrons OR ionisation energy decreases ORA throughout
Chlorine and bromine are elements in Group 7 of the Periodic Table. (a) Chlorine is used in water treatment. State one advantage and one disadvantage of using chlorine in water treatment. advantage: ............................................................................................................................... ................................................................................................................................................... disadvantage:
removes or kills bacteria OR kills germs ORkills micro-organisms OR make it safe to drink OR sterilises water OR disinfects water it is toxic
