Higher Chemistry

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Higher Chemistry

• Unit 1
• Section 4.1 mole and number

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4.1 mole and number

• Look at the mole display which shows one mole of
  each of six substances.
• Although the amount of substance is the same
  (same number of moles), the mass of each
  substance is different because the individual
  atoms and molecules making up each substance have
  different masses.

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Avogadro constant

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• Avogadro constant
• L 6.02 x 10 23 mol -1 formula units
• What are the formula units?
• It refers to the type of particles that make up
• a substance

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Formula units

• Atoms for all elements which are not diatomic
• Molecules for all diatomic elements and covalent
  molecular compounds
• Formula units for all ionic substances and
  covalent networks (simplest ratio)

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One mole of any substance contains 6.02 x 10 23
formula units

• How many ions are present in 1 mole of sodium
  chloride?
• 1 mole NaCl - L formula units
• So Na ions L ions
• Cl- ions L ions
• Total ions 2L (2 X 6.02 x 1023)

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• How many atoms in 1 mole of silicon dioxide SiO2?
• 1 mole SiO2 L formula
  units
• Number of silicon atoms L atoms
• Number of oxygen atoms 2L atoms
• Total number of atoms 3L atoms
• (3 X 6.02 x 10 23)

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Mole and number calculations

• How many molecules in 0.218 mol of water?
• Start with 1 mole formula units
• 1 mole 6.02 x 10 23 molecules
• 0.218 mol 0.218 X 6.02 x 10 23
• 1.31 X 10 23 molecules
• Now try examples on sheet 4.1

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Mass and number calculations

• How many molecules are in 76g of CO2 ?
• Start with
• 1 mole mass (gfm) formula units
• 44 g 6.02 x 10 23molecules
• 76 g 6.02 x 10 23 X 76
• 44
• 1.04 x 10 24molecules
• Now try examples on sheet 4.1

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• Now try the checktest on 4.1
• If time, to help consolidate, try the Home
  Practice Questions for 4.1

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Higher Chemistry

• Unit 1
• Section 4.2 mole and gas volume

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4.2 mole and gas volume

• The molar volume of a gas means the volume
  occupied by one mole of a gas.
• This is measured in l mol -1
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• SCHOLAR - Higher Chemistry Molar volume

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Molar volume simulation

• SCHOLAR - Higher Chemistry Molar volume
• Calculate the molar volume of gases if the mass
  of a measured volume of gas is known.

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Molar volume of methane

• Volume of methane collected 1.0 l
• Initial mass 124.86g
• Final mass 124.14g
• Mass of methane released 0.72 g
• So 0.72 g methane occupies 1.0 l
• 1 g
  1.0
• 
  0.72
• 16g (gfm) 1.0 X 16
• 0.72
• Molar volume of methane is 22.2 l mol -1

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Calculate molar volume for hydrogen if using same
apparatus and initial mass was same and 1 litre
of gas was collected. The final mass was 124.77g

• Volume of hydrogen collected 1.0 litre
• Initial mass 124.86g
• Final mass 124.77g
• Mass of hydrogen released g
• So g hydrogen occupies 1.0 litre
• 1 g occupies
• (gfm) occupies
• Molar volume of hydrogen is

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• This volume is the same for ALL gases under the
  same temperature and pressure
• Molar volume can also be calculated from density
• The data book lists densities at standard
  temperature and pressure (0oC and 1atmosphere)

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Calculate the molar volume of argon at s.t.p

• Density argon 0.0018 g cm -3 in data
  book
• 1.8 g l -1(as 1000cm3 1 l )
• Molar volume gfm
• density careful
  with units!
• 40
• 1.8
• 22.22 l mol-1

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Gas volume and moles or mass

• How many moles of nitrogen in 441 cm3 of the gas
  when molar volume is 24.5 l mol-1
• 24.5 litres 1 mole
• 0.441 1 X 0.441
• 24.5
• 0.018 moles

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• What is mass of hydrogen sulphide gas in 360 cm3
  of the gas when molar volume is 22.6 l mol-1
• 22.6 litres 1 mole 34.1 g
• 0.36 34.1 X 0.36
• 22.6
• 0.54 g
• Now try examples on sheet 4.2 part 2

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Gas volume and number

• If molar volume is 24.08 l mol-1 what is volume
  of CO2 that contains 1.25 x 10 22 molecules
• 6.02 x 10 23 molecules 24.08 litres
• 1.25 x 10 22 24.08 X 1.25 x 1022
• 6.02 x 10 23
• 0.5 litres
• Now try examples on sheet 4.2 part 3

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Higher Chemistry

• Unit 1
• Section 4.3 calculations involving comparisons

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4.3 Comparing quantities by moles

• Equimolar same number of moles same number of
  formula units
• i) gfm 111g 3 moles
• ii) nc x v 2.5
  moles
• iii) 24litres 1 mole 0.25 moles
• Iv) L molecules 1 mole 2.5 moles
• A) i) has greatest number of moles
• B) ii) and iv) are equimolar (same number of
  formula units

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Calculating Excess
Which reactant is in excess if 15g of zinc
carbonate is reacted with 150cm3 of 2 mol l-1
nitric acid. ZnCO3 2 HNO3 ? Zn(NO3)2
CO2 H2O
Find the no moles zinc carbonate present. Find
the no. moles nitric acid present. Use the
equation to find which reactant is in excess.
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• Find the no. moles zinc carbonate present.
• n m/ gfm (always need the gfm)
• Find the no. moles nitric acid present.
• n c x v (volume always in litres)
• Which reactant is in excess? (The one with more
  than you need!)

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Calculating Excess
Which reactant is in excess if 0.98g of magnesium
is reacted with 250 cm3 of 0.2 mol l-1
hydrochloric acid. Mg 2 HCl ?
MgCl2 H2
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• Limiting reactant is the one which is NOT in
  excess and so determines the quantities of the
  products made in the reaction.
• Need to work out first which is limiting reactant
  ie not in excess and then
• Calculate the product from it.

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Calculating Products from reactants using excess
What mass of zinc chloride is made when 5.88 g
of zinc is reacted with 300 cm3 of 0.5 mol
l-1 hydrochloric acid. Zn 2HCl
? ZnCl2 H2
Find the no moles Zn present. Find the no. moles
HCl present. Use the equation to find which
reactant is in excess. Use the no. moles of the
limiting chemical to find the mass of ZnCl2
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• Zn 2HCl ? ZnCl2 H2
• 1 mole 2 moles
• no moles Zn 0.09 moles
• so we need 0.18 moles of HCl
• Calculate how many moles we actually have?
• no moles HCl 0.15 moles not enough so
  limiting factor

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Use the no. moles of the limiting chemical to
find the mass of ZnCl2

• Zn 2HCl ? ZnCl2 H2
• 1 mole 2 moles
• So if 0.15 moles of HCl we need
• 0.075 moles Zn
• Mass n x gfm
• 0.075 x 136.4
• 10.23 g

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Now try example
What mass of nickel chloride is made when 16.4g
of nickel oxide is reacted with 240ml of
1.25moll-1 hydrochloric acid in following
NiO 2 HCl ? NiCl2 H2O
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Higher Chemistry

• Unit 1
• Section 4.4 Gases in reactions

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4.4 One gas from another gas

• The volume of a gas reactant or product can be
  calculated from a balanced equation using the
  number of moles of a reactant or product.

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• Provided the same conditions of temperature and
  pressure are used then the molar volume of the
  gases will be the same. Volumes of solids and
  liquids can be ignored as they are negligible
  compared to equimolar volume of gases.

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Reaction of methane and oxygen

• Balanced equation
• CH4 (g) 2O2(g) CO2(g) 2H2O(l)
• 1 mole 2 mole 1 mole 2
  moles
• 1 volume 2 volumes 1 volume negligible
• So for example
• 10 cm3 methane reacts with 20 cm3 oxygen to
  produce 10 cm3 of carbon dioxide.

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What volume of propane must be burned in oxygen
to produce 60 cm3 of CO2?

• C3H8 (g) 5 O2 (g) 3CO2 (g) 4H2O
  (l)
• 1 mole 5 mole 3 mole
  4 moles
• 1 vol 5 vol 3 vol
• so if 3 vol 60 cm3
• then 1 vol 20 cm3
• 20 cm3 of propane is needed.
• Now try examples on 4.4 part 1

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In almost all reactions one of the reactants is
in excess. Always start by working out which
reactant is in excess.

• If 50 cm3 of oxygen was reacted with 20 cm3 of
  methane which reactant was in excess?
• CH4 (g) 2O2(g) CO2(g) 2H2O(l)
• 1 vol 2 vol
• 20 cm3 methane needs 40cm3 of oxygen
• so oxygen is in excess by 10 cm3
• (If 50 cm3 of oxygen was 2 volumes then you
  would need 25 cm3 of methane)
• Now try examples in 4.4 part 2

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Final total gas volume

• This depends not only on volume of any gases
  produced but also on any unreacted volume which
  is left over from the excess reactant.

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If 50 cm3 of oxygen was reacted with 20 cm3 of
methane at room temperature what was the total
gas volume?

• CH4 (g) 2O2(g) CO2(g) 2H2O(l)
• 20 cm3 40 cm3 20 cm3
• 10 cm3 unreacted oxygen
• Total gas 20 cm3 10 cm3 30 cm3

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If 50 cm3 of oxygen was reacted with 20 cm3 of
methane at 120oC what was the total gas volume?

• CH4 (g) 2O2(g) CO2(g) 2H2O(g)
• 20 cm3 40 cm3 20 cm3 40 cm3
• 10 cm3 unreacted
• Total gas 20 cm3 40 cm3 10 cm3 70 cm3
• Now try examples on 4.4 part 3

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Higher chemistry

• Unit 1
• Section 4.5 chemical energy

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4.5 chemical energy H

• A potential energy diagram can be used to show
  the energy changes for a reaction
• Enthalpy is a measure of the chemical energy
  stored within substances.
• Symbol is H and is measured in kilojoules kJ

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• An exothermic reaction is a chemical change
  which gives out heat energy.
• Draw the enthalpy diagram for an exothermic
  reaction (combustion of methane)
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• An endothermic reaction is a reaction which
  takes in heat energy.
• Draw the enthalpy diagram for an endothermic
  reaction (decomposition of calcium carbonate)
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Enthalpy Changes
A. Combustion of methane CH4 (g) 2O2 (g) ?
CO2 (g) 2 H20 (l)
CH4 (g) 2O2 (g)
H
? H negative, exothermic reaction
CO2 (g) 2 H20 (l)
kJ
reactants
products
B. Cracking of ethane C2H6 (g) C2H4 (g)
H2(g)
C2H4 (g) H2(g)
C2H6 (g)
H
? H positive, endothermic reaction
kJ
reactants
products
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Enthalpy changes

• Enthalpy change is the difference between the
  final total enthalpy of the products and the
  reactants.
• ?H H(products) H(reactants)
• ?H is negative for exothermic reactions
• ?H is positive for endothermic reactions

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?H H(products) H(reactants)

• Copy diagram for combustion of methane and
  calculate the enthalpy change.
• ?H - 894 kJ
• Copy diagram for cracking of ethane and calculate
  the enthalpy change
• ?H 138 kJ

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Higher chemistry

• Unit 1
• Section 4.6 Reactions and enthalpy change

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Neutralisation Reactions

• The enthalpy of neutralisation of an acid is
  defined as the energy change (in kJ) when it is
  neutralised to form 1 mole of water.
• Write out the two equations for neutralising
  hydrochloric acid and sulphuric acid with sodium
  hydroxide quoting the enthalpies for both.

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• The enthalpy values are the same because on
  removing the spectator ions the following is
  left
• H(aq) OH-(aq) H2O(l)
• ?Hneut - 57 kJmol -1

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?Hsol Enthalpy of solution

• When a substance dissolves in water it can cause
  a change in temperature showing that an enthalpy
  change has taken place.
• ?H soln is the enthalpy change that occurs when
  one mole of substance dissolves completely in
  water.

?T
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?H soln is calculated by experimental result

• ?H soln - c m ?T
• c 4.18 kJ kg-1oC-1
• m mass of water in kg
• ?T change in temperature

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Calculate molar enthalpy of solution for ammonium
nitrate.

• By experiment you can calculate the enthalpy
  change for the known mass of substance.
• Note mass of water is 1cm3 0.001kg
• Then work out the enthalpy change for 1 mole
  (gfm) of substance

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• x grams NH4NO3 cm ?T
• 4.18 x 0.05 x ?T
• 
  kJ
• So 1 Mole (80g)
  kJmol-1
• Is this an exothermic or endothermic reaction?
• ?H soln

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?Hc combustion reaction

• Enthalpy of combustion is the enthalpy change
  when 1 mole of a substance burns completely in
  oxygen, all reactants and products being in their
  standard states at 25oC and one atmosphere
  pressure
• Note all combustion reactions are exothermic!

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• Copy out the balanced equation with state symbols
  for the combustion of butane. Write out the
  enthalpy change.
• Write out the equations for the combustion of
  a) ethene and b) hydrogen
• Find values in the data booklet

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• If 1.21g magnesium is burned and the enthalpy
  change is 60 kJ, calculate the enthalpy of
  combustion of magnesium
• Now try the checktest questions