Solids, Liquids and Gases Solids (s): Molecules are held in place by intermolecular interactions....
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Transcript of Solids, Liquids and Gases Solids (s): Molecules are held in place by intermolecular interactions....
Solids, Liquids and Gases• Solids (s): Molecules are held in place by intermolecular
interactions.• Liquids (l): Molecules are held next to one another by
intermolecular interactions, however, these interactions are not strong enough to prevent the molecules from flowingpast one another.
• Gases (s): The intermolecular interactions are too weak to hold the molecules next to one another, so the molecules wander off on their own.
DiffusionDiffusion is the spontaneous spreading out of a substance, and isdue to the natural movement of its particles.
Ink being diffused in water
Cotton wool soaked with
ammonia solution
Cotton wool soaked with
hydrochloric acid Ammonium chloride
Diffusion of ammonia and hydrogen chloride gases
The mass of atoms are very small
• Therefore scientists cannot count individual atoms or molecules directly
• Instead a scientist count there particles by weighing a certain number of them
• Same as a bank cashier, by knowing the weight of one coin they can calculate the number of coins by dividing the total mass by the mass of one coin.
A. What is the Mole?
• A counting number (like a dozen)
• Avogadro’s number (NA)
• 1 mol = 6.02 1023 items
A large amount!!!!
The Mole
• The mole is simply a chemist’s measurement for the amount of a substance.
• It is a very large number (6 x 1023) as it ‘counts’ very small ‘objects’ such as atoms and molecules which have a very small masses.
• This rather strange number was not selected deliberately – instead, it is the number of atoms in a sample of any element that has a mass in grams that is numerically equal to the elements atomic mass.
• It allows chemists to work with masses in the lab which are feasible.
The Mole Definition
A mole of any substance is defined as the amount of the substance that contains as many
particles as 12 g of carbon – 12.
B. Molar Mass
• Mass of 1 mole of an element or compound.
• Atomic mass tells the...– That 6.01 *1023 atoms will weigh this
amount– 6.01 *1023 atoms of carbons will weigh 12
grams– grams per mole (g/mol)– 1 mole of carbon is 12 grams per litre
• Round to 2 decimal places
B. Molar Mass Examples
• carbon
• aluminum
• zinc
12.01 g/mol
26.98 g/mol
65.39 g/mol
B. Molar Mass Examples
• water
• sodium chloride
– 2(1.01) + 16.00 = 18.02 g/mol
– H2O
– NaCl– 22.99 + 35.45 = 58.44 g/mol
B. Molar Mass Examples
• sodium bicarbonate
• sucrose
– NaHCO3
– 22.99 + 1.01 + 12.01 + 3(16.00)
= 84.01 g/mol
– C12H22O11
– 12(12.01) + 22(1.01) + 11(16.00)
= 342.34 g/mol
If a magnesium atom is twice as heavy as an atom of carbon
Then 100 moles of magnesiumAre twice as heavy as 100 moles Of carbonTherefore if we have a piece ofMagnesium that is twice as heavy As a piece of carbon they have an Equal number of atoms
• Atom of carbon weighs
• Mr=12
• Atom of hydrogen weighs
• Mr=1One gram of hydrogen atoms and twelve grams of
Carbon atoms both have the same number of atoms
• Atom of silver weighs
• Ar=108
• Atom of carbon weighs
• Ar=12
If we have 12g of carbon how do we calulate the mass of silver if it has the same number of atoms
12g 24g 27g 40g 56g 63.5 108g
carbon magnesium
aluminium
calcium iron copper
silver
All these masses contain the same number of atoms
Avogadro`s number = 6 X 1023 Symbol L
It is the number of atoms of carbon in 12g of the carbon-12 isotope
The amount of a substance which contains 6 X 1023 particles is called a mole of that
substance
Symbol mol
• One mole of carbon weighs 12grams
• 48g of Mg (one mole weighs 24 g)
• One mole of carbon contains 6 X 1023
atoms
• Is 2 moles of Mg
Converting moles to grams
• Mass of one mole of an element= relative atomic mass in grams
Element Relative atomic mass
H 1
C 12
N 14
Mg 24
Cu 63.5
Fe 56
I 127
The mass of
one mole of
an element
=RelativeAtomic Massin grams
The relative molecular mass of a compound is the sum of the relative atomic masses of all the atoms in the compound
H2O = 2(1) + (16) = 18 (relative molecular mass of H2O)
CuSO4.H2O =(63.5) + (32) +4(16) +5 (18)=249.5
Converting Grams to moles
• 12g of carbon = 1 mole• 1g of carbon = 1/12 moles• 36g of carbon= 36/12=3 moles
How many moles of sulfuric acid (H2SO4) are
there in 12.25 g of sulfuric acid
• Relative mol mass =2(1) + (32) + 4(16) = 98
• Total mass/mass of one mole• 12.25/98=0.125 mole
Converting moles to number of atoms or molecules
• How many atoms of sodium are present in 0.25 moles of a metal
• 1 mole of Na contained 6x1023 atoms• 0.25 x 6x1023 atoms= 1.5 x1023
atoms
232 2
How many molecules are there in 0.5 mols of chlorine gas?How many atoms are there in 2 mols of water?How many electrons are there in 1.5 mols of Calcium?
1 mol of Cl 6 10 molecules [Cl
(i)(ii)(iii)
(i)23 23
2
232 2
23 242
is composed of molecules]
0.5 mols of Cl 0.5 6 10 3 10 molecules
1 mol of H 0 6 10 molecules [H O is composed of molecules]
2mols of H 0 2 6 10 1.2 10 moleculesEach molecule of water contai
(ii)
24 24 24
23
23 232
ns 3 atoms
1.2 10 molecules contains 3 1.2 10 3.6 10 atoms.
1 mol of Ca 6 10 atoms [Ca is composed of atoms]
1.5 mols of H 0 1.5 6 10 9 10 atomsEach atom of Calcium contains 20 electrons
9
(ii)
23 24 2510 atoms contain 23 9 10 2.07 10 electrons.
Molar Volume
• One mole of any gas should occupy the same volume as one mole of any other gas at the same conditions of temperature and pressure.
• Gases are often compared at STP, standard temperature and pressure.Standard pressure = 101325 PaStandard temperature = 273 K
• Molar Volume = 22.4 L = 22,400 cm3 = 2.24 x 10–2 m3
4
3
34
34
What is the volume in litres at STP of 0.01 mols of methane (CH ) gas?
How many mols are there in 280 cm of fluorine gas at STP?
1 mol of CH 22400 cm
0.01mols of CH 0.01 22400 224 cm
1 mol
(i)
(ii)
(i)
(ii) 32
32
32
32
of F 22400 cm1
mols of F = 1cm22400
1280 mols of F = 280cm
224000.0125 molsof F 280cm
Relative Molecular Mass
The relative molecular mass is the average mass of a molecule relative to one twelfth the mass of the carbon 12 atom.The relative molecular mass has no units as it is the ratio of two masses.
2 4 2
2 4
2
Calculate the relative molecular mass of sulfuric acid, (H SO ) Oxygen, (O ).
H SO 2(1) 32 4(16) 98
O 2(16) 32
(i) (a) (b)
(i) (a)
(b)
Molar Mass• The molar mass of a substance is the mass in grams
of one mole of the substance.• The molar mass has the same numerical value as its
relative molecular mass, but its units are grams (g).
2 4
6 12 6
2
2
Sulfuric aicd, H SO 98 98 gGlucose, C H O 180 180 g
Oxygen, O 32 32gSulfur dioxide, SO 64 64 g
Substance Relative molecular mass Molar mass
1 mol of carbon, 12 g
2
2
2
How many moles are there in 990 g of cabon dioxide (CO )The daily intake of calcium for an adult is 800 mg, how many moles of calcium is this?
1 mol of CO 44 g1
mols of CO 1g441
990 mols o44
(i)(ii)
(i)
2
2
3 3 3
3
f CO 990 g
22.5 mols of CO 990 g
1 mol of Ca 40 g1
mols of Ca 1g401
800 10 mols of Ca 800 10 g [800mg 800 10 g]400.02 mols of Ca 800 10 g
(ii)
More Mole Calculations
Moles of X
Volume of X(if a gas at STP) in litres at STP
Number of particles of X
Mass of X in g
÷ 6 x 10 23x 6 x 10 23
÷ 22.4
x 22.4
÷ molar massx molar mass
Notice that there is no direct link from particles to grams, you must first convert to moles. Likewise going from volume to mass.
23
23
2323
What is the mass of one atom of calcium?
1 mol of Ca 40 g
1 mol of Ca 6 10 atoms
6 10 atoms 40 g40
1 atom 6.67 10 g6 10
2
2
2
The Nissan Micra 1.5 Diesel SVE is quoted to have a CO emission figure of 120 g / km.For a 40 km round trip to work calculate:
the mass of CO produced.the number of moles of CO produced.the
(i)(ii)(iii) 2 volume of CO produced at room temperature and pressure.
[Molar volume at room temperature and pressure 24.0 litres]
120 40 4800 km
actual massno. of mols
molecular mass4800
no. of mols mols44
no. of m
(i)
(ii)
2
2
2
ols 109.09 mols
1mol of CO 24.0 L109.09 mols of CO 109.09 24L109.09 mols of CO 2594.16 L
(iii)
How many iron atoms should be consumed daily to meet the recommended daily intake of iron in the diet of 0.014 g?
acutalmassno. of mols
molecular mass0.014
no. of mols56
no. of mols 2.5 10
[H2007, Q4 (e)]
4
3
4 4 3
4 0
mols
1mol 6 10 atoms
2.5 10 mols 2.5 10 6 10 atoms
2.5 10 mols 1.5 10 atoms
Chemical Formulas
Empirical Formula
• The empirical formula of a compound is the formula that gives that simplest whole number ratio in which the atoms of the elements in the compound are present.
• The empirical formula of glucose, C6H12O6
(ratio of atoms is 6:12:6) is CH2O, as this is the lowest ratio (1:2:1) of the atoms C, H and O.
A sample of a brown coloured gas that is a major pollutant is found to contain 2.34 g of Nand 5.34 g of O. What is the simplest formula of the compound?
2.34mols of N atoms 0.167
145.34
mols of O atoms
2
0.33416
Ratio of Nitrogen atoms to Oxygen atoms 1: 2NO
A dry - cleaning fluid composed of carbon and chlorine was found to have the composition 14.5% C, 85.5% Cl. What is the empirical formula of this compound?
C 14r
Element Percentage Percentage SimplestA Ratio
2
.5% 14.5 11.208
12Cl 85.5% 85.5 2
2.40835.5
CCl
A 1.28 g sample of sulfur was allowed to react with an excess of chlorine to produce 4.12 g of a product that contains only sulfur and chlorine. What is the empirical formula of the compound?
Mass of s
2
ulfur consumed = 1.28 gMass of chlorine consumed = 4.12 1.28 2.84 g
1.28Mols of sulfur consumed = 0.04
322.84
Mols of chlorine comsumed = 0.0835.5
Ratio of sulfur atoms to chlorine atoms = 1:2
SCl
Molecular Formula
The molecular formula of a compound is the formula that gives the actual number of atoms of each element present in a molecule of the compound.
2
2
A colourless liquid used in rocket engines, whose empirical formula is NO ,has molecularmass of 92. What is the molecular formula?
Formula mass 46Molecular mass 92
92No. of NO in formula = 2
46Molecular for
2 2 42mula NO N O
Percentage Composition by Mass
If the empirical formula of a compound is known, the percentage by mass of each element present can be calculated. It can be useful to calculate the percentage of a certain element in a compound, for example the percentage of nitrogen in fertilisers.
2 2
r
Calculate the percentage by mass of nitrogen present in urea, CO(NH ) .
mass of nitrogen% of N 100
M of Urea2(14)
% of N 10012 16 2[14 2(1)]28
% of N 100 46.66%60
Structural Formulas
The structural formula of a compound shows the arrangement of the atoms within a molecule of the compound.
Some Examples:
You will come across many more structural formulas in the organic section of the course.
C
H
H
H H
Methane, CH4
C C
H
H
H
H
Ethene, C2H4
C C OH
H
H
H
H
H
Ethanol, C2H5OH
Chemical Equations
Chemical Equations• A chemical equation is a way of representing a chemical change.• It shows reactants and products.• To balance an equation means to change the numbers of each
molecule involved, so that the same number of atoms of each element appear on the reactants side and on the products side.
• Chemical equations balance on an atomic level, not molecular.• You cannot change the formula of a substance, i.e. if the
equation has NH3 you cannot change this you can only put a
number in front of it, 2NH3, increasing the number of N’s and the number of H’s.
• Never change the subscripts (small numbers).• It is possible to write balanced equations for lots of reactions but
that does not mean that the reaction actually takes place.
Formation of Water
(Oxygen does not exist as single atoms)
(This reaction does not take place)
Balancing an Equation
4 2 2 2
Methane reacts with oxygen, forming carbon dioxide and water vapour only.Write a balanced chemical equation for this reaction.
CH O CO H OExamine each element in turn, to have the same no. o
Solution
2
f of each on either side.(Leave the O until last as it appears the most often)Examining C: 1 on LHS and 1 on RHS, leave as they are.
Examining H: 4 on LHS and 2 on RHS, put a 2 in front of the H O, o
atoms
4 2 2 2
2
4 2 2 2
n the RHS.
CH O CO H O
Examining O: 2 on LHS and 4 on RHS, put a 2 in front of the O , on the LHS.
CH O CO2 H O
2
2
Balance the following2 2
2 2
2 2 2
2 2 2
4 10 2 2 2
4 10 2 2 2
2 2 3
2 2 3
Fe HCl FeCl HFe HCl FeCl H
H O H O
H O H O
C H O CO H O
C H O CO H O
Al O Al O
Al O A
2
l O
12
132
32
4 5
1
2
4.
2
.
.
3.
Balancing Redox Equations
• In working out what is oxidised and what is reduced in a reaction, it is important to remember that oxidation numbers are not a charge.
• Write the oxidation numbers below each atom to which it applies, as shown in the examples.
• Oxidation is an increase in oxidation number.• Reduction is a decrease in oxidation number.
1 2 6 22
2
2
12 7H O Cr O OH
2 2 3 32 7 2
2 2 3 3
6 2 2 1 3 3 1 2
6 3 36
2 7 2
3 32
3 2 32
2
3
3
2 3
2
2
Balance the following redox equation
Cr O Fe H Cr Fe H O
Cr O Fe H Cr Fe H O
Cr 3e Cr Cr 6e 2Cr
Fe e Fe Fe e Fe
Cr 6e 2CrF6 e
1.
2.
3.
27 2
3
2 3 32
2 3 32
2 2 3 32 7 2
e FeCr 6Fe 2Cr 6Fe
Cr 6Fe 2C
6 6
O H Hr 6Fe
Cr O 6Fe H 2Cr 6
O
Fe H14 7 O
4.
5.
Assign oxidation numbers to all the atoms in the equation.
Write down half equations of what is oxidised and reduced.Attach subscripts to atoms oxidised and reducedand balance the half equations.
Balance the half equations.
Attach species that were attached to the oxidised and reduced atoms.
Include all the original species and complete the balancing by inspection.
Calculations using Chemical Equations• A balanced equation for a chemical reaction
gives the relative amounts of each reactant and each product involved in the reaction.
• If the amount of one substance is known, based on the molar ratios in the equation, the amounts (masses, particles or volume)of other substances can be calculated.
• Make sure to always work in moles.
A dry - cleaning fluid composed of carbon and chlorine was found to have the composition 14.5% C, 85.5% Cl. What is the empirical formula of this compound?
C 14r
Element Percentage Percentage SimplestA Ratio
2
.5% 14.5 11.208
12Cl 85.5% 85.5 2
2.40835.5
CCl