The Language Of Chemistry

2 The

language of chemistry

2.1 Measurement • Measurements – always have a unit – always an associated uncertainty

• Unit – specific standard quantity of a particular property – used to measure all other quantities of specific property

2.1 Measurement • The seven SI base units Measurement Unit length mass time temperature amount of substance electric current luminous intensity

metre kilogram second kelvin mole ampere candela

Symbol m kg s K mol A Cd

2.1 Measurement • The SI units for ANY physical quantity can be built from the seven base units – For example • area = length × width • length and width are measured in metres • the unit of area is square metre, m2

• Units undergo the same kinds of mathematical operations as the numbers to which they are attached

2.1 Measurement • SI or derived units are sometimes inconvenient • Use prefixes that divide or multiply the unit by a particular power of ten Prefix

Symbol

kilo k decid centi milli m

103 10-1 c 10-3

10-2

Factor

2.1 Measurement • Uncertainties and significant figures – Every measurement has an associated uncertainty

24.3 ºC

24.32 ºC

297.45 K

297.47 K

2.1 Measurement • Uncertainties and significant figures – All figures up to and including the first estimated figure are recorded – These figures are called significant figures

24.3 ºC

24.32 ºC

297.45 K

297.47 K

2.1 Measurement • Uncertainties and significant figures – Scientific notation expresses numbers in terms of powers of ten • 24.3 ºC is 2.43 × 101 ºC • 24.32 ºC is 2.432 × 101 ºC

24.3 ºC

24.32 ºC

297.45 K

297.47 K

2.1 Measurement • Significant figures in calculations – Multiplication or division of measurements 3.14× 2.751 = 13 0.64

– Addition or subtraction of measurements 3.247 41.36 + 125.2 169.8

2.1 Measurement • Precision – Signifies how reproducible a measurement is when made a number of times

2.1 Measurement • Accuracy – refers to how close the value is to the correct value

2.2 Representations of molecules • Chemical formulae – Shows the relative number of each type of atom present in a substance • H2O • C30 H34 AuBClF3N6O2P2PtW

– A molecular formula is a chemical formula that refers to a discrete molecule – There is a standardised way of writing chemical formulae

2.2 Representations of molecules • Binary compounds – Element further to the left of the periodic table appears first • KCl, Al2S3

– Hydrogen is the exception to this rule • LiH, NH3, H2O2, HCl

– If both elements are from the same group the lowest element appears first • SiC, BrF3

2.2 Representations of molecules • Binary compounds – In ionic compounds the cation is written first followed by the anion • NaBr, MgCl2

Note: a subscripted number refers ONLY to the atom immediately preceding it.

2.2 Representations of molecules • Writing chemical formulae for compounds containing more than two elements requires some knowledge of the bonding within the compound. – Ionic compounds – Covalent compounds

2.2 Representations of molecules • Ionic compounds – Cation followed by anion – Total charge zero Ca(NO3)2(s)  Ca2+(aq) + 2NO3-(aq)

– Hydrate formation common among ionic compounds Ca(NO3)2.4H2O

2.2 Representations of molecules • Covalent compounds – Majority are carbon-based organic compounds – Chemical formula often written carbon first, followed by hydrogen and then the remaining elements in alphabetical order, e.g. C2H6O, C4H9BrO – Limited structural information in the chemical formula

2.2 Representations of molecules • Structural formulae – Attempt to show the way in which the atoms in the molecule are bonded together – Chemical symbols still used for each element present – Constituent atoms are placed in the order in which they are bonded together – Bonds between neighbouring atoms are represented as lines

2.2 Representations of molecules • Structural formulae – NH3

H

N H

H

H

N H

H

H

N

H

H

– Structural formulae do not necessarily show the correct geometry of a compound – Presence of lone pairs of electrons is often neglected in depictions of molecules

2.2 Representations of molecules • Organic chemistry – Structural formulae are particularly useful – Carbon atoms bond to themselves — catenation – Carbon is a tetravalent element — it prefers to form a total of 4 bonds within a molecule – These bonds may be single, double or triple

2.2 Representations of molecules • Structural formulae H

H

H

C

C

H H

C

H C

H

C

C

H

H H C2H6

H H C2H4

C2H2

ethane C C

ethene C C

ethyne C C

single bond

double bond

triple bond

2.2 Representations of molecules • Structural formulae can remove ambiguities inherent in the chemical formula

H

H

H

C

C

H

H O

H

H

C

H O

C

H

H H H with the sameCchemical • Isomers areCmolecules 2H6O 2H6O dimethyl ether formula butethanol different structural formula

2.2 Representations of molecules • Two types of shorthand structural formulae – condensed structural formulae – line structures

• Condensed structural formulae – Constituent atoms are arranged in bonded groups, actual bonds not drawn • dimethyl ether • ethanol

CH3OCH3

CH3CH2OH

2.2 Representations of molecules •

Line structures – Are constructed according to the following guidelines: 1. All bonds except C-H bonds are shown as lines. 2. C-H bonds and H atoms attached to carbon are not shown in the line structure. 3. Single bonds are shown as 1 line; double bonds are shown as 2 lines; triple bonds are shown as 3 lines. 4. Carbon atoms are not labelled. All other atoms are labelled with their elemental symbols.

2.2 Representations of molecules – Drawing line structures H H

C

H

H

H3C

C

O

C

H

C

H

H

H

H

2.2 Representations of molecules – Drawing line structures

C

H C H

C

H

C

H C

H3C

C

O

C

H

C

H

H

H

H

O

C

2.2 Representations of molecules – Drawing line structures

C

H C H

C

H

C

O

H C

H3C

C

O

C

H

C

H

H

H

H O

C

2.2 Representations of molecules – Drawing line structures

C

H C H

C

H

C

O

C

O

CH3

H C

H3C

C

O

C

H

C

H

H

H

H

CH3

H3C CH3

2.2 Representations of molecules – Drawing line structures

C

H C H

C

H

C

O

C

H C

H3C

C

O

C

H

C

H

H

H

H

Me

Me

O Me

Me

2.2 Representations of molecules • Three-dimensional structures – To draw a three-dimensional structure add perspective to a structural formula or line structure H

H H H

H

CH3

H

H

H H CH3

cis isomer

H

H

CH3H H

H H H CH3

trans isomer

2.2 Representations of molecules • Three-dimensional structural formulae L

X X

L X X

L

M

L L

L

Tetrahedral

Octahedral

Conventional way to draw 4 bonds around a single carbon atom

Transition metal ion complex ML6

2.2 Representations of molecules • Three-dimensional structural formulae L

X X

X X

L

L

L

L

L

Tetrahedral

Octahedral

Conventional way to draw 4 bonds around a single carbon atom

Transition metal ion complex ML6

2.2 Representations of molecules • Three-dimensional structures

2.3 Nomenclature • Nomenclature — the system for naming of compounds • International Union of Pure and Applied Chemistry (IUPAC) • Some compounds are better known by their common unsystematic name rather than their systematic IUPAC name – The systematic name of water is oxidane

2.3 Nomenclature • Naming Inorganic compounds • Nonmetallic binary compounds – Guidelines for naming:

NO Nitrogen monoxide

NO2 Nitrogen dioxide

N2O5 Dinitrogen pentoxide

1. The element closer to the left of the periodic table appears first. If both elements are from the same group of the periodic table, the lower one appears first.

2.3 Nomenclature • Naming Inorganic compounds • Nonmetallic binary compounds – Guidelines for naming:

NO Nitrogen monoxide

NO2 Nitrogen dioxide

N2O5 Dinitrogen pentoxide

2. The element that appears first retains its elemental name.

2.3 Nomenclature • Naming Inorganic compounds • Nonmetallic binary compounds – Guidelines for naming:

NO Nitrogen monoxide

NO2 Nitrogen dioxide

N2O5 Dinitrogen pentoxide

3. The second element begins with a root derived from its elemental name and ends with the suffix –ide.

2.3 Nomenclature • Naming Inorganic compounds • Nonmetallic binary compounds – Guidelines for naming:

NO Nitrogen monoxide

NO2 Nitrogen dioxide

N2O5 Dinitrogen pentoxide

4. When there is more than one atom of a given element in the formula, the name of the element usually contains a prefix that specifies the number of atoms present.

2.3 Nomenclature • Binary compounds of hydrogen – Hydrogen requires special consideration – It may appear first or second in the chemical formula of a compound, therefore first or second in the name • Lithium hydride LiH • Hydrogen fluoride HF • Hydrogen sulfide H2S • Calcium hydride

CaH2

2.3 Nomenclature • Ionic compounds – Binary ionic compounds are written with the cation first and the anion, which takes the suffix – ide, last • Potassium iodide • Calcium fluoride

KI CaF2

– Ionic compounds may contain polyatomic ions • Ammonium • Nitrate

NH4+ NO3-

2.3 Nomenclature • Ionic compounds – Anions containing a central atom surrounded by oxygen atoms are known as oxoanions. Naming rules: 1. The name has a root taken from the name of the central atom • Carbonate

CO32-

• Nitrite

NO2-

2.3 Nomenclature • Ionic compounds – Anions containing a central atom surrounded by oxygen atoms are known as oxoanions. Naming rules: 2. When an element forms 2 different oxoanions, the one with fewer oxygen atoms ends in –ite, and the other ends in –ate • Sulfite

SO32-

• Sulfate

SO42-

2.3 Nomenclature • Ionic compounds – Anions containing a central atom surrounded by oxygen atoms are known as oxoanions. Naming rules: 3. Chlorine, bromine and iodine each form 4 different oxoanions that are distinguished by prefixes and suffixes • Hypobromite BrOBrO2• Bromite • Bromate

BrO3-

2.3 Nomenclature • Ionic compounds – Anions containing a central atom surrounded by oxygen atoms are known as oxoanions. Naming rules: 4. A polyatomic anion with a charge more negative than 1- may add H+ to give another anion. These anions are named from the parent anion by adding the word hydrogen • Hydrogen carbonate

HCO3-

• Hydrogen phosphate

H2PO4-

2.3 Nomenclature • Organic compounds – Composed primarily of carbon and hydrogen atoms – Naming system based on the number of carbon atoms in a particular molecule – Functional groups • A group of one or more atoms within a molecule bonded in a particular fashion • Usually the point of reaction within a molecule

2.3 Nomenclature • Organic compounds – Named according to their functional groups Functional group in

Name of

hydroxyl alcohols R OH O

carbonyl C aldehydes R

O

H

carbonyl C ketones R

O

R

carboxyl C carboxylic acids R

OH

Found

group

2.3 Nomenclature • Alkanes – Molecules that contain only carbon and hydrogen – Carbon atoms are joined by single bond only – 2 parts to the name of an alkane • Prefix: indicating the number of carbon atoms in the longest chain • Ending: -ane

2.3 Nomenclature • IUPAC rules for naming alkanes: 1. The name of an alkane with an unbranched chain of carbon atoms consists of a prefix showing the number of carbon atoms in the chain and the ending -ane CH3CH2CH2CH3 butane

2.3 Nomenclature • IUPAC rules for naming alkanes: 2. For the branched-chain alkanes, the longest chain of carbon atoms is the parent chain, and its name becomes the root name CH3 CH3CH2CH2CHCH3

5

4

2-methylpentane

3

2

1

2.3 Nomenclature • IUPAC rules for naming alkanes: 3. For an alkane with 1 substituent, number the parent chain so that the carbon atom bearing the substituent is given the lowest possible number CH3 CH3CH2CH2CHCH3

5

4

2-methylpentane

3

2

1

2.3 Nomenclature • IUPAC rules for naming alkanes: 4. Give the substituent on the parent chain a name and number. The number shows the carbon atom of the parent chain to which the substituent is bonded. Use a hyphen to connect the number to the name CH3 CH3CHCH3

1

2-methylpropane

2

3

2.3 Nomenclature • IUPAC rules for naming alkanes: 5. If there are two or more identical substituents, number the parent chain from the end that gives the lower number to the substituent closest to the end of the chain CH3 CH3 CH3CH2CHCH2CH2CH3

6

5

2,4-dimethylhexane

4

3

2

1

2.3 Nomenclature • IUPAC rules for naming alkanes: 6. If there are two or more different substituents, list them in alphabetical order, and number the chain from the end that gives the lower number to the substituent encountered first CH3

1

2

3

CH3CH2CHCH2CHCH2CH3 CH2CH3 3-ethyl-5-methylheptane

4

5

6

7

2.3 Nomenclature • IUPAC rules for naming alkanes: 7. The prefixes di, tri, tetra etc. and the hyphenated prefixes sec- and tert- are disregarded for the purposes of placing the substituents in alphabetical order CH3 CH2CH3 CH3CCH2CHCH2CH3 CH3

1

2

3

4-ethyl-2,2-dimethylhexane

4

5

6

2.3 Nomenclature • Constitutional isomerism in alkanes – Compounds with the same chemical formula but a different order of attachment of the constituent atoms – Usually distinguishable by their differing physical properties

pentane

2-methylbutane 2,2-dimethylpropane

2.3 Nomenclature • General organic nomenclature • The name of any compound with a chain of carbon atoms consists of 3 parts. 1. A prefix: shows the number of carbon atoms in the parent chain 2. An infix: shows the nature of the carboncarbon bonds in the parent chain 3. A suffix: shows the class of compound to which the substance belongs, and therefore the functional group(s) present in the compound

2.3 Nomenclature • General organic nomenclature H3CHC

CH2

a carbon-carbon double bond

propene 3 carbon atoms

a hydrocarbon

O CH3CH2CH2CH2COH only carbon-carbon single bonds

pentanoic acid 5 carbon atoms

a –COOH (carboxyl) group