Use adaptive quiz-based learning to study this topic faster and more effectively.

# Chemistry

## Protons, neutrons and electrons

The atom is the most basic particle of a chemical element that exhibits the properties of the element. Atoms are made of three subatomic particles: protons, neutrons and electrons.

Protons are positively charged, neutrons are uncharged (neutral) and electrons are negatively charged.

Protons and neutrons are also called nucleons, since they make up the atomic nucleus.

A proton and a neutron each have approximately the same mass. An electron has a much smaller mass ($\displaystyle{\frac{1}{1837}}$ of a proton).

The image shows the model of a helium atom ($\ce{He}$). At the centre is the nucleus, with two neutrons and two protons (represented by green and orange circles).

There are two electrons (shown as blue circles) in the electron shell.

## Number of electrons in different shells

The maximum number ($N$) each electron shell can hold depends on its shell number ($n$). The relationship is $N=2n^{2}.$

Shell number Maximum number of electrons
1 2
2 8
3 18

Shell 1 must be completely filled before electrons can occupy the next shell. Similarly, shells 1 and 2 must be filled before shell 3 is filled. Filling of electrons is more complex from shell 3 onwards.

Shells are often drawn as circular "orbits" around a nucleus for simplicity. In reality, an electron shell is a complex region where electrons can be found.

The electrons are each drawn as dots on the circles representing their shells.

A carbon atom has a total of 6 electrons. It has 4 electrons in its outer shell.

The electron configuration of an atom or ion can be also be written down. The number of electrons in each shell, starting from the smallest, is written separated by commas or full stops (periods).

A sodium atom has electron structure 2.8.1

## Dot and cross diagrams

The sharing of electrons in covalent bonds can be illustrated using dot and cross diagrams.

The dots and crosses in the diagrams indicate valence electrons. Electrons represented by dots originate from different elements from those represented by crosses.

In reality, electrons are indistinguishable. The dots and crosses are for illustration purposes only.

It is not possible to tell which electron comes from hydrogen or carbon in the $\ce{C-H}$ bond of methane.

An atom usually shares as many valence electrons as needed to achieve a stable electron configuration.

Carbon bonds with four hydrogen atoms, allowing it to fill its valence shell. Each hydrogen also gains a full shell (2 electrons) by sharing electrons from carbon.

## Molecular formulae

A molecular formula shows the number of the different atoms in a simple covalent molecule.

In a molecular formula, the chemical elements are represented by their symbols (e.g. "$\ce{O}$" for oxygen).

Subscripts (for example the "2" in $\ce{H2O}$) indicate how many atoms of each element are present in a molecule. If there is no subscript, then only one atom of that element is present.

Carbon dioxide has the formula $\ce{CO2}$. It consists of one carbon atom ($\ce{C}$) and two oxygen atoms ($\ce{O}$).

The letter in parentheses after the formulae indicates the state of the compound:

• (s) means the compound is a solid.
• (l) refers to a liquid.
• (g) indicates the substance is a gas.
• (aq) refers to a compound that is dissolved in water (aqueous).
Components of the molecular formula
Component Meaning Example
Element symbol Types of atoms in the molecule ${\definecolor{tred}{RGB}{136,24,28}\color{tred}\ce{CaCO}}\ce{_3}{(s)}$
Subscripts Number of atoms of each element in the molecule $\ce{CaCO}{\definecolor{tred}{RGB}{136,24,28}\color{tred}\ce{_3}}{(s)}$
Letters in parentheses Phase of the compound or element $\ce{CaCO3}\definecolor{tred}{RGB}{136,24,28}\color{tred}{(s)}$

## Calculating molecular formulae

The molecular formula can be determined from the empirical formula if the molar mass of the compound is known.

The following steps are taken to calculate the molecular formula:

• Find the molar mass of the actual compound.
• Calculate the molar mass given by the empirical formula.
• Divide the molar mass of the compound by the molar mass from the empirical formula.
• Multiply the subscripts in the empirical formula using the previous result to obtain the molecular formula

Pentene has an empirical formula of $\ce{CH2}$ and a molar mass of approximately 70 g/mol. The molar mass of the empirical formula is approximately 14 g/mol.

The molecular formula thus must have a molar mass 5 times greater than that of the empirical formula, and the ratio of hydrogen to carbon atoms must stay the same.

The molecular formula is therefore $\ce{C5H10}$.

## Reduction

Reduction occurs when electrons are gained by an atom, molecule or ion.

A copper ion ($\ce{Cu^2+}$) is reduced when it acquires two electrons to become a copper atom ($\ce{Cu}$).

In covalent bonding, an atom is reduced when it gets more than an even share of the electrons it shares in a bond.

Hydrogen atoms exert a weak pull on electrons within molecules. Molecules are thus reduced when they form more bonds with hydrogen.

The carbon atoms in ethene ($\ce{C2H4}$) are reduced when ethene reacts with hydrogen gas to form ethane ($\ce{C2H6}$).

The carbon atoms get more than an even share of electrons in the bonds. The carbon atoms have thus "gained" electrons and are reduced.

Copper ions in blue copper (II) sulphate are reduced to form copper atoms (which are solid). The blue colour fades as solid copper is deposited.

## Endothermic and exothermic reactions

A reaction is classified as either endothermic or exothermic based on the enthalpy change ($\Delta H$) during the reaction.

An endothermic reaction absorbs heat at constant pressure and has a positive $\Delta H$. In such a reaction, heat is converted to chemical energy stored in bonds.

$$\ce{N2 {(g)} -> 2N {(g)}}\hspace{5pt}\Delta H = 472.7\text{ kJ/mol}$$

The splitting of nitrogen gas ($\ce{N2}$) into nitrogen atoms ($\ce{N}$) is endothermic and requires $472.7\text{ kJ/mol}$ of heat. The $\Delta H$ is $472.7\text{ kJ/mol}$.

An exothermic reaction releases heat and has a negative $\Delta H$. In such a reaction, chemical energy from bonds is converted to heat.

$$\ce{C3H8 + 5O2 -> 3CO2 + 4H2O}\hspace{5pt}\Delta H = -2,220\text{ kJ/mol}$$

The combustion of propane ($\ce{C3H8}$) is exothermic and releases $2,220\text{ kJ/mol}$ of heat. The $\Delta H$ is $-2,220\text{ kJ/mol}$.

## Bond energies

The enthalpy change ($\Delta H$) of a reaction is related to the bond energies.

Every bond in a molecule has a set bond energy. A bond with a higher energy is a stronger bond.

The total bond energy of the reactants is found by adding the bond energies of all bonds in the reactants.

Likewise, the total bond energy of the products is found by adding the bond energies of all the bonds in the products.

$\Delta H$ is equal to the difference between the total bond energy of the reactants and the total bond energy of the products. Essentially:

$\Delta H$ = Energy of reactant bonds broken $-$ Energy of product bonds formed

A reaction is therefore endothermic ($\Delta H$ is positive) if the bonds broken are stronger than the bonds formed.

A reaction is exothermic ($\Delta H$ is negative) if the bonds formed are stronger than the bonds broken.

## Activation energy

In an energy profile, the chemical energy must reach at least the energy needed to break all the bonds apart.

This required energy is identified as the peak on the energy profile. Energy decreases after this point because new bonds start to form, releasing energy from the system.

The difference between the peak energy and the energy of the reactants is called the activation energy ($E_A$). The activation energy has to be overcome for the reaction to occur.

After the peak, the energy decreases until all the new bonds are formed. When all products are formed, the reaction ends.

At this point, the energy remains constant and is equal to the energy of the products.

Energy profile of the exothermic combustion of glucose. The lower activation energy peak represents the alternative pathway observed when a catalyst is used.

## Effect of concentration on reaction rate

Concentration influences the rate of reaction in solutions.

At a lower concentration (left), there are fewer collisions while at a higher concentration (right), there are more collisions.

As the concentration (measured in moles of a chemical per unit volume) of reactant molecules increases, collisions between reactant molecules become more frequent.

This increases the likelihood of having many effective collisions. The rate of reaction thus increases with concentration.

Acid rain corrodes statues more quickly than normal rainwater because acid rain has higher concentration of $\ce{H^+}$ ions.

Pressure influences the rate of reaction in gases.

For gases, pressure is directly related to concentration.

As the pressure increases, collisions between molecules become more frequent. The rate of reaction thus increases with pressure.

When ammonia is produced from hydrogen and nitrogen gas, the pressure is set to 200 atmospheres because the reaction is too slow at lower pressures.

## Graphical representation of the reaction rate

A concentration ($\text{mol/}\cdm$) versus time (s) graph charting the changes in concentrations of products and reactants during a reaction.

The concentration of products increases as time passes, and this is shown as a curve with a positive slope. A curve representing the reactants will have a negative slope since its concentration decreases.

When the reaction ends, no more products are formed and no more reactants are consumed. The concentration curves each level off to form a plateau.

The gradient of the graph reflects the rate of reaction.

The product curve is steep (has a higher gradient) at the start of the reaction. This indicates that the reaction is faster.

As the reaction proceeds, the gradient of the product curve gets gentler as the reaction rate declines.

The reaction rate for a given reaction is rarely constant throughout the reaction.

When the reaction is almost complete, the product curve starts to plateau as the concentrations change more slowly.

## Periods

The horizontal rows of the periodic table are called periods. The atomic number increases from left to right in a period.

Atoms in the same period have the same number of electron shells.

Nitrogen ($\ce{N}$), oxygen ($\ce{O}$) and fluorine ($\ce{F}$) all belong to the second period. All three elements have their valence electrons found in the second electron shell.

The period in which an element is found reflects the number of the electron shell holding the valence electrons.

Beryllium is found in the second period, and so its valence electrons are found on the second electron shell. Magnesium and calcium are found in the third and fourth period respectively.

## Trends in physical properties of halogens

Chlorine (left), bromine (centre) and iodine (right) are the first few members of group 7.

Halogens are coloured non-metals with seven valence electrons in group 7. Halogens tend to exist as diatomic molecules (e.g. chlorine exists as $\ce{Cl2}$).

The colour intensity of these elements increases with increasing atomic mass. The physical appearances of the elements are listed below.

• Chlorine ($\ce{Cl2}$): Yellow-green gas
• Bromine ($\ce{Br2}$): Red-brown liquid
• Iodine ($\ce{I2}$): Blue-black solid (that can sublimate to form a purple vapour)

The melting point increases from chlorine to iodine. This is due to increasing strength of intermolecular forces as we go down the group.

The heavier halogens have stronger intermolecular forces than the lighter ones as molecules of halogens get larger.

When molecule size increases, there is more surface area for contact with neighbouring molecules. This results in greater extent of intermolecular forces of attraction.

## Acid definition

An acid is a substance that releases hydrogen ions ($\ce{H^+}$) in an aqueous solution. Acids tend to dissociate (break up) into $\ce{H^+}$ ions and negatively charged ions.

An $\ce{H^+}$ ion can be thought of as a proton. Therefore, the dissociation of $\ce{H^+}$ is often called deprotonation.

The general equation for the dissociation of acids is as follows.

$$\ce{HA -> H^+ + A^-}$$

$\ce{A^-}$ represents the anion of the acid. $\ce{A^-}$ would be the chloride ion ($\ce{Cl^-}$) in the case of hydrochloric acid.

The sour taste in foods is caused by acids. Lemons and vinegar both contain acid.

The characteristic sour taste of citrus fruits is due to the presence of citric acid.

## Strong and weak acids

A strong acid dissociates completely into ions when dissolved in water.

Hydrochloric acid $(\ce{HCl})$ and sulphuric acid $(\ce{H2SO4})$ are examples of strong acids.

$$\ce{HCl {(aq)} -> H^+ {(aq)} + Cl^-{(aq)}}$$

A weak acid does not dissociate completely in water. Some of the hydrogen ions remain attached to the molecule.

Acetic acid $(\ce{CH3COOH})$, formic acid $(\ce{HCOOH})$ and nitrous acid $(\ce{HNO2})$ are examples of weak acids.

In weak acids, the molecules often flow back and forth between dissociated and non-dissociated state.

The equations for dissociation are thus written with the $\leftrightharpoons$ symbol.

These arrows indicate that the reaction is reversible and can go in either direction.

$$\ce{CH3COOH {(aq)} <=> H^+ {(aq)} + CH3COO^- {(aq)}}$$

The venom found in bee stings contains formic acid, a weak acid.

## Reactivity of metals

A metal is more reactive than another metal if it loses its electrons more easily. Each metal has a different reactivity.

Potassium, sodium and calcium are highly reactive and will react with cold water. Magnesium, zinc and iron do not react with cold water but will react with steam to form metal oxides.

$$\ce{2K {(s)} + 2H2O {(l)} -> 2KOH {(aq)} + H2 {(g)}}$$ $$\ce{Mg {(s)} + H2O {(g)} -> MgO {(s)} + H2 {(g)}}$$

Lead and all of the metals listed previously react with dilute hydrochloric acid ($\ce{HCl}$) in water to form salts.

$$\ce{Mg {(s)} + 2HCl {(aq)} -> MgCl2 {(aq)} + H2 {(g)}}$$

Copper and silver each have low reactivity and do not react with cold water, steam or dilute hydrochloric acid ($\ce{HCl}$).

## Alkane definition

A hydrocarbon is an organic molecule consisting only of hydrogen and carbon.

An alkane is a hydrocarbon in which all of the bonds are single bonds (each of the two atoms contributes only one electron to the bond).

Alkanes consisting of just one chain (without any branches) are a homologous series. The general formula for alkanes is: $\text{C}_\text{n}\text{H}_{2\text{n}+2}$

The first four unbranched alkanes are shown in the table below.

Name Diagram Formula
Methane
$\ce{CH4}$
Ethane
$\ce{C2H6}$
Propane
$\ce{C3H8}$
Butane
$\ce{C4H10}$

Alkanes are widely used as the main fuel for cars, planes, gas cookers and heating.

## Complete and incomplete combustion of alkanes

Complete combustion occurs when there is sufficient or excess oxygen present.

There must be enough oxygen atoms to convert all of the alkanes into the products of the reaction: carbon dioxide $(\ce{CO2})$ and water $(\ce{H2O})$.

The complete combustion of methane has the equation: $$\ce{CH4 + 2O2 -> CO2 + 2H2O}$$

Incomplete combustion occurs when there is insufficient oxygen. Such a combustion produces soot (carbon particles) and carbon monoxide $(\ce{CO})$ (in addition to water) instead of carbon dioxide.

The incomplete combustion of methane involves two different reactions: $$\ce{2CH4 + 3O2 -> 2CO + 4H2O}$$ $$\ce{CH4 + O2 -> C + 2H2O}$$

Carbon monoxide is a very poisonous gas.

## Carboxylic acid definition

Carboxylic acids are organic molecules containing the acidic functional group $\ce{-COOH}$. They have the general formula $\text{C}_\text{n}\text{H}_{2\text{n}+1}\text{COOH}.$

The functional group of carboxylic acids, ($\ce{-COOH})$, is called the carboxyl group.

Carboxylic acids are colourless liquids at room temperature and pressure.

The table shows the first four acids in the homologous series.

Name Diagram Formula
Methanoic acid
$\ce{HCOOH}$
Ethanoic acid
$\ce{CH3COOH}$
Propanoic acid
$\ce{C2H5COOH}$
Butanoic acid
$\ce{C3H7COOH}$