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GCSE Chemistry - Bonding and Structure

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Bonding and Structure - Introduction

 

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Bonding and Structure - Ionic Forces

(1) Definition :

When an atom, generally a metal atom, loses one or more of its electrons, a positive ion is formed.

e.g. Na → Na+ + e-

When an atom, generally a non-metal atom, gains one or more electrons, a negative ion is formed.

e.g. Cl2 + 2e- → 2Cl-

Just as with the opposite poles of a magnet, when positive and negative ions approach one another a very strong force of attraction is formed. This is sometimes called an ionic bond, though more accurately it is a strong ionic force of attraction.

e.g. Na+ + Cl- → NaCl

An ionic compound is formed by massive numbers of positive and negative ions. The ions form a very large structure, with positive surrounded by negative ions, and negative ions surrounded by positive ions, in what is called a giant lattice structure.

e.g. for sodium chloride, NaCl -


(2) Properties :

(i) Melting point -

When a compound is melted (or boiled) forces between molecules must be overcome. These physical processes are nothing to do with breaking (covalent) bonds between atoms.

Since there is a massive number of ions present in an ionic compound, the overall value of the force between ions is very great and this makes ionic ompounds have very high melting points.

For example, sodium chloride has a melting point of 800 °C.

(ii) Electrical conductivity -

Electricity is conducted by the movement of charged particles, either positively charged or negatively charged. Therefore, ionic compounds in solid form do not conduct electricity, as the positive and negative ions are held in fixed positions, by the strong ionic forces, unable to move.

However, when the ionic solid is melted, the strong forces holding the ions together is broken and the ions are free to move. So ionic compounds do conduct electricity when molten, i.e. in liquid form.

Another, simpler, way to break the forces between ions is to dissolve the ionic compound in water. The water molecules work their way in between the positive and negative ions, separating them. This causes the ions to move freely in the water and so aqueous solutions of ionic salts conduct electricity.

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Bonding and Structure - Covalent bonding

(1) Definition and Structures:

Covalent bonding involves the sharing of electrons between two atoms, rather than the donation and acceptance of electrons present with ionic compounds (e.g. sodium chloride).

A single covalent bond is formed by the sharing of two electrons, a double bond by the sharing of four electrons and a triple bond by the sharing of six electrons.

Examples -

(i) Methane, CH4:

carbon - element number 6; electron configuration 2.4

hydrogen - element number 1; electron configuration 1

Each carbon atom has four outer shell electrons and so wants to gain, or lose, four electrons, so that it may obtain a full outer shell of electrons (i.e. a noble gas configuration ). The hydrogen atom wants to gain one electron to have a full outer shell.

Solution :

OR,

(ii) Hydrogen, H2 -

(iii) Chlorine, Cl2-

(iv) Hydrogen chloride, HCl -

(v) Water, H2O -

(vi) Methanol, CH3OH -

(vii) Carbon dioxide, CO2 :

(viii) Ethene, C2H4 :

(ix) Nitrogen, N2 -

(2) Properties :

(i) Melting point -

When covalent compounds are melted it is the forces between the molecules that are broken, not the covalent bonds between the atoms.

These intermolecular, i.e. in between molecules, forces are generally weak and so melting points of covalent compounds are low, e.g. the melting point of water is 0 oC.

(ii) Electrical conductivity -

Since covalent molecules are not charged there is no possibility of electricity being conducted, no matter what state of matter the compound is in.

In fact, polymers are used as electrical insulators around electrical wiring to prevent accidental electricution.

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Bonding and Structure - Energy Changes

(1) Definitions :

Most chemical reactions involve some sort of energy change - either energy is taken in, e.g. when ice melts, or energy is given out, e.g. when fuels are burnt.

Reactions that take in energy are called endothermic (endo- meaning inside and -thermic referring to heat) and reactions that give out energy are called exothermic (exo- meaning outside).

The change in energy, which is given the symbol ΔH, can be represented by what is called an energy profile diagram,

Exothermic reaction -

Endothermic reaction -

The energy change is defined as the energy contained in the products minus the energy contained in the reactants. So for exothermic reactions the energy change is given a negative sign and for endothermic reactions the energy change is given a positive sign.

(2) Experimental Determination :

Experimental sheet for determining an energy change.

The energy change for a reaction is not always easy to determine experimentally.

The easiest way to determine an energy change is to measure the change in temperature, during a reaction, with a thermometer. For example, when zinc powder is added to copper(II) sulphate(aq) the change in temperature over the short time the reaction takes gives rise to a graph similar to the one below,

N.B.: The cooling part of the curve must be extrapolated back to when the zinc powder was added to the copper(II) sulphate(aq), so that the theoretical maximum temperature change can be calculated (ΔT in the graph above).

The ΔT value is used to calculate the energy change from this mathematical equation,

ΔH = mass of liquid used x specific heat capacity of liquid x ΔT

This energy change can then be turned into a molar energy change by dividing the energy change calculated above by the lowest amount of reactant used.

Exemplar calculation -

If 0.025 mol of zinc powder is used in 50 cm3 (an excess) of copper(II) sulphate(aq) and the temperature rise is calculated to be 10 °C,

energy change, ΔH = 50 g x 4.2 J °C-1 g-1 x 10 °C
= 2100 J
molar energy change = -2100 J / 0.025 mol
= -84,000 J mol-1 or -84 kJ mol-1

(3) Bond Energy :

(i) Definition -

Energy changes for reactions can also be calculated by using only data contained in tables, listing various values for energy contained in compounds.

At GCSE level the only type of tabulated data that is used are values for the energy contained in covalent bonds. N.B.: Because this applies only to covalent bonds no ionic compounds or reactions are involved in this topic.

When two atoms that form a covalent bond come together, and share their electrons, energy is released. When a covalent bond is broken that same quantity of energy needs to be put into the bond to separate the atoms.

The quantity of energy is called the bond energy and is given the symbol E(A-B), where A and B are the atoms involved in the bond (they may be the same atom e.g. in diatomic molecules such as hydrogen, H2, and oxygen, O2).

Below is a table containing bond energies for some of the common bonds used in GCSE,

Single bonds Energy, kJ mol-1 Double bonds Energy, kJ mol-1 Triple Bonds Energy, kJ mol-1
O-H 464
C-H 413
C-O 358 C=O 805 C≡O 1077
C-C 347 C=C 612 C≡C 838
C-Cl 346
C-Br 290
C-I 228
H-F 568
H-Cl 432
H-Br 366.3
H-I 298.3
N-H 391
H-H 435.9
F-F 158
Cl-Cl 243.4
Br-Br 192.9
I-I 151.2
O-O 144 O=O 498
N-N 158 N=N 410 N≡N 945.4

As can be seen by these data, the more electrons that are shared between the two atoms, the stronger the bond is. So, triple bonds have more energy than double bonds, which have more energy than single bonds.

Note also that there is a great variety of bond energy values even amongst the same class of bonds - the atoms involved in the bond affect the bond energy value greatly, e.g. see the the values for the various single bonds quoted in the table.

(ii) Calculations -

The bond energy data are used to calculate the value for a reaction's energy change by adding up the bond energies for the reactants and subtracting the bond energies for the products.

This is because when a bond is broken, energy must be put into the bond to overcome the covalent bond between the two atoms. This is an endothermic change and hence has a positive value.

When a bond is formed, energy is released as the two atoms come together to form the new covalent bond. This is an exothermic process and hence has a negative value.

Exemplar calculation -

In the hydrogenation of ethene (see alkene hydrogenation in the GCSE organic page) the equation is -

Below is a table showing how the energy change, ΔH, for the reaction can be calculated,

Reactant Bonds Broken Product Bonds Formed
bond energy, kJ mol-1 bond energy, kJ mol-1
4 x E(C-H) = 4 x 413 6 x E(C-H) = 6 x 413
= 1652 = 2478
1 x E(C=C) = 612 1 x E(C-C) = 347
1 x E(H-H) = 436
total = 2700 total = 2825

ΔH = bond energy of reactants - bond energy of products
= +2700 kJ mol-1 - 2825 kJ mol-1
= -125 kJ mol-1

(iii) Practice calculation -

The equation for the combustion of methane is given below,

Using the data in the table above, fill in the boxes below with your answers to calculate the energy change for the reaction.

N.B.: Note the balancing numbers in the equation above.

Reactant Bonds Broken Total Energy
C-H + kJ mol-1
O=O + kJ mol-1
 
Product Bonds Formed Total Energy
C=O - kJ mol-1
O-H - kJ mol-1
∴ ΔH = kJ mol-1

(iv) Further practice calculations -

(1) Balance the following equation for the complete combustion of ethane:

CH3CH3(g) + O2(g)CO2(g) + H2O(g)

Then fill in the boxes below with your answers to the energies associated with bond breaking and bond making for that reaction. Press check to see if you are correct -

total energy of bonds broken = kJ mol-1
total energy of bonds formed = kJ mol-1
∴ ΔH = kJ mol-1

(2) Balance the following equation for the complete combustion of ethene:

CH2CH2(g) + O2(g)CO2(g) + H2O(g)

Then fill in the boxes below with your answers to the energies associated with bond breaking and bond making for that reaction. Press check to see if you are correct -

total energy of bonds broken = kJ mol-1
total energy of bonds formed = kJ mol-1
∴ ΔH = kJ mol-1

(3) Balance the following equation for the complete combustion of ethanol:

CH3CH2OH(g) + O2(g)CO2(g) + H2O(g)

Then fill in the boxes below with your answers to the energies associated with bond breaking and bond making for that reaction. Press check to see if you are correct -

total energy of bonds broken = kJ mol-1
total energy of bonds formed = kJ mol-1
∴ ΔH = kJ mol-1

(4) Balance the following equation for the complete combustion of hydrazine:

NH2NH2(g) + O2(g)N2(g) + H2O(g)

Then fill in the boxes below with your answers to the energies associated with bond breaking and bond making for that reaction. Press check to see if you are correct -

total energy of bonds broken = kJ mol-1
total energy of bonds formed = kJ mol-1
∴ ΔH = kJ mol-1

(5) Balance the following equation for the complete combustion of hydrogen:

H2(g) + O2(g)H2O(g)

Then fill in the boxes below with your answers to the energies associated with bond breaking and bond making for that reaction. Press check to see if you are correct -

total energy of bonds broken = kJ mol-1
total energy of bonds formed = kJ mol-1
∴ ΔH = kJ mol-1
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Bonding and Structure - Metallic bonding

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Bonding and Structure - Giant covalent structures

(1) Graphite:

(2) Diamond:

(3) Buckminsterfullerene:

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written by Dr Richard Clarkson : © Saturday, 1 November 1997

Updated : Thursday, 15th March, 2012

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