TWEED RIVER HIGH SCHOOL - Year 12 Chemistry



TWEED RIVER HIGH SCHOOL

2006

PRELIMINARY CHEMISTRY

Unit 1

The Chemical Earth

Part 4

Energy is required to extract elements from their naturally occurring sources.

• Identify the differences between physical and chemical change in terms of rearrangement of particles.

Matter can undergo two types of changes:

1. Physical change

Physical changes occur without a change in the composition of the particular substance.

Physical changes include changes in volume, density and changes of state, eg solid ( liquid.

Physical changes involve relatively small amounts of energy. For example in a change of state, the intermolecular forces between the molecules need to be broken. These intermolecular forces are weak and require a small amount of energy to break them.

2. Chemical change

Chemical changes are those in which new substances with different compositions and properties are formed.

When a chemical reaction occurs, chemical bonds between the atoms in the molecule need to be broken and reformed to make the new substance.

Bonds between atoms are much stronger than between molecules, therefore, chemical changes require a larger amount of energy.

For example:

|Process |Type of Change |Energy Involved |

| | |(kJmol-1) |

|H2O (l) ( H2O (g) |Physical |+44 |

|H2O (l) ( H2 (g) + ½ O2(g) |Chemical |+286 |

In the space below draw diagrams to show what happens when water changes to water vapour and to hydrogen and oxygen gas.

(Fig 5.1, p68 Chemistry Contexts)

• Identify light, heat and electricity as the common forms of energy that may be released or absorbed during the decomposition or synthesis of substances and identify examples of these changes occurring in everyday life.

• Elements cannot be (chemically) decomposed

• Compounds under certain conditions can be decomposed into their constituent elements or simpler substances.

• Thermal decomposition:

Thermal decomposition is the process by which heat breaks compounds down into simpler substances.

For example, sodium hydrogen carbonate (baking soda) is used in cooking to produce carbon dioxide to make cakes etc rise.

The reaction for this is:

heat

2NaHCO3(s) ( Na2CO3(s) + CO2(g) + H2O(g)

• Decomposition by light.

Light energy can cause the decomposition of some compounds. For example silver salts, such a silver bromide, decompose in the presence of ultraviolet light. The process is used in black and white photography. The reaction for this process is:

uv

1. AgBr(s) ( Ag(s) + Br(s)

• Decomposition by Electricity

These are known as electrolysis reactions.

The most common of these reactions is the decomposition of water as previously discussed. (Draw Fig 5.5, p75 Chemistry Contexts).

Another example is the electrolytic reduction of alumina (Al2O3)

Products of electrolysis, Aluminium and carbon dioxide, are obtained by using:

- a carbon anode and iron container lined with carbon as the cathode.

- a high current density

- a high temperature, 1000(C

Electroplating, eg chromium is also an electrical decomposition reaction.

Homework: Outline the process of electroplating chrome. Include diagrams.

• Plan and safely perform a first-hand investigation to show the decomposition of a carbonate by heat, using appropriate tests to identify carbon dioxide and the oxide as the products of the reaction.

Obtain the practical sheets and using these sheets write up your experiments including the following points:

• Aim

• Risk Assessment

• Method

• Results

• Discussion

• Have your write up approved before you do the practical.

• You will then perform the experiment in the lab.

• Gather information using first-hand or secondary sources to:

- observe the effect of light on silver salts and identify an application of the use of this reaction.

- observe the electrolysis of water, analyse the information provided as evidence that water is a compound and identify an application of the use of this reaction.

Part A – Silver salts:

Obtain the practical sheets and using these sheets write up your experiments including the following points:

• Aim

• Risk Assessment

• Method

• Results

• Discussion

• Have your write up approved before you do the practical.

• You will then perform the experiment in the lab.

Part B – Electrolysis of Water.

Observe and record the set up of the Hofmann voltameter.

Note that in addition to water in the voltameter, dilute sulfuric acid is added as an electrolyte.

This experiment will be performed as a demonstration.

Observe the results as demonstrated and write up a practical report. In your report you are to identify and application of this reaction.

Note: Both reports are to include complete chemical and ionic equations.

• Analyse and present information to model the boiling of water and the electrolysis of water tracing the movements of and changes in arrangements of molecules.

From the unit notes and class discussions, you are to prepare a report covering the above dot point.

• Explain that the amount of energy needed to separate atoms in a compound is an indication of the strength of attraction, or bond between them.

Bond Energy

There are three general types of compounds:

- ionic

- covalent

- covalent network

When these compounds form, they form chemical bonds between the atoms.

To decompose a compound, large amounts of energy are required. This is needed to break the chemical bond. The stronger the attraction between atoms, a larger the amount of energy is required to break the bond.

Examples of bond energies:

Bond energy from several covalent bonds:

|Covalent Bond |Bond Energy (kJmol-1) |

|H-H |436 |

|H-Cl |431 |

|Cl-Cl |242 |

|O-H |463 |

|C-C |346 |

|C-0 |358 |

The ionic lattice energies for several ionic compounds:

|Ionic Compound |Ionic lattice Energy |

| |(kJmol-1) |

|NaCl |788 |

|KCl |718 |

|MgCl2 |2523 |

|CaCl2 |2255 |

|MgO |3800 |

|CaO |3419 |

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