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 Form 4 Chemistry: Energy changes in chemical and physical processes lessons

Meaning of fuels,heating value,causes of fuel pollution and factors to consider when choosing a fuel.

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Answer Text:
- Are compounds which produce a high heat of combustion.
- Fuels can be:-
- Solids; such as charcoal, wood, coal.
- Liquids; such as ethanol, gasoline
- Gaseous; such as methane, water, gas etc.
Basic concepts:
- Calorific value
- Is the energy content of a fuel;
- Is the heat evolved when a given mass of fuel is completely burnt in oxygen;
- Sometimes fuels may undergo incomplete combustion.
- Incomplete combustion of fuel is disadvantageous in
- It reduces the energy content.
- It leads to pollution.
- Heating value:
- Is the amount of heat energy given out when a unit mass or unit volume of a fuel is completely burnt in oxygen.
Fuel pollution.
- Is commonly caused by internal combustion engine.
- Fuel in engine (e.g. petrol) burns completely to water and carbon (IV) oxide, only under ideal conditions.
Causes of fuel pollution:
(i). Incomplete combustion which causes production of (produces) CO and unburnt carbon (soot).
(ii). Some fuels contain sulphur and nitrogen and on combustion release #SO_2# and #NO_2#. These gases are acidic, resulting to acidic rain which
corrodes buildings and affects trees and animals in various ways.
(iii). Fuel additives; e.g. tetraethyl lead, Pb (#C_2H_5)_4# added to petrol to enhance burning efficiency produces volatile lead compounds in the exhaust fumes.
- Lead is very poisonous and affects the nervous system and the brain in children.
Factors to consider when choosing a fuel.
(i) Heating value,
(ii) Ease and rate of combustion.
(iii) Availability
(iv) Ease of transportation
(v) Ease of storage
(vi) Environment effects.