Page 11 - Curriculum Visions Dynamic Book

EQUATION: Dissociation of water
Water ➪ hydrogen + oxygen 2H2O(l) ➪ 2H2(g) + O2(g)
bond: chemical bonding is either a transfer or sharing of electrons by two or more atoms. There are a number of types of chemical bond, some very strong (such as covalent bonds), others weak (such as hydrogen bonds). Chemical bonds form because the linked molecule is more stable than the unlinked atoms from which it formed. For example, the hydrogen molecule (H2) is more stable than single atoms of hydrogen, which is why hydrogen gas is always found as molecules of two hydrogen atoms.
covalent bond: the most common form of strong chemical bonding, which occurs when two atoms share electrons.
dissociate: to break apart. In the case of acids it means
to break up forming hydrogen ions. This is an example of ionisation. Strong acids dissociate completely. Weak acids are not completely ionised and a solution of a weak acid has a relatively low concentration of hydrogen ions.
ion: an atom, or group of atoms, that has gained or lost one or more electrons and so developed an electrical charge.
 This picture shows a detail of the Hoffman’s voltameter, some time after the power has been applied. The indicator measures the amount of hydrogen ions in the water. The hydrogen ions have been converted to hydrogen molecules and form a gas in one tube (above the blue-stained water). The oxygen forms in the tube where the indicator has turned the water red.
Two volumes of hydrogen gas collect for every one volume of oxygen gas, as expected from the formula for water: H2O. The relative volume is also shown by the equation at the foot of the page.
How electrolysis works
Pure water is a poor conductor of electricity.
But water with impurities in it does conduct. Dilute solutions of acids, for example, are excellent conductors of electricity. Dilute sulphuric acid is used in vehicle batteries.
If two electrodes are dipped in a dilute acid solution, the electrical energy will break apart
the hydrogen particles (ions) from the hydroxyl
ions (pairs of hydrogen and oxygen particles). Because opposite charges attract, positively charged hydrogen ions drift through the dilute acid to the negative electrode. There they form hydrogen gas. Negatively charged hydroxyl ions drift to the positive electrode where oxygen gas forms.
The production of hydrogen gas from water comes at a price. Large amounts of electricity have to be used. However, the energy is not wasted, but simply stored in the hydrogen gas as chemical energy. This energy makes hydrogen gas very inflammable and even explosive if mixed with air or oxygen.
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