Chemistry 3 - Water, energy and the perioic table

Newlands and Mendeleev classified the elements by arranging them in atomic weight order

Mendeleev overcame problems in his periodic table by leaving gaps for undiscovered elements.

In the modern periodic table, elements are arranged in atomic number order.

Group 1 elements are low-denisty metals that react with non-metals to form ionic compounds, and with water to release hydrogen. 
Their hydroxides form alkaline solutions.

The transition elements are stronger, harder; have highter melting points, and are less reactive. 
They have ions with different charges, form coloured compounds, and are useful as catalysts.

Group 7 elements react with metals to form ionic compounds.  More reactive halogens displace less reactive halogens from aqueous solutions of their salts.

Hard water contains dissolved compounds of clacium or magnesium. 
Calcium compounds are good for heart, teeth and bones.

Hard water increases costs because more soap is needed, and scale makes settles and heating systems less efficient.

Temporary hard water is softened by boiling. 

Permanent hard water remains hard when it is boiled.

Hard water can be softened by adding sodium carbonate or by passing it through ion exchange columns.

Water is made safte to drink by filtering, and by adding chlorine to reduce microbes. 
Adding fluride may improve dental health.

The energy change of a chemical reaction can be calculated using the equation q=mc∆t.
∆t is the temperature change of water heated by a burrning fuel, or of reacting solutions.

Energy-level diagrams show the elative energies of reactants and products, the activation energy, and then overall energy change of a reaction.

In chemical reactions, energy is supplied to break bonds, and energy is released when new bonds are formed.

Catalysts reduce the minimum amount of energy needed to start a chemical reaction (the activation energy).

Hydrogen can be burnt as a fuel in combustion engines, or used in fuel cells to produce electricity to power vehicles.