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Chemistry · Class 9 · Chapter 8: Chemical Reactivity

8 min

Key points

Chemical reactivity is the tendency of an element to take part in chemical reactions by losing, gaining or sharing electrons
Alkali metals (Group IA) are most reactive metals due to single valence electron which is easily lost
Halogens (Group VIIA) are most reactive non-metals due to seven valence electrons, need only one electron to complete octet
Reactivity of metals increases down the group and decreases across the period from left to right
Reactivity of non-metals decreases down the group and increases across the period from left to right
Noble gases (Group VIIIA) are least reactive due to complete outer electron shell
Activity series arranges metals in order of decreasing reactivity: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au
More reactive metals displace less reactive metals from their salt solutions

Definitions

Chemical Reactivity: Chemical reactivity is the tendency of an element to take part in chemical reactions by losing, gaining or sharing electrons to complete its outermost shell.
Activity Series: Activity series is an arrangement of metals in order of their decreasing reactivity.
Displacement Reaction: A displacement reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its compound.
Noble Gases: Noble gases are elements of Group VIIIA which have complete outermost electron shell and are chemically inert or least reactive.

Worked example

Define chemical reactivity. Explain any three factors that affect the rate of chemical reactions. (6 marks)

Chemical reactivity is the tendency of a substance to undergo chemical change by losing, gaining or sharing electrons with other substances. Three factors affecting rate of chemical reactions are: (1) Temperature - Higher temperature increases kinetic energy of molecules, leading to more frequent collisions and faster reactions. (2) Concentration - Higher concentration of reactants increases collision frequency, thus increasing reaction rate. (3) Surface area - Smaller particle size provides greater surface area for reaction, increasing the rate of reaction.

Common mistakes

Students often confuse reactivity trends - remember metals become MORE reactive down the group, non-metals become LESS reactive
Don't mix up activity series order - learn the sequence K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au
In displacement reactions, always remember more reactive displaces less reactive, not the opposite
Students forget that noble gases are inert due to complete outer shell, not due to single or seven electrons
Don't confuse Group numbers with electron configuration - Group IA has 1 valence electron, not atomic number 1

Quick recap

Chemical reactivity is the tendency to undergo chemical change by electron transfer. Main factors affecting reaction rate include temperature, concentration, surface area, and presence of catalysts. Metals show different reactivity levels as shown in activity series. Higher temperature and concentration always increase reaction rate due to increased collision frequency.

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