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Chemistry

(a) What is a chemical reaction?

(b) State the conditions necessary for a chemical change or reaction.

Chemical Reaction

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Answer

(a) A chemical reaction is a process of breaking chemical bonds of the reacting substances (reactants) and making new bonds to form new substances (products).

(b) Conditions necessary for a chemical change or reaction are :

  1. Mixing (close contact) — Some chemical reactions take place when two substances are mixed in their solid state.
    Pb(NO3)2 (s) + 2KI (s) ⟶ 2KNO3 + PbI2 (s)

  2. Solution — In some cases, a chemical reaction occurs when substances are mixed either in molten or aqueous state.
    AgNO3 + NaCl ⟶ AgCl ↓ + NaNO3

  3. Heat — Some chemical reactions occur only on heating.
    CuCO3 (s) Δ\xrightarrow{\Delta} CuO (s) + CO2 ↑ (g)

  4. Light — Some chemical reactions take place by the action of light. They are called photochemical reaction.
    2AgNO3 sunlight\xrightarrow{\text{sunlight}} 2Ag + 2NO2 + O2

  5. Electricity — Some reactions are caused or accompanied by the passage of an electric current. They are called electrochemical reaction.
    2NaCl electricity\xrightarrow{\text{electricity}} 2Na + Cl2

  6. Pressure — Some chemical reaction take place only when the involved substances are subjected to high pressure.
    N2 + 3H2 above 200 atm\xrightleftharpoons{\text{above 200 atm}} 2NH3

  7. Catalyst — Some chemical reactions need a catalyst to accelerate or decelerate the rate at which they occur.
    2H2O2 MnO2\xrightarrow{\text{MnO}_2} 2H2O + O2

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