Chemical Reactions and Equations | Olympiad Preparation for Class 10 PDF Download

Chemical Reaction

Definition: When two or more substances react and form some new substance, it is called a chemical reaction.
Let us see few activities related to chemical reaction:

• Burning of magnesium ribbon: the magnesium ribbon we bring has a layer of carbonate on it so before burning it needs to be cleaned with sandpaper.
  Products: magnesium oxide (white color_MgO)
  Identification property of reaction: white dazzling flame
• Reacting zinc granules with dilute sulphuric acid
  Chemical reaction: H₂So₄+Zn— ZnSo₄+H₂
  Products: zinc-sulphate and hydrogen gas
  Identification: beaker becomes hot and hydrogen gas is produced which makes match stick burn with pale blue flame and popping sound.
• Reacting barium iodide with lead chloride
  Chemical reaction: BaI+PbCl₂→BaCL₂+Pb
  Products: barium chloride white color and yellow color precipitate of lead iodide.

Characteristics of a chemical reaction

• It is either accompanied by a change in colour.
• It either leads to a change in state.
• Heat energy is produced or absorbed.
• Precipitate is formed.
• Gas is released.

Chemical Equation and Balancing

• Chemical equation: it is the representation of chemical reaction in terms of symbols, molecular formulas, moles, states, etc.
• As we know, all chemical reaction obeys law of chemical combination. Therefore, chemical reactions need to be balanced. It is done by hit and trial method. (For practice, questions are solved in the end).

Types of chemical reactions

1. Combination reaction: when two or more reactants react to form a single product.
i.e A+B→AB (In this one single product is formed)
This reaction can be between element – element, element – compound or compound –  compound.
Let us see an activity to demonstrate a combination reaction:
Burning of magnesium ribbon is an activity of combination reaction

(i) Burning of magnesium ribbon: the magnesium ribbon we bring has a layer of carbonate on it so before burning it needs to be cleaned with sandpaper.
Products: magnesium oxide (white color_MgO)
Identification property of reaction: white dazzling flame

(ii) Decomposition reaction: it is the type of reaction in which a reactant breaks up into its constituents  either by heating or passing current or in presence of light.

2. Photodecomposition (breaking in presence of light)

• Electrolytic decomposition: breaking by passing current
  
• Thermal decomposition: breaking by heating
  

Activity To Demonstrate: heating of ferrous sulphate
FeSO₄.7H₂O→FeSO₄+7H₂O
(green)
FeSO₄-   –heat → Fe₂O₃+SO₂+SO₃

_{(iii) Displacement reaction: it is a reaction between element and a compound in which more reactive element displaces less reactive from its compound}

i.e.

In this we follow reactivity series .

Examples :

Activity To Demonstrate reaction of zinc granules with dilute sulphuric acid is an example of displacement reaction

• Double decomposition or displacement reaction : it  is a reaction between two compounds in which they mutually exchange their ions.
  Examples:
  This is either precipitation reaction or neutralization reaction.
• Precipitation reaction: the reaction in which two compounds mutually react and leads to formation of insoluble substances called precipitates.
  
• Neutralization reaction: a reaction in which two compounds react to form a neutral substance.

  Acid +Base →salt +Water
  HCl+NaOH→NaCl+H₂O
  Examples:
  

Redox Reaction- Rancidity and Corrosion

Redox-reaction: the reaction in which oxidation and reduction both occur simultaneously.

Let us first learn about oxidation and reduction:
On the basis of classical concept

• Oxidation: addition of oxygen  or removal of hydrogen
  Mg+O₂→MgO
  NaH→Na +H₂
• Reduction: addition of hydrogen or removal of oxygen
  Na +H₂ →NaH
  MgO→Mg+O₂
• Oxidizing agent: which oxidizes other and itself gets reduced
• Reducing agent: which reduces others and itself gets oxidized .
  In the first example Na is reducing agent and in the second example Na is oxidizing agent

On the basis of electronic concept

• Oxidation: which donate electrons
• Reduction: which gains electrons
• Oxidising agent: the substance which oxidizes other and itself gets reduced
  In this equation –  Mg+O₂→MgO
  Mg is reducing agent and O is oxidizing agent .
• Reducing agent: the substance which reduces other and itself gets oxidized
  Cuo +H₂→Cu +H₂O
  In this, Cu (Oxidising Agent) Is getting Reduced And Hydrogen (Reducing Agent) Is getting Oxidised

Daily life redox reactions: In daily routine we come across many RedOx reactions. Out of it we are going to learn about two important reactions:

• Rancidity
• Corrosion

(i) Rancidity: We have seen mostly in summers that if we keep any food item at room temperature for longer time, it becomes stale. It is due to  the oxidation of food when it comes in contact with air due to which it starts smelling bad and tastes bad. We can prevent it by taking few precautions like storing food in refrigerator. It will slow down the activity of microorganisms due to which their shelf life increases.
(ii) Prevention :

• Storing at low temperature
• Adding preservative like salting, sugaring, nitrogen gas in potato chips, etc

Corrosion

It is eating away of metal article.

In case of iron it is called rusting.

• Conditions required: moist air
• Chemical reaction: fe+O₂→Fe₂O₃.xH₂O
                                                (brown color rust)

Prevention of rusting:

• Painting
• Lubricating
• Galvanisation: it is the process of depositing thin layer of Zn on iron article. By doing it Zn prevent the reaction of iron with moist air.
• Electroplating: by the process of electrolysis Cu and Cr layers are added on iron articles.
• Alloying: we can use the alloy of iron i.e. stainless steel.