Chapter 1 Chemical Reactions and Equations

Chapter 1 Chemical Reactions and Equations[h]Chemical Reactions[/h]

  • Defination: When two or more substances combines to form a new kind of substance then it is called chemical reaction.

 

Following observations helps us to determine whether a chemical reaction has taken place :-

  • change in state
  • change in colour
  • evolution of a gas
  • change in temperature

Example of chemical Reactions

  • Magnesium ribbon burns with a dazzling white flame and changes into a white powder.
  • This powder is magnesium oxide.
  • It is formed due to the reaction between magnesium and oxygen present in the air.

 

[h]Chemical Equations[/h]

Defination: The symbolic representation of a chemical reaction is called chemical Equations.

The chemical-equation for the Reaction of…  Magnesium Ribbon burning in air   would be –Reactants and Products

Reactants : The substance which undergoes a chemical reaction are called Reactants .

Products : Products are those substance which are formed as a result of a chemical reaction.

Example: The substances that undergo chemical change in the above reaction magnesium and oxygen, are the reactants. The new substance, magnesium oxide, formed during the reaction, is the product

  • A word-equation shows change of reactants to products through an arrow placed between them.
  • The reactants are written on the left-hand side (LHS) with a plus sign (+) between them.
  • Similarly, products are written on the right-hand side (RHS) with a plus sign (+) between them.
  • The arrowhead points towards the products, and shows the direction of the reaction.

Writing a chemical Equation

  • Chemical equations can be made more concise and useful if we use chemical formulae instead of words.
  • A chemical equation represents a chemical reaction. If you recall formulae of magnesium, oxygen and magnesium oxide, the above word-equation can be written as –   Mg + O2 → MgO 

 

Chemical Equations can be divided into two types :-

  1. Balanced Chemical Equations
  2. UnBalanced Chemical Equations
  • UnBalanced chemical Equation is called skeletal Equation.

Balanced Chemical Equations

  • According to the Law of conservation of mass  mass can neither be created nor destroyed in a chemical reaction.
  • That is, the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants.
  • In other words, the number of atoms of each element remains the same, before and after a chemical reaction.

Let’s understand this by taking one chemical Equation and then balancing it.

Zinc + Sulphuric acid → Zinc sulphate + Hydrogen

The above word-equation may be represented by the following chemical equation –

Zn + H2SO4 → ZnSO4 + H2

Let us examine the number of atoms of different elements on both sides of the arrow.

As the number of atoms of each element is the same on both sides of the arrow, is a balanced chemical equation.

How to balance a chemical reaction ?

Here we will learn step by step to balance a chemical Equation.

Let us try to balance the following chemical equation –

Fe + H2O → Fe3O4 + H2

 

Step 1 : To balance a chemical equation, first draw boxes around each formula. Do not change anything inside the boxes while balancing the equation.

 

Step 2 : List the number of atoms of different elements present in the unbalanced equation

Step 3: It is often convenient to start balancing with the compound that contains the maximum number of atoms. It may be a reactant or a product. In that compound, select the element which has the maximum number of atoms. Using these criteria, we select Fe3O4 and the element oxygen in it. There are four oxygen atoms on the RHS and only one on the LHS.

To balance the oxygen atoms –

  • To equalise the number of atoms, it must be remembered that we cannot alter the formulae of the compounds or elements involved in the reactions.
  • For example: to balance oxygen atoms we can put coefficient ‘4’ as 4 H2O and not H2O4 or (H2O)4. Now the partly balanced equation becomes–

 

Step 4: Fe and H atoms are still not balanced. Pick any of these elements to proceed further. Let us balance hydrogen atoms in the partly balanced equation.

To equalise the number of H atoms, make the number of molecules of hydrogen as four on the RHS.

The equation would be –

 

Step 5: Examine the above equation and pick up the third element which is not balanced. You find that only one element is left to be balanced, that is, iron.

To equalise Fe, we take three atoms of Fe on the LHS.

Step 6: Finally, to check the correctness of the balanced equation, we count atoms of each element on both sides of the equation.

  • The numbers of atoms of elements on both sides of above  Eq. (1.9) are equal. This equation is now balanced.
  • This method of balancing chemical equations is called hit-and-trial method as we make trials to balance the equation by using the smallest whole number coefficient.

 

Step VII: Writing Symbols of Physical States Carefully examine the above balanced Eq. (1.9). Does this equation tell us anything about the physical state of each reactant and product? No information has been given in this equation about their physical states.

  • To make a chemical equation more informative, the physical states of the reactants and products are mentioned along with their chemical formulae.
  • The gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s), respectively. The word aqueous (aq) is written if the reactant or product is present as a solution in water.

The balanced Eq. (1.9) becomes

3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)     

  • Note that the symbol (g) is used with H2O to indicate that in this reaction water is used in the form of steam.
  • Usually physical states are not included in a chemical equation unless it is necessary to specify them.
  • Sometimes the reaction conditions, such as temperature, pressure, catalyst, etc., for the reaction are indicated above and/or below the arrow in the equation. For example –

[h]TYPES OF CHEMICAL REACTIONS[/h]

Their are 5 types of chemical Reactions

  1. Combination Reaction
  2. Decomposition Reaction
  3. Displacement Reaction
  4. Double displacement Reaction
  5. Redox Reaction

1. Combination Reaction

Defination: when two or more substances (elements or compounds) combine to form a single product, the reactions are called combination reactions.

Examples: –

1. Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

In this reaction, calcium oxide and water combine to form a single product, calcium hydroxide. Such a reaction in which a single product is formed from two or more reactants is known as a combination reaction.

 

2. Burning of coal

C(s) + O2(g) → CO2(g)

 

3. Formation of water from H2(g) and O2(g)

2H2(g) + O2(g) → 2H2O(l)

 

  • Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.

 

Examples of exothermic reactions are –

(i) Burning of natural gas

CH4(g) + 2O2 (g) → CO2 (g) + 2H2O (g)

 

(ii) respiration is an exothermic process?

  • We all know that we need energy to stay alive. We get this energy from the food we eat. During digestion, food is broken down into simpler substances.
  • For example, rice, potatoes and bread contain carbohydrates.
  • These carbohydrates are broken down to form glucose.
  • This glucose combines with oxygen in the cells of our body and provides energy.

(iii) The decomposition of vegetable matter into compost is also an example of an exothermic reaction.

2. Decomposition Reaction

Defination : a reaction in which single reactant breaks down to give simpler products is called a decomposition reaction.

  • It is opposite of a combination Reaction.

 

Examples 1: 

  • In this reaction you can observe that a single reactant breaks down to give simpler products. This is a decomposition reaction.
  • Ferrous sulphate crystals (FeSO4, 7H2O) lose water when heated and the colour of the crystals changes.
  • It then decomposes to ferric oxide (Fe2O3), sulphur dioxide (SO2) and sulphur trioxide (SO3). Ferric oxide is a solid, while SO2and SO3 are gases.

 

Examples 2: 

  • Decomposition of calcium carbonate to calcium oxide and carbon dioxide on heating is an important decomposition reaction
  • It is used in various industries.
  • Calcium oxide is called lime or quick lime.
  • It has many uses – one is in the manufacture of cement.
  • When a decomposition reaction is carried out by heating, it is called thermal decomposition.

 

Examples 3: 

White silver chloride turns grey in sunlight. This is due to the decomposition of silver chloride into silver and chlorine by light.

These two reactions are used in black and white photography.

Silver bromide also behaves in the same way.

 

  • Decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants.
  • Reactions in which energy is absorbed are known as endothermic reactions.

 

3. Displacement Reaction

Defination: Displacement reaction is a chemicalreaction in which a more reactive element displaces a less reactive element from its compound.

  • Both metals and non-metals take part indisplacement reactions.

Examples of displacement Reaction

1. Reaction between iron and copper sulphate 

  • In this reaction, iron has displaced or removed another element, copper, from copper sulphate solution.

 

2. Reaction of zinc with copper sulphate 

 

2. Reaction of lead with copper chloride

  • Zinc and lead are more reactive elements than copper. They displace copper from its compounds.

 

4. Double Displacement Reaction

Defination: Double displacement Reaction is a type of chemical reaction where two compounds react, and the positive ions (cation) and the negative ions (anion) of the two reactants switch places, forming two new compounds or products.

  • When sodium sulphate solution and barium chloride solution are mixed  a white substance, which is insoluble in water, is formed.
  • This insoluble substance formed is known as a precipitate.
  • Any reaction that produces a precipitate can be called a precipitation reaction.

Example of Double displacement Reaction:-

In the above Reaction The white precipitate of BaSO4 is formed by the reaction of SO42- and Ba2+. The other product formed is sodium chloride which remains in the solution. Such reactions in which there is an exchange of ions between the reactants are called double displacement reactions.

 

5.  Oxidation and Reduction Reaction (Redox Reaction)

Defination: An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species.

  • If a substance gains oxygen during a reaction, it is said to be oxidised.
  • If a substance loses oxygen during a reaction, it is said to be reduced.

 

Example 1

Heat a china dish containing about 1 g copper powder

The surface of copper powder becomes coated with black copper(II) oxide.

This is because oxygen is added to copper and copper oxide is formed.

If hydrogen gas is passed over this heated material (CuO), the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained.

During this reaction the copper(II) oxide is losing oxygen and is being reduced. The hydrogen is gaining oxygen and is being oxidised. In other words, one reactant gets oxidised while the other gets reduced during a reaction. Such reactions are called oxidation-reduction reactions or redox reactions.

 

Example 2

In this reaction carbon is oxidised to CO and ZnO is reduced to Zn.

 

Example 3

In reaction  HCl is oxidised to Cl2 whereas MnO2 is reduced to MnCl2.

 

  • if a substance gains oxygen or loses hydrogen during a reaction, it is oxidised. If a substance loses oxygen or gains hydrogen during a reaction, it is reduced

 

 

[h] THE EFFECTS OF OXIDATION REACTIONS IN EVERYDAY LIFE[/h]

1. Corrosion

Corrosion is a process in which a metal is attacked by substances around it such as moisture, acids, etc.

The black coating on silver and the green coating on copper are other examples of corrosion.

Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, specially those of iron.

2. Rancidity

  • When fats and oils are oxidised, they become rancid and their smell and taste change.
  • Usually substances which prevent oxidation (antioxidants) are added to foods containing fats and oil.
  • Keeping food in air tight containers helps to slow down oxidation.
  • chips manufacturers usually flush bags of chips with gas such as nitrogen to prevent the chips from getting oxidised

 

 

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