Rajasthan Board RBSE Class 10 Science Notes Chapter 6 Chemical Reaction and Catalyst
|Physical Change||Chemical Change|
|There are changes in physical characteristics of substance.||There are changes in chemical characteristics of substance.|
|No new substance is formed after this change.||New substance is formed after this change.|
|Change is reverted when cause of change is removed.||Change does not get reversed even when cause of change is removed.|
|This is a temporary change.||This is usually a permanent change.|
A chemical change is also called a chemical reaction. In other words, a change which results in formation of a new substance is called a chemical change or chemical reaction.
When a chemical reaction is expressed by chemical symbols, it is called chemical equation.
Steps of Writing a Chemical Equation:
- Reactants’ symbols are written, followed by an arrow. A sign of addition (+) is written between formulae of two reactants.
- Products’ symbols are written on RHS of arrow.
- State of matter of reactants and products is shown by s (for solid), l(for liquid) g (for gas) and aq (for aqueous) in brackets, after the formula.
- Special conditions, if any, is written above or below the arrow.
- Number of atoms of each element should be same on both sides of the arrow.
Balancing A Chemical Equation:
When number of atoms of each element is same on both sides of the arrow, the equation is called a balanced equation. This is necessary in order to obey the law of conservation of mass because no new mass is created in a chemical reaction.
Law of Conservation of Mass:
Mass cannot be created and cannot be destroyed.
Steps of balancing a chemical equation:
This is done by trying different combinations of numbers against molecules on both sides of the arrow. While doing so, molecules other than hydrogen and oxygen are given preference.
C3H8 + O2 → CO2 + h2O
Leaving oxygen and hydrogen, let us first try to balance number of carbon atoms.
C3H8 + O2 → 3CO2 + H2O
Let us now balance the number of hydrogen atoms.
C3H8 + O2 → 3CO2 + 4H2O
Let us now balance the number of oxygen atoms.
C3H8 + 5O2 → 3CO2 + 4H2O
Features of chemical equation:
- This shows complete information about reactants and products, e.g. number of molecules, mass, etc.
- This shows physical state of matter.
- This shows the nature of reaction in terms of reversibility or irreversibility, in terms of absorption or release of heat, etc.
Limitations of chemical equation:
- It does not give information about completion of reaction.
- It does not show concentration of reactants or products.
- It does not give mechanism of reaction.
When two or more reactants combine to give one product, the reaction is called combination reaction. Example: When carbon is burnt in air, it gives carbon dioxide.
C + O2 → CO2
When an atom or group of atoms in one reactant displaces another atom of group of atoms from another reactant, the reaction is called displacement reaction. In this case, a more reactive atom or group of atoms displaces a less reactive atom or group of atoms. Example: When zinc granules are kept in copper sulphate solution, the blue colour of copper sulphate fades away. This happens because zinc displaces copper to form zinc sulphate.
CuSO4 + Zn →ZnSO4 + Cu
K > Na > Ca > Mg > Al > C > Zn > Fe > Sn > Pb >
Double displacement reaction:
When an atom or group of atoms in both reactants displaces each other, the reaction is called double displacement reaction. Example: When sodium hydroxide reacts with copper sulphate, then hydroxide and sulphate ions displace each other and copper hydroxide and sodium sulphate are formed.
CuSO4 + 2NaOH → Cu (OH)2 + Na2SO4
When a reactant dissociates to give two or more products, the reaction is called decomposition reaction.
Based on the cause of reaction, decomposition reactions are of following types:
When electric current is passed through molten or aqueous form of a substance and the substance dissociates into two or more products. Example: When electric current is passed through water, the water molecule breaks to give hydrogen and oxygen.
When decomposition reaction takes place due to heat.
Example: When calcium carbonate is heated up to 473 K, we get calcium oxide and carbon dioxide.
When decomposition reaction takes place due to light.
Example: When hydrogen bromide is exposed to light, it dissociates into hydrogen and bromine.
When a reaction happens very quickly, it is called fast reaction. Such reactions are generally ionic reactions, e.g. a reaction between a strong acid and a strong base is completed in one tenth of a second.
NaOH + HCl → NaCI + H2O
Such reaction may take hours, days or even many years for completion. Rusting of iron is an example of slow reaction.
4Fe + 3O2 + 6H2O → 2Fe2O3.3H2O
A reaction which takes place in one direction only is called an irreversible reaction. Concentration of reactants keeps on decreasing and concentration of product keeps on increasing in such reactions.
CH4+2O2 → CO2 + 2H2O
When a reaction takes place in both directions, it is called reversible reaction. In a reversible reaction, the moment a product is formed it starts changing into reactant. Reversible reaction is shown by double arrows.
Oxidation and Reduction:
A substance which causes oxidation is called oxidizing agent. A substance which causes reduction is called a reducing agent. Oxidation and reduction always happen together and can be defined on following bases.
Association or decomposition of oxygen:
Addition of oxygen to a substance is called oxidation and removal of oxygen is called reduction.
When magnesium ribbon is burnt in air, magnesium oxide is produced because of addition of oxygen to magnesium. This is an oxidation reaction in which magnesium is getting oxidized.
2Mg + O2 → 2MgO
Association or decomposition of hydrogen:
Addition of hydrogen to a substance is called reduction and removal of hydrogen from a substance is called oxidation.
In the following reaction, hydrogen is removed from hydrogen sulphide and hence hydrogen sulphide is getting oxidized. Hydrogen is added to oxygen and hence oxygen is getting reduced.
2H2S + O2 2H2O + 2S
Association or dissociation of electropositive elements:
Removal of electropositive element from a substance is called oxidation. Addition of electropositive element to a substance is called reduction.
In following example: potassium (an electropositive element) is removed from potassium iodide and hence oxidation of potassium iodide happens. Here, potassium is added to chlorine and hence reduction of chlorine happens.
2KI + Cl2 → 2KCl + I2
Association or dissociation of electronegative elements:
Addition of electronegative element to a substance is called oxidation. Removal of electronegative element from a substance is called reduction.
In following example; chlorine (an electronegative element) is removed from iron (III) chloride hence reduction of iron (III) chloride happens. Here, chlorine is added to hydrogen and hence oxidation of hydrogen happens.
2 FeCl3 + H2 → 2FeCl2 + 2HC1
Exchange of electrons:
When an element loses electrons, it results in oxidation of that element. When an element gains electrons, its results in reduction of that element.
When an acid reacts with a base, salt and water are formed. This reaction is called neutralization reaction. Example: When hydrochloric acid reacts with sodium hydroxide then sodium chloride and water are formed.
HCl + NaOH →NaCl + H2O
When a strong acid reacts with a strong base, the pH of resulting solution becomes 7. When a strong acid reacts with a weak base, the pH of resulting solution becomes less than 7. When a weak acid reacts with a strong base, the pH of resulting solution becomes more than 7.
A substance which alters the speed of a reaction but does not participate in reaction is called a catalyst. This reaction is called catalysis.
When potassium chlorate is heated in presence of manganese dioxide, it decomposes at lower temperature than usual. The end products of this reaction are potassium chloride and oxygen. Here, manganese dioxide works as a catalyst but does not undergo any change.
Types of catalyst on the basis of physical state:
- Homogeneous Catalyst:
When the catalyst, reactants and products, all are in the same physical state, the catalyst is called homogeneous catalyst. For example,
- Heterogeneous Catalyst:
When the catalyst, reactants and products are in different physical states, the catalyst is called heterogeneous catalyst. For example,
Types of catalyst on the basis of action:
- Positive Catalyst: When a catalyst enhances the speed of reaction, it is called a positive catalyst. Following are some examples of positive catalyst.
- Negative Catalyst: WTien a catalyst reduces the speed of reaction, it is called a negative catalyst.Following are some examples of negative catalyst.
- Auto Catalyst: When a product of a chemical reaction acts like a catalyst for the same reaction, it is called an auto catalyst. For example, the speed of following reaction is initially slow. Once production of acetic acid begins, acetic acid enhances the speed of reaction.
CH3COOC2H5 + H2O → CH3COOH + C2H5OH
- Bio-catalyst: Many substances work as catalyst in biochemical reactions. These are called biocatalyst. There are enzymes which speed up certain biological processes. For example,
A substance which enhances the action of a catalyst but itself is not a catalyst. For example, Mo enhances the action of Fe in the production of ammonia.
A Substance which reduces the action of a catalyst. For example, Co reduces the action of Fe in the production of ammonia.