Chemical Changes And Reactions
Chemical Change
A chemical change is a permanent change in which the chemical composition of a substance is changed and one or more new substances with different chemical compositions and different properties are formed.
A chemical reaction is the process of breaking chemical bonds of the reacting substance and making new bonds to form new substance.
A chemical bond is the force that holds the atoms of a molecule together, as it in a compound.
A chemical occurs when particles collide. Collisions occur when reactants are in closed contacts. Thus one or more of the conditions are necessary for a chemical change:
(i) Mixing: In some cases a chemical reactions occurs when two substance are mixed in their solid states.
(ii) Solutions: In some cases a chemical reactions occurs when substances are mixed in either molten or aqueous state.
(iii) Heat: Some chemical reactions occurs only on heating.
(iv) Light: Some chemical reaction takes place by action of light. These are called as photochemical reactions or photosynthesis. Molecule of the reactants absorb light energy to get activated, and then react rapidly.
(v) Electricity: Certain chemical reactions like the decompositions of certain compounds, occurs only when electricity is passed through the substances.
(vi) pressure: Some chemical reactions takes place only when the involved substances are subjected to high pressure.
(vii) Catalyst: Some chemical reaction need a catalyst to accelerate or decelerate the rate at which they occur. The catalyst themselves do not take part in the reaction and so remains unchanged.
(a) Positive catalyst
When a catalyst accelerates the rate of the reaction, it is known as the positive catalyst.
Promoters: Substance that influence the rate of a chemical reaction by improving the efficiency of the catalyst are called as promoters.
(b) Negative catalyst
A catalyst employed to retard the rate of a reactions is known as a negative catalyst.
Characteristics Of Chemical Reactions
(i) Evolution of gas: In many chemical reactions one of the products is a gas.
(ii) Change of color: Certain chemical reactions are characterized by a change in the color of the reactants.
(iii) Formation of precipitates: Certain chemical reaction are characterized by the formation of insoluble solid substances called precipitates.
(iv) Change of state: In many chemical reactions a change of state is observed. The reaction might start with gaseous or liquid reactants and end up with solid products or vice versa.
Effervescence: The formation of bubbles in a liquid during a reaction is called effervescence. One of the reaction is liquid.
Types of Chemical change or Chemical reaction
1. Direct combination (or synthesis)
2. Decomposition
3. Displacement
4. Double decomposition
1. Direct combination (or synthesis)
A reaction in which two or more substances combine together to form a single substance is called a combination reaction or synthesis.
2. Decomposition
The chemical reaction in which a compound splits into two or more simpler substances (element or compounds) is called decomposition reaction.
In a decomposition reaction usually a single reactants decomposes without the presence of a second substance and very often the action of heat is sufficient to cause the reaction to take place.
Energy Change in Chemical Reactions
Every substance has a fixed amount of stored energy, which is in the form of potential energy, and is referred to as its chemical energy.
Two types of chemical changes or chemical reactions are
1. Exothermic 2. Endothermic
1. Exothermic reaction: A chemical reaction in which heat is given out is called exothermic reaction. It causes a rise in temperature.
2. Endothermic reaction: A chemical reaction in which heat is absorbed is called endothermic reaction. It causes a fall in temperature.
Photochemical reaction
It is a reaction that occurs with absorption of light energy.
Electrochemical reaction
A chemical bond is the force that holds the atoms of a molecule together, as it in a compound.
A chemical occurs when particles collide. Collisions occur when reactants are in closed contacts. Thus one or more of the conditions are necessary for a chemical change:
(i) Mixing: In some cases a chemical reactions occurs when two substance are mixed in their solid states.
(ii) Solutions: In some cases a chemical reactions occurs when substances are mixed in either molten or aqueous state.
(iii) Heat: Some chemical reactions occurs only on heating.
(iv) Light: Some chemical reaction takes place by action of light. These are called as photochemical reactions or photosynthesis. Molecule of the reactants absorb light energy to get activated, and then react rapidly.
(v) Electricity: Certain chemical reactions like the decompositions of certain compounds, occurs only when electricity is passed through the substances.
(vi) pressure: Some chemical reactions takes place only when the involved substances are subjected to high pressure.
(vii) Catalyst: Some chemical reaction need a catalyst to accelerate or decelerate the rate at which they occur. The catalyst themselves do not take part in the reaction and so remains unchanged.
(a) Positive catalyst
When a catalyst accelerates the rate of the reaction, it is known as the positive catalyst.
Promoters: Substance that influence the rate of a chemical reaction by improving the efficiency of the catalyst are called as promoters.
(b) Negative catalyst
A catalyst employed to retard the rate of a reactions is known as a negative catalyst.
Characteristics Of Chemical Reactions
(i) Evolution of gas: In many chemical reactions one of the products is a gas.
(ii) Change of color: Certain chemical reactions are characterized by a change in the color of the reactants.
(iii) Formation of precipitates: Certain chemical reaction are characterized by the formation of insoluble solid substances called precipitates.
(iv) Change of state: In many chemical reactions a change of state is observed. The reaction might start with gaseous or liquid reactants and end up with solid products or vice versa.
Effervescence: The formation of bubbles in a liquid during a reaction is called effervescence. One of the reaction is liquid.
Types of Chemical change or Chemical reaction
1. Direct combination (or synthesis)
2. Decomposition
3. Displacement
4. Double decomposition
1. Direct combination (or synthesis)
A reaction in which two or more substances combine together to form a single substance is called a combination reaction or synthesis.
2. Decomposition
The chemical reaction in which a compound splits into two or more simpler substances (element or compounds) is called decomposition reaction.
In a decomposition reaction usually a single reactants decomposes without the presence of a second substance and very often the action of heat is sufficient to cause the reaction to take place.
Energy Change in Chemical Reactions
Every substance has a fixed amount of stored energy, which is in the form of potential energy, and is referred to as its chemical energy.
Two types of chemical changes or chemical reactions are
1. Exothermic 2. Endothermic
1. Exothermic reaction: A chemical reaction in which heat is given out is called exothermic reaction. It causes a rise in temperature.
2. Endothermic reaction: A chemical reaction in which heat is absorbed is called endothermic reaction. It causes a fall in temperature.
Photochemical reaction
It is a reaction that occurs with absorption of light energy.
Electrochemical reaction
It is a reaction that occurs with absorption of electrical energy.
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