Chemical Reactions And Equations Class 10 Science Chapter 1 Notes

Chemical Reactions and Equations Class 10 Science Chapter 1 Notes are available here. These notes were carefully prepared by our team of subject experts to make it easier for you to grasp the concepts. We hope you like these notes and have a great time studying with them.

Chemical Reactions and Equations Class 10 Science Chapter 1 Notes

Introduction

Chemical reactions are when one substance changes into another substance.
Examples of chemical reactions are rusting of iron, milk turning into curd, digestion of food, and breathing.
In a chemical reaction, a new substance with different properties is formed.
It’s like a magic trick where the atoms rearrange themselves to make something new.
The things that join together to start a chemical reaction are called reactants.
The new things that are made after the reaction are called products.

Burning of Magnesium

Burning magnesium is a chemical reaction that changes magnesium into something called magnesium oxide.
We can write the chemical reaction like this: 2Mg(s) + O₂(g) → 2MgO(s).
Before burning, the magnesium ribbon is cleaned by rubbing it with sandpaper. It’s like giving it a little scrub.
This cleaning is important because it removes a special layer called basic magnesium carbonate from the surface of the magnesium ribbon.
This layer protects the magnesium, but we want it to react and make something new, so we have to clean it off.
After the cleaning, we can burn the magnesium and see it turn into magnesium oxide.

Reactants and Products

When substances join together to have a chemical reaction, we call them reactants.
Reactants are like the ingredients in a special recipe for making something new.
For example, when we burn magnesium, the reactants are magnesium (we can write it as “Mg”) and oxygen (we can write it as “O₂”).
After the chemical reaction happens, something new is made, and we call it a product.
In the case of burning magnesium, the product is called magnesium oxide, and we can write it as “MgO”.
So, reactants are the things that start the chemical reaction, and products are the new things made after the reaction.

Characteristics of Chemical Reactions

1. Evolution of Gas:

Sometimes, when different things come together, they create a gas as a result.
For example, when zinc and dilute sulphuric acid mix, they produce a gas called hydrogen.
We can write it like this: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) ↑.

2. Change in Colour:

Chemical reactions can make things change their colour, just like magic!
When copper sulphate solution reacts with iron, the colour changes from blue to green.

3. Change in State of Substance:

Chemical reactions can make things change from one form to another.
When hydrogen reacts with oxygen water will form in this reaction both the reactants are glasses but the product is liquid.

4. Change in Temperature:

Some chemical reactions can make things feel hot or cold.
When calcium oxide reacts with water to form calcium hydroxide it gets hotter.
The same happens when zinc granules mix with dilute sulphuric acid.

Chemical Equations

A chemical equation is like a special format that shows us how substances react and change into new substances.
It uses symbols and formulas to represent the substances involved in a chemical reaction.
For example, we can write a chemical equation like this: A + B → C + D.
In this equation, A and B are the reactants, and C and D are the products.
The arrow tells us the direction of the reaction, showing us how things change.
Sometimes, there may be conditions or special requirements for the reaction, and we write them above the arrow.

Let’s see an example:

When hydrogen and oxygen react, they make water.
We can represent this reaction with a chemical equation like this: Hydrogen + Oxygen → Water.
And using symbols and formulas, it looks like this: H₂ + O₂ → H₂O.
The above first equation uses words, and the second one uses symbols to make it easier to understand.

1. Balanced Chemical Equations

In a balanced chemical equation, the number of atoms for each element is the same on both sides.
It’s like having the same number of ingredients on both sides of a recipe.
For example, let’s look at this equation: Zn + H₂SO₄ → ZnSO₄ + H₂.
If we count the atoms, we see that the number of zinc, hydrogen, and sulphur are the same on both sides.
So, this is a balanced chemical equation.

The Law of Conservation of Mass:

There’s a special rule called the Law of Conservation of Mass in chemistry.
It says that mass cannot be created or destroyed in a chemical reaction. It remains the same.
To follow this rule, the total mass of elements in the reactants must be equal to the total mass of elements in the products.

2. Unbalanced Chemical Equations:

Sometimes, the number of atoms of each element in the reactants is not the same as the number in the products.
When this happens, we call it an unbalanced chemical equation.
Let’s look at an example: Fe + H₂O → Fe₃O₄ + H₂.
In this equation, the number of atoms is not the same on both sides.
Reactant side Fe is 1 whereas it is 3 on product side

Steps to Balance a Chemical Equation:

Write the number of atoms of each element in the reactants and products in a table.
Find the element with the maximum number of atoms on either side.
To balance that element, multiply the atoms on the other side to make them equal.
Repeat this process for any remaining unbalanced elements until the number of atoms is the same on both sides.

Let’s balance the equation together:

We start with Fe + H₂O → Fe₃O₄ + H₂.
We see that oxygen has the maximum number of atoms on the right side, which is 4.
So, we multiply the oxygen on the left side by 4 to make it equal.
Now we have 3Fe + 4H₂O → Fe₃O₄ + H₂.
Next, we see that hydrogen has 8 atoms on the left side, which is more than the 2 atoms on the right side.
To balance it, we multiply the hydrogen on the right side by 4.
Now we have 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
Finally, we see that iron has 3 atoms on the left side and 3 atoms on the right side. It is already balanced.
After balancing, the equation becomes: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
At the end we should also mention the physical state of reactants and products: 3Fe(s) + 4H₂O(l) → Fe₃O₄(s) + 4H₂(g).
Now this is the final and complete balanced chemical equation.

Making Equations More Informative

1. Symbols for Physical States:

We can use symbols to show whether a substance is a gas, liquid, solid, or dissolved in water.
When a substance is a gas, we write (g) next to its formula.
When it’s a liquid, we write (l) next to the formula.
For solids, we use (s) next to the formula.
When a substance is dissolved in water, we write (aq) next to the formula.

2. Writing the Reaction Condition:

Sometimes, it’s important to mention the conditions under which a reaction takes place.
We can write the condition above or below the arrow in a chemical equation.

3. Drawing upward or downward arrows:

Upward arrow is used after the gas produced in the reaction.
Downward arrow is used after the precipitate formed in the reaction.

Types of Chemical Reactions

1. Combination Reactions

Combination reactions happen when two or more things combine to make one new thing.
We can write a general combination reaction as: A + B → AB.
Examples:
Magnesium + Oxygen → Magnesium Oxide:

Magnesium burns in the air (oxygen) to form magnesium oxide.
We can write it as: Mg + O₂ → MgO.
Carbon + Oxygen → Carbon Dioxide:

Carbon burns in oxygen (air) to form carbon dioxide.
We can write it as: C + O₂ → CO₂.

2. Decomposition Reactions

Decomposition reactions happen when one compound breaks down into two or more substances.
We can write a general decomposition reaction as: AB → A + B.
Here are some examples to understand it better:

Calcium Carbonate → Calcium Oxide + Carbon Dioxide:

When calcium carbonate is heated, it breaks down into calcium oxide and carbon dioxide.
We can write it as: CaCO₃(s) → CaO(s) + CO₂(g).
Calcium carbonate is found in things like chalk or seashells.

Ferric Hydroxide → Ferric Oxide + Water:

When ferric hydroxide is heated, it decomposes into ferric oxide and water.
We can write it as: 2Fe(OH)₃(s) → Fe₂O₃(s) + 3H₂O(l).
It’s like ferric hydroxide getting hot and transforming into ferric oxide and water.
Ferric hydroxide is a compound containing iron and hydroxide ions.

Thermal Decomposition Reaction:

Thermal decomposition happens when a substance breaks down due to heating.
For example, when lead nitrate is heated, it decomposes into lead oxide, nitrogen dioxide, and oxygen.
We can write it as: 2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g).
It’s like lead nitrate getting heated and splitting into its parts.

Electrolytic Decomposition Reaction:

Electrolytic decomposition happens when compounds break down into simpler substances due to the passing of electricity.
For instance, when electricity is passed through water, it decomposes into hydrogen and oxygen gases.
We can write it as: 2H₂O(l) → 2H₂(g) + O₂(g).
It’s like water being split apart by electricity into its components.

Photolysis or Photo Decomposition Reaction:

Photolysis or photo decomposition happens when a compound breaks down due to sunlight.
For example, when silver chloride is exposed to sunlight, it decomposes into silver metal and chlorine gas.
We can write it as: 2AgCl(s) + Sunlight → 2Ag(s) + Cl₂(g).
It’s like sunlight making silver chloride turn into silver metal and chlorine gas.

3. Displacement Reactions

Displacement reactions happen when a more reactive element takes the place of a less reactive element in a compound.
It’s like a competition where the stronger element replaces the weaker one!
We can represent a general displacement reaction using the equation: A + BC → AC + B.
Here are a couple of examples to understand it better:

Zinc + Hydrochloric Acid → Zinc Chloride + Hydrogen Gas:

When zinc reacts with hydrochloric acid, zinc displaces hydrogen from the acid.
We can write it as: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g).
Zinc is more reactive than hydrogen, so it can push hydrogen out.

Zinc + Copper Sulphate → Zinc Sulphate + Copper Metal:

When zinc reacts with copper sulphate, zinc displaces copper from the compound.
We can write it as: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s).
Zinc is more reactive than copper, so it can replace it.

4. Double Displacement Reactions

Double displacement reactions happen when ions from two different substances exchange places to form new compounds.
We can represent a general double displacement reaction using the equation: AB + CD → AC + BD.
Here are a couple of examples to understand it better:

Barium Chloride + Sodium Sulphate → Barium Sulphate + Sodium Chloride:

When the solution of barium chloride mixes with the solution of sodium sulphate, a white precipitate of barium sulphate is formed, along with sodium chloride.
We can write it as: BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) (Precipitate) + 2NaCl(aq).
It’s like barium and sodium switching partners to create barium sulphate and sodium chloride.
The white precipitate is like a solid that forms and settles down.

Sodium Hydroxide + Hydrochloric Acid → Sodium Chloride + Water:

When sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed.
We can write it as: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l).
It’s like sodium and hydrogen switching places to make sodium chloride and water.
Sodium hydroxide is a base, and hydrochloric acid is an acid.

Precipitation Reaction:

In a precipitation reaction, when we mix two solutions of salts, an insoluble solid called a precipitate is formed.
Example: When we mix sodium chloride (NaCl) with silver nitrate (AgNO₃), a white solid called silver chloride (AgCl) is formed.
We can write it as: NaCl + AgNO₃ → AgCl + NaNO₃.
It’s like sodium and silver switching places to create silver chloride and sodium nitrate.

Neutralisation Reaction:

In a neutralisation reaction, an acid reacts with a base to form salt and water.
It’s like mixing something sour with something bitter to make something tasty and neutral.
Example: When we mix sodium hydroxide (NaOH), which is a base, with hydrochloric acid (HCl), they react to form sodium chloride (NaCl) and water (H₂O).
We can write it as: NaOH(aq) + HCl(aq) → NaCl + H₂O.
It’s like sodium, hydrogen, and oxygen coming together to create sodium chloride and water.

5. Oxidation And Reduction Reaction

Oxidation Reaction:

Oxidation happens when oxygen or a non-metal element is added to a compound or when hydrogen or a metallic element is removed from a compound.
It’s like adding oxygen or taking away hydrogen from a special chemical recipe!
When a substance undergoes oxidation, we say it is being oxidised.
Example: If we add oxygen to a compound or remove hydrogen from it, it is undergoing oxidation.

Reduction Reaction:

Reduction happens when hydrogen or a metallic element is added to a compound or when oxygen or a non-metal element is removed from a compound.
It’s like adding hydrogen or taking away oxygen from the chemical recipe!
When a substance undergoes reduction, we say it is being reduced.
Example: If we add hydrogen to a compound or remove oxygen from it, it is undergoing reduction.

Oxidising Agent:

An oxidising agent is a substance that provides oxygen for oxidation or removes hydrogen from a compound.
It’s like a special helper that brings oxygen or takes away hydrogen from the reaction.
A substance that provides oxygen or removes hydrogen is called an oxidising agent.

Reducing Agent:

A reducing agent is a substance that provides hydrogen for reduction or removes oxygen from a compound.
It’s like another special helper that brings hydrogen or takes away oxygen from the reaction.
A substance that provides hydrogen or removes oxygen is called a reducing agent.

6. Redox Reactions:

Redox (reduction-oxidation) reactions occur when both oxidation and reduction happen simultaneously.
Example: When copper oxide is heated with hydrogen, copper metal and water are formed.
We can write it as: CuO + H₂ → Cu + H₂O.
In this reaction, copper oxide is reduced (oxygen is removed), and hydrogen is oxidised (oxygen is added).

Exothermic Reactions:

Exothermic reactions are reactions that release or give off energy.
It’s like a reaction that produces heat or energy, like a firecracker exploding!
Example: Respiration is an exothermic reaction that happens in our bodies when we breathe.
We can write it as: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy.
It’s like our bodies breaking down sugar to release energy for us to move and play.
Another example is when quicklime (CaO) is added to water, and it releases energy.
We can write it as: CaO + H₂O → Ca(OH)₂.
It’s like quicklime and water having a reaction and giving off energy.

Endothermic Reactions:

Endothermic reactions are reactions that absorb or take in heat energy from their surroundings.
It’s like a reaction that needs heat or energy to happen, like an ice cube melting!
Example: The decomposition of calcium carbonate is an endothermic reaction.
We can write it as: CaCO₃(s) → CaO(s) + CO₂(g).
It’s like calcium carbonate needing heat energy to break down into calcium oxide and carbon dioxide.
It absorbs heat from its surroundings to make the reaction happen.

Effects of Oxidation Reactions in Everyday Life:

Corrosion:

Slow conversion of metals into undesirable compounds due to their reaction with oxygen, water, acids, or gases in the atmosphere.
Example: Rusting of iron (Fe₂O₃.nH₂O)

Rancidity:

Undesirable change in the taste and odour of food containing fats and oils due to their oxidation.
Preventive methods: Adding antioxidants, using airtight containers, nitrogen gas flushing, and refrigeration.