IAA ACADEMY – Lesson 10: Chemical Equations and Calculations

4.8 Chemical Equations and Chemical Reactions (کیمیائی مساوات اور کیمیائی رد عمل – کیمیائی مساوات اور کیمیائی رد عمل)

Chemists always want to understand how chemical changes happen and what factors control them. To do this effectively, we need a clear and precise way to represent these changes.

Fortunately, chemists have developed a very suitable method: representing a chemical change in terms of symbols of elements and formulas of compounds.

Representing a chemical change in this way is called a chemical equation (کیمیائی مساوات – کیمیائی مساوات).

Purpose of a Chemical Equation (کیمیائی مساوات کا مقصد – کیمیائی مساوات کا مقصد)

A chemical equation tells us:

• The elements or compounds which are reacting (وہ عناصر یا مرکبات جو رد عمل کر رہے ہیں – وہ عناصر یا مرکبات جو رد عمل کر رہے ہیں). These are called reactants (ری ایکٹنٹ – ری ایکٹنٹ).
• And those which are being produced (جو بن رہے ہیں – جو بن رہے ہیں) as a result of the chemical change. These are called products (پروڈکٹ – پروڈکٹ).

It is customary (روایتی – روایتی) to write:

• Reactants on the left-hand side (بائیں ہاتھ کی طرف – بائیں ہاتھ کی طرف)
• Products on the right-hand side (دائیں ہاتھ کی طرف – دائیں ہاتھ کی طرف)

An arrowhead (→) drawn from reactants to products separates the two, indicating the direction of the reaction.

Example:

4Al (s) + 3O₂ (g) → 2Al₂O₃ (s)
        

In this example:

• Reactants: Al (s) [Aluminum solid], O₂ (g) [Oxygen gas]
• Products: Al₂O₃ (s) [Aluminum Oxide solid]

Points to Remember While Writing a Chemical Equation (کیمیائی مساوات لکھتے وقت یاد رکھنے کے نکات – کیمیائی مساوات لکھتے وقت یاد رکھنے کے نکات)

For a chemical equation to be correct and useful, it must follow certain rules:

• A chemical equation must obey the law of conservation of mass (ماس کے تحفظ کا قانون – ماس کے تحفظ کا قانون). This means no atom should be destroyed or produced during a chemical change. The total number and type of atoms must remain the same (ایٹموں کی کل تعداد اور قسم یکساں رہنی چاہیے – ایٹموں کی کل تعداد اور قسم یکساں رہنی چاہیے) on both sides of the equation. This is why equations must be balanced!
• The formulas of elements and compounds must be written correctly. (جیسا کہ ہم نے پچھلے سبق میں سیکھا)
• A chemical equation must determine the correct mole ratio (صحیح مول کا تناسب – صحیح مول کا تناسب) among the reactants, the products, and between the two. This is crucial for calculations!
• A chemical equation must also indicate the direction in which the change is proceeding (جس سمت میں تبدیلی ہو رہی ہے – جس سمت میں تبدیلی ہو رہی ہے).
• It is a usual practice to show the normal physical states (عام طبعی حالتیں – عام طبعی حالتیں) of reactants and products using symbols:
  • Solid = (s)
  • Liquid = (l)
  • Gas = (g)
  • Aqueous = (aq) (meaning dissolved in water, forming solvated ions)

Experimental Verification (تجرباتی تصدیق – تجرباتی تصدیق)

A chemical equation can only be written if all the above points are experimentally verified (تجرباتی طور پر تصدیق شدہ – تجرباتی طور پر تصدیق شدہ). For example, the nature of the products and their correct formulas must first be ascertained (معلوم کرنا – معلوم کرنا) through experiments before writing a chemical equation.

Example of a Complete and Verified Chemical Equation

Zn (s) + H₂SO₄ (aq) → ZnSO₄ (aq) + H₂ (g)
        

According to this equation:

• Zinc (solid) reacts with sulphuric acid (aqueous solution) to give zinc sulphate (aqueous solution) and hydrogen gas.
• The equation tells the mole ratio between reactants and products: One mole of zinc reacts with one mole of sulphuric acid to produce one mole of zinc sulphate and one mole of hydrogen gas.

Reversible Reactions (قابل واپسی رد عمل – قابل واپسی رد عمل)

Sometimes, a chemical reaction can move both ways (دونوں سمتوں میں – دونوں سمتوں میں). That is, reactants react to form products, and products can also react to form the reactants again!

These are called reversible reactions (قابل واپسی رد عمل – قابل واپسی رد عمل) and are indicated by the symbol (⇌).

Example:

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)
        

This equation shows that nitrogen gas and hydrogen gas can react to form ammonia, but ammonia can also break down to form nitrogen and hydrogen again under certain conditions.

Ionic Reactions (آئنک رد عمل – آئنک رد عمل)

Reactions involving ions may also be shown in the form of a chemical equation.

Example: Both AgNO₃ (silver nitrate) and NaCl (sodium chloride) are ionic compounds and are soluble in water. When they are mixed in water, they react to form products:

AgNO₃ (aq) + NaCl (aq) → AgCl (s) + Na⁺ (aq) + NO₃⁻ (aq)
        

Here, AgCl (silver chloride) is insoluble (حل نہ ہونے والا – حل نہ ہونے والا) in water, so it comes out of the aqueous solution as a white precipitate (تہ نشین – تہ نشین – insoluble solid). The Na⁺ and NO₃⁻ ions remain dissolved in the solution.

4.9 Calculations Based on Chemical Equation (کیمیائی مساوات پر مبنی حسابات – کیمیائی مساوات پر مبنی حسابات)

This is where chemical equations become incredibly powerful for chemists and in industry!

Understanding Stoichiometric Ratios (اسٹویچیومیٹرک تناسب کو سمجھنا – اسٹویچیومیٹرک تناسب کو سمجھنا)

A complete and balanced chemical equation tells us the:

• Mole ratio (مول کا تناسب – مول کا تناسب) or
• Molar mass ratio (مولر ماس کا تناسب – مولر ماس کا تناسب) between the reactants and products.

With the help of this ratio:

• We can find out the molar masses of products if we know the molar masses of reactants.
• Similarly, the molar masses of reactants can be found if we know the molar masses of products.

Example Equation

Let’s consider this equation, which describes the reaction of limestone (calcium carbonate) with hydrochloric acid:

CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)
        

This equation tells us the following relationships in terms of moles and mass (using approximate molar masses: CaCO₃=100 g/mol, HCl=36.5 g/mol, CaCl₂=111 g/mol, H₂O=18 g/mol, CO₂=44 g/mol):

Notice that the total masses of the reactants (100 g + 73 g = 173 g) equal the total masses of the products (111 g + 18 g + 44 g = 173 g). This confirms the law of conservation of mass!

Examples Based on Chemical Calculations (کیمیائی حسابات پر مبنی مثالیں – کیمیائی حسابات پر مبنی مثالیں)

Let’s work through some problems to see how we use chemical equations for calculations.

CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)

C₂H₅OH (l) + 3O₂ (g) → 2CO₂ (g) + 3H₂O (l)
            

4Al (s) + 3O₂ (g) → 2Al₂O₃ (s)
            

2H₂ (g) + O₂ (g) → 2H₂O (l)
            

Exercise: Put Your Skills to the Test!

Now it’s your turn to practice calculations based on chemical equations. Click to reveal the answers!