This MCQ module is based on: Chemical Reactions and Writing Balanced Equations
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Chemical Reactions and Writing Balanced Equations
🎓 Class 10
Science
CBSE
Theory
Ch 1 — Chemical Reactions and Equations
⏱ ~24 min
🌐 Language: [gtranslate]
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[myaischool_lt_science_assessment grade_level="class_10" science_domain="chemistry" difficulty="intermediate"]
Introduction: What Are Chemical Reactions?
Look around you and think about the many changes happening every day. Milk left out too long turns sour. An iron gate left exposed to rain and air slowly develops a reddish-brown coating. The food you eat gets broken down inside your body to provide energy. Grapes left for several days begin to ferment. In every one of these cases, the original substances have transformed into entirely new substances with different properties. These transformations are examples of chemical reactions.
But how can we tell that a chemical reaction has actually taken place? Scientists look for certain telltale signs: a change in colour, a change in state (such as a gas being produced or a solid forming in a liquid), evolution of a gas, or a change in temperature. Let us explore some reactions through activities and learn how to represent them using chemical equations.
Key Idea: In a chemical reaction, the chemical composition of substances changes. New substances with new properties are formed. This is different from a physical change (like melting ice) where no new substance is created.
Exploring Chemical Reactions Through Activities
Activity 1.1 — Burning a Magnesium Ribbon
L3 Apply
Predict first: What do you think will happen when a magnesium ribbon is heated over a flame? What kind of substance will form?
- Take a clean magnesium ribbon (about 2 cm long) and rub it with sandpaper to remove any oxide coating.
- Hold the ribbon with a pair of tongs and bring it near a Bunsen burner flame.
- Hold the burning ribbon over a watch glass so that the product collects on it.
- Observe the flame colour and the substance formed on the watch glass.
Observations:
The magnesium ribbon burns with a dazzling white flame and produces a white powder (magnesium oxide, MgO) that collects on the watch glass.
Chemical equation:
The magnesium ribbon burns with a dazzling white flame and produces a white powder (magnesium oxide, MgO) that collects on the watch glass.
Chemical equation:
\(2\text{Mg}(s) + \text{O}_2(g) \xrightarrow{\text{heat}} 2\text{MgO}(s)\)
Magnesium reacts with the oxygen present in air to form magnesium oxide. This is a chemical change because a new substance (MgO) with different properties has been formed.
Activity 1.2 — Lead Nitrate and Potassium Iodide
L3 Apply
Predict first: When you mix two colourless solutions together, do you always get a colourless mixture? What if a new substance forms?
- Take a small amount of lead nitrate solution in a test tube.
- Add potassium iodide solution to it using a dropper.
- Observe the change that takes place.
Observations:
A bright yellow precipitate of lead iodide (PbI2) forms immediately when the two solutions are mixed.
Chemical equation:
A bright yellow precipitate of lead iodide (PbI2) forms immediately when the two solutions are mixed.
Chemical equation:
\(\text{Pb(NO}_3)_2(aq) + 2\text{KI}(aq) \rightarrow \text{PbI}_2(s)\downarrow + 2\text{KNO}_3(aq)\)
The formation of a yellow precipitate is clear evidence that a chemical reaction has occurred.
Activity 1.3 — Zinc Granules with Dilute Acid
L3 Apply
Predict first: When a metal reacts with an acid, what kind of gas do you think might be produced? How would you test for it?
- Take a few zinc granules in a conical flask or test tube.
- Add dilute hydrochloric acid (HCl) or dilute sulphuric acid (H2SO4) carefully.
- Observe what happens on the surface of the zinc granules.
- Bring a burning matchstick near the mouth of the test tube. Listen for a sound.
Observations:
Bubbles of gas rise from the surface of the zinc granules. When a burning matchstick is brought near the mouth of the test tube, you hear a pop sound. This confirms the gas is hydrogen (H2).
Chemical equation:
Bubbles of gas rise from the surface of the zinc granules. When a burning matchstick is brought near the mouth of the test tube, you hear a pop sound. This confirms the gas is hydrogen (H2).
Chemical equation:
\(\text{Zn}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{H}_2(g)\uparrow\)
Signs That a Chemical Reaction Has Taken Place
How do we know a chemical reaction has occurred? We can look for one or more of the following changes:
| Sign | Example |
|---|---|
| Change in colour | Blue CuSO4 solution turns green when iron is added |
| Change in state | A solid precipitate forms in a liquid solution |
| Evolution of gas | Bubbles of H2 when zinc reacts with acid |
| Change in temperature | Quicklime + water becomes hot (exothermic) |
1.1 Chemical Equations
A chemical equation is a shorthand way of representing a chemical reaction. Instead of writing out the full names of all substances involved, we use their chemical formulae.
From Words to Symbols
Consider the reaction where magnesium burns in oxygen to form magnesium oxide. We can represent this in different ways:
Word equation:
Magnesium + Oxygen → Magnesium oxide
Chemical equation (using formulae):
\(\text{Mg} + \text{O}_2 \rightarrow \text{MgO}\)
In a chemical equation, the substances on the left side of the arrow are called reactants, and those on the right side are called products. The arrow (→) is read as "gives" or "yields".
Reading a Chemical Equation:
Reactants → Products
"Reactants give/yield Products"
Reactants → Products
"Reactants give/yield Products"
Balancing Chemical Equations
According to the Law of Conservation of Mass, matter cannot be created or destroyed in a chemical reaction. This means the total number of atoms of each element must be the same on both sides of the equation. An equation that satisfies this condition is called a balanced chemical equation.
Look at this unbalanced equation:
\(\text{Mg} + \text{O}_2 \rightarrow \text{MgO}\) (unbalanced)
Left side: 1 Mg, 2 O. Right side: 1 Mg, 1 O. The oxygen atoms are not balanced! We fix this by placing coefficients (whole numbers) in front of the formulae:
\(2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}\) (balanced)
Now: Left side: 2 Mg, 2 O. Right side: 2 Mg, 2 O. All atoms are balanced.
Important Rule: When balancing equations, you can only change the coefficients (the numbers in front of formulae). You must NEVER change the subscripts within a chemical formula, as that would change the substance itself. For example, changing H2O to H2O2 changes water into hydrogen peroxide — a completely different compound!
Step-by-Step Balancing: Hit-and-Trial Method
Let us balance the reaction of iron with steam:
\(\text{Fe} + \text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + \text{H}_2\) (unbalanced)
1Write the unbalanced equation with correct formulae
\(\text{Fe} + \text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + \text{H}_2\)
2Count atoms on each side
| Element | Reactant side | Product side |
|---|---|---|
| Fe | 1 | 3 |
| H | 2 | 2 |
| O | 1 | 4 |
3Start with the compound having the most atoms (Fe3O4). Balance Fe first by placing coefficient 3 before Fe.
\(3\text{Fe} + \text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + \text{H}_2\)
4Balance O: we need 4 O on the left, so place coefficient 4 before H2O. This gives 8 H on the left.
\(3\text{Fe} + 4\text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + \text{H}_2\)
5Balance H: we have 8 H on the left (from 4H2O), so place coefficient 4 before H2 on the right.
\(3\text{Fe} + 4\text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2\)
6Verify: count all atoms on both sides
| Element | Reactant side | Product side | Balanced? |
|---|---|---|---|
| Fe | 3 | 3 | Yes |
| H | 8 | 8 | Yes |
| O | 4 | 4 | Yes |
Writing Symbols for Physical States
To make chemical equations more informative, we include the physical state of each substance using special symbols in parentheses:
| Symbol | Meaning | Example |
|---|---|---|
| (s) | Solid | Fe(s), CaCO3(s) |
| (l) | Liquid | H2O(l) |
| (g) | Gas | O2(g), CO2(g) |
| (aq) | Aqueous (dissolved in water) | NaCl(aq), HCl(aq) |
We also write conditions over or under the arrow: the symbol △ (or "heat") for heating, the name of a catalyst, and so on.
The fully detailed balanced equation for the burning of magnesium therefore becomes:
\(2\text{Mg}(s) + \text{O}_2(g) \xrightarrow{\Delta} 2\text{MgO}(s)\)
And the iron-steam reaction:
\(3\text{Fe}(s) + 4\text{H}_2\text{O}(g) \xrightarrow{\Delta} \text{Fe}_3\text{O}_4(s) + 4\text{H}_2(g)\)
Interactive: Equation Balancer L3 Apply
Balance the following equation by entering the correct coefficients. Leave a box as 1 if no coefficient is needed.
Balance: H2 + O2 → H2O
H2
+
O2
→
H2O
Competency-Based Questions
Ravi performed an experiment in the lab. He took a clean magnesium ribbon, held it with tongs, and burnt it over a watch glass. He then collected the white residue and dissolved it in water. He tested the resulting solution with litmus paper.
Q1. L1 Remember What is the white powder formed when magnesium ribbon burns in air?
Answer: B. Magnesium oxide (MgO). When magnesium burns in air, it reacts with oxygen to form a white powder called magnesium oxide.
Q2. L2 Understand Write the balanced chemical equation for the burning of magnesium ribbon. Include state symbols. (2 marks)
Answer: \(2\text{Mg}(s) + \text{O}_2(g) \xrightarrow{\Delta} 2\text{MgO}(s)\)
Two atoms of solid magnesium react with one molecule of gaseous oxygen to form two formula units of solid magnesium oxide.
Two atoms of solid magnesium react with one molecule of gaseous oxygen to form two formula units of solid magnesium oxide.
Q3. L4 Analyse Why must we balance a chemical equation? What fundamental law does balancing obey? (2 marks)
Answer: A chemical equation must be balanced to satisfy the Law of Conservation of Mass, which states that matter can neither be created nor destroyed during a chemical reaction. The total mass (and hence the total number of atoms of each element) on the reactant side must equal that on the product side.
Q4. L3 Apply Balance the following equation: \(\text{Fe} + \text{Cl}_2 \rightarrow \text{FeCl}_3\). Show your working. (3 marks)
Answer:
Unbalanced: Fe + Cl2 → FeCl3
Fe: 1 on each side (already balanced).
Cl: 2 on left, 3 on right (not balanced).
Place 3 before Cl2 and 2 before FeCl3: Fe + 3Cl2 → 2FeCl3
Now Fe: 1 vs 2 (not balanced). Place 2 before Fe:
Balanced: \(2\text{Fe} + 3\text{Cl}_2 \rightarrow 2\text{FeCl}_3\)
Check: Fe: 2=2; Cl: 6=6. Balanced!
Unbalanced: Fe + Cl2 → FeCl3
Fe: 1 on each side (already balanced).
Cl: 2 on left, 3 on right (not balanced).
Place 3 before Cl2 and 2 before FeCl3: Fe + 3Cl2 → 2FeCl3
Now Fe: 1 vs 2 (not balanced). Place 2 before Fe:
Balanced: \(2\text{Fe} + 3\text{Cl}_2 \rightarrow 2\text{FeCl}_3\)
Check: Fe: 2=2; Cl: 6=6. Balanced!
Q5. L5 Evaluate A student wrote the equation for the formation of water as: H2 + O → H2O and claimed it is balanced. Identify the error and write the correct balanced equation. (3 marks)
Answer: The student's error is writing "O" instead of "O2". Oxygen exists as a diatomic molecule (O2), not as a single atom. The correct formula for oxygen gas is O2.
The correct balanced equation is:
\(2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l)\)
This satisfies the law of conservation of mass: H: 4=4, O: 2=2.
The correct balanced equation is:
\(2\text{H}_2(g) + \text{O}_2(g) \rightarrow 2\text{H}_2\text{O}(l)\)
This satisfies the law of conservation of mass: H: 4=4, O: 2=2.
Assertion-Reason Questions
Assertion (A): In a balanced chemical equation, the number of atoms of each element is the same on both sides.
Reason (R): Chemical reactions follow the Law of Conservation of Mass.
Answer: A. Both are true. The Law of Conservation of Mass (R) requires that mass is conserved, which means the total number of atoms of each element must be equal on both sides (A). The Reason correctly explains the Assertion.
Assertion (A): The symbol (aq) in a chemical equation stands for a substance dissolved in water.
Reason (R): "Aqueous" is derived from the Latin word "aqua" meaning water.
Answer: A. Both are true. The symbol (aq) stands for "aqueous", meaning dissolved in water. The word "aqueous" indeed comes from the Latin "aqua" (water), and this etymology explains why the symbol is used.
Assertion (A): We can change the subscripts in a chemical formula to balance an equation.
Reason (R): Changing subscripts changes the identity of the compound.
Answer: D. The Assertion is false -- we must NEVER change subscripts to balance an equation. The Reason is true: changing subscripts changes the substance (e.g., H2O becomes H2O2). We can only change coefficients.
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