Chapter 11: Chemical Changes — Comprehensive Notes
Section A: Choose the correct answer
1. Which one of the following representation of a chemical change is INCORRECT?
A
B
C
D
(Note: The transcript lists options A–D but does not specify which diagram/representation is incorrect.)
2. Which of the following is NOT a balanced chemical equation?
A 5\mathrm{C} + 2\mathrm{SO2} \rightarrow \mathrm{CS2} + 4\mathrm{CO}
B \mathrm{C2H4} + 3\mathrm{O2} \rightarrow 2\mathrm{H2O} + 2\mathrm{CO_2}
C 2\mathrm{ZnS} + 3\mathrm{O2} \rightarrow 2\mathrm{ZnO} + 2\mathrm{SO2}
D 2\mathrm{CuSO4} + \mathrm{Na2CO3} \rightarrow \mathrm{Na2SO4} + 2\mathrm{CuCO3}
(Note: The prompt asks which is NOT balanced; no explicit correct option is provided in the transcript.)
3. Which of the following substances reacts with an acid to produce a gas that extinguishes a lighted splint with a ‘pop’ sound?
A copper carbonate
B oxygen gas
C magnesium ribbon
D sodium hydroxide
(Answer: C – magnesium ribbon)
4. In which reaction is carbon dioxide NOT produced?
A complete combustion of carbon-containing fuel
B hydrochloric acid reacting with sodium carbonate
C mixing magnesium with sulfuric acid
D thermal decomposition of copper(II) carbonate
(Answer: C)
5. The word equation shows a chemical reaction. substance S + sulfuric acid → carbon dioxide + salt + water. What could substance S be?
A calcium carbonate
B iron
C magnesium
D sodium hydroxide
(Answer choices provided; the correct option would be calcium carbonate to release CO₂.)
Section B: Short answer and classification
1. State whether each of the following changes is a physical or a chemical change.
A piece of platinum wire becomes red-hot when held in a Bunsen flame, but returns to its original silver colour on cooling.
Answer: Physical (temporary change in colour/temperature; the metal returns to original state after cooling)
A silvery piece of magnesium ribbon burns with a dazzling white light to form a white ash.
Answer: Chemical (combustion forming magnesium oxide as a new substance)
An iron nail forms a layer of reddish-brown flaky substance on the surface when left in a damp place.
Answer: Chemical (formation of rust, Fe2O3·nH_2O; a new substance forms)
When copper(II) sulfate crystals are stirred in water, the crystals dissolve forming a blue solution.
Answer: Physical (dissolution; no new substance formed; the blue solution is still CuSO4 in solution)
A white solid formed when a salt solution is heated to dryness.
Answer: Physical (evaporation/crystallization; solid remains same substance—assuming no chemical change is implied)
2. Name the type of chemical change that has taken place in each of the following changes. (C and D represent two items in a table that asks for the name of the process.)
Change (C): Name of process:
Change (D): Name of process:
Section C: Mass change during a reaction
3. When 25 g of substance A was added to 55 g of substance B in a conical flask, effervescence was observed with lots of gas bubbles released. At the end of the reaction the mass of the substances remaining in the flask is 70 g. Explain why the mass of the products is not the same as the mass of the reactants.
Answer: There was a loss of 10 g mass due to the escape of gas into the atmosphere (mass is conserved if all products are weighed; if gases leave, the measured mass of the system decreases). In this example, reactants: 25 g + 55 g = 80 g; products: 70 g (gas mass left the system during the reaction).
Section D: Acids, bases, and neutralisation
4. Stomach acid neutralisation tablet (calcium carbonate + magnesium hydroxide) in excess HCl
(a) Name the two types of salts produced when the tablet reacts with hydrochloric acid in the stomach.
Answer: calcium chloride and magnesium chloride
(b) Write a word equation to represent the reaction of hydrochloric acid with calcium carbonate.
Word equation: hydrochloric acid + calcium carbonate → calcium chloride + carbon dioxide + water
(c) Name the reaction that occurred when hydrochloric acid reacts with magnesium hydroxide.
Answer: neutralisation
5. When magnesium is added to hydrochloric acid, a salt and a gas are produced.
(a) Name the gas formed.
Answer: hydrogen
(b) Describe how you would test and identify the gas.
Test: place a lighted splint at the mouth of the test tube; if the flame extinguishes with a ‘pop’ sound, it is hydrogen gas.
(c) Write a word equation for the reaction.
Word equation: magnesium + hydrochloric acid → magnesium chloride + hydrogen
(d) Write a balanced chemical equation for the reaction.
Balanced: ext{Mg} + 2\mathrm{HCl} \rightarrow \mathrm{MgCl2} + \mathrm{H2}
Section E: Acid–base reactions and gas tests
6. Hydrogen chloride gas dissolves in water to form an acidic solution. (a) Explain why hydrogen chloride gas forms an acidic solution when dissolved in water.
Explanation: Hydrogen chloride molecules react with water to form hydronium ions: \mathrm{HCl + H2O \rightarrow H3O^+ + Cl^-}. The presence of \(H_3O^+\) gives the solution its acidic property.
(b) Describe how sodium carbonate can be used to confirm that the aqueous solution is acidic.
If the solution is acidic, adding sodium carbonate will release carbon dioxide gas: \mathrm{Na2CO3 + 2H^+ \rightarrow 2\,Na^+ + CO2 + H2O} and the gas will form a white precipitate with limewater.
(c) Write a word equation for the reaction in (b).
Word equation: sodium carbonate + hydrochloric acid → sodium chloride + carbon dioxide + water
(d) Write a balanced chemical equation for the reaction in (b).
Balanced: \mathrm{Na2CO3 + 2HCl \rightarrow 2NaCl + CO2 + H2O}
Section F: Extraction of iron from limestone (calcium carbonate)
7. (a) Limestone (calcium carbonate) is used in iron extraction. When heated, it breaks down into calcium oxide and carbon dioxide.
(i) Write a word equation to represent the reaction.
Word equation: calcium carbonate → calcium oxide + carbon dioxide
(ii) State the type of chemical change that has occurred.
Answer: thermal decomposition
(b) The calcium oxide formed from heating limestone reacts with silica (an impurity) to form calcium silicate.
(i) Write a word equation to represent the reaction.
Word equation: calcium oxide + silica → calcium silicate
(ii) The chemical equation for the reaction is given below. Balance the equation.
Provided skeleton: _32\u00a0CaO + _96\u00a0SiO2 \rightarrow _32\u00a0Ca2SiO_4
Balanced form: 2\mathrm{CaO} + \mathrm{SiO2} \rightarrow \mathrm{Ca2SiO_4}
Section G: Optional SEC – Electronic configurations and dot-and-cross diagrams
(Optional for SEC) Write the electronic configurations of the atoms of the compounds below and that of the nearest noble gas or ion. Hence draw the dot-and-cross diagrams of the following substances. Only valence electrons needed.
(a) Hydrogen gas
Atom: Electronic configuration of atom: H = 1s^1
Nearest noble gas: He = 1s^2 (noble gas configuration is 2 electrons in 1s)
(b) Oxygen gas
Atom: O has valence electrons 2,6 (overall 8 electrons in neutral O)
Nearest noble gas: Ne = 2,8
(c) Nitrogen gas
Atom: N has 2,5
Nearest noble gas: Ne = 2,8
(d) Potassium chloride (KCl)
K: Atom configuration 2,8,8,1; Cl: 2,8,7; Ion formed: K^+ (2,8,8); Cl^- (2,8,8)
(e) Calcium oxide (CaO)
Ca atom: 2,8,8,2; O: 2,6; Ions formed: Ca^{2+} (2,8,8); O^{2-} (2,8)
(f) Aluminium fluoride (AlF_3)
Al atom: 2,8,3; F: 2,7; Ions formed: Al^{3+} (2,8,8); F^{-} (2,8,7)
Key: The notes show how to map valence electron configurations to ionic charges and to construct dot-and-cross diagrams. The provided data includes various electron counts for the atoms and their nearest noble gas configurations. (Due to formatting, some rows are presented in compact form; the essential idea is to identify valence electrons and how they are transferred/shared in compounds.)
Connections and key concepts (summary)
Chemical changes involve the formation of new substances with different properties, as seen in combustion, neutralisation, precipitation, gas evolution, and decomposition.
Physical changes may involve phase changes or dissolving, without formation of new substances (e.g., dissolving copper sulfate crystals in water).
Gas evolution and flame tests: hydrogen gas can be tested with a squeaky pop when exposed to a burning splint.
Mass conservation in reactions: total mass before and after can differ in a closed system if gases escape; mass accounting must consider gaseous products leaving the system.
Indicators and pH: different indicators change color at different pH thresholds; used to determine acidity/basicity and end-points in reactions.
Neutralisation: acid + base (or acid + carbonate) forms a salt + water (and CO_2 when carbonate is involved).
Acid in the stomach and antacids: reactions between acids and carbonate/hydroxide bases produce salts, CO_2, and water.
Extraction of iron from ore: limestone (CaCO3) decomposes to CaO + CO2; CaO then reacts with silica to form calcium silicate; balancing of the final equation yields the appropriate coefficients.
Electron configurations and ionic/covalent bonding: dot-and-cross diagrams illustrate valence electron transfer or sharing; knowledge of noble gas configurations helps in predicting ion formation and compound structure.
Formulas and equations (recap in LaTeX)
Combustion and redox-like equations (examples from Section A):
5\mathrm{C} + 2\mathrm{SO2} \rightarrow \mathrm{CS2} + 4\mathrm{CO}
\mathrm{C2H4} + 3\mathrm{O2} \rightarrow 2\mathrm{H2O} + 2\mathrm{CO_2}
2\mathrm{ZnS} + 3\mathrm{O2} \rightarrow 2\mathrm{ZnO} + 2\mathrm{SO2}
2\mathrm{CuSO4} + \mathrm{Na2CO3} \rightarrow \mathrm{Na2SO4} + 2\mathrm{CuCO3}
Neutralisation and acid/base reactions:
Word: hydrochloric acid + calcium carbonate → calcium chloride + carbon dioxide + water
Balanced: \mathrm{Na2CO3 + 2HCl \rightarrow 2NaCl + CO2 + H2O}
Hydration/acid dissociation:
\mathrm{HCl + H2O \rightarrow H3O^+ + Cl^-}
Ionic equations for salts in reactions:
\mathrm{Mg + 2HCl \rightarrow MgCl2 + H2}
Quick practice prompts (for exam prep)
Identify which reaction releases CO_2 in a chosen list.
Balance equations for limestone decomposition and the subsequent reaction with silica to form calcium silicate.
Explain why an open-system reaction may show a decrease in mass despite total mass conservation in a closed system.
Draw dot-and-cross diagrams to illustrate electron transfer in ions like Na^+, Cl^-, Ca^{2+}, O^{2-}, and in compounds like KCl, CaO, and AlF_3.