Chemical Reactions and Equations-QnA

Q1: What is a chemical reaction?

A process in which one or more substances are converted into new substances with different properties.

Q2: Give one example of a chemical reaction.

Burning of magnesium ribbon in air to form magnesium oxide.

Q3: What is a balanced chemical equation?

An equation in which the number of atoms of each element is equal on both sides.

Q4: How is a chemical equation balanced?

By adjusting the coefficients to equalize the number of atoms of each element.

Q5: State one observable indication of a chemical reaction.

Change in color.

Q6: Name the gas evolved when zinc reacts with dilute sulphuric acid.

Hydrogen gas.

Q7: What is the chemical formula of quick lime?

CaO (Calcium oxide).

Q8: What type of reaction is photosynthesis?

Endothermic.

Q9: What is a precipitate?

An insoluble solid formed during a chemical reaction.

Q10: What does the arrow \(\rightarrow\) in a chemical equation represent?

The direction of the reaction: reactants forming products.

Q11: Why should a magnesium ribbon be cleaned before burning in air?

Magnesium ribbon is cleaned to remove its surface oxide layer, so that pure magnesium reacts with oxygen to give a bright, correct result.

Q12: A solution of a substance ‘X’ is used for whitewashing. (i) Name the substance ‘X’ and write its formula.

Substance ‘X’ is Calcium Oxide. Its formula is CaO.

Q13: (ii) Write the reaction of the substance ‘X’ named in (i) above with water.

\(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\)

Q1: Why is it necessary to balance a chemical equation?

It ensures the law of conservation of mass is followed. Balancing shows atoms for each element are equal on both sides, reflecting no loss or gain of atoms.

Q2: Describe the difference between a chemical change and a physical change with an example.

Chemical change forms new substances and is usually irreversible (e.g., rusting of iron). Physical change does not form new substances and is often reversible (e.g., melting of ice).

Q3: What happens when lead nitrate reacts with potassium iodide? Observation?

A yellow precipitate of lead iodide forms and potassium nitrate remains in solution. It is a double displacement and precipitation reaction.

Q4: What is meant by a displacement reaction? Give one example.

When a more reactive element displaces a less reactive element from its compound (e.g., \(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\)).

Q5: State two observations when iron filings are added to copper sulphate solution.

The blue color of solution fades (iron sulphate forms), and a brown deposit of copper appears on iron.

Q6: What is oxidation? Give an example.

Addition of oxygen or removal of hydrogen during a reaction (e.g., \(\mathrm{Mg + O_2 \rightarrow 2MgO}\)).

Q7: State the significance of the symbol (aq) in a chemical equation.

It denotes that a substance is dissolved in water, i.e., an aqueous solution.

Q8: What is a combination reaction? Example from the chapter?

Two or more substances combine to form one product. Example: \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\).

Q9: Why do some substances change color during a chemical reaction?

Because new chemical substances with different properties (and colors) are formed.

Q10: What do you observe when silver chloride is left in sunlight?

Silver chloride turns grey due to decomposition into silver metal and chlorine gas.

Q11: Write the balanced equation for: Hydrogen + Chlorine ? Hydrogen chloride

\(\mathrm{H_2 + Cl_2 \rightarrow 2HCl}\)

Q12: Write the balanced equation for: Barium chloride + Aluminium sulphate ? Barium sulphate + Aluminium chloride

\(\mathrm{3BaCl_2 + Al_2(SO_4)_3 \\\\\qquad\rightarrow 3BaSO_4 + 2AlCl_3}\)

Q13: Write the balanced equation for: Sodium + Water ? Sodium hydroxide + Hydrogen

\(\mathrm{2Na + 2H_2O \rightarrow 2NaOH + H_2}\)

Q14: Write a balanced chemical equation with state symbols: Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.

\({BaCl_2(aq) + Na_2SO_4(aq) \\\\\qquad\rightarrow BaSO_4(s) + 2NaCl(aq)}\)

Q15: Write a balanced chemical equation with state symbols: Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

\(\mathrm{NaOH(aq) + HCl(aq) \\\\\qquad\rightarrow NaCl(aq) + H_2O(l)}\)

Q16: Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Iron is more reactive than copper. When an iron nail is dipped in copper sulphate solution, iron displaces copper to form iron sulphate, and copper is deposited. The blue colour of copper sulphate fades as iron sulphate (which is nearly colourless) forms.

Q17: Identify the substances that are oxidised and the substances that are reduced in the following reactions. (i) \(\mathrm{4Na(s) + O_2(g) \rightarrow 2Na_2O(s)}\)

Oxidised: Sodium \(\mathrm{(Na)}\) is oxidised to sodium oxide \(\mathrm{(Na_2O)}\). Reduced: Oxygen \(\mathrm{(O_2)}\) is reduced to oxide ions in \(\mathrm{(Na_2O)}\).

Q18: Identify the substances that are oxidised and the substances that are reduced in the following reactions. (ii) \(\mathrm{CuO(s) + H_2(g) \rightarrow Cu(s) + H_2O(l)}\)

Oxidised: Hydrogen \(\mathrm{(H_2)}\) is oxidised to water \(\mathrm{(H_2O)}\) as it gains oxygen. Reduced: Copper oxide (CuO) is reduced to copper (Cu) as it loses oxygen.

Q1: What are chemical equations? Explain types of chemical reactions with examples from textbook.

Chemical equations represent reactions symbolically.

Combination: \(\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}\);

Decomposition: \(\mathrm{2Pb(NO3)_2 \rightarrow 2PbO + 4NO_2 + O_2}\);

Displacement: \(\mathrm{Zn + CuSO_4 \rightarrow ZnSO_4 + Cu}\);

Double displacement: \(\mathrm{AgNO_3 + NaCl \rightarrow AgCl + NaNO_3}\);

Redox: \(\mathrm{CuO+H_2\xrightarrow{Heat}Cu+H_2O}\)
\(\mathrm{H_2}\) Oxidised to \(\mathrm{H_2O}\) and \(\mathrm{CuO}\) reduced to \(\mathrm{Cu}\)

Equations must be balanced and indicate states.

Q2: Illustrate the law of conservation of mass with an example from the chapter.

Law: mass is neither created nor destroyed in a chemical reaction. E.g., \(\mathrm{Mg + O_2 \rightarrow MgO}\): Total mass before reaction equals total mass after reaction. Both atoms and mass are conserved in balanced equations.

Q3: What are double displacement and precipitation reactions? Give a suitable experiment from textbook.

Double displacement is exchange of ions between two compounds. Precipitation produces insoluble solid. E.g., mixing lead nitrate and potassium iodide forms yellow PbI2 precipitate and potassium nitrate \(\mathrm{(Pb(NO_3)_2 + 2KI \rightarrow PbI_2 + 2KNO_3)}\).

Q4: Explain the role of heat, light, or electricity in decomposition reactions, one example each.

\(\mathrm{CaCO_3 \xrightarrow{Heat} CaO + CO_2}\) (thermal);

\(\mathrm{2AgCl \xrightarrow{Light} 2Ag + Cl_2}\) (photolytic);

\(\mathrm{2H_2O \xrightarrow{Electricity} 2H_2 + O_2 }\) (electrolytic).

Energy is needed to break down compounds in these reactions.

Q5: Discuss the importance of balancing chemical equations. What errors occur if not balanced?

Balancing ensures conservation of mass/atoms; unbalanced equations misrepresent quantities, giving wrong stoichiometry and yields. E.g., \(\mathrm{H_2 + O_2 \rightarrow H_2O}\) is unbalanced; correct: \(\mathrm{2H_2 + O_2 \rightarrow 2H_2O}\). Balancing is vital for scientific accuracy.