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23.Write any two observations in an activity which may suggest that a chemical reaction has taken place. Give an example in support of your answer
Solution:
(i) Change in temperature
When calcium oxide reacts vigorously with water to produce calcium hydroxide a large amount of heat is evolved and thus temperature of the system increases hence it confirms that a chemical reaction has been taken place.
(ii) Gas evolution
When calcium carbonate is heated it decomposes into calcium oxide and carbon dioxide. Here carbon dioxide is a gas and thus confirms a chemical reaction
24. (a) Aluminium hydroxide reacts with sulphuric acid to form aluminium sulphate and water. Write a balanced equation for this reaction.
(b) Balance the following chemical equation:
MnO2 + HCl → MnCl2 + Cl2 + H2O
Solution:
(a) 2Al(OH)3 + 3H2SO4 → Al2(SO4)3 + 6H2O
(b) MnO2 + 4HCl → MnCl2 + Cl2 + 2H2O
25. Write the balanced chemical equations for the following reactions, and add the state symbols:
(a) Magnesium carbonate reacts with hydrochloric acid to produce magnesium chloride, carbon dioxide and water.
(b) Sodium hydroxide reacts with sulphuric acid to produce sodium sulphate and water.
Solution:
(a) MgCO3 (s) + 2HCl (aq) →MgCl2 (aq) + CO2 (g) + H2O (l)
(b) 2NaOH (aq) + H2SO4 (aq) →Na2SO4 (aq) + 2H2O (l)
26.Carbon monoxide reacts with hydrogen under certain conditions to form methanol (CH3OH). Write a balanced chemical equation for this reaction indicating the physical states of reactants and product as well as the conditions under which this reaction takes place.
Solution:
CO(g) + 2H2(g) → CH3OH
Pressure: 300atm
Temperature: 300℃
Catalyst: zinc oxide and chromium oxide
27.(a) Potassium chlorate (KClO3) on heating forms potassium chloride and oxygen. Write a balanced equation for this reaction and indicate the evolution of gas. (b) Rewrite the following information in the form of a balanced chemical equation:
Magnesium burns in carbon dioxide to form magnesium oxide and carbon.
Solution:
(a) 2KClO3 (s) → 2KCl (s) + 3O2 (g)
(b) 2Mg + CO2 → 2MgO + C
28. (a) Substitute formulae for names and balance the following equations:
Calcium carbonate reacts with hydrochloric acid to produce calcium chloride, water and carbon dioxide gas.
(b) Write balanced chemical equation with state symbols for the following reaction:
Sodium hydroxide solution reacts with hydrochloric acid solution to produce sodium chloride solution and water.
Solutions:
(a) CaCO3 + 2HCl → CaCl2 + H2O + CO2
(b) NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l)
29.Ammonia reacts with oxygen to form nitrogen and water. Write a balanced chemical equation for this reaction. Add the state symbols for all the reactants and products.
Solution:
4NH3 (g) + 3O2 (g) → 2N2 (g) + 6H2O (l)
30. Write a balanced chemical equation for the process of photosynthesis giving the physical states of all the substances involved and the conditions of the reaction.
Solution:
6CO2 (g) + 6H2O (l) → C6H12O6 (aq.) + 6O2 (g) in the presence of sunlight
31.Translate the following statement into chemical equation and then balance it:
‘Barium chloride solution reacts with aluminium sulphate solution to form a precipitate of barium sulphate and aluminium chloride solution’.
Solution:
3BaCl2 (aq) + Al2(SO4)3 (aq) → 3BaSO4 (s) + 2AlCl3 (aq)
32.When potassium nitrate is heated, it decomposes into potassium nitrite and oxygen. Write a balanced equation for this reaction and add the state symbols of the reactants and products.
Solution:
2KNO3 (s)→ 2KNO2 (s) + O2 (g)
Long answer type questions
33.(a) What is meant by a chemical reaction? Explain with the help of an example.
(b) Give one example each of a chemical reaction characterised by:
(i) Evolution of a gas
(ii) Change in colour
(iii) Formation of a precipitate
(iv) Change in temperature
(v) Change in state.
Solutions:
(a) A chemical reaction results in new substances by undergoing certain reactions and rearrangements. There will be reactants on the left hand side and products on the right hand side.
An example of combustion is methane + oxygen forms carbon dioxide and water. This can be written as a balanced symbol equation:
CH4 + 2O2 →CO2 + 2H2O
(b) (i) Evolution of gas
Reaction of Zinc and Hydrochloric acid
Zn + 2HCl → ZnCl2 + H2(g)
Hydrogen gas is released in the form of diatomic molecule
(ii) Change in colour
Cu(s) + O2(g) + H2O → Cu(OH)2(s) + O2(g)
Copper is in reddish brown colour and when reacts with oxygen and water it forms hydrated copper carbonate which is green in colour
(iii) Formation of precipitate
AgNO3(aq) + KCl(aq) →AgCl(s) + KNO3
When silver nitrate and potassium chloride in its aqueous form reacts a precipitate of potassium nitrate is obtained
(iv) Change in temperature
CaO + H20 →Ca(OH)2
When Quick lime is added to water to form slaked lime a change in temperature is obtained as it reacts vigorously and produce heat which is an exothermic reaction
(v) Change in state
NH3(g) + HCl(g) → NH4Cl(s)
Ammonia and Hydrogen chloride gas reacts to form Ammonium chloride which is a solid