Chemical Reactions and Equations – Lecture Review

These flashcards cover definitions, examples, equations, observations, and key principles from the chapter on Chemical Reactions and Equations, including balancing equations, reaction types (combination, decomposition, displacement, double displacement, redox), energy changes, corrosion, and rancidity.

What general change happens to the nature and identity of a substance during a chemical reaction?

The original substance changes into one or more new substances with different properties.

List four common observations that indicate a chemical reaction has taken place.

Change in state, change in colour, evolution of a gas, change in temperature.

In a word-equation, on which side are reactants written and what symbol separates them from products?

Reactants are written on the left-hand side and are separated from products by an arrow (→).

Give the word-equation for burning magnesium in air.

Magnesium + Oxygen → Magnesium oxide.

How does a skeletal chemical equation differ from a balanced chemical equation?

A skeletal equation shows correct formulas but unequal numbers of atoms; a balanced equation has equal numbers of each atom on both sides.

State the law used to justify balancing chemical equations.

Law of conservation of mass – mass is neither created nor destroyed in a chemical reaction.

What is the balanced chemical equation for the reaction of zinc with dilute sulphuric acid?

Zn + H₂SO₄ → ZnSO₄ + H₂.

In balancing equations, why can coefficients be changed but subscripts within formulas cannot?

Changing coefficients adjusts the number of whole molecules without altering the identity of the substances; changing subscripts would create new substances.

Write the fully balanced chemical equation (with states) for the reaction: Fe + H₂O (steam) → Fe₃O₄ + H₂.

3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g).

What notation is used to indicate an aqueous solution in a chemical equation?

(aq)

Define a combination (synthesis) reaction.

A reaction in which two or more reactants combine to form a single product.

Give the balanced equation for the formation of slaked lime from quicklime.

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat.

Why is the quicklime–water reaction considered exothermic?

Because it releases a large amount of heat as the product forms.

Write the balanced equation for burning of methane.

CH₄ + 2O₂ → CO₂ + 2H₂O + heat.

State why respiration is classified as an exothermic reaction.

It releases energy when glucose reacts with oxygen in cells.

Define a decomposition reaction.

A reaction in which a single reactant breaks down into two or more products.

What type of energy supply drives thermal decomposition?

Heat.

Write the thermal decomposition equation of calcium carbonate.

CaCO₃(s) → CaO(s) + CO₂(g).

What colour change is observed when ferrous sulphate is thermally decomposed?

Green ferrous sulphate crystals turn brown due to formation of ferric oxide.

Give the balanced equation for electrolysis of water (include states).

2H₂O(l) → 2H₂(g) + O₂(g).

During electrolysis of water, why is twice as much hydrogen collected as oxygen?

Because water contains two hydrogen atoms for every oxygen atom.

Define a photolytic decomposition reaction and give an example.

Decomposition initiated by light; e.g., 2AgCl(s) → 2Ag(s) + Cl₂(g) under sunlight.

What is a displacement reaction?

A reaction in which a more reactive element displaces a less reactive element from its compound.

Write the displacement reaction between iron and copper(II) sulphate.

Fe + CuSO₄ → FeSO₄ + Cu.

Explain why the blue colour of CuSO₄ fades when an iron nail is added.

Iron displaces copper, forming colourless/greenish FeSO₄ and depositing brown copper metal.

Define a double-displacement (precipitation) reaction.

A reaction where two compounds exchange ions to form two new compounds, often producing a precipitate.

Give the balanced equation for mixing aqueous solutions of sodium sulphate and barium chloride.

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq).

What is a precipitate?

An insoluble solid that forms in, and separates from, a solution during a reaction.

Define oxidation in terms of oxygen transfer.

Gain of oxygen or loss of hydrogen by a substance.

Define reduction in terms of oxygen transfer.

Loss of oxygen or gain of hydrogen by a substance.

Why is the reaction CuO + H₂ → Cu + H₂O a redox reaction?

CuO is reduced to Cu (loses O), while H₂ is oxidised to H₂O (gains O).

Identify the oxidising and reducing agents in the reaction: ZnO + C → Zn + CO.

ZnO is the oxidising agent (it oxidises C), and C is the reducing agent (it reduces ZnO).

What is corrosion?

The gradual destruction of metals by chemical reaction with substances in the environment (e.g., moisture, air).

Name the reddish brown product that forms when iron corrodes.

Hydrated iron(III) oxide, commonly called rust.

What term describes the spoiling of fats and oils due to oxidation?

Rancidity.

How do food manufacturers slow down rancidity in packaged chips?

By flushing the packets with nitrogen gas to limit oxygen exposure.

State the difference between exothermic and endothermic reactions.

Exothermic reactions release heat whereas endothermic reactions absorb heat from the surroundings.

Provide one example each of exothermic and endothermic reactions from the chapter.

Exothermic: combustion of methane; Endothermic: thermal decomposition of CaCO₃.

What is meant by balancing a chemical equation?

Adjusting coefficients so that the number of atoms of each element is equal on both sides of the equation.

During balancing, why is it convenient to start with the compound containing the maximum number of atoms?

Because changing its coefficient affects the counts of several atoms at once, simplifying subsequent steps.

Write the balanced equation for the thermite reaction: Fe₂O₃ + Al → Al₂O₃ + Fe.

Fe₂O₃ + 2Al → Al₂O₃ + 2Fe (displacement; highly exothermic).

Give an example of a double-displacement reaction that forms a coloured precipitate other than BaSO₄.

Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) (yellow precipitate) + 2KNO₃(aq).

Why must magnesium ribbon be cleaned before burning?

To remove the protective layer of magnesium oxide so it burns readily.

State one industrial use of quicklime produced by decomposition of limestone.

Quicklime is used in cement manufacture.

What role do catalysts, temperature or pressure labels above/below an arrow play in equations?

They indicate specific reaction conditions required for the reaction to proceed.

Which gas is usually evolved when dilute acids react with active metals like Zn?

Hydrogen gas (H₂).

Explain ‘skeletal equation’ using Mg + O₂ → MgO as an example.

It shows reactants and products in formulas but is not balanced; Mg + O₂ → MgO has unequal O atoms.

What is indicated by the symbol (g) next to H₂O in 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)?

Water is present as steam (gaseous state) in the reaction.

Why are antioxidants added to food items containing oil?

To delay oxidation and prevent rancidity, thereby extending shelf life.

What general change happens to the nature and identity of a substance during a chemical reaction?

The original substance changes into one or more new substances with different properties.

List four common observations that indicate a chemical reaction has taken place?

Change in state, change in colour, evolution of a gas, change in temperature.

In a word-equation, on which side are reactants written and what symbol separates them from products?

Reactants are written on the left-hand side and are separated from products by an arrow (→).

Give the word-equation for burning magnesium in air.

Magnesium + Oxygen → Magnesium oxide.

How does a skeletal chemical equation differ from a balanced chemical equation?

A skeletal equation shows correct formulas but unequal numbers of atoms; a balanced equation has equal numbers of each atom on both sides.

State the law used to justify balancing chemical equations.

Law of conservation of mass – mass is neither created nor destroyed in a chemical reaction.

What is the balanced chemical equation for the reaction of zinc with dilute sulphuric acid?

Zn + H₂SO₄ → ZnSO₄ + H₂.

In balancing equations, why can coefficients be changed but subscripts within formulas cannot?

Changing coefficients adjusts the number of whole molecules without altering the identity of the substances; changing subscripts would create new substances.

Write the fully balanced chemical equation (with states) for the reaction: Fe + H₂O (steam) → Fe₃O₄ + H₂.

3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g).

What notation is used to indicate an aqueous solution in a chemical equation?

(aq)

Define a combination (synthesis) reaction.

A reaction in which two or more reactants combine to form a single product.

Give the balanced equation for the formation of slaked lime from quicklime.

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + heat.

Why is the quicklime–water reaction considered exothermic?

Because it releases a large amount of heat as the product forms.

Write the balanced equation for burning of methane.

CH₄ + 2O₂ → CO₂ + 2H₂O + heat.

State why respiration is classified as an exothermic reaction.

It releases energy when glucose reacts with oxygen in cells.

Define a decomposition reaction.

A reaction in which a single reactant breaks down into two or more products.

What type of energy supply drives thermal decomposition?

Heat.

Write the thermal decomposition equation of calcium carbonate.

CaCO₃(s) → CaO(s) + CO₂(g).

What colour change is observed when ferrous sulphate is thermally decomposed?

Green ferrous sulphate crystals turn brown due to formation of ferric oxide.

Give the balanced equation for electrolysis of water (include states).

2H₂O(l) → 2H₂(g) + O₂(g).

During electrolysis of water, why is twice as much hydrogen collected as oxygen?

Because water contains two hydrogen atoms for every oxygen atom.

Define a photolytic decomposition reaction and give an example.

Decomposition initiated by light; e.g., 2AgCl(s) → 2Ag(s) + Cl₂(g) under sunlight.

What is a displacement reaction?

A reaction in which a more reactive element displaces a less reactive element from its compound.

Write the displacement reaction between iron and copper(II) sulphate.

Fe + CuSO₄ → FeSO₄ + Cu.

Explain why the blue colour of CuSO₄ fades when an iron nail is added.

Iron displaces copper, forming colourless/greenish FeSO₄ and depositing brown copper metal.

Define a double-displacement (precipitation) reaction.

A reaction where two compounds exchange ions to form two new compounds, often producing a precipitate.

Give the balanced equation for mixing aqueous solutions of sodium sulphate and barium chloride.

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq).

What is a precipitate?

An insoluble solid that forms in, and separates from, a solution during a reaction.

Define oxidation in terms of oxygen transfer.

Gain of oxygen or loss of hydrogen by a substance.

Define reduction in terms of oxygen transfer.

Loss of oxygen or gain of hydrogen by a substance.

Why is the reaction CuO + H₂ → Cu + H₂O a redox reaction?

CuO is reduced to Cu (loses O), while H₂ is oxidised to H₂O (gains O).

Identify the oxidising and reducing agents in the reaction: ZnO + C → Zn + CO.

ZnO is the oxidising agent (it oxidises C), and C is the reducing agent (it reduces ZnO).

What is corrosion?

The gradual destruction of metals by chemical reaction with substances in the environment (e.g., moisture, air).

Name the reddish brown product that forms when iron corrodes.

Hydrated iron(III) oxide, commonly called rust.

What term describes the spoiling of fats and oils due to oxidation?

Rancidity.

How do food manufacturers slow down rancidity in packaged chips?

By flushing the packets with nitrogen gas to limit oxygen exposure.

State the difference between exothermic and endothermic reactions.

Exothermic reactions release heat whereas endothermic reactions absorb heat from the surroundings.

Provide one example each of exothermic and endothermic reactions from the chapter.

Exothermic: combustion of methane; Endothermic: thermal decomposition of CaCO₃.

What is meant by balancing a chemical equation?

Adjusting coefficients so that the number of atoms of each element is equal on both sides of the equation.

During balancing, why is it convenient to start with the compound containing the maximum number of atoms?

Because changing its coefficient affects the counts of several atoms at once, simplifying subsequent steps.

Write the balanced equation for the thermite reaction: Fe₂O₃ + Al → Al₂O₃ + Fe.

Fe₂O₃ + 2Al → Al₂O₃ + 2Fe (displacement; highly exothermic).

Give an example of a double-displacement reaction that forms a coloured precipitate other than BaSO₄.

Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) (yellow precipitate) + 2KNO₃(aq).

Why must magnesium ribbon be cleaned before burning?

To remove the protective layer of magnesium oxide so it burns readily.

State one industrial use of quicklime produced by decomposition of limestone.

Quicklime is used in cement manufacture.

What role do catalysts, temperature or pressure labels above/below an arrow play in equations?

They indicate specific reaction conditions required for the reaction to proceed.

Which gas is usually evolved when dilute acids react with active metals like Zn?

Hydrogen gas (H₂).

Explain ‘skeletal equation’ using Mg + O₂ → MgO as an example.

It shows reactants and products in formulas but is not balanced; Mg + O₂ → MgO has unequal O atoms.

What is indicated by the symbol (g) next to H₂O in 3Fe(s) + 4H₂O

Water is present as steam (gaseous state) in the reaction.