Chemical Reactions

Question 1

What is the general formula for the reaction between

Alkali metal and water

All the alkali metals react strongly with water.

Answer 1

A + H₂O→ AOH + H₂

alkali metal + water → metal hydroxide + hydrogen

(A represents the alkali metal.)

• 2Na + 2H₂O → 2NaOH + H₂
  sodium + water → sodium hydroxide + hydrogen
• 2K + 2H₂O → 2KOH + H₂
  potassium + water → potassium hydroxide + hydrogen

Question 2

What is the general formula for a

Synthesis reaction

Synthesis reactions occur where two or more substances react together to form just one product. Most synthesis reactions are exothermic.

Answer 2

A + B→ AB

(A and B represent two elements.)

• 2H₂ + O₂ → 2H₂O
  hydrogen + oxygen → water
• 2Na + Cl₂ → 2NaCl
  sodium + chlorine → sodium chloride

Question 3

What is the general formula for a

Decomposition reaction

Decomposition reactions have just one reactant, which breaks down to give two or more simpler products. All decomposition reactions are endothermic.

Answer 3

AB → A + B

(A and B represent two elements.)

• NaOH → Na₂O + H₂O
  sodium hydroxide → sodium oxide + water
• H₂CO₃ → CO₂ + H₂O
  carbonic acid → carbon dioxide + water

Question 4

What is the general formula for a

Neutralisation reaction

Neutralisation reactions occur when acids react with bases or alkalis. Their acidity is destroyed, they are neutralised and a salt.

Answer 4

HA + BOH→AB + H₂O
acid + base → salt + water

(HA represents an acid. BOH represents an base. AB is a salt.)

• HCl + NaOH → NaCl + H₂O
  hydrochloric acid + sodium hydroxide
• H₂SO₄ + CuO → CuSO₄ + H₂O
  sulfuric acid + copper(II) oxide → copper(II) sulfate + water
  Note that copper(II) oxide is a weak base when dissolved in water.

Question 5

What is the general formula for the reaction between

Acid and metal

This reaction is violent and dangerous with the very reactive metals such as sodium or calcium. No reaction occurs with metals which are less reactive than lead.

Answer 5

HA + B → AB + H₂
acid + metal → salt + hydrogen

(HA represents an acid. B represents a metal. AB is a salt.)

• Mg + 2HNO₃ → Mg(NO₃)₂ + H₂
  magnesium + nitric acid → magnesium nitrate + hydrogen
• Zn + 2HCl → ZnCl₂ + H₂
  zinc + hydrochloric acid → zinc chloride + hydrogen

Question 6

What is the general formula for the reaction between

Acid and metal carbonate

All acids react with carbonates in similar ways.

Answer 6

HA + BCO₃ → AB + H₂O + CO₂
acid + metal carbonate → salt + water + carbon dioxide

(HA is an acid. BCO₃ is a metal carbonate. AB is a salt.)

• 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂
  hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide
• 2HNO₃ + Na₂CO₃ → 2NaNO₃ + H₂O + CO₂
  nitric acid + sodium carbonate → sodium nitrate + water + carbon dioxide

Question 7

What is the general formula for a

Precipitation reaction

Precipitation reactions involve the formation of an insoluble solid product.

Answer 7

AX + BY → AY + BX

(A, B, X and Y are all ions in an aqueous solution.)

• AgNO₃ + KCl → AgCl + KNO₃
  silver nitrate + potassium chloride → silver chloride + potassium nitrate
• 2KOH + CaCl₂ → Ca(OH)₂ + 2KCl
  potassium hydroxide + calcium chloride → calcium hydroxide + potassium chloride

Question 8

What is the general formula for a

Displacement reaction

A displacement reaction occurs because a more reactive element will displace a less reactive one from a solution of one of its compounds.

Answer 8

A + BC → AC + B

(A, B and C are three elements. A is more reactive than B.)

• Cl₂ + 2KI → 2KCl + I₂
  chlorine + potassium iodide → potassium chloride + iodine
• Zn + CuSO₄ → ZnSO₄ + Cu
  zinc + copper(II) sulfate → zinc sulfate + copper(II)

Question 9

What is the general formula for a

Combustion reaction

(For carbon-based fuels.)

This is the reaction of a substance with oxygen causing the release of energy; it is exothermic and often involves a flame.
Note that when dissolved in water, the product of a combustion reaction is acidic if the fuel was a non-metal, and alkaline if the fuel was a metal. See pages 272 and 273 in textbook for more information and exceptions.

Answer 9

A + O₂ → CO₂ + H₂O
fuel + oxygen → carbon dioxide + water

(A represents a carbon-based fuel.)

• CH₄ + 2O₂ → CO₂ + 2H₂O
  methane + oxygen → carbon dioxide + water
• C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
  glucose + oxygen → carbon dioxide + water