Chemical Changes and Reactions Quiz

Chemical Changes and Reactions

• A chemical reaction involves breaking bonds in reactants and forming new bonds in products.
• A chemical bond holds atoms together in a molecule or compound.

Conditions Necessary for a Chemical Change

• Close Physical Contact (Mixing): Reactions can occur when solids are mixed, such as iodine and sulfur exploding when mixed.
• Solution: Reactions can also occur when substances are mixed in solution, like sodium carbonate and tartaric acid.
• Heat: Some reactions require heat to proceed. For example, copper(II) carbonate decomposes into copper(II) oxide and carbon dioxide when heated.
• Light: Photochemical reactions occur with light energy. Photosynthesis is an example, where carbon dioxide and water react to form glucose and oxygen with sunlight.
• Electricity: Some reactions happen only when electric current passes through the substance. Water decomposes into hydrogen and oxygen gas when electrolyzed.
• Pressure: High pressure is sometimes required for a reaction to occur.
• Catalyst: A catalyst speeds up or slows down a reaction without being consumed. A catalyst works by changing the reaction rate.

Characteristics of a Chemical Reaction

• Evolution of Gas: A gas is produced as a product in some reactions.
• Change of Colour: A change in colour might indicate a reaction.
• Formation of Precipitate: An insoluble solid that forms is called a precipitate.
• Change of State: A change in the phase of matter (solid, liquid or gas) from reactant to product

Types of Chemical Changes/Reactions (Direct Combination/Synthesis)

• Forming a single product from two or more reactants
  • Combination of two elements: Two iron atoms and one sulfur atom combine to form iron sulfide.
  • Combination of an element and compound: Two carbon monoxide molecules and one oxygen molecule combine to form two carbon dioxide molecules.
  • Combination of two or more compounds: Lead(II) oxide and sulfur dioxide react to form lead(II) sulfate.

Types of Chemical Changes/Reactions (Decomposition)

• A single compound breaks down into two or more simpler substances.
  • Decomposition by heat: Mercuric oxide breaks down into mercury and oxygen gas when heated.
  • Decomposition by electricity: Water breaks down into hydrogen and oxygen gas when an electric current is passed through it.

Thermal decomposition of metal compounds

• Metal hydroxides are generally stable to heat but decompose when heated to a given temperature to give metal oxide and water.
• Metal carbonates decompose when heated to form metal oxide and carbon dioxide.

Types of Chemical Changes/Reactions (Other types)

• Metal bicarbonates/hydrogen carbonates: These decompose to give metal carbonate, water, and carbon dioxide.
• Metal Nitrates: When heated, these decompose, forming a different metal oxide, nitrogen dioxide, and often some oxygen.

Reversible Reactions

• A reversible reaction can proceed in either direction, and the conditions can change the direction.

Displacement Reactions

• A more reactive element displaces a less reactive element in a compound.
• Zinc displaces copper in copper sulfate.

Double Displacement Reactions

• A reaction where ions from two compounds exchange places to form two new compounds.
  • Sodium hydroxide and hydrochloric acid produce sodium chloride and water.

Double Decomposition/Metathesis

• A type of reaction where two compounds in a solution react to form two new compounds by the exchange of radicals. These reactions might form a precipitate.

Neutralisation Reactions

• A reaction between an acid and a base that produces a salt and water.
• NaOH + HCl → NaCl + H2O(This is a neutralization reaction).

Hydrolysis

• A reaction where a salt reacts with water, forming an acidic or basic solution. Different combinations of solutions will form solutions with different pH levels.

Energy Changes in Chemical Reactions

• Exothermic: Reactions that release heat.
• Endothermic: Reactions that absorb heat.

Photochemical Reactions

• Reactions that proceed with light energy absorption.

Electrochemical Reactions

• Reactions that proceed with the absorption of electricity.