Year 9 Chemical Reactions Comprehensive Study Guide

Fundamental Principles of Chemical Reactions

• Definition of a Chemical Reaction: A chemical reaction is best defined as a process in which atoms rearrange and new substances form. This process involves some existing chemical bonds breaking and new chemical bonds forming. It is distinct from a physical change, such as melting ice, where the substance (water) undergoes a state change but does not form a new substance and the process can be reversed.

• Evidence of Chemical Change: Reliable signs that a chemical reaction has occurred include:

  • Colour change: A visible shift in the hue of the substances.

  • Temperature change: The production of heat (exothermic) or absorption of heat (endothermic), such as a fire warming a person's face.

  • Odour release: The production of a new smell.

  • Precipitation: The formation of an insoluble solid when two liquids or solutions are mixed.

  • Production of light: Such as the bright bursts seen in fireworks or a sparkler.

  • Production of sound: Noises like sizzling or popping indicate gas production or energy release.

  • Gas production: Indicated by bubbles or effervescence.

  • Disappearance of reactants: The visible loss of the starting materials.

Reactants, Products, and Chemical Equations

• Reactant: A starting substance that is used up during a chemical reaction.

• Product: A new substance produced by a chemical reaction.

• General Equation Layout: The standard format for representing a chemical reaction is:     $\text{reactants} \rightarrow \text{products}$

• Word Equation Examples:

  • Reaction of sodium and water: $\text{sodium} + \text{water} \rightarrow \text{sodium hydroxide} + \text{hydrogen}$

  • Thermite-style reaction: $\text{iron oxide} + \text{aluminium} \rightarrow \text{aluminium oxide} + \text{iron}$

  • Synthesis of salt: $\text{sodium} + \text{chlorine} \rightarrow \text{sodium chloride}$

The Law of Conservation of Mass

• Core Principle: Mass cannot be created or destroyed in a chemical reaction. In a closed system, the total mass of the reactants must equal the total mass of the products.

• Calculations:

  • If $12\,g$ of one reactant reacts with $6\,g$ of another, the total product mass is $18\,g$.

  • If a reaction produces $14\,g$ of product $X$ and $9\,g$ of product $Y$, the total starting mass of reactants was $23\,g$.

  • If a $25\,g$ reactant decomposes and produces $7\,g$ of gas, the remaining solid mass is $18\,g$ ($25\,g - 7\,g = 18\,g$).

• Open Systems: If a reaction in an open beaker appears to lose mass, it is usually because a gaseous product has escaped into the atmosphere. The law of conservation of mass still holds because the total mass, including the escaped gas, remains unchanged.

Classification of Reaction Patterns

• Combination Reactions: A process where two or more reactants join together to form a single product. Its general pattern is:     $A + B \rightarrow AB$

  • Example: $H_2 + Cl_2 \rightarrow 2HCl$

  • Example: $2NO + O_2 \rightarrow 2NO_2$

  • Example: $2Mg + O_2 \rightarrow 2MgO$

• Decomposition Reactions: A process where one reactant splits or breaks down into two or more simpler products. Its general pattern is:     $AB \rightarrow A + B$

  • Example: $H_2CO_3 \rightarrow H_2O + CO_2$

  • Example: $2H_2O_2 \rightarrow 2H_2O + O_2$

  • Example: $CaCO_3 \rightarrow CaO + CO_2$

Acid Reactions and Gas Tests

• Reaction Patterns with Acids:

  • Neutralisation: $\text{acid} + \text{base} \rightarrow \text{salt} + \text{water}$

  • Acid + Metal: $\text{acid} + \text{metal} \rightarrow \text{salt} + \text{hydrogen}$

  • Acid + Carbonate: $\text{acid} + \text{carbonate} \rightarrow \text{salt} + \text{water} + \text{carbon dioxide}$

• Practical Gas Identification:

  • Hydrogen ($H_2$): Identified by holding a lit match or burning splint at the mouth of a test tube. A positive result is a "squeaky pop."

  • Carbon Dioxide ($CO_2$): Identified by bubbling the gas through limewater. A positive result is the limewater turning cloudy or milky.

• Salts and Neutralisation:

  • Hydrochloric acid always produces salts ending in "-chloride."

  • Reaction with sodium hydroxide produces sodium chloride.

  • Reaction with magnesium produces magnesium chloride.

  • Safety Application: A wasp sting, which is alkaline, can be treated with vinegar because vinegar is a weak acid that safely neutralises the alkalinity.

Aqueous Chemistry and Precipitation

• Solubility Terms:

  • Soluble: A substance that is able to dissolve in a solvent, such as water.

  • Insoluble: A substance that does not readily dissolve in a solvent.

  • Dissociation: The process where an ionic compound separates into its individual ions when dissolved in water.

• Precipitation Reactions: A reaction where two clear solutions (aqueous) are mixed together to form an insoluble solid product called a precipitate.

  • Cloudiness: The mixture becomes cloudy because the insoluble solid particles are suspended in the liquid.

  • Identification: If sodium hydroxide and silver nitrate (both clear) form a brown solid, the appearance of a new insoluble substance with different properties confirms a chemical reaction occurred.

Redox, Combustion, and Corrosion

• Common Redox Definitions:

  • Oxidation: The gain of oxygen or a reaction with oxygen.

  • Reduction: The loss or removal of oxygen.

  • Redox Pairing: Oxidation and reduction occur together because when one substance gains oxygen, another must supply it. They are paired parts of a single chemical process.

• Combustion: A rapid redox reaction that produces significant heat and light. For example, when methane burns, it is oxidised as it reacts with oxygen to form carbon dioxide and water.

• Corrosion: A slow oxidation process that damages metal surfaces. A common example is the rusting of a bicycle. Unlike combustion, corrosion is slow and results in a discoloured or damaged surface rather than intense heat and flame.

Metal Displacement Reactions

• Definition: A reaction where a more reactive metal replaces a less reactive metal from its compound.

• Example: When iron is placed in a copper sulfate solution ($\text{iron} + \text{copper sulfate} \rightarrow \text{iron sulfate} + \text{copper}$):

  • The iron metal begins to dissolve.

  • Solid copper metal forms.

  • This demonstrates that iron is more reactive than copper.

• Predicting Reactivity: A less reactive metal cannot displace a more reactive metal. For instance, copper will not displace iron from iron sulfate because copper is less reactive than iron.

Chemical Formulas and State Symbols

• State Symbols:

  • $(s)$: Solid

  • $(l)$: Liquid

  • $(g)$: Gas

  • $(aq)$: Aqueous (dissolved in water)

• Equation Components:

  • Arrow ((\rightarrow)): Means "reacts to form" or "produces."

  • Plus Sign (+): Used to separate different substances on the same side of the equation.

• Formula Equation Example:

  • The reaction between lead nitrate solution and potassium iodide solution produces solid lead iodide and potassium nitrate solution:     $Pb(NO_3)_2(aq) + 2KI(aq) \rightarrow PbI_2(s) + 2KNO_3(aq)$

  • In this equation, the $(s)$ symbol on $PbI_2$ identifies it as the precipitate.

Questions & Discussion

• Question: Why doesn't a negative pop test prove no reaction occurred?

• Response: A negative result only proves that hydrogen was not produced. A reaction could have still occurred that produced a different gas or no gas at all (such as a colour change or precipitate formation).

• Question: In a sealed flask containing $11\,g$ of reactant $A$ and $15\,g$ of reactant $B$, if $8\,g$ of product $C$ is present after the reaction, what is the mass of product $D$?

• Response: The starting mass is $11\,g + 15\,g = 26\,g$. According to the law of conservation of mass, the final mass must also be $26\,g$. Therefore, product $D$ must have a mass of $18\,g$ ($26\,g - 8\,g = 18\,g$).