Unit 8 - chem reactions notes

Chemical Reactions

• Definition: A chemical reaction involves a change in composition of substances.
    - Chemical Equation: Represents a chemical reaction using symbols and formulas.
    - Reactants: The original substance(s) undergoing change.
    - Products: The new substance(s) created after the reaction.
    - Example: A silver spoon tarnishes. The silver (Ag) reacts with sulfur (S) in the air to produce silver sulfide (Ag2S), the black material known as tarnish.
      - Chemical Equation: $2 ext{Ag} + ext{S} <br>ightarrow ext{Ag}_2 ext{S}$

Understanding Chemical Equations

• Formation of Equations:
    - Example: $2 ext{H}_2(g) + ext{O}_2(g) <br>ightarrow 2 ext{H}_2 ext{O}(l)$
    - Symbols:
        - Reactants: Substances on the left-hand side of the arrow (e.g., H2 and O2).
        - Products: Substances on the right-hand side of the arrow (e.g., H2O).
        - Arrow ($ightarrow$): Separates reactants from products.

• Physical States Indicated in Equations:
    - (s): Solid substance
    - (l): Liquid substance
    - (g): Gaseous substance
    - (aq): Aqueous solution (substance dissolved in water)

Types of Chemical Reactions

Synthesis Reactions

• Form: $A + B <br>ightarrow AB$
    - Definition: Two materials (elements or compounds) combine to create a single product.
    - Examples:
      - Hydrogen gas and oxygen gas react to form water: $2 ext{H}_2 + ext{O}_2 <br>ightarrow 2 ext{H}_2 ext{O}$
      - Sulfur trioxide reacts with water to produce sulfuric acid: $ext{H}_2 ext{O} + ext{SO}_3 <br>ightarrow ext{H}_2 ext{SO}_4$

Decomposition Reactions

• Form: $XZ <br>ightarrow X + Z$
    - Definition: A single reactant breaks down into two or more products.
    - Examples:
      - Potassium chlorate when heated decomposes into oxygen gas and potassium chloride: $2 ext{KClO}_3 <br>ightarrow 2 ext{KCl} + 3 ext{O}_2$
      - Heating sodium bicarbonate releases water, carbon dioxide, and sodium carbonate: $6 ext{NaHCO}_3 <br>ightarrow 3 ext{Na}_2 ext{CO}_3 + 3 ext{H}_2 ext{O} + 3 ext{CO}_2$

Single Replacement Reactions

• Also called Single Displacement Reactions.
    - Example: $2 ext{KI} + ext{Cl}_2 <br>ightarrow 2 ext{KCl} + ext{I}_2$
    - Mechanism: Involves the replacement of one ion in a compound by another ion.
      - Positive ions will replace other positive ions.
      - Negative ions will replace other negative ions.

Double Replacement Reactions

• Also called Double Displacement Reactions.
    - Example: $ext{AgNO}_3 + ext{KCl} <br>ightarrow ext{AgCl}(s) + ext{KNO}_3$
    - Mechanism: Involves the exchange of partners between two compounds; positive and negative ions switch.
      - Important Note: The positive ion is always written first.

Combustion Reactions

• Definition: Occurs between a hydrocarbon and oxygen.

• Products: Always produce carbon dioxide and water.
    - Example: $ext{CH}_4 + 2 ext{O}_2 <br>ightarrow ext{CO}_2 + 2 ext{H}_2 ext{O}$

Balancing Chemical Equations

• Procedure: Balance equations by adding whole number coefficients in front of each compound or element's symbol.

• Tips for Balancing:
    - Delay balancing hydrogen and oxygen until last.
    - When polyatomic ions appear on both sides, treat them as a single unit.
    - Keep in mind that balancing one element might unbalance another.

Reaction Rates

• Factors affecting reaction rates include:
    - Temperature: Higher temperatures increase the rate of reaction.
    - Concentration: More molecules per unit volume increase the likelihood of collision, thus increasing the rate.
    - Activation Energy: The minimum energy required for a reaction to occur.

Collision Theory

• Definition: A chemical reaction can only occur when particles collide.

• Effective Collisions: Not all collisions lead to a reaction; only effective collisions determine if particles will react.

Predicting Products of Reactions

• Method: Predict products by classifying the reaction and examining the reactants.

Double Replacement Recap

• Procedure: Assign oxidation numbers and cross charges (similar to nomenclature) - partners swap on the product side. Always place positive ions first.

• Solubility Rules: Understanding which compounds are soluble in aqueous solutions is vital for knowing if a reaction will result in a precipitate (an insoluble solid that forms from a liquid solution).

Single Replacement Recap

• Activity Series: A ranked list of elements based on their ability to undergo single replacement reactions, organized from most to least reactive.

• Replacement Principle: An element can replace those beneath it in the activity series but not those above it.

• Group Reactivity: An element can react with anything in its group and everything below.

Diatomic Molecules

• List of Diatomic Elements: F2, Cl2, Br2, H2, I2, N2, O2

• Formation: Diatomic molecules occur when two atoms of the same element bond to achieve greater stability through sharing unpaired electrons.

Further Examples of Reactions

• Example Reactions:
    - For $ext{Al} + ext{ZnCl}_2$: This reaction proceeds to completion because aluminum is above zinc in the activity series and can replace it.
    - For $ext{Co} + ext{NaCl}$: No Reaction (NR); cobalt cannot replace sodium since sodium is above it in the series.
    - For $ext{NaCl} + ext{I}_2$: No Reaction (NR); iodine is below chlorine in the halogen activity series and cannot replace it.
    - For $ext{Al} + ext{H}_2 ext{O}(l)$: No Reaction (NR); aluminum requires water in the form of steam for reaction, not liquid water.
    - For $ext{Li} + ext{HCl}$: This reaction proceeds to completion because lithium can react with acids.
      

Summary of Synthesis and Decomposition

• Tips: Use a “cheat sheet” or gas ion chart to understand how elements or compounds combine or decompose.

• Combustion Products: In combustion reactions, the products should always be carbon dioxide and water.

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Synthesis Reactions

• Definition: Two materials combine to create a single product.

• Steps:
    1. Identify the two reactants (e.g., A and B).
    2. Combine them to form one product (e.g., AB).
    3. Write the equation: $A + B \rightarrow AB$

Decomposition Reactions

• Definition: A single reactant breaks down into two or more products.

• Steps:
    1. Identify the single reactant (e.g., XZ).
    2. Break it apart into its components (e.g., X and Z).
    3. Write the equation: $XZ \rightarrow X + Z$

Single Replacement Reactions

• Definition: One element replaces another in a compound.

• Steps:
    1. Identify the reactants (e.g., KI and Cl2).
    2. Recognize which ion will be replaced.
    3. Write the equation showing the replacement: $2KI + Cl_2 \rightarrow 2KCl + I_2$

Double Replacement Reactions

• Definition: Two compounds exchange partners.

• Steps:
    1. Identify the two compounds (e.g., AgNO3 and KCl).
    2. Swap the cations and anions.
    3. Write the equation: $AgNO_3 + KCl \rightarrow AgCl(s) + KNO_3$

Combustion Reactions

• Definition: Typically involves a hydrocarbon and oxygen, producing CO2 and H2O.

• Steps:
    1. Identify the hydrocarbon (e.g., CH4) and oxygen.
    2. Recognize the products: CO2 and H2O.
    3. Write the equation: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$