Chemical Equilibrium and Hydrocarbons

Introduction to Equilibrium

  • Equilibrium studies the extent of a chemical reaction, especially when it's reversible.

Quantitative vs. Reversible Reactions

  • Quantitative Reactions: Proceed until the limiting reactant is consumed, proceeding in one direction only.

  • Reversible Reactions: Can proceed in both forward and reverse directions.

    • Example: H2+I2⇌2HI

      • Forward reaction: Hydrogen and iodine form hydrogen iodide.

      • Reverse reaction: Hydrogen iodide converts back into hydrogen and iodine.

  • Both forward and reverse reactions occur simultaneously, competing until a balanced (equilibrium) state is reached.

  • The rates of both forward and reverse reactions are the same at equilibrium.
    * Reactants convert to products at the same rate as products revert to reactants.

  • Equilibrium is about the extent a reaction goes to completion when balance is achieved.

  • Reversible reactions use bi-directional arrows in chemical equations (e.g., H<em>2+I</em>2⇌2HIH<em>2 + I</em>2 \rightleftharpoons 2HI).

  • Other reversible reactions examples:

    • N2(g)+3H2(g)⇌2NH3​(g)

    • H2O(l)+H2O(g)⇌H

Reaction Yields

  • Actual yields in stoichiometry are often lower than theoretical yields.

  • Reversibility of chemical reactions is a critical reason for lower actual yields.

  • Reactions with forward and reverse reactions never reach 100% yield.
    * Products are simultaneously consumed as they are produced.

Dynamic Equilibrium

  • Dynamic equilibrium is when forward and reverse reaction rates are equal.

    • Reactants and products are present, with no further concentration changes over time.

    • To the human eye, it may appear nothing is happening.

    • Microscopically, reactants convert to products, and products convert to reactants at the same speed.

Activation Energies (For Reverse Reactions)

  • Most chemical reactions are reversible.

  • Exothermic reactions with large ΔH\Delta H may be difficult to reverse, acting as one-directional.

    • Ea(reverse)E_a(reverse) is very large, creating a high energy barrier.

  • Reactions with a large Ea(reverse)E_a(reverse) can still occur with enough energy (e.g., heating) or a catalyst.

  • Equilibrium can be achieved from either direction of a chemical reaction.

Le Châtelier’s Principle

  • Explains how a system at equilibrium responds to disturbances, counteracting the disturbance to re-establish equilibrium, but with slightly offset concentrations of reactants and products (approx. 10%).

  • Change in Reactant or Product Concentrations

    • Increasing reactant or product amount: equilibrium shifts away from the increase.

    • Decreasing reactant or product amount: equilibrium shifts towards the decrease.

    • Example:

      • N2(g)+3H2(g)⇌2NH

        • Adding H2H_2 gas to the system:

          • The system moves to consume (decrease) H2H_2.

          • Equilibrium "shifts to the right" (forward reaction rate increases).

          • N2 is consumed with H2H2, and NH3NH3​ isproduced.

          • [H2] and [N2][N2] decrease, [NH3][NH3​] increases until a new equilibrium is established.

  • Effects of Volume and Pressure

    • Increasing pressure: equilibrium shifts to counteract the increase and generate fewer moles of gas.

    • Decreasing pressure: equilibrium shifts to counteract the decrease and generate more moles of gas.

    • Volume and pressure are inversely related.

    • Example:

      • N2(g)+3H2(g)⇌2NH3​(g)+46kJmol−1

        • Increasing pressure: Favors direction with fewer moles of gas; amount of NH3NH_3 increases at equilibrium.

    • Pressure has no effect if the number of product and reactant moles of gas are the same.

  • Effect of Temperature Changes

    • For endothermic reactions (\Delta H > 0), heat is a reactant.

    • For exothermic reactions (\Delta H < 0), heat is a product.

    • Increasing temperature:

      • Shifts equilibrium away from the heat.

      • If \Delta H > 0, it favors the forward reaction.

      • If \Delta H < 0, it favors the reverse reaction.

    • Decreasing temperature:

      • Shifts equilibrium towards the heat.

      • If \Delta H > 0, it favors the reverse reaction.

      • If \Delta H < 0, it favors the forward reaction.

    • Example:

      • Co(H2O)6(aq)+4Cl−(aq)⇌CoCl42−(aq)+6H2O(l)ΔH>0Adding heat: equilibrium shifts right (forward reaction) to remove heat.

        • Removing heat: equilibrium shifts left (reverse reaction) to produce heat.

    • Example:

      • NH4Cl(s)+629kJ⇌NH4+(aq)+Cl−(aq)

        • Increasing temperature: shifts equilibrium to the right to use excess energy.

        • Decreasing temperature: shifts equilibrium to the left to replace lost energy.

    • Equilibrium calculations involve an equilibrium constant that is temperature-dependent.

      • Changing temperature affects the equilibrium direction and final concentrations.

Effect of a Catalyst on Equilibria

  • Adding a catalyst does not shift equilibrium position.

  • Catalysts help a system reach equilibrium sooner by lowering the activation energy for both forward and reverse reactions.

  • Catalysts do not influence which reaction is favored.

Organic Chemistry and Hydrocarbons

  • Organic chemistry is the study of compounds where carbon is the principal element.

  • Key aspects of Carbon:

    1. Naturally abundant on Earth.

    2. Versatile:

      • Has 4 valence electrons, forming several possible covalent bonds (single, double, triple).

      • Forms chains, rings, sheets, and tubes of varying sizes.

  • Carbon-based compounds are stable due to stable C-C and C-H bonds.

  • The simplest family of organic compounds are hydrocarbons.

Introduction to Hydrocarbons

  • Hydrocarbons contain only carbon and hydrogen.

  • Hydrocarbons can be classified:

    • Aliphatic: straight, branched chains, or rings of carbon-to-carbon bonds.

      • Cyclic hydrocarbons: carbons arranged in a ring.

    • Aromatic: Contains a ring of carbon with alternating double-single bonds, differs from aliphatic cyclic structures.

*Classification by Saturation:
* Saturated: Contain the maximum number of hydrogens; only single bonds between carbons.
* Unsaturated: Contain 1 or more double or triple bonds.

Helpful Definitions

  • Molecular Formula: States the number of atoms of each element in a compound. Example: Butane

  • Fully Expanded Structural Formula: Shows all atoms and bonds. Example: Butane

  • Fully Condensed Structural Formula: Omits bonds, side chains written in brackets.Example Butane

  • Line-bond structures show all the C and H atoms and the bonds

  • Skeletal structures use lines to represent the carbon-atom framework

Aliphatic Families

  • Alkanes

    • Contain only single bonds (saturated hydrocarbons).

    • Formula for straight and branched chains: CnH2n+2 (nn = # carbons).

    • Ringed alkanes are called cycloalkanes.

    • Formula for cycloalkanes: CnH2n (nn = # carbons).

    • Non-polar compounds with only C-C or C-H bonds.

    • Very low melting/boiling points due to London dispersion forces (LDF).

    • Melting/boiling points increase with molecular size (more LDF).
      *Straight-chain Alkanes: Carbons are in a row.
      *Branched-chain Alkanes: Contain a carbon chain that has branching off.
      *Cycloalkanes: Carbons are in a ring

  • Alkenes

    • Have at least one carbon-carbon double bond

    • Formula for straight chains: CnH2n (nn = # carbons).

  • Alkynes

    • Have at least one carbon-carbon triple bond

    • Formula for straight chains: CnH2n−2 (nn = # carbons).

    • Alkenes and alkynes are unsaturated since they do not contain the maximum number of hydrogen atoms.

Naming Hydrocarbons

  • IUPAC (International Union of Pure and Applied Chemists) has internationally accepted rules for naming organic compounds.

  • Prefix – Parent – Suffix
    *Substituent: An atom or group other than hydrogen on a molecule, may be a carbon chain or a functional group.

  • Table 01 - Names for # of Carbon atoms in main chain

    • # of C: Parent

    • 1: Meth-

    • 2: Eth-

    • 3: Prop-

    • 4: But-

    • 5: Pent-

    • 6: Hex-

    • 7: Hept-

    • 8: Oct-

    • 9: Non-

    • 10: Dec-

  • Table 02 – Names of alkyl branches

    • Side Chain Length In Carbons: Prefix Name, Side chain name, Alkyl Formula CnH3(2n+1)

    • 1: meth, methyl, –CH3

    • 2: eth, ethyl, –C2H5

    • 3: prop, propyl, –C3H7

    • 4: but, butyl, –C4H9

    • 5: pent, pentyl, –C5H11

    • 6: hex, hexyl, –C6H13

Naming Alkanes

*Suffix "-ane"
*Naming differs slightly in whether they are straight or branched.

  • Straight-Chain Alkane

    *Combine a parent for the number of carbons present in the chain, followed by -ane. Straight-chain alkanes don’t have any substituents so there is no need for a prefix.

  • Branched Chain Alkanes

      1.  Identify the parent hydrocarbon. This is done by finding the longest carbon chain (NOT ALWAYS STRAIGHT!!!)

    • If two different chains of equal length are present, choose the one with the larger number of branch points. * Ex: Use… Hexane with two branches NOT!!! Hexane with one substituent

      1. Number the carbons in the primary chain

      • Starting at the end closer to the first branch point, number each carbon in the parent chain

        • Ex: Number as… NOT!!!
          Number as… NOT!!!

        1. Identify # the substituents

        2. Write the name. Separate numbers with commas and words with hyphens

  • If two or more different substituents are present, list them in alphabetical order (e.g., di-, tri-, etc.) But don’t use the prefixes to alphabetize.

  • Ex.3-methylhexane; 3-Ethyl-4,7-dimethylnonane; 3-Ethyl-2-methylhexane

  • 4-Ethyl-3-methylheptane; 4-Ethyl-2,4-dimethylhexane; 4,5,5-triethyl-3,6,6-trimethylnonane
    5. Name a complex substituent as though it were itself a compound. To name the compound fully, the complex substituent (one with sub-branches) must be named first.

    • Begin numbering the complex substituent (carbon #1) at the point of attachment to the parent chain and identify it as a 2-methylpropyl group.

      • The substituent is alphabetized according to the first letter of its complete name (including any numerical prefix) and is set off in parentheses (brackets) when naming the complete molecule.

  • Ex: 5-(1,2-Dimethylpropyl)-2-methylnonane
    Naming Alkenes & Alkynes
    *Alkenes have the family suffix “-ene”, while alkynes have the suffix “-yne”

  • Both are named using similar rules for alkanes.
    1. Identify the parent hydrocarbon. Use the longest carbon chain containing the double bond or triple bond!Name as a pentene NOT!!!
    2. Number the carbons in the chain. Starting at the end closer to the double/triple bond or, if the double/triple bond is the same distance from both ends, begin at the end closer to the first branch point.

    3.  Write the full name. Indicate the position of the double/triple bond by giving the number of the first alkene/alkyne carbon and placing that number in between the parent name and the “-ene” suffix.

    *If more than one double bond is present, indicate the position of each and use one of the suffixes -diene, -triene, etc.

  • Ex: Hex-2-ene; 2-Methylhex-3-ene; 2-Ethylpent-1-ene;

  • 2-Methylbuta-1,3-diene;6-Methyloct-3-yne

Naming Cyclic Alkanes

*Identify the parent hydrocarbon. Then, count the number of carbons in the ring and the number of carbons in the largest straight chain.
*If the number of carbons in the ring is equal or greater than the number in the substituent, then the ring is the parent chain!
*If the ring has fewer carbons than the largest straight chain, then the straight chain is the parent chain!
Cycloalkanes contain the prefix “cyclo-“

*For a substituted cycloalkane, start at a point of attachment as Carbon 1 and number the substituents on the ring so that the second substituent has as low a number as possible.

  • If ambiguity still , number lowest point is found.

*When two or more different straight chains could potential receive the same numbers, number them by alphabetically priority.

  • EX: Number as… 1-Ethyl-2-methylcyclopentane

Cycloalkenes & Cycloalkynes

*If the ring structure has a double or triple bond, then the ring structure is the parent chain.
Double/triple bond must be between Carbon 1 and 2

  • 3, 4-dimethylcyclopentyne

  • 4-ethyl-5-methylcyclohexene

Non-Systematic Alkyl Groups

*Some alkyl groups have common names instead of the systematic IUPAC names.
• The prefix iso- is not hyphenated and considered part of the alkyl-group name for alphabetizing.
• The prefixes sec- and tert- are NOT considered part of the name, so they are hyphenated and are not used for alphabetizing.
*Ex: sec-butyl & tert-butyl are both alphabetized as the letter “b” in naming.

*The above alkyl groups, plus a few others that are not mentioned here, are so common that the IUPAC system allows for them to be used. Ex: 4-(1-Methylethyl)heptane or 4-isopropylheptane

*The sec- and tert- designations come from the number of carbons bound to the carbon attached to the parent chain.
o Sec- = secondary = 2 carbons attached
o Tert= = tertiary = 3 carbons attached

The designation n- is sometimes used to indicate that an alkyl chain is straight.

Naming Aromatic Compounds with Benzene

*Most aromatic compounds contain a benzene ring(C<em>6H</em>6)(C<em>6H</em>6)

  • Benzene is a planar (flat) molecule
    Contains 2 resonance structures since the electrons in the double bonds are continuously moving, resulting in no permanent double bonds forming.

*Most of the time, benzene is the parent molecule. Number the substitutions such that they have the lowest numbers possible.
*Sometimes when the side chains are difficult to name, it may be easier to treat the benzene as a substituent instead. When benzene is used as a substituent, it is named “phenyl”

  • Ortho, Meta, Para Positioning

    • Orth = 1,2 positions

    • Meta = 1, 3 positions

    • Para = 1, 4 positions

Drawing Hydrocarbons

Follow these steps
Look at the parent name and draw its carbon structure
Find the substituents and place them on the proper carbons.
Add hydrogens to complete the structure