FS-Physical-Sciences-Grade-12-SUMMARIES-Terms-and-Definitions-Paper-2-YEAR-2023

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• This document is an extra resource to help Grade 12 learners perform well in Physical Sciences.
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ORGANIC MOLECULES

• Homologous Series:
  • Alkanes
  • Alkenes
  • Alkynes
  • Haloalkanes
  • Alcohols
  • Aldehydes
  • Ketones
  • Carboxylic acids
  • Esters
• General Formula:
  • Alkanes: $C<em>nH</em>{2n+2}$
  • Alkenes: $C<em>nH</em>{2n}$
  • Alkynes: $C<em>nH</em>{2n-2}$
  • Haloalkanes: $C<em>nH</em>{2n+1}X$ (X = F, Cl, Br, or I)
  • Alcohols: $C<em>nH</em>{2n+1}OH$
  • Aldehydes: $C<em>nH</em>{2n}O$
  • Ketones: $C<em>nH</em>{2n}O$
  • Carboxylic acids: $C<em>nH</em>{2n}O_2$
  • Esters: $C<em>nH</em>{2n}O_2$
• Functional Group:
  • Alkanes: Only C-H and C-C single bonds
  • Alkenes: Carbon-carbon double bond
  • Alkynes: Carbon-carbon triple bond
  • Haloalkanes: Halogen atom bonded to a saturated C atom
  • Alcohols: Hydroxyl group bonded to a saturated C atom
  • Aldehydes: Formyl group
  • Ketones: Carbonyl group bonded to two C atoms
  • Carboxylic acids: Carboxyl group
  • Esters: Ester group
• Chemical Reactions:
  • Alkanes: Oxidation, Substitution
  • Alkenes: Addition
  • Alkynes: Addition
  • Haloalkanes: Substitution, Elimination
  • Alcohols: Substitution, Elimination, Esterification
  • Aldehydes: Oxidation
  • Ketones: Oxidation
  • Carboxylic acids: Esterification
  • Esters: Hydrolysis
• Intermolecular Forces:
  • Alkanes, Alkenes, Alkynes: London forces
  • Haloalkanes, Aldehydes, Ketones, Esters : Dipole-dipole forces
  • Alcohols, Carboxylic acids: Hydrogen Bonding

NOMENCLATURE OF ORGANIC COMPOUNDS

• Alcohol: An organic compound in which H atoms in an alkane have been substituted with hydroxyl groups (-OH groups). General formula: $C<em>nH</em>{2n + 1}OH$
• Aldehydes: Organic compounds having the general structure RCHO where R = H or alkyl. General formula: RCHO (R = alkyl group)
• Alkane: An organic compound containing only C-H and C-C single bonds. General formula: $C<em>nH</em>{2n + 2}$
• Alkene: A compound of carbon and hydrogen that contains a carbon-carbon double bond. General formula: $C<em>nH</em>{2n}$
• Alkyl group: A group formed by removing one H atom from an alkane.
• Alkyne: A compound of carbon and hydrogen that contains a carbon-carbon triple bond.
• Carbonyl group: Functional group of ketones (>C=O)
• Carboxyl group: Functional group of carboxylic acids $(-COOH)$
• Carboxylic acid: An organic compound containing a carboxyl group $(-COOH)$ group). General formula: $C<em>nH</em>{2n + 1}COOH$ (or RCOOH)
• Chain isomers: Compounds with the same molecular formula, but different types of chains.
• Condensed structural formula: A formula that shows the way in which atoms are bonded together in the molecule but DOES NOT SHOW ALL bond lines.
• Functional group: A bond or an atom or a group of atoms that determine(s) the physical and chemical properties of a group of organic compounds.
• Functional isomers: Compounds with the same molecular formula, but different functional groups.
• Haloalkane (Alkyl halide): An organic compound in which one or more H atoms in an alkane have been replaced with halogen atoms. General formula: $C<em>nH</em>{2n + 1}X$ (X = F, Cℓ, Br or I)
• Homologous series: A series of organic compounds that can be described by the same general formula and that have the same functional group. OR A series of organic compounds in which one member differs from the next with a $CH_2$ group.
• Hydrocarbon: Organic compounds that consist of hydrogen and carbon only.
• IUPAC naming: A chemical nomenclature (set of rules) created and developed by the International Union of Pure and Applied Chemistry (IUPAC) to generate systematic names for chemical compounds.
• Molecular formula: A chemical formula that indicates the type of atoms and the correct number of each in a molecule, e.g. $CH_4$.
• Organic chemistry: Chemistry of carbon compounds.
• Positional isomer: Compounds with the same molecular formula, but different positions of the side chain, substituents, or functional groups on the parent chain.
• Primary alcohol: The C atom bonded to the hydroxyl group is bonded to ONE other C atom.
• Primary haloalkane: The C atom bonded to the halogen is bonded to ONE other C atom.
• Saturated compounds: Compounds in which there are no multiple bonds between C atoms in their hydrocarbon chains. OR Compounds with only single bonds between C atoms in their hydrocarbon chains.
• Secondary alcohol: The C atom bonded to the hydroxyl group is bonded to TWO other C atoms.
• Secondary haloalkane: The C atom bonded to the halogen is bonded to ONE other C atom.
• Structural formula: A structural formula of a compound shows which atoms are attached to which within the molecule. Atoms are represented by their chemical symbols and lines are used to represent ALL the bonds that hold the atoms together.
• Structural isomer: Organic molecules with the same molecular formula, but different structural formulae.
• Substituent (branch): A group or branch attached to the longest continuous chain of C atoms in an organic compound.
• Tertiary alcohol: The C atom bonded to the hydroxyl group is bonded to THREE other C atoms.
• Tertiary haloalkane: The C atom bonded to the halogen is bonded to THREE other C atoms.
• Unsaturated compounds: Compounds in which there are multiple bonds (double or triple bonds) between C atoms in their hydrocarbon chains.

WRITING IUPAC NAMES OF ORGANIC COMPOUNDS

The name of each organic molecule has three parts: prefix - parent - suffix

• Type and position of substituents

• Number of C atoms in the longest chain?

• Type of functional group or homologous series
  Step 1: Suffix

• Determine the functional group in the structure of the given compound or the homologous series to which the compound belongs.

• The functional group or homologous series determines the suffix (last part of the name).
  Step 2: Parent name

• The number of C atoms in the longest carbon chain that contains the functional group determines the parent name.

• Count the number of C atoms in the longest chain containing the functional group.

  • Number of carbon atoms: 1, 2, 3, 4, 5, 6, 7, 8
  • Parent name: meth, eth, prop, but, pent, hex, hept, oct

• Alkanes and haloalkanes: Number from the side that will give the substituents the smallest numbers.

• Alkenes, alkynes, alcohols, ketones: Number from the side that will the functional group the smallest number. The functional group receives a number that is written between parent name and suffix.

• Aldehydes and carboxylic acids: Number from the C atom that forms part of the functional group.

• Esters: To determine the first part of the name, count the C atoms attached to the single-bonded O atom of the functional group. Add –yl to this part e.g. ethyl. To determine the last part of the name, number from the C atom bonded to the O atom with a double bond. Add the -anoate to this part e.g. butanoate.
  Step 3: Prefix

• Identify substituents on the parent chain.

• Substituents can be methyl (one C atom i.e. –$CH<em>3$) or ethyl (2 C atoms i.e. –$CH</em>2CH_3$).

• Use numbers on the parent chain to indicate the position of the substituents on the parent chain.

• Arrange substituents in alphabetical order in the IUPAC name (bromo, chloro, ethyl, methyl)

• If two or more of the same substituents occur, use di- and tri- in front of the name of the substituent e.g. dimethyl or tribromo. (Di- and tri are ignored when arranging substituents in alphabetical order.)

• When there are two (or more) identical groups on the same C atom, the number of the C atom is repeated with commas between the numbers e.g. 2,4,4-trimethylhexan-3-one

• Final IUPAC names, except those of esters, are written as one word with COMMAS BETWEEN NUMBERS and HYPHENS BETWEEN NUMBERS AND WORDS e.g. 2,4,4-trimethylhexan-3-one. IUPAC names of esters and carboxylic acids are written as two words e.g. ethyl methanoate and pentanoic acid.

WRITING STRUCTURAL FORMULAE FROM IUPAC NAMES

1. Identify the parent name in the IUPAC name. Draw a carbon skeleton with the number of C atoms indicated by the parent name.
2. Identify the functional group (suffix) or homologous series to which this compound belongs. Use the number in front of the functional group (suffix) to place the functional on the correct C atom.
3. Identify the substituents (prefix). Use the number in front of each substituent to place the substituents on the correct C atoms.
4. Ensure that each C atom is surrounded by 4 bonds (lines indicating bonds).
5. Include H atoms at all open bonds after ensuring that each C atom is surrounded by 4 bonds.
6. All bonds should be shown. Do not draw any part of the molecule condensed e.g. –$CH_3$.
7. As a final check ensure all C atoms form 4 bonds, all O atoms 2 bonds and all H atoms, Br atoms, and Cℓ atoms 1 bond.

FORMULAE FOR REPRESENTING MOLECULES

• Molecular formula: A chemical formula that indicates the type of atoms and the correct number of each in a molecule.
• Condensed structural formula: Shows the way in which atoms are bonded together in a molecule but DOES NOT SHOW ALL bond lines.
• Structural formula: Shows which atoms are attached to which within the molecule. Atoms are represented by chemical symbols and lines are used to represent ALL the bonds that hold atoms together. Structural formulae usually do NOT depict the actual geometry/shape of molecules.

NOTE: When drawing condensed structural formulae, the following conventions are used:

• All atoms are drawn in, but the bond lines are usually omitted
• Atoms are usually drawn next to atoms to which they are bonded
• Brackets are used around similar groups bonded to the same atom

STRUCTURAL ISOMERS

Same molecular formula, different structural formulae.

• Chain isomers: Same molecular formula, different chains.
• Positional isomers: Same molecular formula, different positions of functional group or side chain.
• Functional isomers: Same molecular formula, different functional groups.

TYPICAL QUESTIONS

(Followed by numerous example questions from past papers, spanning from November 2014 to November 2022. These include multiple-choice and structured questions, covering IUPAC naming, structural formulas, homologous series, functional groups, isomerism, and reaction types. Specific questions and answers are available in the original document.)

PHYSICAL PROPERTIES

• Factors affecting melting point, boiling point, and vapor pressure:
  • Strength of intermolecular forces:
    • London forces (weak)
    • Dipole-dipole forces (stronger)
    • Hydrogen bonding (strongest)
      • Alcohols – one site for hydrogen bonding
      • Carboxylic acids – two sites for hydrogen bonding
  • Functional group
  • Homologous series
  • Chain length: longer chain → stronger IMF → higher boiling point, lower vapor pressure, higher melting point
  • Branching: more branched → weaker IMF → lower boiling point, higher vapor pressure, lower melting point

EXPLAINING DIFFERENCES IN PHYSICAL PROPERTIES

STEP 1: State the DIFFERENCE IN STRUCTURE (chain length/branching/functional group) responsible for the difference in boiling point/melting point/vapour pressure.

STEP 2: State the EFFECT of this factor ON INTERMOLECULAR FORCES.

STEP 3: State the EFFECT ON ENERGY NEEDED TO OVERCOME INTERMOLECULAR FORCES.

TERMS AND DEFINITIONS

• Boiling point: The temperature at which the vapor pressure of a liquid equals atmospheric pressure.
• Dipole-dipole force: Intermolecular forces found between polar molecules i.e. molecules in which there is an uneven distribution of charge so that the molecule has a positive and a negative side.
• Hydrogen bond: A strong intermolecular force found between molecules in which an H atom is covalently bonded to either an N, O, or F atom.
• Intermolecular force: Forces between molecules that determine physical properties of compounds.
• London force: A weak intermolecular force between non-polar molecules.
• Melting point: The temperature at which the solid and liquid phases of a substance are at equilibrium.
• Van der Waals forces: A combined name used for the different types of intermolecular forces.
• Vapor pressure: The pressure exerted by a vapor at equilibrium with its liquid in a closed system.
• Volatility: The tendency of a substance to vaporize.

TYPICAL QUESTIONS

(Followed by numerous example questions from past papers, spanning from November 2014 to November 2022. These include multiple-choice and structured questions, covering influences on boiling points, vapor pressure, intermolecular forces, and structural effects.)

ORGANIC REACTIONS

• Reactions of Alkanes:
  • Oxidation (Combustion) : Alkane + oxygen → carbon dioxide + water + energy. Reaction conditions: Burns in EXCESS oxygen
  • Substitution: Alkane → haloalkane. Type of substitution: halogenation. Reaction conditions: heat OR sunlight. Reactants: alkane + $X_2$ (F, Cℓ, Br, I). Products: haloalkane + HX
  • Elimination: Alkane → alkene(s) + alkane with the shorter chain. Type of elimination: cracking. Reaction conditions: heat + high pressure OR catalyst. Reactant: alkane. Products: alkene(s) + alkane / alkene + $H_2$
  • Reactions of Alkenes:
    • Addition: Alkene → alkane .Type of addition: hydrogenation. Reaction conditions: Pt, Pd, or Ni as catalyst. Reactants: alkene + $H_2$. Product: alkane.
    • Addition: Alkene → haloalkane. Type of addition: halogenation. Reaction conditions: unreactive solvent. Reactants: alkene + $X_2$ (X = Cℓ, Br). Product: haloalkane.
    • Addition: Alkene → haloalkane. Type of addition: hydrohalogenation. Reaction conditions: no water; unreactive solvent. Reactants: alkene + HX (X = I, Br, Cℓ). Product(s): haloalkane(s). Major product: H atom bonds to the C atom already having the greater number of H atoms.
    • Addition: Alkene → alcohol(s). Type of addition: hydration. Reaction conditions: excess $H<em>2O$; a small amount of acid ($H</em>2SO<em>4$) as a catalyst. Reactants: alkene + $H</em>2O$. Product: alcohol(s). Major product: H atom bonds to the C atom already having the greater number of H atoms.
• Reactions of Alcohols:
  • Substitution: Alcohol → haloalkane. Conditions: heat Reactants needed: alcohol + HX. Primary & secondary alcohols: NaBr + $H<em>2SO</em>4$ used to make HBr in reaction flask. Tertiary alcohols: water free HBr (or HCℓ) Products: haloalkane + $H_2O$
  • Esterification: Alcohol → ester. Type of reaction: esterification. Conditions: concentrated sulphuric acid as catalyst + heat. Reactants: alcohol + carboxylic acid. Products: ester + water Acid catalysed $H<em>2SO</em>4$
  • Elimination: Alcohol → alkene .Type of elimination: dehydration. Conditions: dehydrating agent ($H<em>2SO</em>4/H<em>3PO</em>4$) + heat. Reactants: alcohol + $H<em>2SO</em>4$. Products: alkene(s) + $H_2O$. Major product: The one where the H atom is removed from the C atom with the least number of H atoms.
• Reactions of Haloalkanes:
  • Elimination: Haloalkane→ alkene. Type of elimination: dehydrohalogenation. Conditions: concentrated strong base (NaOH, KOH, LiOH) in ethanol + heatReactants: haloalkane + concentrated strong base. Products: alkene + NaBr + $H_2O$. Major product: The one where the H atom is removed from the C atom with the least number of H atoms (most substituted double bond forms i.e. double bond with most alkyl groups)
  • Substitution: Haloalkane → alcohol. Type of substitution: hydrolysis. Conditions: dilute strong base (NaOH/KOH/LiOH) + mild heat. Reactants: haloalkane + dilute strong base. Products: alcohol + NaBr/KBr/LiBr
  • Substitution: Haloalkane → alcohol. Type of substitution: hydrolysis. Conditions: excess $H<em>2O$ + mild heat. Reactants: haloalkane + $H</em>2O$. Products: alcohol + HBr

TERMS AND DEFINITIONS

• Addition reaction: A reaction in which a double bond in the starting material is broken and elements are added to it.
• Cracking: The chemical process in which longer chain hydrocarbon molecules are broken down to shorter, more useful molecules.
• Dehydration: Elimination of water from a compound, usually an alcohol.
• Dehydrohalogenation: The elimination of hydrogen and a halogen from a haloalkane.
• Elimination reaction: A reaction in which elements of the starting material are “lost” and a double bond is formed.
• Esterification: The preparation of an ester from the reaction of a carboxylic acid with an alcohol.
• Halogenation: The reaction of a halogen ($Br<em>2, Cℓ</em>2$) with a compound.
• Hydration: The addition of water to a compound.
• Hydrogenation: The addition of hydrogen to an alkene
• Hydrohalogenation: The addition of a hydrogen halide to an alkene.
• Hydrolysis: The reaction of a compound with water.
• Substitution reaction: A reaction in which an atom or a group of atoms in a molecule is replaced by another atom or group of atoms.

TYPICAL QUESTIONS

(Followed by numerous example questions from past papers, spanning from November 2014 to November 2022. These include flow diagrams, reaction conditions, and product identification.)

REACTION RATE AND ENERGY IN CHEMICAL REACTIONS

ENERGY IN CHEMICAL REACTIONS

• HEAT OF REACTION ($\Delta H$): The energy absorbed or released in a chemical reaction. EXOTHERMIC REACTIONS: Reactions that release energy. ENDOTHERMIC REACTIONS: Reactions that absorb energy.

  • For exothermic reactions: \Delta H < 0
  • For endothermic reactions: \Delta H > 0

• Activation energy ($E_a$): The minimum energy needed for a reaction to take place.

• Activated complex: The unstable transition state from reactants to products.

• EFFECT OF A CATALYST Catalyst: Increases reaction rate without undergoing a permanent change. Mechanism of catalyst: Increase reaction rate by lowering the total activation energy

• Practical skills:

  • Independent variable The variable that is changed e.g. an increase in temperature.
  • Dependent variable The variable that changes due to a change in the independent variable e.g., reaction rate changes due to a change in temperature.
  • Controlled variable The variable(s) that are kept constant e.g. concentration and surface area are kept constant to measure the effect of temperature on reaction rate.
  • Investigative question A question about the relationship between the dependent and independent variables.
  • Hypothesis A prediction on the answer to the investigative question prior to the investigation.
  • Conclusion The conclusion is drawn after the investigation and answers the investigative question.
    REACTION RATE Change in concentration of reactants or products per unit time

• Factors affecting reaction rates

  1. The nature of reactants
  2. Concentration – higher concentration, faster rate
  3. Surface Area – greater surface area, faster rate
  4. Temperature – higher temperature, faster rate
  5. Catalyst – increases reaction rate without undergoing a permanent change

• Calculating Reaction Rate:

  • Determine rate in terms of products

  Rate =
  \frac{\Delta c}{\Delta t} \ \
  Rate = \frac{\Delta m}{\Delta t} \ \
  Rate = \frac{\Delta V}{\Delta t} \ \
  Rate = \frac{\Delta n}{\Delta t}

  • Determine rate in terms of reactants

  Rate =
  -\frac{\Delta c}{\Delta t} \ \
  Rate = -\frac{\Delta m}{\Delta t} \ \
  Rate = -\frac{\Delta V}{\Delta t} \ \
  Rate = -\frac{\Delta n}{\Delta t}

  • Unit of reaction rate: Any unit of the above quantities per second Examples: mol·dm-3·s-1 OR g·s-1 OR dm-3·s-1 OR mol·s-1

MEASURING REACTION RATE

• MEASURING LOSS IN MASS OF REACTANTS PER TIME
• MEASURING THE TIME FOR THE FORMATION ON AN AMOUNT OF PRECIPITATE
  $Na<em>2S</em>2O_3(aq) + HCℓ(aq)$
• MEASURING VOLUME OF GAS RELEASED PER TIME

THE COLLISION THEORY

The collision theory explains the factors influencing reaction rate.

• States that for a chemical reaction to occur, the reacting particles must collide with one another.
• The rate of the reaction depends on the frequency of collisions i.e. the number of collisions per unit time.
• For collisions to be successful or effective, reacting particles must:
  • Collide with sufficient kinetic energy
  • Have the correct orientation

BOLTZMANN-MAXWELL DISTRIBUTION CURVE OR ENERGY DISTRIBUTION CURVE

• Particles in any system represent a variety of kinetic energies.
• The area under the graph is a measure of the total number of particles, e.g. molecules, present.
• The magnitude of the activation energy is indicated on the Maxwell-Boltzmann distribution curve as a line at the specific kinetic energy.
• Only a few numbers of particles have sufficient kinetic energy i.e. kinetic energy equal to or greater than the activation energy.

EPLANATIONS IN TERMS OF THE COLLISION THEORY

• EFFECT OF INCREASE IN TEMPERATURE ON REACTION RATE
  • At a higher temperature, the average KINETIC ENERGY of particles INCREASES.
  • More particles have sufficient /enough kinetic energy.
  • More effective collisions take place per unit time.
  • Reaction rate increases.
• EFFECT OF A CATALYST ON REACTION RATE
  • The catalyst provides a path of LOWER ACTIVATION ENERGY.
  • More particles have sufficient kinetic energy.
  • More effective collisions take place per unit time.
  • Reaction rate increases.
• EFFECT OF INCREASE IN CONCENTRATION ON REACTION RATE
  • At a higher concentration, there are more particles s per unit volume.
  • More particles will have correct orientation./More collisions per unit time.
  • More effective collisions take place per unit time.
  • Reaction rate increases.
• EFFECT OF INCREASE IN SURFACE AREA ON REACTION RATE
  • With a greater surface area/state of division, more particles are exposed per unit volume.
  • More particles will have correct orientation./More collisions per unit time.
  • More effective collisions per (unit) time.
  • Reaction rate increases.

TERMS AND DEFINITIONS

• Mole: One mole of a substance is the amount of substance having the same number of particles as there are atoms in 12 g carbon-12.
• Molar gas volume at STP: The volume of one mole of a gas. (1 mole of any gas occupies 22,4 dm3 at 0 °C (273 K) and 1 atmosphere (101,3 kPa).
• Molar mass: The mass of one mole of a substance. Symbol: M Unit: g∙mol-1
• Avogadro’s Law: Under the same conditions of temperature and pressure, the same number of moles of all gases occupy the same volume.
• Concentration: The amount of solute per liter/cubic decimeter of solution. In symbols: $c = \frac{n}{V}$. Unit: mol∙dm-3
• Empirical formula: The simplest positive integer ratio of atoms present in a compound.
• Percentage yield: $\frac{actual \ mass \ obtained}{calculated \ mass} \times 100$
• Percentage purity: $\frac{mass \ of \ pure \ chemical}{total \ mass \ of \ sample} \times 100$
• Percentage composition: $\frac{mass \ contributed \ by \ component}{mass \ of \ all \ components} \times 100$
• Limiting reagents: The substance that is totally consumed when the chemical reaction is complete.
• Heat of reaction $(ΔH)$: The energy absorbed or released in a chemical reaction.
• Exothermic reactions: Reactions that release energy. (ΔH < 0)
• Endothermic reactions: Reactions that absorb energy. (ΔH > 0)
• Activation energy: The minimum energy needed for a reaction to take place.
• Activated complex: The unstable transition state from reactants to products.
• Reaction rate: The change in concentration of reactants or products per unit time.
  $Rate \ at \ which \ reactants \ are \ used: Rate = -\frac{\Delta c}{\Delta t} Unit: mol\cdot dm^{-3}\cdot s^{-1}$
  $Rate \ at \ which \ products \ are \ formed: Rate = \frac{\Delta c}{\Delta t} Unit: mol\cdot dm^{-3}\cdot s^{-1}$
• Collision theory: A model that explains reaction rate as the result of particles colliding with a certain minimum energy.
• Catalyst: A substance that increases the rate of a chemical reaction without itself undergoing a permanent change.
• Factors that affect reaction rate: Nature of reacting substances, surface area, concentration (pressure for gases), temperature and the presence of a catalyst.

TYPICAL QUESTIONS

(Followed by numerous example questions from past papers, spanning from November 2014 to November 2022. These include calculations of reaction rates, and relating potential energy diagrams to reaction exothermic/endothermic properties.)

CHEMICAL EQUILIBRIUM

CHEMICAL EQUILIBRIUM

• Dynamic equilibrium:

  • Rate of forward reaction equals the rate of the reverse reaction.
  • Amount of reactants and amount of products remain constant.

• Requirements

  • A REVERSIBLE chemical reaction
  • A CLOSED system

• Factors Affecting Equilibrium:

  • Concentration
    • Increase: Reaction that DECREASES concentration is favored.
    • Decrease: Reaction that INCREASES concentration is favored.
  • Temperature
    • Increase: Endothermic reaction is favored.
    • Decrease: Exothermic reaction is favored.
  • Pressure (Gases only)
    • Increase: Reaction that produces the smallest number of moles of gas is favored.
    • Decrease: Reaction that produces the greatest number of moles of gas is favored.
  • A CATALYST has NO EFFECT on the equilibrium position. It only shortens the time it takes to reach equilibrium.

• Predict and explain the effect on equilibrium

  Le Chatelier’s principle

When the equilibrium in a closed system is disturbed, the system will re-instate a new equilibrium by favouring the reaction that opposes the disturbance.

TERMS AND DEFINITIONS

• Open system: A system that continuously interacts with the environment – it exchanges matter and energy with its environment.
• Closed system: A system that only exchanges energy with its surroundings, but it does not exchange matter with its surroundings.
• Reversible reaction: A reaction is reversible when products can be converted back to reactants.
• Chemical equilibrium: Dynamic equilibrium when the rate of the forward reaction equals the rate of the reverse reaction.
• Factors that influence the equilibrium position: Pressure (gases only), concentration and temperature.
• Le Chatelier's principle: When the equilibrium in a closed system is disturbed, the system will re-instate a new equilibrium by favouring the reaction that will oppose the disturbance.

SOLVING PROBLEMS INVOLVING KC CALCULATIONS

The best way to solve Kc calculations is to use a table. * Draw a table with SIX rows. The number of columns will depend on the number of reactants and products in the balanced equation.

• 1st row: Reactants and products in the balanced equation
• 2nd row: Ratio in which reactants react and products form in balanced equation
• 3rd row: Initial quantities (number of moles) of reactants and products
• 4th row: Change i.e., the decrease in the number of moles of reactants and increase in the number of moles of products during the reaction
• 5th row: Equilibrium quantities, nequilibrium (number of moles)
• 6th row: Equilibrium concentrations, cequilibrium

WRITING AN EXPRESSION FOR THE EQUILIBRIUM CONSTANT (Kc)

• For the reaction $aA(g) + bB(g) \rightleftharpoons cC(g) + dD(g)$, $K_c = \frac{[C]^c[D]^d}{[A]^a[B]^b}$
• Only gases (g) and solutions (aq) appear in the Kc expression – no solids (s) and pure liquids (ℓ) are included.
• The equilibrium constant does not have a unit.
• Large Kc values: Reactions in which the concentration of products is high in comparison to that of reactants. Such a reaction proceeded well to form products.
• Small Kc values: Reactions in which the concentration of products is low in comparison to that of reactants. Such a reaction did not proceed well to form products.
• Only temperature can change the Kc value. Therefore, the Kc value for a reaction is given at a specific temperature.
  ${a\over b} [reac tan ts] \over [products]$ is NOT a Kc expression!

1 STEPS WHEN EXPLAINING IN TERMS OF LE CHATELIER’S PRINCIPLE

When explaining in terms of Le Chatelier’s principle, the following steps should be used:

1. Identify the disturbance e.g. increase in temperature.
2. State that the system will act to oppose this disturbance e.g. the system will decrease the temperature.
3. State how the system will manage to oppose the disturbance e.g. the increase in temperature will favor the endothermic reaction.
4. State which reaction will be favored when opposing the disturbance e.g. the reverse will be favored.
5. State the effect on the number of moles of products/reactants e.g. the number of moles of products will decrease or number of moles of reactants will increase.

CHEMICAL EQUILIBRIUM GRAPHS

• Rate versus time graph
  • Initially, only reactants are present, and the rate of the forward reaction is high but decreases with time as reactants are being used up. As products are formed, the rate of the reverse reaction increases from zero.
    At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction.
    hence the rates graph should indicate that both the forward and reverse rates are equal.
• Concentration