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Chemistry Test 240
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Chemistry Test 240
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  • Question 1/10
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    All fluorine atoms are in same plane in

    Solutions

    CONCEPT:

    Arrangement of Fluorine Atoms in Molecules

    • In molecular geometry, the arrangement of atoms affects the spatial orientation of atoms and bonds, which determines the shape of the molecule.
    • The position of atoms, particularly those in the same plane, is influenced by the type of bonding and the geometry around the central atom.
    • For certain compounds, like XeOF4, the geometry allows for some atoms (like fluorine) to be in the same plane, while others (like oxygen) may be above or below this plane.

    EXPLANATION:


     

    • In XeOF4, the xenon atom is surrounded by four fluorine atoms and one oxygen atom. The four fluorine atoms are in the same plane, forming a square planar arrangement, while the oxygen atom is positioned above or below this plane.

    CONCLUSION:

    • The correct answer is Option 3: XeOF4, where all fluorine atoms are in the same plane.

     

  • Question 2/10
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    The incorrect order of bond angle

    Solutions

    CONCEPT:

    Bond Angles and Molecular Geometry

    • Bond angles are the angles between adjacent bonds in a molecule, which influence the overall molecular shape and properties.
    • The geometry of a molecule is determined by the arrangement of electron pairs (bonding and lone pairs) around the central atom according to the Valence Shell Electron Pair Repulsion (VSEPR) theory.
    • The expected order of bond angles is determined by the type of geometry: linear (180°), trigonal planar (120°), tetrahedral (109.5°), and octahedral (90°).

    • The expected order should be XeF(180°) > XeF4 (90°) and XeCl4 (90°). However, this option presents an incorrect relationship, as XeF2 should not be less than XeF4 or XeCl4. Therefore, this order is incorrect.

    CONCLUSION:

    • The correct answer is Option 3, which contains the incorrect order of bond angles.

     

  • Question 3/10
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    What will be the effect on the equilibrium constant on increasing temperature, if the reaction neither absorbs heat nor releases heat?

    Solutions

    CONCEPT:

    Equilibrium Constant and Temperature Effects

    • The equilibrium constant (K) quantifies the ratio of concentrations of products to reactants at equilibrium for a given reaction.
    • The value of K is temperature-dependent; it changes based on whether the reaction is exothermic or endothermic.
    • In thermally neutral reactions (where the reaction neither absorbs nor releases heat), the equilibrium constant remains constant with temperature changes.

    EXPLANATION:

    • Le Chatelier’s Principle: This principle states that if a system at equilibrium is disturbed by a change in temperature, pressure, or concentration, the system will shift in a direction that counteracts the change.
    • Exothermic Reactions: For reactions that release heat, increasing the temperature will shift the equilibrium to favor the reactants, decreasing the value of K.
    • Endothermic Reactions: For reactions that absorb heat, increasing the temperature will shift the equilibrium to favor the products, increasing the value of K.

    Additional Information

    • A thermally neutral reaction has a heat of reaction (ΔH) of zero.
    • For such reactions, temperature changes do not affect the equilibrium position or the value of the equilibrium constant (K).
    • Thus, if the temperature is changed, K will remain constant.

    CONCLUSION:

    • The correct answer is Option 1: Equilibrium constant will remain constant.

     

  • Question 4/10
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    Which of the following is incorrect statement?

    Solutions

    CONCEPT:

    Understanding Reaction Orders and Equilibrium Statements

    • The order of a reaction indicates how the rate is affected by the concentration of reactants, which can be determined experimentally.
    • Zero-order reactions have a constant rate that is independent of the concentration of reactants, meaning the rate is determined by factors other than concentration.
    • Stoichiometry describes the proportions of reactants and products in a reaction but does not necessarily provide information about the reaction order or whether it is elementary.

    EXPLANATION:

    • Statement 1: Stoichiometry of a reaction tells about the order of the elementary reactions.
      • This statement is incorrect because stoichiometry describes the quantitative relationships between reactants and products but does not directly indicate the reaction order, which is determined experimentally.
    • Statement 2: For a zero-order reaction, rate and the rate constant are identical.
      • This statement is correct. In zero-order reactions, the rate is equal to the rate constant (k), meaning the rate does not depend on the concentration of reactants.
    • Statement 3: A zero-order reaction is controlled by factors other than the concentration of reactants.
      • This statement is correct. Zero-order reactions are typically controlled by factors such as temperature, catalysts, or surface area, rather than the concentration of reactants.
    • Statement 4: A zero-order reaction is an elementary reaction.
      • This statement is somewhat misleading. While some zero-order reactions can be elementary, not all are. Zero-order can also result from a complex mechanism. Thus, it’s not universally true that all zero-order reactions are elementary.

    CONCLUSION:

    • The incorrect statement is Option 1: Stoichiometry of a reaction tells about the order of the elementary reactions.

     

  • Question 5/10
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    If there are no intermolecular forces of attraction then the volume occupied by the molecules of 4.5 kg of water at STP will be

    Solutions

    CONCLUSION:

    • The volume occupied by 4.5 kg of water is 0.0045, m3 or 4.5 liters.

     

  • Question 6/10
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    When propionic acid is treated with aqueous sodium bicarbonate, CO2 is liberated. The ‘C’ of CO2 comes from

    Solutions

    CONCEPT:

    Reaction of Propionic Acid with Sodium Bicarbonate

    • Propionic acid (CH3CH2COOH) is a carboxylic acid, which means it has a functional group known as a carboxyl group (-COOH).
    • Sodium bicarbonate (NaHCO3), commonly known as baking soda, is a weak base that can react with acids to produce carbon dioxide (CO2), water (H2O), and a salt.
    • When propionic acid reacts with sodium bicarbonate, a typical acid-base reaction occurs, leading to the liberation of CO2 gas.

    EXPLANATION:

    • The reaction proceeds as follows:
      • When propionic acid is added to sodium bicarbonate, the bicarbonate ion (HCO3-) reacts with the hydrogen ion (H+) from the carboxylic acid group of propionic acid.
      • This interaction leads to the formation of carbonic acid (H2CO3), which is unstable and decomposes rapidly into water (H2O) and carbon dioxide (CO2).
    • The overall reaction can be summarized as:

      CH3CH2COOH + NaHCO3 → CH3CH2COONa + H2O + CO2

    • In this reaction, the CO2 produced comes specifically from the bicarbonate ion (HCO3-), which contains the carbon atom that is released as a gas.
    • Therefore, the answer to the original question regarding the source of the 'C' in CO2 is that it comes from the bicarbonate (option 4). The methyl group (CH3), methylene group, and the carboxylic acid group do not contribute the carbon that is released as CO2.

    CONCLUSION:

    The correct answer is bicarbonate.

     

  • Question 7/10
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    Which one of the following acids is thermally most unstable

    Solutions

    CONCEPT:

    Thermal Stability of Organic Acids

    • Thermal stability refers to the ability of a compound to withstand heat without decomposing.
    • Factors affecting thermal stability include molecular structure, the presence of functional groups, and the ability to lose small molecules (like water or carbon dioxide).
    • Generally, compounds with longer carbon chains and fewer reactive functional groups are more thermally stable.

    EXPLANATION:

    • This compound features an even longer -carbon chain, enhancing its thermal stability compared to the previous options.

    CONCLUSION:

    The correct answer is Option 1: CH3 – CO – COOH.

     

  • Question 8/10
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    Which of the following compounds does not evolve brown coloured gas on heating?

    Solutions

    CONCEPT:

    Decomposition of Metal Nitrates and the Evolution of NO2

    • When metal nitrates are heated, they generally decompose to form metal oxides, nitrogen dioxide (NO2), and oxygen (O2).
    • Nitrogen dioxide (NO2) is a toxic, reddish-brown gas, and its evolution is a key indicator of thermal decomposition of nitrates from certain metals.
    • The thermal decomposition of metal nitrates depends on the metal cation. Nitrates of heavier metals like lead (Pb) and calcium (Ca) release NO2 gas upon decomposition.

    EXPLANATION:

    • Pb(NO3)2 (Lead Nitrate)
      • Lead nitrate undergoes thermal decomposition as follows:

        2 Pb(NO3)2 → 2 PbO + 4 NO2 + O2

      • As seen from the reaction, lead nitrate produces nitrogen dioxide (NO2), which is a brown-colored gas.
    • LiNO3 (Lithium Nitrate)
      • Lithium nitrate decomposes as:

        4 LiNO3 → 2 Li2O + 4 NO2 + O2

      • Here again, nitrogen dioxide (NO2) is evolved, which appears as a brown gas.
    • Ca(NO3)2 (Calcium Nitrate)
      • Calcium nitrate undergoes decomposition as follows:

        2 Ca(NO3)2 → 2 CaO + 4 NO2 + O2

      • Again, nitrogen dioxide (NO2) is produced as a brown gas.
    • KNO3 (Potassium Nitrate)
      • When potassium nitrate is heated, it does not decompose into nitrogen dioxide (NO2). Instead, the decomposition reaction proceeds as:

        2 KNO3 → 2 KNO2 + O2

      • This reaction produces potassium nitrite (KNO2) and oxygen (O2), but no nitrogen dioxide (NO2) is released.
      • Since no NO2 is produced, no brown gas is evolved when potassium nitrate is heated.

    CONCLUSION:

    The correct answer is Option 4: KNO3.

     

  • Question 9/10
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    Which of the following is liable to be oxidised by periodic acid?

    Solutions

    CONCEPT:

    Oxidation by Periodic Acid (HIO4)

    • Periodic acid (HIO4) is a selective oxidizing agent used to cleave compounds that have vicinal diols (two hydroxyl groups attached to adjacent carbon atoms) or 1,2-diketones (two carbonyl groups on adjacent carbons).
    • The mechanism involves the cleavage of the C-C bond between the vicinal diols or 1,2-diketones, breaking the bond and forming carbonyl compounds (aldehydes or carboxylic acids).
    • This oxidation method is widely used in organic chemistry to identify the presence of such functional groups in molecules.

    EXPLANATION:

    Additional Information

    • Periodic acid is highly specific for vicinal diols and 1,2-diketones because of its ability to form a cyclic intermediate during the reaction, leading to the cleavage of the C-C bond.
    • This reaction is commonly used in carbohydrate chemistry to break down sugars into smaller components by cleaving the carbon backbone.

    CONCLUSION:

    All three compounds are oxidized by periodic acid (Option 4)

     

  • Question 10/10
    4 / -1

    Acetyl chloride does not react with

    Solutions

    CONCEPT:

    Reactivity of Acetyl Chloride (CH3COCl)

    • Acetyl chloride is an acyl chloride, which is highly reactive due to the presence of the electronegative chlorine atom attached to the carbonyl group.
    • It undergoes nucleophilic acyl substitution reactions, where the chlorine atom is substituted by a nucleophile.
    • Acetyl chloride readily reacts with nucleophilic substances like water, active metals like sodium, and alkenes in the presence of appropriate catalysts.

    EXPLANATION:

    Additional Information

    • Acyl chlorides like acetyl chloride are highly reactive due to the electron-withdrawing effect of the chlorine atom, which makes the carbonyl carbon highly electrophilic and prone to attack by nucleophiles.
    • Because acetyl chloride can react with nucleophiles like water, metals, and alkenes (in the presence of a catalyst), it is considered one of the most reactive members of the carboxylic acid derivative family.

    CONCLUSION:

    The correct answer is option 4. It reacts with all the three.

     

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