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Chemistry Notes Chapter-10 short question
Table of Contents
Q.1) When a clear liquid is placed in a beaker. How can you identify it as an acid, base or neutral (water)?
Answer: Litmus paper can be used to identify the pH of a clear liquid. If its an acid blue litmus paper will turn red. If its a base red litmus paper will turn blue. If it’s a neutral liquid the colour of litmus paper will not change.
Q.2) Justify H+ ion as Lewis acid?
Answer: Lewis acid is a specie that accepts or tends to accept a pair of electron. H+ ion act as a lewis acid because they protonate by accepting an electron pair from non-bonding electrons on a base such as OH–, NH3, etc.
Q.3) Distinguish strong acid from weak acids? Give two examples of each?
A strong acid is one that ionizes completely in an aqueous solution and gives a higher concentration of H+ ions.
The acids which do not completely dissociate in an aqueous solution and give lower concentration of H+ ions are called weak acids.
Q.5) A carbonated drink has [H+]= 3.2 x 10-3 M, classify the drink as neutral, acidic or basic with reason.
Answer: When the [H+] is greater than 1.0 x 10 -7M the solution is acidic so a carbonated drink has [H+]= 3.2 x 10-3M is acidic. The reason is CO2 is found in carbonated drinks. When carbon dioxide interacts with water carbonic acid is formed CO2 + H2O ⇌ H2CO3 Then carbonic acid dissociates as H2CO3 → H+ + HCO3– Releasing the H+ ions responsible for acidity of carbonated drink.
Q.6) Write the chemical name of an acid present in the following: a) Apple b) Grape juice c) Lemon Juice d) Sour milk
Q.7) What determines the strength of a base? Give one example of each solution that are strongly and weakly basic.
Answer: The strength of the base is determined by their dissociation in aqueous solution. Strong base completely dissociates in aqueous solution whereas a weak base not completely dissociate in an aqueous solution. NaOH is a strong base which completely dissociates into Na+ and OH– ions. NaOH + (aq) ⇌ Na+ + OH– Ammonia does not completely dissociate hence it forms a weakly basic solution. NH3(aq)+H2O(l) ⇌ NH+4(aq)+OH−(aq)
Q.8) Calculate the pH and pOH of 0.5M solution of HCl.
Answer: NaOH is a strong electrolyte it will dissociate completely NaOH ⇌ Na+ + OH– OH– conc. of NaOH = 0.5 So, pOH = -log[OH–] pOH = -log(0.2) pOH= 0.6989 pH + pOH = 14 pH = 14- pOH pH = 14 – 0.6989 pH = 13.30
Long Questions Chemistry Class 10 Notes 2021
Q.1 a) What is salt?
Answer: Salt: Salt is an ionic compound formed when an acid is neutralized with an alkali, a base, a carbonate or a metal. It is made up of two groups of oppositely charged ions. The ion with a positive charge is called a cation, and the ion with a negative charge is called an anion. For example, sodium chloride (NaCl) is a salt composed of sodium (metal) positive ion Na+ and chloride (non-metal) negative ion Cl–. Salts have very high melting and boiling points owing to the ionic bond which tightly binds the negative and positive ions together. However, the resulting molecule is neutral because positive and negative charges cancel out. The shape of the crystals is due to the water of crystallization in salts. Other examples of salts are: potassium sulphate (K2SO4), ferric phosphate (FePO4), silver bromide (AgBr), etc.
Q.1 b) Write down the different types of salts with examples?
Answer: Types of salts: Following are the main types of salts: (i) Normal or neutral salts A normal or neutral salt is formed when all of the hydrogen ions (H+) of acid have been replaced by positive metal ions or ammonium ions (NH4+). These salts are neutral to litmus paper. Examples: HCl + NaOH → NaCl + H2O H2SO4 + ZnO → ZnSO4 + H2O H3PO4 + NaOH → Na3PO4 + H2O (ii) Acidic salts Acidic salt is formed when hydrogen ions of acid have been partially replaced by positive metal ions. Acidic salts turn blue litmus red. Examples: H2SO4 + KOH → KHSO4 + H2O H3PO4 + NaOH → NaH2PO4 + H2O These salts react with bases to produce normal or neutral salts. For example: KHSO4 + KOH → K2SO4 + H2O NaH2PO4 + 2NaOH → Na3PO4 + 2H2O (iii) Basic salts A basic salt is formed by the partial neutralization of a base by an acid. These salts turn red litmus blue. Examples: Pb(OH)2 + HNO3 → Pb(OH)NO3 + H2O Al(OH)3 + HCl → Al(OH)2Cl + H2O These salts react with acids to produce normal or neutral salts. For example: Pb(OH)NO3 + HNO3 → (PbNO3)2 + H2O Al(OH)2Cl + 2HCl → AlCl3+ 2H2O
Chapter 10 Chemistry Part 2 Notes Matric
Q.2 a) Define the auto-ionization of water. How can you find the pH of water?
Answer: Autoionization of water: In the self ionization of water or autoionization, two molecules of water produce a hydronium ion(H3O+) and a hydroxide ion (OH–) by the transfer of a proton. Pure water contains equal number of hydronium ion (H3O+) and hydroxide ion (OH–). The following reaction takes place H2O + H2O → H3O+ + OH– ” The reaction in which two water molecules produce ions is called as the self ionization or autoionization of water” pH of Water: The pH of water is expressed by the following equation. pH = -log [H+] According to this scale, pH of water is calculated as, pH = – log [H+] Putting the value of [H+], the concentration of H+ ions in pure water is always 1.0 x 10-7 mol/dm3 of water at 25oC pH = – log [1.0 x 10-7] pH = -(-7.0) log 10 (log 10=1) pH = 7
Q.2 b) Why some acids are called monoprotic, diprotic and polyprotic acids. Explain your answer with suitable examples?
Answer: Acids can be classified in terms of number of protons that can be given by per molecule of acid when added to water. Based on this, acid is classified as monoprotic, diprotic and polyprotic. The acids which produce one proton per molecule are called monoprotic acids. For example, HCl, HNO3, HCN, CH3COOH, etc. The acids which produce two proton per molecule are called diprotic acids. For example, H2SO4, H2CO3 etc. The acids which produce more than one proton per molecule are called polyprotic acids, For example, H3PO4 , etc
Q.3 a) Discuss the concept of Lewis acids and bases with examples.
Answer: Lewis acids and bases In 1923, G.N. Lewis from UC Berkeley proposed a theory to describe acids and bases. His theory gave a generalized explanation of acids and bases based on structure and bonding. His theory used electrons instead of proton transfer. According to this concept: Acid is any substance that can accept a pair of non-bonding electrons. A Lewis acid is, therefore, an electron-pair acceptor. Base is any substance that can donate a pair of non-bonding electrons. A Lewis base is, therefore, an electron-pair donor. For example, when ammonia reacts with boron trifluoride, ammonia donates a pair of electron to boron trifluoride as follows:
As shown above, the reaction of a Lewis acid and a Lewis base produces a coordinate covalent bond. A coordinate covalent bond is a type of covalent bond in which one atom donates a pair of electrons and the other atom accepts a pair of electrons. In this case, NH3 donates its electrons to the BF3. Cations act as Lewis acids. For example, H+ acts as an acid when it reacts with ammonia, which acts as a base.
Thus, Lewis acid is a species that accepts a pair of electrons and will have vacant orbital. All cations are Lewis acids as they are able to accept electrons e.g. H+, Cu2+, Fe2+, etc. An atom, ion, or molecule with an incomplete octet of electrons is a Lewis acid e.g. BF3, AlF3, FeCl3, etc. Lewis base is a species that donates a pair of electrons and will have lone-pair of electrons. Anions are Lewis bases as they are able to donate electrons e.g. OH–, CN–, CH3COO– etc. An atom, ion, or molecule with a lone-pair of electrons is a Lewis base e.g. :NH3, H2O: , etc.
Q.3 b) Give the Bronsted-Lowery definition of acids and bases. Write equations that explain the definition.
Answer: Bronsted and Lowry concept: In 1923, the Danish Chemist J. N. Bronsted and an English chemist T. M. independently expanded the Arrhenius acid and base definitions. According to this concept, A Bronsted-Lowery acid is a substance (molecule or ion) that is a proton (H+) donor, whereas Bronsted-Lowery base is a substance (molecule or ion) that is a proton (H+) acceptor. HCl + NH3 → NH4+ + Cl– Hydrochloric acid acts as a Bronsted-Lowery acid when it reacts with ammonia. In this example, HCl donates a proton and acts an acid, while ammonia (NH3+) accepts a proton to form ammonium ion (NH4+) and serve as a base.
Water can also act as Bronsted-Lowery acid. For example the following reaction, in which the water molecule donated a proton to the ammonium molecule. H2O + NH3 → NH4+ + OH– In another example, water acts as a Bronsted-Lowery base as it accepts a proton from HCl, which act as an acid. HCl + H2O ⇔ H3O+ + Cl– Therefore, water molecule acts both as an acid as well as base.
Q.4) Below are two equations showing how two alkalis react with water.
NaOH + H2O ⇌ Na+ + OH– NH3 + H2O ⇌ NH4+ + OH– a) Name both the alkalis b) Which is classified as weak alkali and why? c) What is the likely pH of each alkali?
Answer: a) Sodium Hydroxide (NaOH) and ammonia (NH3) b) NH3 is weak alkali does not completely dissociate in an aqueous solution and give a lower concentration of OH– ions c) pH of NaOH is 13 and pH of ammonium solution is between 11 and 12.
Q.5 a) Write the balanced neutralization reaction of,
i) Strong acid and strong base ii) Strong acid and weak base iii) Weak acid and strong base iv) Weak acid and weak base
Answer: i) Strong acid and strong base HCl + NaOH →NaCl + H2O ii) Strong acid and weak base HCl + NO2– → HNO2 + Cl–iii) Weak acid and strong base HNO2 + NaOH →H2O + NaNO2 iv) Weak acid and weak base HF + CH3CO2– → F– + CH3CO2H
Answer: pH: The pH of a solution is defined as the negative logarithm of the molar hydrogen ion concentration. pH = -log [H]+ pOH: The pOH of a solution is defined as the negative logarithm of the molar hydroxide ion concentration. pOH = – log [OH]–
Q.6 a) According to your understanding which one of the three acid definitions is the broadest? Explain.
Answer: The Arrhenius and Bronsted-Lowry concepts of acids and bases are limited to substances that contain protons. G.N. Lewis (1923) proposed a more general and broader concept of acids and bases. According to Lewis Concept of Acids An acid is a substance (molecule or ion) that can accept a pair of electrons, while a base is a substance (molecule or ion) that can donate a pair of electrons. For example, a reaction between ammonia and boron trifluoride takes place by forming a coordinate covalent bond between ammonia and boron trifluoride by donating an electron pair of ammonia and accepting that electron pair by boron trifluoride.
According to Lewis concept, the following species can act as Lewis acids: (i) Molecules in which the central atom has incomplete octet. For example, in BF3, AICI3, FeCl3, the central atoms have only six electrons around them, therefore, these can accept an electron pair. (ii) Simple cations can act as Lewis acids. All cations act as Lewis acids since they are deficient in electrons. However, cations such as Na+, K+, Ca2+ ions, etc., have a very little tendency to accept electrons. While the cations like H+, Ag+ ions, etc., have a greater electron-accepting tendency, therefore, act as Lewis acids.
Q.6 b) Write the uses of any three salts?
Answer: Uses of Salts: 1. Sodium sulphate is used in the manufacture of paper, detergents and glass etc. 2. Potassium nitrate is used as fertilizer and for the manufacture of flint glass 3. Magnesium sulphate is used as antacid and laxative. This salt is also used in dye industry.