(a)(i) What is an acid-base indicator?
(ii) Give one example of an acid-base indicator.
(b) State the property exhibited by nitrogen(IV) oxide in each of the following equations:,
(i) 4Cu + 2NO\(_2\) -> 4CuO + N\(_2\) (ii) H\(_2\)O + 2NO\(_2\) –> HNO\(_3\) + HNO\(_2\)
(c)(i) Define enthalpy of combustion..
(ii) State why the enthalpy of combustion is always negative.
(d)(i) Distinguish between a primary cell and a secondary cell.
(ii) Give an example of each of the cells stated in I (d)(i).
(e) Define the term mole.
(f) Calculate the amount of hydrochloric acid in 40.0 cm\(^3\) of 0.40 moldm\(^{-3}\) dilute HCl.
(g) Name two substances which can be used as electrodes during the electroylsis of acidified water.
(h) List two forces of attraction that can exist between covalent molecules.
(i) Name the products formed when butane undergoes incomplete combustion.
(j) Write the electron configuration of \(_{26}\)Fe\(^{3+}\)
Explanation
a)(i) An acid-base indicator is either a weak organic acid or base which gives different colours in an acidic or basic mediurn. (Organic dye which changes colour according to the pH/hydrogen ion concentration of the solution).
(ii) Example of an acid-base indicator is: (i) phenolphthalein. (ii) methyl red. (iii) methyl orange. (iv) methyl yellow (v) alizarin yellow. (vi) litmus paper/solution. (vii) methyl violet. (viii) bromothymol blue (ix) bromocresol green.
(b) Property exhibited by nitrogen (IV) oxide in each of the following equations:
(i) 4Cu + 2NO\(_2\) —> 4CuO + N\(_2\), NO\(_2\) is an oxidizing agent
(ii) H\(_2\)O + 2NO\(_2\) —> HNO\(_3\) + HNO\(_3\), NO\(_2\) is oxidizing agent as well as a reducing agents or NO\(_2\) serves as acid anhydride or mixed anhydride.
(c)(i) Enthalpy of combustion is the amount of heat evolved or liberated when one mole of a substance is burnt mpletely in excess oxygen.
(ii) Enthalpy of combustion is always negative because heat is always given out or the action is exothermic.
(d) To distinguish between primary cell and secondary cell.
(i) (a) Primary cells cannot be recharged while secondary cells can be recharged.
(b) Primary cells do not last long while secondary cells last long.
(c) Reactions in primary cells are not reversible while reactions in secondary cells are reversible.
(ii) Examples of each of the cells in 1(d)(i). Primary cell: radio battery, dry cell, alkaline battery, button battery, Le'clanche cells, Daniel cell, torch battery, clock battery, bicycle lamp battery (any of these).
Secondary cell: lead-acid (storage - cell), lead-acid accumulator, lithium-ion battery, battery in rechargeable lamp, car battery, nickel-metal hydride cell, phone battery, nickel-cadmium cell, (hydrogen-oxygen) fuel cell, aluminium air cell/battery (any of these).
(e) Mole is defined as the amount of substance containing 6.02 x10\(^{23}\) particles of that substance as present in 12g of carbon-12 or the amount of substance containing as many elementary entities as there are atoms in 12g of carbon-12.
(f) Amount of hydrochloric acid in 40.0cm\(^{3}\) of 0.40 mo/dm\(^{3}\) dilute HCl
n = CV; C = concn in mo/dm\(^{3}\)
n = 0.40 x \(\frac{40}{1000}\); V = volume in dm\(^{3}\)
n(mol) = 0.016mol; n = amount in mol
(g) Two substances used as electrodes during electrolysis of acidified water are: graphite (carbon) and platinum.
(h) Two forces of attraction that can exist between covalent molecules
(1) Van der waals' forces or induced dipole-dipole forces.
(2) Hydrogen bonds.
(3) Permanent dipole-dipole interactions or dipolar forces (any of the two):
(i) Products formed when butane undergoes incomplete combustion are carbon (ii) oxide and water.
(j) Electron configuration of \(_{26}\)Fe\(^{3+}\)
\(_{26}\)Fe; Fe - 3\(\bar{e}\) \(\to\) Fe\(^{3+}\) lost of 3 electrons
\(_{26}\)Fe 26 protons + 26 electrons
\(_{26}\)Fe\(^{3+}\) 26 protons + 23 electrons
\(_{26}\)Fe\(^{3+}\) \(\to\) 1s\(^2\), 2s\(^2\), 2p\(^6\), 3s\(^2\), 3P\(^2\), 3d\(^5\)