Chemistry

(c) Carius method

In the Carius method the organic compound is heated with fuming nitric acid (with AgNO$_3$ for halogens, or with BaCl$_2$ for sulphur). It is weighed as AgX or BaSO$_4$. Dumas and Kjeldahl are for nitrogen, and Liebig is for carbon and hydrogen.

(b) $\Delta H$ is positive

An endothermic reaction absorbs heat from the surroundings, so the enthalpy of the products is greater than that of the reactants:

$\Delta H = H_{products} - H_{reactants} > 0$

(a) s

Atomic number 11 is sodium, with electron configuration $1s^2 2s^2 2p^6 3s^1$. The last electron enters the 3s orbital, so it is an s-block element (alkali metal).

(b) Quick lime

Heating limestone in a kiln (about 900 °C) decomposes it:

$CaCO_3 \xrightarrow{\Delta} CaO + CO_2$

CaO is quick lime. Adding water gives slaked lime, $Ca(OH)_2$.

(a) NH$_4$Cl

NH$_4$Cl is a salt of a strong acid (HCl) and a weak base (NH$_4$OH). Its cation hydrolyses:

$NH_4^+ + H_2O \rightleftharpoons NH_4OH + H^+$

This makes the solution acidic. NaCl and KNO$_3$ are neutral salts, and NH$_4$OH is a base.

(d) Ag

A metal can displace Cu from solution only if it is more reactive than Cu. The order is Zn > Fe > Pb > Cu > Ag. Zn, Fe and Pb displace copper, but Ag is less reactive and cannot.

(b) 7

Milli-equivalents of NaOH $= 20 \times 0.1 = 2$. Milli-equivalents of HCl $= 10 \times 0.2 = 2$.

They are equal, so the mixture is exactly neutralised, leaving a solution of NaCl (a neutral salt) with pH $= 7$.

(a) Efflorescent

Efflorescence is the loss of water of crystallisation when a hydrated salt is exposed to air, for example $Na_2CO_3\cdot10H_2O$. Deliquescent substances absorb so much moisture that they dissolve, and hygroscopic substances absorb moisture.

(a) Normality

Normality is the number of gram equivalents of solute per litre of solution:

$N = \frac{\text{gram equivalents of solute}}{\text{volume of solution (L)}}$

Molarity is moles per litre of solution, and molality is moles per kg of solvent.