(a)The table below gives the volume/pressure data for a particular sample of a gas-at a given temperature.
Volumedm\(^3\) (V)
4.00
2.00
1.00
Pressure/atm (P)
1.00
2.00
4.00
(i) Deduce a mathematical relationship between volume (V) and pressure (P). (ii) Name law that can be deduced from the data. (iii) Calculate the pressure of the gas when the volume is 3.20 dm\(^3\)
(b)(i) What is the role of a salt bridge in an electrochemical cell? (ii) What type of ions must flow into the cathode? Give a reason for your answer. (iii)A standard galvanic cell constructed with Ag\(^+_{(aq)}\) Ag\(_{(s)}\) and Zn\(^{2+}_{(aq)}\) couple is discharged until 3.3 g of Ag forms. I. Write the overall cell reaction and standard cell potential? II. How many moles of electrons flowed through the circuit during the discharge? III. How many coulombs of charges flowed through the circuit? Ag\(^+_{(aq)}\) + e\(^-\) \(\rightleftharpoons\) Ag\(_{(s)}\); E° = + 0.80V Zn\(^{2+}_{(aq)}\) + 2\(^-\) \(\rightleftharpoons\) Zn\(_{(s)}\), E° = – 0.76 V [Ag = 108]
(c)(i) Define each of the following terms: I. Activation energy; II. Exothermic reaction. (ii) Give one example of an endothermic process. (iii) What is the significance of activated complex in a chemical reaction?
Explanation
(a)(i) Volume (V) decreases as pressure (P) increases or V a PV = K where K is a constant of proportionality (ii) Boyles law (iii) P\(_1\)V\(_1\) = P\(_2\)V\(_2\) \(\frac{P_1V_1}{V_2}\) = \(\frac{1 \times 4}{3.2}\) \(\frac{2 \times 42}{3.2}\) = \(\frac{4 \times 1}{3.2}\) = 12.5 atm
(b)(i) Salt bridge is to maintain electrical neutrality in each cell / to allow cations and anions move from o electrode compartment to the other. (ii) Negative ions Because the cathode is positively charged. (iii) (I) 2 Ag\(^+_{(aq)}\) + Zn\(_{(s)}\) \(\to\) 2Ag\(_{(s)}\) + ZnAg\(^+_{(aq)}\) E° = E° cathode — E° Anode = + 0.80 – (-0.76V) = +1.56V (II) Ag\(^+\) + e\(^-\) \(\to\) Ag 108g of Ag = 1 mole of e\(^-\) Thus 3.3g of Ag = \(\frac{1}{108}\) = 0.03mol (III) 1 mole e\(^-\) = 96500C 0.03 mole e\(^-\) = 96500 x 0.03 = 2.39 x 10\(^3\)C.
c)(i) (I) Activation energy is the minimum amount of energy required to effect a chemical reactioniminim amount of energy required by reactants to form products. (II) Exothermic reaction is a reaction in wh energy is released to the surrounding. (ii) Ammonium chloride dissolved in water. –Thermal decomposit of CaCO\(_3\) (iii) Activated complex must be attained / formed for reactants to form products.
