Kinetics Self-Test 1

Always use pen & paper to record your calculations, as some questions involve using figures already calculated

You must enter your answers as decimals and NOT in scientific notation, for example
0.000374 NOT 3.74E-4 or other scientific notations.

You MUST give your answer to THREE significant figures - remember that preceding zeros don't count to the number of significant figures. For numbers less than one always include a zero before the decimal point.

Just enter the number, not the units.

Answer to Kinetics Lecture 1, Prob. 1, p2

a) 1.05 x10-4 mol/s
b) 0.52 mol.m-3.s-1 = 5.23 x 10-4mol.L-1.s-1

## 1

The decomposition of p-methoxyphenyldiazonium ions in solution at 25C can be followed by measuring the N2 gas produced.

CH3O.C6H4.N2+soln = CH3O.C6H4+soln + N2,gas

Note that 1 atm = 101325 Pa

If this reaction produces 510 cm3 N2 (at 25C, 1 atm pressure) in 5 minutes, calculate the rate in moles of reaction per second

## 2

If this reaction occurs in 250 cm3 of solution, calculate the rate in mol.L-1.s-1

## 3

What is the rate calculated in the previous question, when expressed in mol.m3.s-1 ?

Answer to Prob. 2, Lecture 1, p2

(i) 0.005 mol .m-3.s-1

(ii) -0.010 mol .m-3.s-1

## 4

If the reaction between thiosulphate and iodine in solution

2S2O32- + I2 = S4O62- + 2I-

goes at 1000 mol.L-1.s-1, how fast will thiosulphate disappear?

## 5

How fast will tetrathionate, S4O62-, appear (in mol.L-1.s-1) ?

Answer to Prob. 3, Lecture 1, p5.

a) ¼ mm if to the nearest 0.5 mm. But if the lines are thin and the scale and your eyesight are good, then maybe 0.1 mm.

b) Rate of reaction of sucrose:

fastest = 2.043 x 10-4 mol.L-1.s-1
nominal = 1.690 x 10-4
slowest = 1.337 x 10-4
so rate = (1.69 +/- 0.35) x 10-4 mol.L-1.s-1.

## 6

Precision :
i.e. the difference between the maximum possible value, consistant with the data, and the nominal or 'best' value.

Calculate the precision on the amount (in moles) of N2 produced in question 1 if the pressure of the N2 is 101 325 ± 300 Pa, the volume is 510 ± 0.5 cm3, and the temperature is 25 ± 0.1C. Just give the numerical answer, not the units, to ONE significant figure.

Answer to Prob. 4, Lecture 1, p6.

Accuracy is how near the measurement is to giving the true value of a quantity. Inaccuracies may be due to the wide precision of the measuring scale used, or errors in the theory, or in the measuring device used.

Precision is the fineness of the measuring scale used together with the reproducibility of the measurement. The better the precision, the lower the scatter or spread of repeated measurements.

Precise measurements may not be accurate ones.