A sample contains very long covalent molecules. It softens when heated, does not conduct electricity and becomes solid again on cooling.
Explain these observations using a polymer model.
[6 marks]OCR Gateway GCSE Chemistry June 2019 Paper 4
5 questions on this paper's topics
29:001 of 5
A reaction uses the same solution concentration at 20 °C and 35 °C. A student explains the faster reaction at 35 °C by saying particles are larger and the activation energy has decreased.
Evaluate the explanation.
[4 marks]Three repeated demonstrations with equal samples of a group 1 metal collect hydrogen volumes of 47, 49 and 48 cm³. A separate run collects 31 cm³ after the gas tube becomes loose.
.1
Calculate the mean volume from the three valid repeated demonstrations.
[3 marks].2
Explain why the low run cannot show that the metal was less reactive.
[2 marks]A tank initially holds 2.0 dm³ of solution at 18 g/dm³. A 0.50 dm³ portion is removed after mixing, then 0.50 dm³ of water is added.
.1
Calculate the final concentration in the tank.
[4 marks].2
Explain why restoring the initial volume does not restore the initial concentration.
[2 marks]An exothermic synthesis gives the following measured data. At 250 °C: equilibrium yield 80%, time to equilibrium 40 min. At 350 °C: yield 60%, time 8 min. A plant must reach equilibrium within 10 min and obtain at least 55% yield.
.1
Calculate the factor by which equilibration is quicker at 350 °C.
[2 marks].2
Justify a temperature choice from these two settings and explain why the highest yield alone is not the best criterion.
[2 marks]