Lesmahagow High School Higher Chemistry Enthalpy - Tutorial Questions
Energy Changes Tutorial Questions 1 1. A pupil found the enthalpy of combustion of propan-1-ol using the following apparatus: a. In addition to the initial and final temperatures of the water, what other measurements would the pupil make? b. The table below shows the enthalpies of combustion of three alcohols. Compound Enthalpy of combustion / kjmol -1 methanol -715 ethanol -1371 propan-1-ol -2010 Why is there a regular increase in enthalpies of combustion from methanol to ethanol and propan-1-ol? Energy Changes Tutorial Questions 2 1. Which reaction cannot be described as an enthalpy of formation? A Si (s) + 4Cl (g) SiCl 4(l) B Mg (s) + ½ O 2(g) MgO (s) C C (s) + 2H 2(g) + ½O 2(g) CH 3 OH (l) D 2C (s) + 3H 2(g) C 2 H 6(g) 2. Hess s Law states that: The total enthalpy change of a chemical reaction is independent of the intermediate steps between reactants and products. It can be verified using the enthalpy changes for the reaction between sodium hydroxide and hydrochloric acid. Reaction 1 NaOH (s) + aq NaOH (aq) Reaction 2 NaOH (aq) + HCl (aq) NaCl (aq) + H 2 O (l)
A pupil carried out reaction 2 by adding 50cm 3 NaOH (aq) to 50cm 3 HCl (aq). Each solution had a concentration of 2.0moll -1. The temperature rise was 13.5 o C. a) What measurements would the pupil have to make to obtain the temperature rise? b) Use the results from reaction 2 to calculate the enthalpy of neutralisation for the reaction. Energy Changes Tutorial Questions 3 1 a. Draw a labelled diagram of the assembled apparatus you could use to find the enthalpy of combustion of butan-1-ol. b. In their experiment a group of students found that 0.10 mol of butan-1-ol released 143 kj on burning. Use this value to calculate the enthalpy of combustion of butan-1-ol. c. The enthalpies of combustion of methanol, ethanol and propan-1-ol are given in the data booklet. Use these values to predict the enthalpy of combustion of butan-1-ol. d. In addition to heat loss, give another reason to explain why the experimental value for the enthalpy of combustion of butan-1-ol is significantly lower than the value given in data books.
Energy Changes Tutorial Questions 4 1. Ethanol (C 2 H 5 OH) has a different enthalpy of combustion from dimethyl ether (CH 3 OCH 3 ). This is because the compounds have different A B C D molecular masses bonding within the molecules products of combustion boiling points 2. A calorimeter, like the one shown, can be used to measure the enthalpy of combustion of ethanol. The ethanol is ignited and burns completely in the oxygen gas. The heat energy released in the reaction is taken in by the water as the hot product gases are drawn through the coiled copper pipe by the pump. a. Why is the copper pipe coiled as shown in the diagram? b. The value for the enthalpy of combustion of ethanol obtained by the calorimeter method is higher than the value obtained by the typical school laboratory method. One reason for this is that more heat is lost to the surroundings in the typical school laboratory method. Give one other reason for the value being higher with the calorimeter method. c. In one experiment the burning of O.980g of ethanol resulted in the temperature of 400 cm 3 of water rising from 14.2 C to 31.6 C. Use this information to calculate the enthalpy of combustion of ethanol. Energy Changes Tutorial Questions 5 1. The enthalpy of neutralisation in an acid/alkali reaction is always the energy released in A B C D the neutralisation of one mole of acid the neutralisation of one mole of alkali the formation of one mole of water the formation of one mole of salt.
2. The following results are taken from the notebook of a student who was trying to confirm Hess's Law. a. In experiment 1, calculate the energy released during the reaction. b. Calculate T for experiment 2. Energy Changes Tutorial Questions 6 1. When 3.6g of butanal (relative formula mass = 72) was burned, 134 kj of energy was released, From this result, what is the enthalpy of combustion, in kjmol -1? A -6.7 B +6.7 C -2680 D +2680
2. The Thermit Process involves the reaction between aluminium and iron(iii) oxide to produce iron and aluminium oxide. This highly exothermic reaction, which generates so much heat that the temperature of the mixture rises to around 3000 C, is used for repairing cracked railway lines as shown in the diagram below. Suggest why this process is suitable for repairing cracked railway lines.