Lesmahagow High School

Transcription

1 Lesmahagow High School Higher hemistry Enthalpy - Tutorial Questions

2 Energy hanges Tutorial Questions 1 1. What is the relationship between a, b, c and d? S (s) + H 2(g) H 2 S (g) H = a H 2(g) + ½ O 2(g) H 2 O (l) H = b S (s) + O 2(g) SO 2(g) H = c H 2 S (g) + 1½ O 2(g) H 2 O (l) + SO 2(g) H = d a = b + c d a = d b c a = b c d a = d + c b 2. 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. ompound Enthalpy of combustion / kjmol -1 methanol -715 ethanol propan-1-ol Why is there a regular increase in enthalpies of combustion from methanol to ethanol and propan-1-ol? c. The equation for the enthalpy of formation of propan-1-ol is: 3 (s) + 4H 2(g) + ½O 2(g) 3 H 7 OH (l) Use information on enthalpies of combustion from the data booklet to calculate the enthalpy of formation of propan-1-ol.

3 Energy hanges Tutorial Questions 2 1. Which reaction cannot be described as an enthalpy of formation? Si (s) + 4l (g) Sil 4(l) Mg (s) + ½ O 2(g) MgO (s) (s) + 2H 2(g) + ½O 2(g) H 3 OH (l) 2 (s) + 3H 2(g) 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) + Hl (aq) Nal (aq) + H 2 O (l) a. Write an equation for a third reaction which, when taken with the above two, could be used to verify Hess s Law. b. pupil carried out reaction 2 by adding 50cm 3 NaOH (aq) to 50cm 3 Hl (aq). Each solution had a concentration of 2.0moll -1. The temperature rise was 13.5 o. i. What measurements would the pupil have to make to obtain the temperature rise? ii. Use the results from reaction 2 to calculate the enthalpy of neutralisation for the reaction. c. The enthalpy change involved in reaction 1 can be shown by the following diagram which is not drawn to scale. Na + (g) + OH - (g) hydration kj enthalpy -850 kj NaOH (s) Na + (aq) + OH - (aq) i. Name the enthalpy change involved in process.

4 ii. alculate the enthalpy of solution of NaOH (s), in kj Energy hanges Tutorial Questions 3 1. The enthalpies of combustion of (s), H 2(g) and 4 H 9 OH (l), are as follows: (s) + O 2(g) O 2(g) H = a H 2(g) + ½O 2(g) H 2 0 (l) H = b 4 H 9 0H (l) (g) 4O 2(g) + 5H 2 0 (l) H = c The enthalpy of formation of butan-1-ol is given by the equation: 4 (s) + 5H 2(g) + ½O 2(g) 4 H 9 0H (l) Which of the following can be used to calculate the enthalpy of formation of butan-1-ol? 4a + 5b - c 2a + 10b - c c - 4a - 5b 2a + 5b + c 2a. raw 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.

5 Energy hanges Tutorial Questions 4 1. N 2{g) (g) 2NO 2(g) H = +88kJ N 2{g) (g) N 2 O 4(g) H = +10kJ The enthalpy change for the reaction 2NO 2(g) N 2 O 4(g) will be +98 kj +78kJ -78kJ -98 kj. 2. Some rockets have a propellant system which combines dinitrogen tetroxide with methyl hydrazine. 5N 2 O 4 + 4H 3 NHNH 2 xn 2 + yh 2 O + zo 2 a. State the values of x, y and z required to balance the above equation. b. raw the full structural formula for methyl hydrazine. c. Methyl hydrazine burns according to the following equation. H 3 NHNH 2(l) + 2O 2(g) O 2(g) + 3H 2 O (l) + N 2(g) H = -1305kJmol -1 Use this information, together with information from page 9 of the data booklet, to calculate the enthalpy change for the following reaction. (s) + N 2(g) + 3H 2(g) H 3 NHNH 2(l)

6 Energy hanges Tutorial Questions 5 1. Ethanol ( 2 H 5 OH) has a different enthalpy of combustion from dimethyl ether (H 3 OH 3 ). This is because the compounds have different molecular masses bonding within the molecules products of combustion boiling points 2. Potassium hydroxide can be used in experiments to verify Hess's Law. The reactions concerned can be summarised as follows. a. omplete the list of measurements that would have to be made in order to calculate H 2. i ii iii iv Mass of potassium hydroxide b. What solution must be added to the potassium hydroxide solution in order to calculate H 3?

7 Energy hanges Tutorial Questions 6 1. onsider the reaction pathway shown below: W H = -210kJmol -1 Z H = -50 kjmol -1 X H = -86 kjmol -1 Y ccording to Hess's Law, the H value, in kjmol -1, for reaction Z to Y is 2. 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 to Use this information to calculate the enthalpy of combustion of ethanol.

8 Energy hanges Tutorial Questions 7 1. The enthalpy of neutralisation in an acid/alkali reaction is always the energy released in 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, find by calculation which reactant is in excess. b. In experiment 1, calculate the energy released during the reaction. c. alculate T for experiment 2. d. Outline a third experiment which would have to be carried out in order to confirm Hess's Law.

9 Energy hanges Tutorial Questions 8 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? 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, is used for repairing cracked railway lines as shown in the diagram below. a. Suggest why this process is suitable for repairing cracked railway lines. b. The enthalpy changes for the formation of one mole of aluminium oxide and one mole of iron(iii) oxide are shown below. 2l (s) + 1½ O 2(g) l 2 O 3(s) H = kjmol -1 2Fe (s) + 1½ O 2(g) Fe 2 0 3(s) H = -825 kjmol -1 Use the above information to calculate the enthalpy change for the reaction: 2l (s) + Fe 2 0 3(s) 2Fe (s) + l 2 O 3(s)