Heat energy change revision questions

Transcription

1 Name: Heat energy change revision questions Date: Time: Total marks available: 63 Total marks achieved:

2 Q1. A student uses this apparatus to find the increase in temperature of water when methanol, CH 3OH, is burned. (a) There are several reasons why the increase in temperature is less than expected. (i) One reason is the incomplete combustion of methanol to form only carbon monoxide and water. Write the chemical equation for this incomplete combustion. (ii) State another reason why the increase in temperature is less than expected. (b) The student records these results. (i) Calculate the heat energy change (Q), in joules, in this experiment using the expression Q = m 4.2 ΔT where m is the mass of water in grams and ΔT represents the increase in temperature. Q =... J

3 (ii) The relative molecular mass of methanol is 32 Use this information and your value for Q to calculate the molar enthalpy change, ΔH, for the combustion of methanol. Give your answer in kj/mol. (4) ΔH =... kj/mol (iii) The student draws an energy level diagram for the complete combustion of methanol. Identify the two mistakes in his diagram

4 (c) The student is given this table of average (mean) bond energies. The equation for the complete combustion of methanol is Use this equation and the information in the table to calculate another value for the molar enthalpy change, ΔH, for the combustion of methanol. (4) ΔH =... kj/mol (Total for question = 15 marks) Q2. A student does some experiments to find the heat energy released when natural gas burns. She uses this apparatus.

5 (a) The diagram shows the thermometer readings in one of her experiments. Use these readings to complete the table, entering all values to the nearest 0.1 C. (3) (b) The student repeats the experiment three times. The table shows her results. (i) Calculate the amount, in moles, at room temperature and pressure, of methane burned in experiment 1. Assume that natural gas contains only methane. (The volume of 1 mol of a gas at room temperature and pressure is cm 3 ) amount =... mol

6 (ii) The quantity of heat energy released in experiment 1 is J. Calculate the molar enthalpy change, in kj/mol, for the combustion of methane. molar enthalpy change =... kj/mol (iii) The temperature rise in experiment 2 is 41.2 C. Calculate the heat energy change in experiment 2 using the expression heat energy change = volume of water 4.2 temperature change (in J) (in cm 3 ) (in C) heat energy change =... J (iv) The student uses the results from experiment 3 to calculate the molar enthalpy change, in kj/mol, for the combustion of methane. She compares her value with the value in a data book. Which is the best explanation for the large difference between these two values? A natural gas contains other gases that release heat energy when burned B not all of the heat energy is transferred to the water C some of the water evaporates during the experiment D the student measures the gas by volume instead of by mass (c) The student uses a table of average bond energies to calculate another value for the molar enthalpy of combustion of methane.

7 The equation for the combustion can be shown using displayed formulae. (i) Use values from the table to calculate the energy taken in when the bonds in the reactants are broken. energy taken in =... kj (ii) Use values from the table to calculate the energy given out when the bonds in the products are formed. energy given out =... kj (iii) Use your answers to (i) and (ii) to calculate the molar enthalpy change for the combustion of methane. molar enthalpy change =... kj/mol (Total for question = 15 marks) Q3. A student uses this apparatus to measure the temperature change when lithium iodide dissolves in water.

8 He measures the steady temperature of the water before adding the lithium iodide. He then adds the lithium iodide, stirs the mixture until all the solid dissolves and records the maximum temperature reached. The diagram shows the thermometer readings before and after dissolving the lithium iodide. (a) Use the readings to complete the table. (3) (b) In a second experiment, using the same mass of water, the student records a temperature increase of 4.9 C. (i) Use this expression to calculate the heat energy change in this experiment. heat energy change =... J (ii) In this experiment, 6.3 g of lithium iodide were used. Calculate the amount, in moles, of lithium iodide in 6.3 g. [M r of lithium iodide = 134] amount of LiI =... mol

9 (c) In a third experiment the student obtains these results. (i) Calculate the molar enthalpy change, in kj/mol, in this experiment. molar enthalpy change =... kj/mol (ii) The temperature change in this experiment shows that dissolving lithium iodide in water to form lithium iodide solution is an exothermic process. Complete the energy level diagram to show the position of the lithium iodide solution. Label the diagram to show ΔH, the molar enthalpy change. (Total for question = 11 marks)

10 Q4. A student does an experiment to investigate how the temperature changes as different masses of solid potassium nitrate are dissolved in water. She looks at this graph to help her decide the masses of water and potassium nitrate to use in her experiment. (a) The student decides to use a mass of 50 g of water at a temperature of 25 C. From the graph, find the maximum mass of potassium nitrate that dissolves in this experiment. (b) The student prepares six samples of potassium nitrate, each with a mass of 2.0 g. She pours 50 cm 3 of water into a 100 cm 3 beaker and records the temperature of the water. She then uses this method to find the change in temperature as she adds each sample of potassium nitrate. add the first sample of potassium nitrate to the beaker and stir until the sample dissolves record the temperature of the solution add the second sample of potassium nitrate to the solution in the beaker and stir until the sample dissolves record the new temperature of the solution repeat until all six samples of potassium nitrate have been added

11 The table shows her results. (i) Plot the student's results on the grid. Draw a straight line of best fit. (3) (ii) From the graph, find the mass of potassium nitrate that would be needed to produce a temperature change of 10.0 C. (iii) Explain how the student's results show the type of heat change that occurs when potassium nitrate dissolves in water.

12 (iv) Complete the energy level diagram for this experiment. (c) The student repeats the experiment and obtains these results. mass of water (m) = 50 g total mass of potassium nitrate added = 15 g starting temperature = 32 C final temperature = 13 C Calculate the heat energy change (Q), in joules, using the expression [ΔT is the temperature change] heat energy change (Q) =... J (Total for question = 10 marks)

13 Q5. Hydrogen is used as a fuel. The equation for its combustion is (a) Explain, in terms of the energy changes involved in breaking and making bonds, why this reaction gives out heat (3) (b) Hydrogen is often described as a clean fuel because the only product of its combustion is water. (i) Anhydrous copper(ii) sulfate can be used to show the presence of water. State the colour change of anhydrous copper(ii) sulfate when water is added to it, and write a chemical equation for the reaction. Colour change... to... Equation. (ii) A physical test can be used to show that a sample of the water formed is pure. State the test and the result for pure water. Test. Result. (4) (c) The water formed by burning hydrogen as a fuel is often slightly acidic because it is contaminated with nitric acid. The presence of acid in this water can be detected using methyl orange or by measuring the ph of the water. (i) State the colour of methyl orange in water contaminated with a small amount of nitric acid.. (ii) Suggest why universal indicator is more suitable than methyl orange for comparing the acidities of samples of water... (iii) Suggest a possible ph value for water contaminated with a small amount of nitric acid.. (Total for question = 12 marks)