5.1. Enthalpy change, \(\Delta H\)
A subsection of Chemistry, 9701, through 5. Chemical energetics
Listing 10 of 305 questions
For some chemical reactions, such as the thermal decomposition of potassium hydrogencarbonate, KHCO3, the enthalpy change of reaction cannot be measured directly. In such cases, the use of Hess’ Law enables the enthalpy change of reaction to be calculated from the enthalpy changes of other reactions. State Hess’ Law. In order to determine the enthalpy change for the thermal decomposition of potassium hydrogencarbonate, two separate experiments were carried out. experiment 1 30.0 cm3 of 2.00 mol dm–3 hydrochloric acid (an excess) was placed in a conical flask and the temperature recorded as 21.0 °C. When 0.0200 mol of potassium carbonate, K2CO3, was added to the acid and the mixture stirred with a thermometer, the maximum temperature recorded was 26.2 °C. Construct a balanced equation for this reaction. Calculate the quantity of heat produced in experiment 1, stating your units. Use relevant data from the Data Booklet and assume that all solutions have the same specific heat capacity as water. Use your answer to to calculate the enthalpy change per mole of K2CO3. Give your answer in kJ mol–1 and include a sign in your answer. Explain why the hydrochloric acid must be in an excess. experiment 2 The experiment was repeated with 0.0200 mol of potassium hydrogencarbonate, KHCO3. All other conditions were the same. In the second experiment, the temperature fell from 21.0 °C to 17.3 °C. Construct a balanced equation for this reaction. Calculate the quantity of heat absorbed in experiment 2. Use your answer to to calculate the enthalpy change per mole of KHCO3. Give your answer in kJ mol–1 and include a sign in your answer. When KHCO3 is heated, it decomposes into K2CO3, CO2 and H2O. 2KHCO3 K2CO3 + CO2 + H2O Use Hess’ Law and your answers to and to calculate the enthalpy change for this reaction. Give your answer in kJ mol–1 and include a sign in your answer.
9701_w11_qp_21
THEORY
2011
Paper 2, Variant 1
View
For some chemical reactions, such as the thermal decomposition of potassium hydrogencarbonate, KHCO3, the enthalpy change of reaction cannot be measured directly. In such cases, the use of Hess’ Law enables the enthalpy change of reaction to be calculated from the enthalpy changes of other reactions. State Hess’ Law. In order to determine the enthalpy change for the thermal decomposition of potassium hydrogencarbonate, two separate experiments were carried out. experiment 1 30.0 cm3 of 2.00 mol dm–3 hydrochloric acid (an excess) was placed in a conical flask and the temperature recorded as 21.0 °C. When 0.0200 mol of potassium carbonate, K2CO3, was added to the acid and the mixture stirred with a thermometer, the maximum temperature recorded was 26.2 °C. Construct a balanced equation for this reaction. Calculate the quantity of heat produced in experiment 1, stating your units. Use relevant data from the Data Booklet and assume that all solutions have the same specific heat capacity as water. Use your answer to to calculate the enthalpy change per mole of K2CO3. Give your answer in kJ mol–1 and include a sign in your answer. Explain why the hydrochloric acid must be in an excess. experiment 2 The experiment was repeated with 0.0200 mol of potassium hydrogencarbonate, KHCO3. All other conditions were the same. In the second experiment, the temperature fell from 21.0 °C to 17.3 °C. Construct a balanced equation for this reaction. Calculate the quantity of heat absorbed in experiment 2. Use your answer to to calculate the enthalpy change per mole of KHCO3. Give your answer in kJ mol–1 and include a sign in your answer. When KHCO3 is heated, it decomposes into K2CO3, CO2 and H2O. 2KHCO3 K2CO3 + CO2 + H2O Use Hess’ Law and your answers to and to calculate the enthalpy change for this reaction. Give your answer in kJ mol–1 and include a sign in your answer.
9701_w11_qp_22
THEORY
2011
Paper 2, Variant 2
View
A student calculated the standard enthalpy change of formation of ethane, C2H6, using a method based on standard enthalpy changes of combustion. He used correct values for the standard enthalpy change of combustion of ethane (–1560 kJ mol–1) and hydrogen (–286 kJ mol–1) but he used an incorrect value for the standard enthalpy change of combustion of carbon. He then performed his calculation correctly. His final answer was –158 kJ mol–1. What did he use for the standard enthalpy change of combustion of carbon?
9701_w12_qp_11
MCQ
2012
Paper 1, Variant 1
View
A student calculated the standard enthalpy change of formation of ethane, C2H6, using a method based on standard enthalpy changes of combustion. He used correct values for the standard enthalpy change of combustion of ethane (–1560 kJ mol–1) and hydrogen (–286 kJ mol–1) but he used an incorrect value for the standard enthalpy change of combustion of carbon. He then performed his calculation correctly. His final answer was –158 kJ mol–1. What did he use for the standard enthalpy change of combustion of carbon?
9701_w12_qp_12
MCQ
2012
Paper 1, Variant 2
View
Use of the Data Booklet is relevant to this question. A student carried out an experiment to determine the enthalpy change for the combustion of methanol. The following results were obtained by the student. start temperature of the water 20 °C final temperature of the water 53 °C mass of alcohol burner before burning 259.65 g mass of alcohol burner after burning 259.15 g mass of glass beaker plus water 150.00 g mass of glass beaker 50.00 g How much of the heat energy produced by the burning of methanol went into the water?
9701_w13_qp_11
MCQ
2013
Paper 1, Variant 1
View
Use of the Data Booklet is relevant to this question. A student carried out an experiment to determine the enthalpy change for the combustion of methanol. The following results were obtained by the student. start temperature of the water 20 °C final temperature of the water 53 °C mass of alcohol burner before burning 259.65 g mass of alcohol burner after burning 259.15 g mass of glass beaker plus water 150.00 g mass of glass beaker 50.00 g How much of the heat energy produced by the burning of methanol went into the water?
9701_w13_qp_12
MCQ
2013
Paper 1, Variant 2
View
Questions Discovered
305