Fig. 1 shows part of a field kit designed to warm water for washing equipment at a remote survey site. A sealed inner container holds calcium oxide powder. ...
Question 1 Report
Fig. 1 shows part of a field kit designed to warm water for washing equipment at a remote survey site. A sealed inner container holds calcium oxide powder. When the divider is opened, water enters the powder and a reaction forms calcium hydroxide. The container is placed inside an insulated box around a metal water bottle. A student records the temperature of 5.0 kg of water before and after the reaction. No electrical energy is supplied. The temperature increase is used to judge whether the kit can replace a gas heater.
Fig. 1
The water temperature rises from 16.0 degrees C to 34.0 degrees C. The specific heat capacity of water is 4.2 kJ/kg degrees C.
(a) Name the type of energy transfer taking place from the reacting chemicals to the water. [1]
(b) Use the temperature data and the specific heat capacity to calculate the thermal energy gained by the water. Give your answer in kJ. [3]
(c) Describe two features of the box in Fig. 1 that reduce energy transfer to the surroundings. [2]
(d) Explain why the reaction between calcium oxide and water is exothermic. [2]
(e) What change could the manufacturer make to transfer more thermal energy to the water, without changing the mass of calcium oxide? [2]
Answer Details
(a) Thermal energy transfer, or heating, [1] takes place from the reacting chemicals to the water.
(b) [3] First find the temperature change:
\[\Delta T=34.0-16.0=18.0\,^{\circ}\text{C}\]
Use \(E=mc\Delta T\):
\[E=5.0\text{ kg}\times4.2\text{ kJ kg}^{-1}\,^{\circ}\text{C}^{-1}\times18.0\,^{\circ}\text{C}=378\text{ kJ}\]
The water gains 378 kJ of thermal energy.
(c) [2] Two acceptable features are: the thick insulating material reduces energy transfer by conduction; trapped air in insulation is a poor conductor; a lid or closed box reduces convection; or a shiny inner surface reduces energy transfer by infrared radiation.
(d) [2] The reaction is exothermic because energy is released when new bonds form in calcium hydroxide. More energy is released in bond formation than is taken in to break bonds in the reactants, so the surroundings, including the water, warm up.
(e) [2] Two suitable changes are to use a thinner metal bottle wall, increase the contact area between the reaction container and the bottle, add a conducting metal plate between them, or reduce the gap between them. Each reduces resistance to thermal energy transfer to the water without increasing the calcium oxide mass.
Download The App On Google Playstore
Everything you need to excel in your exams
