Exothermic and endothermic reactions and calorimetryIB MYP Chemistry: Subtopic test

1. 1
   A hiker in Chile carries a disposable hand warmer. When she opens the sachet, the iron powder inside reacts with oxygen from the air and the sachet warms to about 55 °C for several hours. Her friend carries an instant cold pack for sports injuries. When the pack is squeezed, a solid dissolves in water inside it and the pack quickly becomes cold.

   (a)

   Which term describes a reaction that transfers energy to the surroundings and makes the temperature of the surroundings rise?

   [1 mark]

   • AEndothermic
   • BDecomposition
   • CExothermic
   • DReversible

   (b)

   Which of these is an endothermic change?

   [1 mark]

   • AThe combustion of methane
   • BThe neutralisation of an acid by an alkali
   • CIron rusting in a hand warmer
   • DThe thermal decomposition of calcium carbonate

   (c)

   Explain, in terms of energy transfer, why the hand warmer feels warm but the cold pack feels cold.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student in Kuala Lumpur burns ethanol in a spirit burner to heat 100 g of water in a metal can. The temperature of the water rises from 20.0 °C to 52.0 °C. The mass of the spirit burner falls from 85.40 g to 84.65 g. The specific heat capacity of water is 4.18 J/g/°C.

   (a)

   Which equation is used to calculate the energy transferred to the water?

   [1 mark]

   • AQ = m × c × ΔT
   • BQ = m ÷ (c × ΔT)
   • CQ = c ÷ (m × ΔT)
   • DQ = m + c + ΔT

   (b)

   What is the temperature change of the water?

   [1 mark]

   • A72.0 °C
   • B32.0 °C
   • C20.0 °C
   • D52.0 °C

   (c)

   Calculate the energy transferred to the water, in joules.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Students at a school in Lagos compare two liquid fuels, ethanol and propan-1-ol, to find which releases more energy per gram. They burn each fuel in a spirit burner to heat 100 g of water in a metal can. Burning 0.80 g of ethanol raises the temperature of the water by 30.0 °C. Burning 0.70 g of propan-1-ol raises the temperature of the water by 33.0 °C. The specific heat capacity of water is 4.18 J/g/°C. The data book value for propan-1-ol is 33.6 kJ/g.

   (a)

   State two variables that the students should control to make the comparison fair, and one safety precaution they should take.

   [3 marks]

   (b)

   Calculate the energy released per gram of propan-1-ol, in kJ/g. Suggest one reason why the result is lower than the data book value, and one improvement to the method.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   After floods in Pakistan, a relief agency supplies self-heating meal packs. Each pack has a sealed compartment of 14.0 g of calcium oxide next to a compartment of water. When the pack is pressed the two mix and react: CaO + H₂O → Ca(OH)₂. Each mole of calcium oxide that reacts releases 64 kJ of energy. Assume that all of this energy heats 150 g of food, which has the same specific heat capacity as water, 4.18 J/g/°C, and starts at 20.0 °C. The packs are used once and then thrown away. The agency could instead give families small gas stoves with fuel canisters.

   (a)

   Calculate the final temperature of the food in a pack. Relative atomic masses: Ca = 40, O = 16.

   [6 marks]

   (b)

   Discuss the benefits and drawbacks of using self-heating meal packs rather than gas stoves, and evaluate which the agency should supply after floods.

   [6 marks]

   Total for question 4: 12 marks

End of questions