*In 2001, Nescafe launched a self-heating can of coffee, called Hot When You Wan
ID: 1056202 • Letter: #
Question
*In 2001, Nescafe launched a self-heating can of coffee, called Hot When You Want. To heat up the coffee, a button was pressed which mixed the heating ingredients in a shell outside the can containing the coffee. The can warmed up 210 cm3 of coffee by 40 °C. A single step mixed calcium oxide and water to produce calcium hydroxide and generate heat according to the reaction: CaO (s) + H2O (l) ? Ca(OH)2 (aq) ?H = -82 kJ/mol a) Assuming that the heat capacity for coffee is the same as that of water, 4.18 Jg-1K-1 , calculate the energy needed to warm 210 cm3 of coffee by 40 °C. b) Use this value to hence calculate the minimum mass of CaO in the can for it to function as specified.
Explanation / Answer
(a) the energy needed to warm 210 cm3 of coffee by 40 °C is , Q = mcdt
Where
m = mass of coffee = 210 cm3 x 1g/cm3 = 210 g
c = specific heat capacity = 4.18 J/(g-K)
dt = Change in temperature = 40 oC = 40 K
Plug the values we get Q = 210x4.18x40 = 35.112x103 J
= 35.1 kJ
(b) The balanced equation is : CaO (s) + H2O (l) Ca(OH)2 (aq) H = -82 kJ/mol
Molar mass of CaO = 40+16 = 56 g/mol
From the balanced reaction,
1 mole= 56 g of CaO produces 82 kJ of heat
Mg of CaO produces 35.1 kJ of heat
M = ( 35.1x56) / 82
= 24.0 g of CaO
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