A calorimeter contains 20.0 mL of water at 15.0 C . When 1.90 g of X (a substanc

Question

A calorimeter contains 20.0 mL of water at 15.0 C . When 1.90 g of X (a substance with a molar mass of 78.0 g/mol ) is added, it dissolves via the reaction X(s)+H2O(l)X(aq) and the temperature of the solution increases to 30.0 C . Calculate the enthalpy change, H, for this reaction per mole of X. Assume that the specific heat of the resulting solution is equal to that of water [4.18 J/(gC)], that density of water is 1.00 g/mL, and that no heat is lost to the calorimeter itself, nor to the surroundings. Express the change in enthalpy in kilojoules per mole to three significant figures.

Determine the masses of titanium and iodine that react if 1.59×103 kJ of heat is emitted by the reaction.

Express your answer to three significant figures and include the appropriate units

Explanation / Answer

Since the temperature of the solution is increased, heat is absorbed by the solution and hence heat is released by the reation. Hence the reaction is exothermic.

Mass of water, m = Vxd = 20.0 mL x (1.00 g/mL) = 20.0 g

Change in temperature, dT = 30.0 DegC - 15.0 DegC = 15.0 DegC

Hence heat absorbed by water, Q = mxsxdT = 20.0 g x(4.18 J/(gC) x15.0 C = 1254 J

Hence 1.9 g of X on dissolution releases 1254 J of heat.

Molar mass of X = 78.0 g/mol

Hence the amount of heat released when 78.0 g of X is dissoluted = (1254 J / 1.9 g) x 78.0 g = 51480 J

= 51480J x (1 KJ / 1000J) = 51.5 KJ

Hence enthalpy change, H, for this reaction per mole of X = 51.5 KJ (answer)

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