Tuesday, April 30, 2013

A coffee cup calorimeter contains 0.0375 mol HCl dissolved in 1.50 x 10^2 mL of water at 20.51 C. When 1.75 g of solid zinc metal is placed in the calorimeter, the temperature rises to 29.02 C. Assume that no heat is lost to the surroundings.

A coffee cup calorimeter contains 0.0375 mol HCl dissolved in 1.50 x 10^2 mL of water at 20.51 C. When 1.75g of solid zinc metal is placed in the calorimeter, the temperature rises to 29.02 °C.  Assume that no heat is lost to the surroundings.

Write the balanced equation for the reaction that takes place in the calorimeter.  1pt
Balanced reaction
Zn (s)  + 2 HCl (aq) --> ZnCl2(aq) + H2 (g)

Find the limiting reactant. Show your work. 2 pts.
Number of moles of Zn             = given mass of Zn / molar mass of Zn
                                                  = 1.75 g Zn / 65.3.9 g/mol
                                                  = 0.02676 moles Zn
In equation 1 mole of Zn react with 2 mol of HCl
Hence  0.02676 moles Zn react with        = 2  x 0.02676
                                                             = 0.05352 mol of HCl 
But required moles of HCl (0.05352 mol) > given moles (0.0375)
Hence HCl is limiting reagent

Find the amount of heat lost or gained by the calorimeter. Show your work. 2pts.
Formula of heat change, ∆T       = T2 - T1 = 29.02 - 20.51  = 8.51 ° C
Specific heat capacity for water , C = 4.184 J/g-C
Mass of water m                            = 1.50 x 102  ml = 150 g  (for water 1 ml = 1 g )
Use formula of enthalpy change ∆H = m * C *∆T
Plug the values
                                               ∆H = 150 g  x 4.184 J/g-C  x 8.51 C
H = 5341 Joules         
Divide by 1000 to convert in kJ 
                                                H = 5.34 KJ
Temperature is increasing so that it is a exothermic reaction
And for all exothermic reaction H is always negative 
energy lost by reaction and gained by solution.  hence calorimeter gain this energy
 So final answer will H = - 5.34 KJ