Heat of the reaction?

Question

mass of calorimeter: 11.60g

mass of calorimeter and HCI: 61.42g

mass of Mg added: 0.25g

Initial temperature of solution (before adding Mg) 49.39C

highest temperature reached after adding Mg :58.37C

how do i calculate heat of the reacion?

Answer 1

You need to give as the constant for the reaction, namely enthalpy of the reaction

Answer 2

Start with the basic law of conservation of energy.

q(lost) = -q(gained)

the heat lost by the "reaction" is equal to the heat gained by the water in the solution.

q(reaction) = mcDT (water)

q(reaction) = mass of water x specific heat of water x change in temperature of the water

q(reaction) = 49.82 g x 4.18 J/gC x 8.98 C = 1870 J

That is 1870 J given off for 0.25 grams of Mg.  Now convert that mass of Mg to moles go get the joules per mole.

0.25 g Mg x (1 mol Mg / 24.3 g Mg) = 0.0103 mol Mg

-1870 J / 0.0103 mol = -181,764 J / mol

or, the heat of reaction is

-181.8 kJ / mol

The heat of reaction is negative, since the reaction is exothermic.