finding final temperature from the heat of a reaction.

Question

1.20 grams of Mg is burned producing MgO. The heat from the reaction is used to warm 500 grams  water at 5 degrees Celsius. Find the final temperature of the water.

Answer 1

Hi Kaylee!

 

Let me help you with this.

 

First, let's write the balance chemical reaction for the burning of Mg:

 

2 Mg  +  O₂ --> 2 MgO

 

 

Second, convert the quantity of magnesium from grams to moles:

 

1.20 g Mg (1mole Mg/24.3 g ) = 0.0493 mol  Mg

 

Third, we have the calorimetry:

 

q = m(C_p)ΔT, where ΔT is always final T minus initial T

 

The question gives the mass of water, the C_p of water is 4.184 J/g-C, and the initial T of the water is 5 C.  But what about q?  In the chemical equation, MgO is made from its elements, so we can use the enthalpy of formation from a thermodynamics table in a textbook.  My text gives the ΔH as -601.8 kJ/mole MgO.

 

But that quantity has to be converted to joules and is per mole of MgO.  So,

 

601.8 kJ/mole (1000 joules/kJ)(0.0493 mole) =29669 joules 

 

Plug in for q:

 

29699 joules = (500 g)(4.184 J/g-C) (T_f - 5)

 

Take it from here and solve.

 

I hope this was helpful.  Message me if you have any questions.