specific heat capacity of liquid

Question

The temperature of the solution increases from 21.3 °C to 38.2 °C.

0.468 g Mg reacting

0.0193 mol Mg reacting

80.844 g HCl solution

The energy released is given by

q = specific heat × mass × temperature change

The solution is almost all water, so we use the specific heat capacity of water, 4.18 J/g•°C. What is the quantity of heat released? Enter your answer as a positive value.

 J released

Answer 1

Hello, Natalie,

Start with a balanced equation so that we can insure there is enough HCl to complete the reaction of all the Mg. I see the reaction needs twice as many moles of HCl as Mg. There is more than enough HCl to completely react will all the Mg. This is why the "Mg reacting" moles of Mg is exactly the total amount of Mg reacting.

Mg + 2HCl = MgCl2 + H2

The temperature change is +16.9 C. It might seem we have enough information to determine the amount of heat released, but we're missing a key piece of information. We need the mass of the water solution. Was this done in a 50ml beaker? Or in the YMCA Olympic Pool? The latter experiment would tell us there is a HUGE amount of heat released to warm that much water. Not as impressive with a small beaker. So we can't go any further until we know the mass of water that was heated. When we know that, input the data in the equation q = (4.18 J/g*C)*(16.9C)*( g solution) to determine the total amount of heat released.

I hope this helps,

Bob