please help me HURRY I need the answer

Question

A student placed 50.0 Ml of 2.05 mol/L NaOH in a coffee cup calorimeter at 20.4 degress celcius, and added 50.0 mL of 1.20 mol/L H2SO4 also at 20.4 degrees Celsius. After quickly stirring the mixture, its temperature rose to 28.2 . Determine the enthalpy change Hrxn for the reaction:

2 NaOH (aq) + H2SO4 (aq) + 2 H2O (i) + Na SO4 (aq)

Write a therefore station, explain whether the reaction is endothermic or exothermic and the direction of heat transfer.

Answer 1

Hello, Lala,

First, the equation needs to be balanced. The NaOH reactions with H2SO4 to produce Na2SO4 and H2O. The balanced equation is therefore:

2NaOH + H2SO4 = Na2SO4 + 2H2O

Next, let's calculate the moles of the reactants. Molar is defined as moles/liter. 2.05 moles/liter for the NaOH. 50 ml are used, but we need liters in the calculation. Then multiply the number of liters of NaOH by the Molarity (moles/liter) to get moles:

50ml*(1liter/1000ml)*2.05M = 0.1025 moles NaOH

For H2SO4:

50ml*(1 liter/1000ml)*1.20M = 0.06 moles H2SO4

Our equation tells us we need twice as much NaOH as H2SO4, but we don't have enough NaOH. So NaOH becomes the limiting reagent and the heat released will be based on the amount of NaOH, assuming it reacts completely.

The experiment ends up with 100ml of solution, primarily water. It's temperature rises by 7.8C. We can use the equation for specific heat:

q = cmDt

where q is the heat released/consumed, c is the specific heat (of water in this case), and Dt is the temperature change. m is the mass of the water, which for 100ml should be 100g, or 0.1 kg. The specific heat of water is 4.18kJ/kgK. The difference in Celsius temperature will be the same value in K, so we can use the difference of 7.8C and call it 7.8K.

q = (4.18kJ/kgK)(0.1kg)(-7.8K)

q = -3.26 kJ

The sign is negative since the reaction gave up heat that was captured by the water. It is an exothermic reaction.

Note that this is the total energy released by a specific amount of reactants. It would be preferable to express it in units of energy/mass or moles, to give it some perspective. 3,260 Joules sounds like a lot, but was it a tanker load or a measly fraction of a mole? [It is a lot, so don't try it at home!].

The question is which mass do we use to express the result. Do we use the mass of a reactant or of the product. To use the product, calculate the resulting mass of Na2SO4, assuming the NaOH is the limiting reagent. It takes two moles of NaOH to make 1 mole of Na2SO4. That means we made 0.105/2 moles of Na2SO4. Calculate the mass of that many moles, convert it to kg, and then divide that into the heat we obtained.

Bob