Reactions - CONFUSED, urgent

Question

A chemist measures the energy change during the following reaction:

2NH3(g) --> N2(g) + 3H2(g) DeltaH: 160.kJ

Use the information to answer the following questions.

1. Is the reaction endo- or exo- thermic?
2. Suppose 24.7g of NH3 react. Will heat be released or absorbed?
3. If you said heat will be released or absorbed in the second part of this question, calculate how much heat will be released or absorbed. Be sure your answer has the correct number of significant digits.

Answer 1

Here is an example problem that I will go through to help you answer your question:

Suppose someone measures the energy change in the decomposition of hydrogen peroxide to water and oxygen gas:

2H₂O₂ --> 2H₂O + O₂ ΔH = -45 kJ

1. Is the reaction endo- or exothermic?

Assuming constant pressure, ΔH is a measure of the change of heat energy during a reaction. Since ΔH is negative, energy was released during the reaction therefore this reaction is exothermic.

1. Suppose 20 g of H2O2 react. Will heat be released or absorbed?

Any non-zero amount of reactant that is consumed will release heat, and since this is an exothermic reaction heat will be released.

1. If you said heat will be released or absorbed in the second part of this question, calculate how much heat will be released or absorbed. Be sure your answer has the correct number of significant digits.

We can calculate the energy released in kilojoules using what we know from the given thermochemical equation:

We know that there is a mole ratio for the reactant and the energy released:

2H₂O₂ --> 2H₂O + O₂ ΔH = -45 kJ i.e. 2 moles H₂O₂ is proportional to -45 kJ of energy released.

We can use this to calculate the energy released when x amount of moles of H₂O₂ reacts:

x moles H₂O₂ × (-45 kJ ÷ 2 moles H₂O₂) = energy released when x moles H₂O₂ reacts

We solve for x by converting 20 g H₂O₂ into moles H₂O₂:

20 g H₂O₂ ÷ (34.01 g H₂O₂/mole H₂O₂) = 0.59 moles H₂O₂

Plugging this into our energy equation:

0.59 moles H₂O₂ × (-45 kJ ÷ 2 moles H₂O₂) = -13 kJ

Therefore, -13 kJ of heat will be released when 20 g H₂O₂ reacts.

Answer 2

Hello, Abigail,

My responses are in bold.

A chemist measures the energy change during the following reaction:

2NH3(g) --> N2(g) + 3H2(g) DeltaH: 160.kJ

Use the information to answer the following questions.

1. Is the reaction endo- or exo- thermic? Endothermic - it takes in energy. The Delta H is positive. If it were negative, it would be exothermic, since energy leaves the system. [I changed this on 10/14 in response to Edward L.'s comment, below. Sorry if this caused confusion - Bob]

Suppose 24.7g of NH3 react. It depends on what it reacts with, but I'll assume it is the same decomposition reaction for illustration. Will heat be released or absorbed? Released.

1. If you said heat will be released or absorbed in the second part of this question, calculate how much heat will be released or absorbed. Be sure your answer has the correct number of significant digits.
2. I don't see a reference to a "standard" Delta H, which is the energy released per a specific amount of reactants. Perhaps the number above is standard, but I believe ΔH = 45.6 kJ/molNH₃ is the correct number for this reaction. This suggests that the original reaction consisted of more than 3 moles of NH3. Using 45.6 kJ/molNH3, we get:

Molar mass NH3 = 17

24.7 grams NH3 = 1.45 moles

1.45 moles x 45.6 kJ/mol = 66.3 kJ (3 sig figs)

The Delta H is positive, so it is endothermic.



Bob