3.4 Enthalpy Changes

Nitrogen and oxygen react under certain conditions.

The following reaction forms, N₂O, and is a reversible reaction.

2N₂ (g)   +   O₂ (g)     2N₂O (g)

The enthalpy profile diagram for the reaction is shown below. 

Another oxide of nitrogen can be used to release large amounts of energy in rockets. 

The equation for this reaction is:

4CH₃NHNH₂ (l) + 5N₂O₄ (l) → 4CO₂ (g) + 12H₂O (g) + 9N₂ (g)

Use the data to calculate the enthalpy change of reaction.

Explain, in terms of bond breaking and forming, why this reaction is exothermic.

This question is about enthalpy of combustion. 

A student carried out the following method to determine the enthalpy of combustion of cyclohexane. 

• Place a pure sample of cyclohexane in a spirit burner
• Place the spirit burner under a beaker containing 50 g of water 
• Ignite the cyclohexane 
• Extinguish the flame after a few minutes 

The results for the table are below: 

The student determined from this experiment that the enthalpy of combustion of cyclohexane is -1216 kJ mol^-1.

Use the data to calculate the final temperature of the water in this experiment.

The specific heat capacity of water = 4.18 J K^–1 g^–1

The relative molecular mass (M_r) of cyclohexane = 84.0

A 50.0 g sample of water was used in the experiment. 

Explain how you could measure out this mass of water without using a balance. 

The table gives some standard enthalpies of combustion.

Use these to calculate the enthalpy change for the following reaction:

6C (s)   +   6H₂ (g) →    C₆H₁₂ (l)

Other than heat loss, identify three major sources of error in the experiment. 

You must not refer to the precision of equipment in your answer.

A student investigated the temperature change for the neutralisation of malonic acid,  HOOCCH₂COOH, and sodium hydroxide solution according to the following method.

1. Measure 25.0 cm³ of 0.400 mol dm^–3 of malonic acid into a beaker.
2. Record the temperature every minute for three minutes.
3. On the fourth minute, add 50.0 cm³ of 0.500 mol dm^–3 sodium hydroxide solution and stir.
4. Record the temperature every minute for eight minutes.

The results are shown below. 

Explain why the student’s temperature change of 3.5 ^oC is not an accurate temperature change.

Another student completed the same investigation and recorded a maximum temperature of 22.4 ^oC. The student calculated the heat energy, q, for the reaction to be 9.719 x 10^-1 kJ.

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[1]

Write a balanced symbol equation for the neutralisation of malonic acid with sodium hydroxide solution. 

Calculate the enthalpy change in this reaction per mole of water. 

Use the information in part (a) and (c) to help you.

This question is about the reactions of ethyne, C₂H₂.

Ethyne is also known as acetylene, and can be used to cut through steel when used in an oxy-acetylene flame. 

The equation for the complete combustion of ethyne is:

Calculate the enthalpy change of combustion of ethyne using the mean bond enthalpies in the table below. 

Bond	Mean bond enthalpy / kJ mol-1
C=O	+805
O-H	+464
C-H	+412
O=O	+498
CC	+837

Give one reason why the value of enthalpy of combustion calculated from bond enthalpies in part (a) is not entirely accurate. 

Ethyne can also react with hydrogen fluoride, shown in the equation below. 

Bond	C-H	CC	H-F	C-C
Mean bond enthalpy / kJ mol-1	+412	+837	+562	+348

Use the data in the table to calculate a value for the bond enthalpy of a C-F bond in difluoroethane.

Draw the dot and cross diagram for ethyne, C₂H₂.

Define the term standard enthalpy of formation, Δ_fH^ϴ.

Ammonia is a colourless gas which has a very pungent smell. It also has a number of important chemical applications. 

Ammonia reacts with hydrogen chloride, to produce solid ammonium chloride. 

Write the equation, including state symbols, for this reaction.

Using the enthalpy of formation data provided, calculate the enthalpy change for the reaction between ammonia and hydrogen chloride in part (b). Show your working.

Ammonia is produced via the Haber process, where hydrogen and nitrogen gas are reacted together. 

State why the enthalpy of formation of hydrogen would be zero.

A teacher instructs a class to complete a calorimetry practical, to calculate the enthalpy change that occurs when hydrochloric acid and sodium hydroxide react together. 

Each student was given roughly 60 cm³ of 0.35 mol dm^-3 hydrochloric acid, roughly 60 cm³ of 0.35 mol dm^-3 sodium hydroxide, a polystyrene cup and access to all standard laboratory equipment.  

Draw a diagram to demonstrate the practical set up that the students would need to use to determine the enthalpy change during this neutralisation reaction, and state the key measurements that the students would have to make.

The students then completed the practical from part (a), using their own method and measurements that they had chosen.

One student found that when they reacted 35.0 cm³ of the hydrochloric acid with 35.0 cm³ of the sodium hydroxide, the temperature rose from 19.6 ^oC to 22.3 ^oC. 

Calculate the enthalpy change for this reaction in kJ mol^-1. 

Assume that both solutions have a density of 1.00 g cm^-3 and a specific heat capacity of 4.18 J g^-1 K^-1.

Explain why the value that you have calculated for the students’ practical in part (b), might be different from the correct value given in a data book.

State how the students' practical could be improved to allow the students to calculate a more accurate value which is closer to the correct value given in data books.

Propane gas is commonly used as a fuel for outdoor cooking. It can be produced in a number of ways, including from the addition reaction of propyne gas with hydrogen. Propyne has the formula CHCCH₃ and includes a triple bond.  

Write an equation for the formation of propane from propyne.

The table below contains some key enthalpy of combustion data. 

Calculate the enthalpy change in kJ mol^-1 for the reaction in part (a) using the data provided. Show all working.

Hydrazine, N₂H₄ can react with hydrogen peroxide in an exothermic reaction, according to the following equation. 

N₂H₄ (g) + 2H₂O₂ (l) → N₂ (g) + 4H₂O (g)   Δ_rH^ϴ = -789 kJ mol^-1 

Some enthalpy data for a different chemical reaction is shown below.

Using the reaction equation and the data in the table above, calculate the value of the N-H bond in hydrazine.

Often when a bond enthalpy is calculated, it is a different value to the one which is quoted in a data book. State a reason for this.

Alcohols are often used as fuels. They can be used to fill spirit burners, which can be used in calorimetry practicals by students.
A teacher set a class a calorimetry practical to calculate the enthalpy of combustion of ethanol, propan-1-ol and butan-1-ol. The set up that the student used for the experiment is shown. 

The table below shows the change in mass when using the ethanol spirit burner. 

The temperature increased by 34.5 ^oC during the reaction.

The specific heat capacity of the water is 4.18 J g^-1 K^-1.

Using the students results calculate the enthalpy of combustion for ethanol in this experiment.

Suggest two reasons, excluding heat transfer, why the value which you have calculated from the student’s data in part (a), might not be the same as a value provided in a data book.

The student repeated the experiment in part (a) with propan-1-ol and butan-1-ol.  

Write an equation for the complete combustion of one mole of butan-1-ol.

Enthalpy data for butanol, carbon dioxide and water is shown in the table below.

Use the data in the table and your equation in part (c), to calculate the enthalpy of combustion of butan-1-ol.