Enthalpy of formation questions

Enthalpy of formation questions

Is the Hope diamond just a very expensive pencil? A pencil can be purchased for less than a dollar. Both items contain carbon, but there is a big difference in how that carbon is organized. The diamond was formed under very different reaction conditions than the graphite, so it has a different heat of formation. A relatively straightforward chemical reaction is one in which elements are combined to form a compound.

Sodium and chlorine react to form sodium chloride see video below. Hydrogen and oxygen combine to form water. Like other reactions, these are accompanied by either the absorption or release of heat.

For example, iron is a solid, bromine is a liquid, and oxygen is a gas under those conditions. The standard heat of formation of an element in its standard state is by definition equal to zero. The graphite form of solid carbon is its standard state withwhile diamond is not its standard state.

Some standard heats of formation are listed in the Table below. Watch a video of the reaction between sodium metal and lg tv demo mode gas. Read the material at the link below and answer the questions:. Skip to main content. Search for:. List some factors affecting the standard heat of formation.Answer the following to the best of your ability.

Questions left blank are not counted against you. A new page will appear showing your correct and incorrect responses. If you wish, you may return to the test and attempt to improve your score. If you are stumped, answers to numeric problems can be found by clicking on "Show Solution" to the right of the question. Do NOT type units into the answer boxes, type only the numeric values.

Do NOT use commas or scientific notation when entering large numbers. Answer all non-integer questions to at least 3 significant figures. State whether the following processes exothermic or endothermic: a.

enthalpy of formation questions

Sublimation X s X g Exothermic Endothermic b. Condensation X g X l Exothermic Endothermic c. Deposition X g X s Exothermic Endothermic d. Fusion X s X l Exothermic Endothermic e. Evaporation X l X g Exothermic Endothermic f. Solidification X l X s Exothermic Endothermic Determine which of the following is not an element in its standard state: a. Evaporating 3. Condensing Mass b. Volume c.


Density d. Specific Heat b. Enthalpy c. Heat Capacity d. Moles of Matter b. Temperature c. Pressure d. Density e. Sublimation X s X g. Condensation X g X l. Evaporation X l X g. Solidification X l X s.When a chemical reaction is represented graphically, we see that the enthalpy change is reversed between the forward and reverse reactions. If a reaction produces energy in a forward process, it will require an input of energy in the reverse process, and vice versa.

A catalyst only affects the rate of a chemical reaction; it does not affect the equilibrium. Finally, exothermic reactions are not always spontaneous, but will have lower activation of energies compared to endothermic reactions.

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in enthalpy of a reaction. The net reaction is. Hess's law states that the change in enthalpy for a total reaction can be considered equal to the sum of the enthalpy changes for every step involved in the reaction.

In other words, we can determine the enthalpy change for nitrogen dioxide by adding the enthalpy changes for both steps involved in its formation.

enthalpy of formation questions

This gives us the total change in enthalpy for the listed reaction. Because the question asks for the enthalpy change for four moles of nitrogen dioxide, the value must be doubled.

The reaction only produces two moles of nitrogen dioxide. In this case the known reactions are given. For this particular reaction, since there are two moles of product, the enthalpy of formation for must be multiplied by two. Since we have the enthalpies of formation, we can find the enthalpy of the reaction using the following equation:. Since oxygen gas is elemental, it does not have an enthalpy of formation, and is omitted from the equation.

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Your Infringement Notice may be forwarded to the party that made the content available or to third parties such as ChillingEffects.The standard enthalpy of formation refers to the enthalpy change when one mole of a compound is formed from its elements.

In chemistry, the standard state of a material, be it a pure substance, mixture, or solution, is a reference point used to calculate its properties under different conditions.

In principle, the choice of standard state is arbitrary, although the International Union of Pure and Applied Chemistry IUPAC recommends a conventional set of standard states for general use. A standard pressure of 1 bar Strictly speaking, temperature is not part of the definition of a standard state; the standard state of a gas is conventionally chosen to be 1 bar for an ideal gas, regardless of the temperature.

However, most tables of thermodynamic quantities are compiled at specific temperatures, most commonly Standard states for atomic elements are given in terms of the most stable allotrope for each element. For example, white tin and graphite are the most stable allotropes of tin and carbon, respectively.

Therefore, they are used as standard states or reference points for calculations of different thermodynamic properties of these elements. Tin : White tin on the left is the most stable allotrope of tin, and is used as its standard state for thermodynamic calculations. The standard state should not be confused with standard temperature and pressure STP for gases, or with the standard solutions used in analytical chemistry.

Graphite : Graphite is the most stable state of carbon and is used in thermochemistry to define the heat of formation of carbon compounds. The standard enthalpy of formation, or standard heat of formation, of a compound is the change in enthalpy that accompanies the formation of one mole of the compound from its elements in their standard states.

For example, the standard enthalpy of formation for carbon dioxide would be the change in enthalpy for the following reaction:. The standard enthalpy of reaction is the enthalpy change that occurs in a system when a chemical reaction transforms one mole of matter under standard conditions. The change in enthalpy does not depend upon the particular pathway of a reaction, but only upon the overall energy level of the products and reactants; enthalpy is a state function, and as such, it is additive.

In order to calculate the standard enthalpy of a reaction, we can sum up the standard enthalpies of formation of the reactants and subtract this from the sum of the standard enthalpies of formation of the products.

Stated mathematically, this gives us:. In order to calculate the standard enthalpy of reaction, we need to look up the standard enthalpies of formation for each of the reactants and products involved in the reaction. These are typically found in an appendix or in various tables online. For this reaction, the data we need is:.

Next, we sum up our standard enthalpies of formation. This law states that if a reaction takes place in several steps, then the standard reaction enthalpy for the overall reaction is equal to the sum of the standard enthalpies of the intermediate reaction steps, assuming each step takes place at the same temperature.

Since enthalpy is a state function, the change in enthalpy between products and reactants in a chemical system is independent of the pathway taken from the initial to the final state of the system. Negative enthalpy change for a reaction indicates exothermic process, while positive enthalpy change corresponds to endothermic process.

However, we can see that the net reaction is a result of A being converted into B, which is then converted into C, which is finally converted into D.

Turning graphite into diamond requires extremely high temperatures and pressures, and therefore is impractical in a laboratory setting. The change in enthalpy for this reaction cannot be determined experimentally. Our intermediate steps are as follows:. In order to get these intermediate reactions to add to our net overall reaction, we need to reverse the second step. Sometimes, you will need to multiply a given reaction intermediate through by an integer. Restating the first equation and flipping the second equation, we have:.The standard enthalpy of formation is also known as the standard heat of formation of a compound.

It refers to the change of enthalpy during the formation of 1 mole of a substance from its constituents when all the substances are in their standard states. The enthalpy of combustion of propane will be calculated by adding all the above two equation that is 1 and 2by first multiply e Questions are typically answered within 1 hour. A: Given, grams of Aluminum. Q: In one experiment you reacted A: Hi, since there are multiple sub-parts posted we will provide you with answers for the first three s Q: A chemistry graduate student is given Q: Write a balanced equation for the combustion of C3H8 g propane -i.

Q: The change in internal energy for the combustion of 1. Q: In the reaction of silver nitrate with sodium chloride, how many grams of silver chloride will be pr A: The balanced equation and the moles of silver nitrate formed can be given as.

Enthalpy questions, 10 points..?

The mixture is ignited producing CO2 and H2O. A: According to the balanced chemical reaction shown as follows:. Subscribe Sign in. Operations Management. Chemical Engineering. Civil Engineering. Computer Engineering. Computer Science. Electrical Engineering. Mechanical Engineering. Advanced Math. Advanced Physics. Earth Science. Social Science. Asked Oct 31, Step 1. The equation used to calculate the enthalpy of reaction is shown as follows:. Step 2. Step 3.

Want to see the full answer? Want to see this answer and more? Tagged in.Syllabus ref: 5. As the definition is according to standard states, these must be included in the equation. Once again, the key substance is the one being formed, so the elements themselves may, and often do, appear with fractional coefficients.

The state symbols are important. Example: Give the equation for the enthalpy of formation of sulfuric acid. Example: The enthalpy of formation of calcium carbonate is represented by the following equation:. The sign of the enthalpy of formation of a compound gives us the relative stability of the compound being formed with respect to its constituent elements.

A negative value tells us that it is an exothermic compound compared to its elements. Similarly, a positive enthalpy of formation value tells us that the compound is relatively unstable with respect to its elements, it is an endothermic compound. Once again there is no implication regarding the possibility of direct formation from the elements. Ethyne is an endothermic compound. The energy of ethyne with respect to its constituent elements in their standard states has a positive value.

NOTE: The elements are at zero energy by definition. In reality, this does not mean that they have no chemical energy, just that we have decided to measure from this point. It should be noted that the energy change 'labels' are given that best describes the specific change taking place.

Usually this is unambiguous, but there are occasions where the enthalpy of formation of one substance is also the enthalpy of combustion of another. For example, when sulfur is burned in excess oxygen under standard conditions, the enthalpy change for the process is:.

This equation represents the standard combustion enthalpy of sulfur. However, it also represents the standard enthapy of formation of sulfur IV oxide.

Both descriptions are equally correct. Beware, though as this is not always the case. For example, the standard enthalpy of formation of sodium oxide is NOT the same as the standard combusion enthalpy of sodium. The enthalpy of formation is the energy change when one mole of a substance is made from its constituent elements in their standard states.

The standard enthalpy of formation Standard enthalpy of formation equations Endothermic and exothermic compounds Overlap between combustion enthalpy and formation enthalpy.Since the formation of a bond is an exothermic process the individual atoms have a higher enthalpy than the atoms bonded together.

The negative sign means that to obtain non-bonded atoms I you would have to supply heat "enthalpy" to increase the enthalpy of the system to zero.

In this calculation the enthalpy of the individual unbonded atoms or of the elements in their standard states, see answer to 6b below is considered to be zero. The bond enthalpies given in the question are presumably also relative to the elements C, H, O in their standard states.

Enthalpies of Formation (Example)

It is important that all the enthalpy data used in a calculation is relative to the same standard states of the elements to get the right answer. The enthalpies of formation given for C3H6, H2O and CO2 are however specific for these compounds and probably originally calculated from measurements of the enthalpy of combustion! You are incredible. I was struggling with this set of questions today! I need more information. Enthalpy calculations are quite simple you look at Luis structures of what the starting materials are and then look at what the products are, which bonds are different?

Then you simply take the difference between the two and you have the enthalpy of formation. However, I don't know your starting materials so I can't calculate any of your answers.

enthalpy of formation questions

The answer to d is that the calculations using bond energies are based on averages. Not every C-H bond is alike. For example, the C-H bond energy of a group close to an carbon-oxygen bond are not the same as those for C-H bonds next to other carbon-hydrogen group.

In fact, energies for breaking C-H bonds in CH4 are not the same. It takes less energy to break the first C-H bond is less than the second or third. Surprisingly, the energy cost of breaking the 4th C-H bond on methane is less than the third.

Update: the thing being combusted is the propene missed that out, sorry! Answer Save. Mata N Lv 4. Favorite Answer. So the enthalpy of the reactants relative to the individual non-bonded atoms is: The negative sign means that to obtain non-bonded atoms I you would have to supply heat "enthalpy" to increase the enthalpy of the system to zero.

Chris 5 years ago Report. What is the compound being combusted? C because you would subtract the products enthalpy from the reactants enthalpy. Still have questions? Get your answers by asking now.


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