Energetics I

Question 1

What is enthalpy?

Answer 1

Enthalpy is a measure of the total energy of a system. It includes the internal energy (the energy required to create the system). Enthalpy = internal energy + pressure x volume

Question 2

How can you determine the enthalpy of a system?

Answer 2

You cannot directly measure the enthalpy, but you can measure the enthalpy change during a physical or chemical change.

Question 3

What is enthalpy change?

Answer 3

The enthalpy change of a process is the heat energy that is transferred between the system and the surroundings at constant pressure.

Question 4

What are exothermic reactions?

Answer 4

Exothermic reactions are reactions in which heat energy is transferred from the system to the surroundings.

Question 5

What are endothermic reactions?

Answer 5

Reactions in which heat energy is transferred from the surroundings to the system.

Question 6

What is the standard enthalpy change of reaction?

Answer 6

The enthalpy change which occurs at 100kPa and a stated temperature (usually 298K), when the reaction proceeded as per the stoichiometric equation (as written).

Question 7

What is the standard enthalpy change of reaction?

Answer 7

The enthalpy change which occurs at 100kPa and a stated temperature (usually 298K), when the reaction proceeded as per the stoichiometric equation (as written).

Question 8

What is the standard enthalpy change of combustion?

Answer 8

The enthalpy change measured at 100kPa and a stated temperature (usually 298K), when one mole of a substance is completely burned in oxygen.

Question 9

What is the standard enthalpy change of neutralisation?

Answer 9

The enthalpy change which occurs at 100kPa and a stated remote (298K), when one mole of water is produced by the neutralisation of an acid with an alkali.

Question 10

What is the standard enthalpy change of formation?

Answer 10

The enthalpy when that occurs at 100kPa and a stated temperature (298K), when one mole of a substance is formed from its elements in their standard states.

Question 11

What is an enthalpy level diagram?

Answer 11

An enthalpy level diagram shows the enthalpy of the reactants and products, with the arrow between them showing the enthalpy change of the reaction. For an exothermic reaction, the reactants are at higher enthalpy than the products and the opposite it’s true for endothermic reactions. This is different to energy level diagrams which show the activation energy (there is no peak in an enthalpy level diagram, just straight lines).

Question 12

How would you determine the enthalpy change of combustion experimentally?

Answer 12

• weigh a spirit burner containing the liquid you are going to combust
• fill a copper can with a known volume of water and use a thermometer to measure its initial temperature
• put a lid on the copper can and set up a fraught shield to prevent heat loss
• light the spirit burner and place it underneath a tripod and gauze with the copper can on top
• measure the temperature change in a given time
• extinguish the spirit burner and weigh its final mass

Question 13

How do you calculate enthalpy change of combustion from experimental results?

Answer 13

You need to know the volume of liquid being heated, the specific heat capacity of that liquid, the temperature rise, the mass of the other liquid that was burned, and the molar of of the liquid that was burned.
Q=mcdeltaT heat energy=mass of water x specific heat capacity of water x change in temperate
For exothermic reactions (such as combustion) deltaH=-Q/n change in enthalpy= -heat energy / number of moles of fuel burnt

Question 14

What are some sources of error in determining the enthalpy change of combustion experimentally?

Answer 14

• heat energy loss to the surroundings rather than the water
• some incomplete combustion occurs, which is not accounted for
• not standard conditions

Question 15

How do you determine the enthalpy change of neutralisation experimentally?

Answer 15

Mix a known volume of acid and alkali together in a polystyrene cup with a lid. Measure the initial temperature then measure the temperature at fixed time intervals (e.g. every minute). The temperature change is the difference between the initial temperature and the maximum temperature reached.

Question 16

How do you calculate the enthalpy change of neutralisation from experimental results?

Answer 16

Q=mcdeltaT
Heat energy = mass of solution x specific heat capacity of water x change in temperature
Exo: deltaH=-Q/n
Endo: deltaH=Q/n
n = number of moles of water produced
mass of solution can be determined using the volumes and densities of each solution added.

Question 17

What are the errors in the experimental determination of enthalpy of neutralisation?

Answer 17

• heat loss to the surroundings
• measurement errors

Question 18

What is Hess’ Law?

Answer 18

Hess’ Law states that the enthalpy change of a reaction is independent of the pathway it takes (provided the initial and final conditions are the same). This allows enthalpy changes which cannot be measured directly to be calculated.

Question 19

How do you construct a Hess cycle?

Answer 19

Write the equation for which you are determining the enthalpy change at the top of the cycle. For enthalpy of formation, write the elements in their standard states at the bottom and draw arrows up to each side. For combustion, put the combustion products at the bottom and draw arrows down to the products.

Question 20

What is bond enthalpy and mean bond enthalpy?

Answer 20

Bond enthalpies are the enthalpy changes which occur when one mole of a specific bond is broken in the gas phase. The mean bond enthalpy I’d the same, but averaged out over many compounds (these give the average bond enthalpy for all C-C bonds, for example), whereas bond enthalpies specifically give the bond enthalpy for a particular bond in a particular compound (e.g. a C-C bond in butane).

Question 21

How can you calculate the enthalpy change of a reaction using mean bond enthalpies?

Answer 21

Multiply the mean bond enthalpy of each type of bond by the number of bonds present. Enthalpy change = total bond enthalpy of the reactants-total bond enthalpy of the products.

Question 22

What are some limitations of using mean bond enthalpies to calculate the enthalpy change of a reaction?

Answer 22

Sometimes, the reactants/products are not in their standard states, but the mean bind enthalpies are calculated for compounds in their standard states. Also, the mean bond enthalpies are averaged out over many compounds and may not reflect the actual bond enthalpies involved, which are specific to the compounds.