Thermochemistry

Question 1

What is Thermochemistry?

Answer 1

The study of heat changes that occur during chemical reactions at constant pressure.

Question 2

Define Enthalpy

Answer 2

The potential energy of a substance stored in the chemical bonds.

It has symbol H.

Question 3

What is Enthalpy change?

Answer 3

The heat transferred at constant pressure by a closed system during a chemical reaction.

It has symbol ∆H and is measured in kJmol-1.

Question 4

True or false
An open system is a system that allows transfer of both energy and matter across its boundaries.

Answer 4

True

Question 5

True or false
A closed system is a system that allows transfer of energy but prevents transfer of matter across its boundaries.

Answer 5

True

Question 6

True or false
An isolated system is a system prevents the transfer of only energy but not matter across its boundaries.

Answer 6

False
An isolated system is a system prevents the transfer of both energy and matter across its boundaries.

Question 7

What factors affect the magnitude of enthalpy change?

Answer 7

1. The amount of reactants used up. The higher the amount of reactants used up, the higher the enthalpy change
2. The physical states of the reactants and products. The magnitude of the enthalpy change also depends on the initial and final states of the reactants and products.
3. The temperature at which the reaction is carried out.

For example the heat absorbed to convert a fixed mass of ice at -5oC is to water is more what is needed the ice is at 0oC.

4. Different allotropes of an element. Different allotropes of an element when reacted under the same conditions give different enthalpy changes.

So it is important to indicate the allotrope involved.

Question 8

What are the Standard conditions for pressure, temperature, concentration during enthalpy reactions?

Answer 8

Pressure: 1 atmosphere or 101325 Nm-2 or 760mmHg
Temperature of 25 °C or 298 K
A concentrations of 1 moldm−3

If carbon involved, then it is assumed to be in the form of graphite (its most stable allotrope at a temperature of 25oC and a pressure of 1 atmosphere)

Question 9

What is an Exothermic reaction?

Answer 9

An exothermic reaction is a reaction that releases heat to the surroundings.

In an exothermic reaction:
- Temperature of the surroundings increases
- The products are at a lower energy level than the reactants
- The heat absorbed to break the bonds of the reactants is less than the heat released when the bonds of the products are formed.
- The sign of the enthalpy change, ∆H is negative.

Question 10

What is an Endothermic reaction?

Answer 10

An endothermic reaction is a reaction that absorbs heat to the surroundings.

In an endothermic reaction:
- Temperature of the surroundings decreases
- The products are at a higher energy level than the reactants
- The heat absorbed to break the bonds of the reactants is higher than the heat released when the bonds of the products are formed.
- The sign of the enthalpy change, ∆H is positive.

Question 11

Define standard state

Answer 11

This is the most stable physical state of a substance at a temperature of 25oC and a pressure of 1 atmosphere.

Eg
Water- liquid
Carbon dioxide- gas
Carbon- Solid

Question 12

Define Standard enthalpy change of a reaction (∆HӨr)

Answer 12

This is the heat absorbed or given out when molar amounts of reactants, as shown in the stoichiometric equation, react together under standard conditions to give products.

∆𝐻𝑟⊖ = ∑△ 𝐻𝑓⊝ (𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠) − ∑△ 𝐻𝑓⊝ (𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠)

Question 13

Define Standard enthalpy of formation (∆HӨf)

Answer 13

This is the heat absorbed or evolved when one mole of a compound is formed from its elements in their standard states under standard conditions.

Question 14

True or false
The enthalpy of formation of an element is 0kJ mol-1.

Answer 14

True

Question 15

Define Standard enthalpy of combustion (∆HӨc)

Answer 15

This is the heat given out when one mole of a pure substance is completely burnt in oxygen under standard conditions.

Question 16

Describe the procedure for the experiment to determine the enthalpy of combustion of ethanol

Answer 16

Procedure

• A known mass of water is put into a thin walled copper calorimeter and its initial steady temperature measured by a thermometer and recorded.
• Some ethanol is put in a spirit burner, the burner and its contents weighed and the initial mass recorded.
• The wick is lit and the flame kept steady under the copper calorimeter containing water which is constantly stirred with a thermometer.
• After a reasonable temperature rise of the water, the flame is extinguished and maximum temperature attained by the water is recorded.
• The burner is allowed to cool, reweighed and its mass after combustion recorded.

Question 17

What assumptions are made during the experiment to determine the enthalpy of combustion of ethanol?

Answer 17

• Heat loss to the surroundings is negligible.
• The ethanol was pure.
• There was complete combustion of the ethanol.
• Heat absorbed by the calorimeter is negligible

Question 18

What are the sources of error in the experiment to determine the enthalpy of combustion of ethanol?

Answer 18

• Heat loss to the surroundings
• Incomplete combustion of ethanol
• Some ethanol could have evaporated before burning.

Question 19

Describe the procedure for the experiment to determine the enthalpy of combustion of a substance (solid) using a bomb calorimeter

Answer 19

Procedure:
- A known mass (mg) of the substance under investigation is placed in a crucible in a bomb calorimeter.

• The steel bomb calorimeter is filled with oxygen gas maintained at a pressure of 20 - 25 atmospheres and the calorimeter tightly closed using a screw.
• The bomb calorimeter is placed in a metallic bucket containing a known mass of water (Wg) which is constantly stirred and fitted with an accurate thermometer.
• The whole setup is well lagged to minimize heat losses using an insulating jacket.
• The initial temperature of the water is measured using a thermometer and recorded for about three minutes before ignition of the sample takes place.
• The substance/sample is ignited by the ignition coil and the water constantly stirred. The temperature of water is measured and recorded every minute until when a significant drop in temperature is realized.
• A graph of temperature against time is plotted.
• The maximum temperature is obtained by constructing a vertical line AB at the time of ignition and it meets the extrapolated line BC.
• The maximum temperature is the temperature on the graph that corresponds to the meeting point of these two lines.

Question 20

What is Standard enthalpy of atomization (𝜟𝑯⊖ 𝒂𝒕)

Answer 20

This is the heat absorbed when one mole of gaseous atoms is formed from an element in its standard state under standard conditions.

𝜟𝑯⊖ is always a positive value (endothermic reaction).

Question 21

True or false

The standard enthalpy of atomization of the noble gases is zero

Answer 21

True
because under standard conditions, noble gases already exist as independent gaseous atoms.

Question 22

Define first ionization energy, (𝜟𝑯⊖ or IE1 𝑰𝑬(𝟏))

Answer 22

This is the minimum energy absorbed when one electron is removed from each atom in one mole of free gaseous atoms under standard conditions.

Ionization energy is always endothermic.

Question 23

Define second ionization energy, (𝜟𝑯⊖ or IE2 𝑰𝑬(𝟐))

Answer 23

This is the energy absorbed when one electron is removed from each ion in one mole of free gaseous ions with a single positive charge under standard conditions.

Question 24

What is first electron affinity, (𝜟𝑯⊖ 𝑬𝑨(𝟏))?

Answer 24

This is the heat given out when one electron is added to each atom in one mole of free gaseous atoms under standard conditions.

The first electron affinity is always exothermic.

Question 25

Define second electron affinity, (𝜟𝑯⊖ 𝑬𝑨(𝟐))

Answer 25

This is the heat absorbed when one electron is added to each ion in one mole of free gaseous ions with a single negative charge under standard conditions.

The second electron affinity is always endothermic.

Question 26

Why is the second electron affinity always endothermic?

Answer 26

This is because a lot of heat energy is absorbed to overcome the repulsion between the incoming negatively charged electron and the already negatively charged ion.

Question 27

Define bond enthalpy/ Bond Energy/ Bond dissociation energy

Answer 27

This is the heat absorbed when one mole of a specific covalent bond, in a gaseous molecule, is broken under standard conditions.

Question 28

What is Average bond energy (Mean bond energy)?

Answer 28

This is the average amount of energy needed to break one mole of a particular covalent bond in a range of molecules in their gaseous state under standard conditions.

Note: If the same bond exists in different environments, then average of these bond energies is used in calculations involving that particular bond.

Question 29

What factors affect average bond enthalpies?

Answer 29

1. Bond length
   The larger the atoms joined by a particular covalent bond, the longer the bond length and lower the bond energy.

This is because large atoms have many electrons which results in an increase in repulsion between the electron shells of each atom.

In addition, the nucleus of each atom is more effectively shielded.

Both of these effects result into the weakening of the bond.

2. Number of bonding electrons

The more electrons present in a bond, the greater the strength of the bond.

This is because the more bonded electrons, the stronger the electrostatic forces of attraction between the bonded electrons and the positively charged nuclei

3. Bond polarity
   Bonds become more polar as the difference in electronegativity between the two bonded atoms increases.

The higher the bond polarity, the higher ionic character of the bond and this increases the bond energy of the covalent bond.

Question 30

Why is the F – F bond energy lower than expected?

Answer 30

This is due to strong lone pair–lone pair repulsion caused by the small atomic radii and closeness of the two bonded fluorine atoms.

Question 31

True or false
When all reactants and products are gaseous molecules, the enthalpy change for the reaction can be calculated using:
∆𝐻𝑟 ⊖ = ∑( 𝑏𝑜𝑛𝑑 𝑒𝑛𝑒𝑟𝑔𝑖𝑒𝑠 𝑜𝑓𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠) − ∑( 𝑏𝑜𝑛𝑑 𝑒𝑛𝑒𝑟𝑔𝑖𝑒𝑠 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠)

Answer 31

False
∆𝐻𝑟 ⊖ = ∑( 𝑏𝑜𝑛𝑑 𝑒𝑛𝑒𝑟𝑔𝑖𝑒𝑠 𝑜𝑓 𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠) − ∑( 𝑏𝑜𝑛𝑑 𝑒𝑛𝑒𝑟𝑔𝑖𝑒𝑠 𝑜𝑓 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠)

Note: Bond energies are only used for gaseous reactants and products.

Question 32

True or false
Bond breaking is exothermic while bond formation is endothermic.

Answer 32

False
Bond breaking is endothermic while bond formation is exothermic.

Note: Energy needed to form a bond is referred as bond formation energy.

Question 33

What is Hess’s Law?

Answer 33

The enthalpy change of a chemical reaction is constant and independent of the reaction route between the initial states of reactants and final states of the products.

Question 34

What is Standard enthalpy of sublimation, (𝜟𝑯⊖ 𝒔𝒖𝒃)?

Answer 34

This is the heat absorbed to convert one mole of a pure solid into a gas under standard conditions.

Question 35

Define Standard enthalpy of fusion, (𝜟𝑯⊖𝒇𝒖𝒔)

Answer 35

This the heat absorbed to convert one mole of a pure solid into a liquid under standard conditions.

Question 36

What is standard enthalpy of vapourisation, (𝜟𝑯⊖ 𝒗𝒂𝒑)?

Answer 36

This the heat absorbed to convert one mole of a pure liquid into a gas under standard conditions.

Question 37

Define Standard enthalpy change of solution, (𝜟𝑯⊖)

Answer 37

This is the enthalpy change when one mole of solute is dissolved in enough pure solvent to form an infinitely dilute solution under standard conditions.

Question 38

What is an infinitely dilute solution?

Answer 38

One that does not produce any further enthalpy change when more solvent is added.

Question 39

Describe the procedure for the experiment to determine the standard enthalpy change of solution of ammonium chloride

Answer 39

• A known mass of distilled water is put into a well-insulated copper calorimeter fixed with a stirrer and a thermometer.
• The water is constantly stirred and its initial steady temperature measured using a thermometer and recorded.
• A know mass of finely divided ammonium chloride is carefully added to the water in the copper calorimeter and the mixture is thoroughly stirred until the thermometer registers no further drop in temperature indicating no further heat change is taking place.
• The minimum temperature of the solution is measured and recorded.

Question 40

What is Standard enthalpy change of hydration, (𝜟𝑯⊖ 𝒉𝒚𝒅)?

Answer 40

This is the heat liberated when one mole of specified gaseous ions is surrounded by sufficient water molecules to form an infinitely dilute solution.

Or
This is the heat liberated when one mole of specified gaseous ions dissolves in sufficient water to form a very dilute solution.

Question 41

What factors affect hydration energy ?

Answer 41

(a) Ionic radius
The smaller the ionic radius, the greater (more exothermic) is the hydration energy.

This is because the water molecules can get closer to the centre of the charge so the attractive forces are increased.

(b) Charge on the ion
The greater the charge on the ion, the greater the affinity for water molecules and the more exothermic the hydration process will be.

Question 42

Define Standard enthalpy change of neutralization, (𝜟𝑯⊖𝒏)

Answer 42

This is the heat given out when one mole of hydrogen ions is completely neutralized by an alkali under standard conditions.

Or
This is the heat given out when one mole of water molecules is formed by the reaction between an acid and an alkali under standard conditions.

Question 43

Describe the procedure of an experiment to determine the standard enthalpy change of neutralization of hydrochloric acid

Answer 43

• A known volume of standard hydrochloric acid is put in a well-insulated copper calorimeter and the initial steady temperature of the acid is measured using a thermometer and recorded.
• To the acid is added an equal volume of sodium hydroxide solution of slightly higher molarity than the acid.
• The reaction mixture is thoroughly stirred using the thermometer and the highest temperature attained is measured and recorded.

Question 44

What is 𝑺𝒕𝒂𝒏𝒅𝒂𝒓𝒅 𝒆𝒏𝒕𝒉𝒂𝒍𝒑𝒚 𝒐𝒇 𝒅𝒊𝒔𝒑𝒍𝒂𝒄𝒆𝒎𝒆𝒏𝒕, (𝜟𝑯⊖ 𝒅𝒊𝒔𝒑)?

Answer 44

This is the heat given out when one mole of a metal is displaced from an aqueous solution of its salt by a more electropositive metal.

Question 45

Describe the procedure for an experiment to determine standard change of displacement of copper by zinc

Answer 45

• 50cm3 of 0.2M copper(II) sulphate solution is measured and transferred into a polystyrene cup.
• Using a thermometer the temperature of the solution is measured and recorded every 30 seconds for up to 3 minutes until a constant temperature is achieved.
• At 3 minutes, 1.3g of zinc powder (slightly in excess) is added immediately and stirring of the solution is started.
• The mixture is stirred vigorously and the temperature recorded every thirty seconds for five minutes.
• A temperature – time graph is plotted.
• The graph is extrapolated to determine the maximum temperature change if no heat was lost to the surroundings (cooling correction).

Question 46

What is 𝑳𝒂𝒕𝒕𝒊𝒄𝒆 𝒆𝒏𝒕𝒉𝒂𝒍𝒑𝒚, (𝜟𝑯⊖ 𝒍𝒂𝒕𝒕)?

Answer 46

This is the heat absorbed when one mole of a solid ionic compound is broken apart into its constituent gaseous ions under standard conditions.

Or
This is the heat evolved when one mole of a solid ionic compound is formed from its constituent gaseous ions under standard conditions

Question 47

What factors affect the magnitude of lattice energy?

Answer 47

1. Charge on the ions involved
   The higher the charges on the ions, the stronger the electrostatic forces of attraction between them and the greater the lattice enthalpy.
2. Ionic radius
   The smaller the ionic radius of the two ions involved, the greater the lattice energy.

This is because the smaller the ions, the closer they approach each other and the stronger the electrostatic forces of attraction between them

Question 48

What assumptions are made during the experiment to determine the standard enthalpy change of solution of ammonium chloride?

Answer 48

• No heat is lost to the surroundings
• The specific heat capacity of the solution is equal to that of water.
• The mass of ammonium chloride is negligible compared to that of water.