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Question 1

What is enthalpy change, AH, in a chemical reaction?

Answer 1

The change in energy, normally heat, at constant pressure

Enthalpy change is a key concept in thermodynamics, representing the heat content of a system.

Question 2

What happens to the surroundings in an exothermic reaction?

Answer 2

The temperature of the surroundings increases

In exothermic reactions, heat is released to the surroundings.

Question 3

What is the effect on the surroundings in an endothermic reaction?

Answer 3

The temperature of the surroundings decreases

Endothermic reactions absorb heat from the surroundings.

Question 4

How can enthalpy change be expressed mathematically?

Answer 4

AH = AU + pAV

Where AU is the change in internal energy, p is pressure, and AV is the change in volume.

Question 5

What sign is associated with AH for exothermic reactions?

Answer 5

Negative

This indicates that the enthalpy of the products is lower than that of the reactants.

Question 6

What sign is associated with AH for endothermic reactions?

Answer 6

Positive

This indicates that the enthalpy of the products is higher than that of the reactants.

Question 7

What is activation energy?

Answer 7

The minimum collision energy needed for a reaction to occur

Activation energy is a critical factor in determining the rate of a chemical reaction.

Question 8

How do catalysts affect activation energy?

Answer 8

They provide an alternative reaction mechanism with lower activation energy

This allows for a greater proportion of particles to react, speeding up the reaction.

Question 9

What role do transition metal complexes play in reactions?

Answer 9

They can act as catalysts

Transition metal complexes have a central metal ion with attached molecules or ions via dative covalent bonds.

Question 10

What is one industrial application of heat released from exothermic reactions?

Answer 10

It is used to speed up other reactions in a factory

This practice helps to reduce energy waste in industrial processes.

Question 11

Why is the use of catalysts important in industrial reactions?

Answer 11

They allow reactions to occur at acceptable rates with lower temperatures

This results in significant energy savings.

Question 12

What catalyst is used in the Haber process for ammonia production?

Answer 12

Iron

In this process, reactants adsorb onto the catalyst’s surface, react, and then desorb as products.

Question 13

What catalyst is used in the contact process for sulfuric acid production?

Answer 13

Vanadium(V) oxide

It catalyzes the oxidation of SO2 to form SO3.

Question 14

What is the equation for calculating heat energy (Q) in a reaction?

Answer 14

Q = mcΔT

Where m is mass, c is specific heat capacity, and ΔT is the change in temperature.

Question 15

What are the standard conditions for measuring enthalpy changes?

Answer 15

Pressure, concentration, temperature

These conditions ensure consistent and comparable results in thermodynamic measurements.

Question 16

Fill in the blank: In an exothermic reaction, energy is transferred ______ the system.

Answer 16

to

This transfer leads to an increase in the temperature of the surroundings.

Question 17

Fill in the blank: In an endothermic reaction, energy is transferred ______ the system.

Answer 17

from

This leads to a decrease in the temperature of the surroundings.

Question 18

What is the standard enthalpy change of formation, AH°?

Answer 18

The enthalpy change that occurs when one mole of a substance is formed from its elements in their standard state, under standard conditions.

Standard conditions typically refer to a temperature of 298 K and a pressure of 1 atm.

Question 19

What is the standard enthalpy change of combustion, ΔH°c?

Answer 19

The enthalpy change that occurs when one mole of a substance, in its standard state, is burnt completely in excess oxygen, under standard conditions.

Combustion reactions typically produce carbon dioxide and water.

Question 20

Give an example of a reaction that illustrates the standard enthalpy change of formation.

Answer 20

Ca(s) + C(s) + 1.50g(9) → CaCO3(s)

This reaction shows the formation of calcium carbonate from its elements.

Question 21

Give an example of a reaction that illustrates the standard enthalpy change of combustion.

Answer 21

C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l)

This reaction shows the combustion of ethanol.

Question 22

What is the formula to calculate the heat energy change, Q?

Answer 22

Q = mcΔT

Where m is the mass, c is the specific heat capacity, and ΔT is the temperature change.

Question 23

What data is recorded to measure enthalpies of combustion?

Answer 23

• Mass of fuel burnt (g)
• Maximum temperature change (K)
• Mass of water in calorimeter (g)

These measurements help calculate the amount of heat released during combustion.

Question 24

What materials are recommended for a calorimeter used in combustion experiments?

Answer 24

A material with high thermal conductivity, such as copper.

This ensures maximum heat transfer to the water.

Question 25

Calculate Q for burning 0.400 g of ethanol, given that the temperature of 100 g of water increased by 25 K.

Answer 25

Q = 100 x 4.18 x 25 = 10450 J = 10.45 kJ. ## Footnote The specific heat capacity of water is 4.18 J/g·K.

Question 26

How do you calculate the number of moles of ethanol burned?

Answer 26

Number of moles = mass of ethanol / molar mass of ethanol. ## Footnote For ethanol, the molar mass is approximately 46.0 g/mol.

Question 27

What is the calculated standard enthalpy change of combustion for ethanol based on the previous calculations?

Answer 27

ΔH° = -10.45 kJ / 0.00870 mol = -1200 kJ mol-1. ## Footnote Standard enthalpy changes of combustion are always negative as they release heat.

Question 28

True or False: The true value of ΔH° for ethanol is greater than the experimental value calculated.

Answer 28

False ## Footnote The true value is -1371 kJ mol-1, which is greater in magnitude than the experimental value due to heat loss and incomplete combustion.

Question 29

What factors can lead to discrepancies between experimental and true values of enthalpy changes?

Answer 29

* Heat loss to the atmosphere * Loss of unburnt ethanol when extinguished * Incomplete combustion producing carbon monoxide or carbon ## Footnote These factors can affect the accuracy of experimental measurements.

Question 30

Fill in the blank: Standard enthalpy changes of combustion always have _______ signs.

Answer 30

negative ## Footnote This indicates that heat is released during the combustion process.

Question 31

What is the change in mass of the butane camping gas burner used to heat 500 cm³ of water from 20°C to 80°C?

Answer 31

2.80 g ## Footnote This information is relevant for calculating the enthalpy of combustion for butane.

Question 32

What is the process called when ionic solids dissolve in water?

Answer 32

Hydration ## Footnote Ions interact with water molecules during this process.

Question 33

What happens to ions when soluble ionic solids dissolve in water?

Answer 33

Ions break away from each other in the ionic lattice ## Footnote This process requires energy.

Question 34

Is the process of breaking the ionic lattice endothermic or exothermic?

Answer 34

Endothermic ## Footnote Energy is required to break the lattice.

Question 35

What type of energy interaction occurs when newly separate ions interact with water molecules?

Answer 35

Exothermic ## Footnote This process releases energy.

Question 36

Why is water considered a good solvent for many ionic compounds?

Answer 36

The energy released during ion-water interactions compensates for the energy required to break the lattice.

Question 37

What is the standard enthalpy change of hydration (ΔH°) defined as?

Answer 37

Enthalpy change when one mole of gaseous ions is dissolved in water to make an infinitely dilute solution.

Question 38

What is the standard enthalpy change of hydration for sodium ion (Na+)?

Answer 38

-406 kJmol-1 ## Footnote This value represents the energy change when Na+ is dissolved in water.

Question 39

What is the standard enthalpy change of hydration for chloride ion (Cl-)?

Answer 39

-378 kJmol-1 ## Footnote This value represents the energy change when Cl- is dissolved in water.

Question 40

What are the two key factors that affect the standard enthalpy change of hydration?

Answer 40

* Charge on the ion * Size of the ion

Question 41

How does the charge on the ion affect the enthalpy of hydration?

Answer 41

The greater the charge, the stronger the electrostatic interactions with water, resulting in a more exothermic enthalpy of hydration.

Question 42

What effect does the size of the ion have on the enthalpy of hydration?

Answer 42

Larger ions have weaker electrostatic interactions with water, resulting in a less exothermic enthalpy of hydration.

Question 43

What is the formula to calculate the enthalpy change of solution (ΔHsol) when ionic solids dissolve?

Answer 43

ΔHsol = -ΔLEH + [ΔH° (cation) + ΔH° (anion)]

Question 44

What is the lattice enthalpy (ΔLEH)?

Answer 44

The energy released when gaseous ions combine to form the ionic lattice.

Question 45

What is the lattice enthalpy for sodium chloride (NaCl)?

Answer 45

-787 kJmol-1

Question 46

What does a positive value for ΔHsol indicate?

Answer 46

The enthalpy change is slightly endothermic when sodium chloride dissolves in water.

Question 47

Calculate the enthalpy change of solution for ammonium nitrate (NH4NO3) using the given data.

Answer 47

ΔHsol = -(-646) + (-307 - 314) ## Footnote Use provided enthalpy values for NH4+ and NO3-.

Question 48

What is the formula to calculate ΔH° for a reaction?

Answer 48

ΔH° = [ΔH° (products) - ΔH° (reactants)] ## Footnote This formula is used to find the standard enthalpy change of a reaction.

Question 49

How is ΔH° for the breaking down of sodium hydrogencarbonate calculated?

Answer 49

ΔH° = (-1131 - 286 - 394) - (2 × -951) = +91 kJ mol-1 ## Footnote The values correspond to the standard enthalpy of formation for each compound involved.

Question 50

What are the units for enthalpy changes?

Answer 50

kJ mol-1 ## Footnote This unit measures the energy change per mole of substance.

Question 51

What is the formula to calculate ΔH° of a compound?

Answer 51

ΔH° (compound) = [ΔH° (elements) - ΔH° (compound)] ## Footnote This relationship is used to determine the standard enthalpy of formation of a compound.

Question 52

What is the balanced equation for the formation of one mole of methane from its elements?

Answer 52

C (s) + 2H2(g) → CH4(g) ## Footnote This equation represents the reaction where carbon and hydrogen combine to form methane.

Question 53

What is the standard enthalpy of formation of methane (ΔH° (CH4))?

Answer 53

-890 kJ mol-1 ## Footnote This value is used in calculations to determine the enthalpy changes associated with reactions involving methane.

Question 54

How is the ΔH° for the formation of methane calculated?

Answer 54

ΔH° = [(-394) + (2 × -286)] - (-890) = -76 kJ mol-1 ## Footnote This calculation includes the enthalpy of formation for carbon and hydrogen gas.

Question 55

True or False: The ΔH° value for H2 is multiplied by two in calculations.

Answer 55

True ## Footnote This is because two moles of hydrogen gas are involved in the formation of methane.

Question 56

What is the reaction for producing hydrogen gas from methanol?

Answer 56

CH3OH(l) + H2O(l) → CO2(g) + 3H2(g) ## Footnote This reaction shows the conversion of methanol and water into carbon dioxide and hydrogen gas.

Question 57

What is the standard enthalpy of formation of methanol (ΔH° (CH3OH))?

Answer 57

-238 kJ mol-1 ## Footnote This value is necessary for calculating enthalpy changes in reactions involving methanol.

Question 58

What is the significance of calculating ΔH° for reactions like the methanol to hydrogen conversion?

Answer 58

To choose conditions for optimum rate and yield for the reaction ## Footnote Understanding enthalpy changes helps in optimizing industrial processes.

Question 59

Fill in the blank: ΔH° for elements is always ______.

Answer 59

zero ## Footnote This is a standard convention in thermochemistry.