9) Enthalpy *****

Question 1

What is enthalpy?

Answer 1

Enthalpy (H) is a measure of the heat energy stored in a chemical system. It is thought of as the energy stored within bonds. It cannot be measured directly, but enthalpy changes can.

Question 2

Describe an exothermic reaction in terms of energy transfer and the sign of ΔH.

Answer 2

Heat energy is transferred from the system to the surroundings. The surroundings’ temperature increases. ΔH is negative.

Question 2

What is the equation for enthalpy change?

Answer 2

ΔH = H(products) - H(reactants)

If ΔH is negative, the reaction is exothermic (products have less energy).

If ΔH is positive, the reaction is endothermic (products have more energy).

Question 3

Describe an endothermic reaction in terms of energy transfer and the sign of ΔH.

Answer 3

Heat energy is transferred from the surroundings to the system. The surroundings’ temperature decreases. ΔH is positive.

Question 4

What is activation energy?

Answer 4

The minimum energy required to start a reaction by breaking the bonds in the reactants. It is always an energy barrier that must be overcome.

Question 5

What are the standard conditions for enthalpy changes?

Answer 5

Pressure: 100 kPa

Temperature: 298 K (25 °C)

Concentration: 1 mol dm⁻³ (for solutions)

Substances must be in their standard states.

Question 6

Define the standard enthalpy change of formation.

Answer 6

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions.

ΔfH for any element in its standard state is 0 kJ mol⁻¹.

Question 7

Define the standard enthalpy change of neutralisation (ΔₙₑᵤₜH°)

Answer 7

The energy change when an acid and a base react to form 1 mole of water (H₂O(l)) under standard conditions.

For strong acids and bases: ΔₙₑᵤₜH° ≈ -57 kJ mol⁻¹

Ionic Eq: H⁺(aq) + OH⁻(aq) → H₂O(l)

Question 7

Define the standard enthalpy change of combustion (Δ꜀H°).

Answer 7

The enthalpy change when 1 mole of a substance reacts completely with oxygen under standard conditions.

Question 8

What is the equation for calculating the energy change in a calorimetry experiment?

Answer 8

q = m × c × ΔT

q = energy change (J)

m = mass of substance changing temperature (g)

c = specific heat capacity (J g⁻¹ K⁻¹). For water, c = 4.18 J g⁻¹ K⁻¹

ΔT = temperature change (K or °C)

Question 9

After finding q (in kJ), how do you calculate the molar enthalpy change (ΔH)?

Answer 9

ΔH = q / n

where n is the amount (in moles) of the limiting reactant that reacted.

Question 10

List three sources of error in simple calorimetry experiments (e.g., spirit burner).

Answer 10

Heat loss to the surroundings (air, apparatus).

Incomplete combustion of the fuel.

Evaporation of fuel or water.

Question 11

What is the purpose of plotting a cooling curve and extrapolating the line?

Answer 11

To correct for heat loss during the reaction. By extrapolating the cooling section back to the time of mixing, you can find the theoretical maximum temperature if no heat was lost to the surroundings, giving a more accurate ΔT.

Question 12

Define average bond enthalpy

Answer 12

The energy required to break one mole of a specific type of bond in a gaseous molecule.

Bond breaking is always endothermic (+ΔH).

It is an average because the exact value depends on the chemical environment.

Question 13

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

Answer 13

ΔH = Σ(Bond enthalpies broken) - Σ(Bond enthalpies formed)

Energy In: Sum of all bond enthalpies for bonds broken (endothermic, positive).

Energy Out: Sum of all bond enthalpies for bonds formed (exothermic, negative).

Question 14

What are two limitations of using average bond enthalpies?

Answer 14

They are average values from many compounds and may not reflect the exact value in a specific molecule.

Calculations assume all reactants and products are in the gaseous state, whereas standard enthalpy changes often involve liquids or solids.

Question 15

State Hess’s Law.

Answer 15

The total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same. The total enthalpy change for the reaction is the sum of all changes

Question 16

What is the formula for calculating ΔHᵣₓₙ using standard enthalpies of formation?

Answer 16

The correct formula to determine the standard enthalpy change (ΔHᵣₓₙ) for the reaction is given by ΔHᵣₓₙ = ∑ΔHₚ (products) - ∑ΔHₚ (reactants), where ΔHₚ refers to the standard enthalpy of formation.

Question 17

What is the key principle when constructing a Hess’s Law cycle?

Answer 17

Ensure the same starting and ending points are connected by two different routes. The sum of the enthalpy changes for one route equals the sum for the other route.

Question 17

What is ΔHᵣₓₙ

Answer 17

the change in enthalpy (heat) of the chemical reaction

For formation
ΔHᵣₓₙ = ΣΔfH°(products) - ΣΔfH°(reactants)

For Combustion
ΔHᵣₓₙ = ΣΔ꜀H°(reactants) - ΣΔ꜀H°(products)

Question 18

What should be noted when measuring the Enthalpy of Combustion?

Answer 18

the starting temperature of the water
the final temperature of the water
the mass of the burner before the experiment
the mass of the burner after heating
the temperature change

Question 19

What is the Specific Heat Capacity of Water

Answer 19

4.18 J/g°C

Question 20

Why do chemists use Hess’s Law?

Answer 20

It allows us to indirectly determine enthalpy changes for reactions that are difficult or impossible to measure directly in the lab (e.g., the formation of benzene from carbon and hydrogen, which would produce many products).