Energetics

Question 1

⦵

How can we identify a reaction as exothermic? (2) Give an example of an exothermic reaction. (1)

Answer 1

⇒ -ΔH ((reaction emits energy(EXO=EXIT)) (1)
⇒ Energy reactants > Energy products on an Enthalpy profile diagram. (1)

⇒ N₂ + 3H₂ ⇌ 2NH₃ (Formation of Ammonia) (1)

Question 2

How can we identify an endothermic reaction? (2) Give an example of an endothermic reaction (1)

Answer 2

⇒ +ΔH ((reaction absorbs energy(EN-DO=EN-TRANCE)) (1)
⇒ Energy products > Energy reactions on an Enthalpy profile diagram (1)

⇒ 2NH₃ ⇌ N₂ + 3H₂ (Decompostion of Ammonia) (1)

Question 3

What happens when a reaction takes? (1)

Answer 3

⇒ Reactant bonds are broken, and product bonds are formed. (1)

Question 4

What type of reaction occurs when a bond is broken and vice vera (4)

Answer 4

⇒ To break bonds energy is required, thus making an endothermic reaction (1)
⇒ Stronger bonds require larger amounts of energy (+ΔH) (1)
⇒ To engineer bonds, energy is released, thus an exothermic reaction (1)
⇒ Stronger bonds release more energy when they form (-ΔH) (1)

Question 5

Define: Enthalpy Change, and state its units (3)

Answer 5

⇒ Enthalpy change is the amount of heat energy taken in or given out during any change in a system (1)
⇒ provided the pressure is constant. (1)
⇒ KJ mol⁻¹ (1)

Question 6

What are the two standard conditions for enthalpy change, hence state its symbol (3)

Answer 6

⇒ 100 kPa pressure (1)
⇒ 298 K (room temperature or 25 °C) (1)
⇒ Standard Conditions symbol: ΔH^⦵ (1)

Question 7

Define the term standard enthalpy of formation. (3)

Answer 7

⇒ enthalpy change when 1 mole of a substance (1)
⇒ is formed from its elements (1)
⇒ and all substances are in their standard states (1)

Question 8

Define the term standard enthalpy of combustion. (3)

Answer 8

⇒ Enthalpy change when 1 mole of a compound (1)
⇒ is completely burnt in oxygen (1)
⇒ under standard conditions (1)

Question 9

Define standard enthalpy change of neutralisation? (3)

Answer 9

⇒ Enthalpy change when an acid and an alkali react together (1)
⇒ under standard conditions (1)
⇒ to give one mole of water. (1)

Question 10

What is the general rule for engineering chemical reactions that are undergoing Standard enthalpy change of combustion? (1)

Answer 10

⇒ Since 1 mole of a substance is burned, we need to treat the substance as a constant and complete the chemical equation in any way that allows the substance to remain as having one mol (1)
e.g. ⇒ C₄H₁₀₍ᵩ₎ + 6.5O₂₍ᵩ₎ → 4CO₂₍ᵩ₎ + 5H₂O

Question 11

What is the general rule for engineering chemical reactions that are undergoing Standard enthalpy change of Neutralisation? (1)

Answer 11

⇒ Water must stay as having 1 mol. (1)

⇒ e.g: NaOH(aq) + HCl(aq) → NaOH(aq) + H2O

Question 12

What is the general rule for engineering chemical reactions that are undergoing Standard enthalpy change of Formation?

Answer 12

⇒ The Product can only have 1 mole (1)

⇒ e.g: Na(s) + 1/2Cl₂(g) → NaCl(s)

Question 13

What is the equation for heat change, 𝓆?

Answer 13

𝓆 =mc∆T
𝓂 = mass of water (not substance we are investigating)
𝒸 = specific heat capacity (water)
∆𝒯 = temperature change (of the water)

Question 14

What is the equation for molar enthalpy change for a reaction, ∆ℋ?

Answer 14

∆ℋ = -𝓆/moles

Question 15

State Hess’s Law (1)

Answer 15

⇒ The enthalpy change in a reaction is independent of the route taken. (1)

Question 16

Where do arrows originate from and where do they point in combustion and formation for Hess’s cycle? (2)

Answer 16

⇒ Formation: From the elements towards the main equation (upwards)
⇒ Combustion: From the main equation, towards the elements (going down)

Question 17

What should be in the toolbox of the cycle if all data given is ∆cH⦵? (2)

Answer 17

⇒ CO2 (1)

⇒ H2O (1)

Question 18

What is the equation for the standard enthalpy of combustion? (1)

Answer 18

⇒ [Σ∆cH reactants] - [Σ∆cH products] (1)

Question 19

What is the equation for the standard enthalpy of formation? (1)

Answer 19

⇒ ∆H = [Σ∆fH products]-[Σ∆fH reactants] (1)

Question 20

Define mean bond enthalpy. (2)

Answer 20

⇒ enthalpy to break 1 mole of a bond (1)

⇒ averaged over different molecules (1)

Question 21

What should the states of all substances be when breaking bonds? (1)

Answer 21

⇒ Gaseous form/state (1)

Question 22

Why does mean bond enthalpy differ from those determined using Hess’s Law? (2)

Answer 22

⇒ They are mean values (1)

⇒ Across a range of different compounds/substances containing that bond. (1)

Question 23

What is the equation for Bond enthalpy? (1)

Answer 23

⇒ [Σ∆H bonds broken]-[Σ∆H bonds made] (1)

Question 24

What is the formula for percentage uncertainty? (1)

Answer 24

% uncertainty = (uncertainty/measured value)x100

e.g: The resistance of a filament lamp is given as 5.0 ± 0.4Ω. Calculate the percentage uncertainty
→ (0.4/5.0)x100 = 8%

Question 25

What is the formula for calculating the overall percentage error?

Answer 25

((actual value-acquired value)/actual value)) x100
e.g:
acquired value: -1920
Accurate value: -2422
percantage error: ((-2422-(-1920))/-2422) x 100
= 20.7%

Question 26

Suggest four improvements that would reduce errors due to heat in a calorimetry experiment? (4)

Answer 26

⇒ Reduce the distance between the flame and the beaker (1)
⇒ Put a sleeve around the flame to protect from drafts (1)
⇒ Add a lid (1)
⇒ Use a copper calorimeter rather than a Pyrex beaker (1)

Question 27

Suggest one source of error in a student’s experiment, without considering: heat loss, apparatus error, or student error (1)

Answer 27

Incomplete combustion

Question 28

A student concluded that his temperature rise recorded was smaller than it should have been. Suggest a practical reason for this. (1)

Answer 28

⇒ Thermal energy / heat loss
⇒ Incomplete combustion
⇒ Evaporation

Question 29

Why is it important to stir reaction mixture before recording the temperature? (1)

Answer 29

⇒ ensures that all of the solution is at the same temperature (1)

Question 30

In calorimetry how could, the analysis of the results, be improved in order to determine a more accurate value for the enthalpy of reaction.
Do not refer to the precision of the measuring equipment. Do not change the amounts or the concentration of the chemicals. (6)

Answer 30

⇒ Insulate the beaker or use a polystyrene cup or a lid
→To reduce heat loss.
⇒ Record the temperature for a suitable time before adding the metal
→To establish an accurate initial temperature
⇒ Record temperature values at regular time intervals
→To plot the temperature results against time on a graph
⇒ Extrapolate the cooling back to the point of addition
→To establish a maximum temperature

Question 31

State how your answer (-778KJmol-1) is likely to differ from the value quoted in reference sources.
Give one reason for your answer. (2)

Answer 31

⇒ Less negative than the reference (1)

⇒ Heat loss not taken into account (1)

Question 32

Suggest why a value for the Na-Cl bond enthalpy is not found in any data book. (1)

Answer 32

⇒ Is an ionic bond / not covalent (1)

Question 33

Mean bond enthalpy = -789
Enthalpy change of formation = -777
Explain why the mean bond value obtained is different from that obtained by Hess’s law? (1)

Answer 33

⇒ Mean bond enthalpies are not exact (1)

Question 34

The enthalpy of combustion determined experimentally is less exothermic than that calculated using enthalpies of formation
Give one possible reason for this, OTHER THAN HEAT LOSS. (1)

Answer 34

⇒ Incomplete combustion (1)