Energetics Flashcards
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How can we identify a reaction as exothermic? (2) Give an example of an exothermic reaction. (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)
How can we identify an endothermic reaction? (2) Give an example of an endothermic reaction (1)
⇒ +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)
What happens when a reaction takes? (1)
⇒ Reactant bonds are broken, and product bonds are formed. (1)
What type of reaction occurs when a bond is broken and vice vera (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)
Define: Enthalpy Change, and state its units (3)
⇒ 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)
What are the two standard conditions for enthalpy change, hence state its symbol (3)
⇒ 100 kPa pressure (1)
⇒ 298 K (room temperature or 25 °C) (1)
⇒ Standard Conditions symbol: ΔH^⦵ (1)
Define the term standard enthalpy of formation. (3)
⇒ enthalpy change when 1 mole of a substance (1)
⇒ is formed from its elements (1)
⇒ and all substances are in their standard states (1)
Define the term standard enthalpy of combustion. (3)
⇒ Enthalpy change when 1 mole of a compound (1)
⇒ is completely burnt in oxygen (1)
⇒ under standard conditions (1)
Define standard enthalpy change of neutralisation? (3)
⇒ Enthalpy change when an acid and an alkali react together (1)
⇒ under standard conditions (1)
⇒ to give one mole of water. (1)
What is the general rule for engineering chemical reactions that are undergoing Standard enthalpy change of combustion? (1)
⇒ 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
What is the general rule for engineering chemical reactions that are undergoing Standard enthalpy change of Neutralisation? (1)
⇒ Water must stay as having 1 mol. (1)
⇒ e.g: NaOH(aq) + HCl(aq) → NaOH(aq) + H2O
What is the general rule for engineering chemical reactions that are undergoing Standard enthalpy change of Formation?
⇒ The Product can only have 1 mole (1)
⇒ e.g: Na(s) + 1/2Cl₂(g) → NaCl(s)
What is the equation for heat change, 𝓆?
𝓆 =mc∆T
𝓂 = mass of water (not substance we are investigating)
𝒸 = specific heat capacity (water)
∆𝒯 = temperature change (of the water)
What is the equation for molar enthalpy change for a reaction, ∆ℋ?
∆ℋ = -𝓆/moles
State Hess’s Law (1)
⇒ The enthalpy change in a reaction is independent of the route taken. (1)
Where do arrows originate from and where do they point in combustion and formation for Hess’s cycle? (2)
⇒ Formation: From the elements towards the main equation (upwards)
⇒ Combustion: From the main equation, towards the elements (going down)
What should be in the toolbox of the cycle if all data given is ∆cH⦵? (2)
⇒ CO2 (1)
⇒ H2O (1)
What is the equation for the standard enthalpy of combustion? (1)
⇒ [Σ∆cH reactants] - [Σ∆cH products] (1)
What is the equation for the standard enthalpy of formation? (1)
⇒ ∆H = [Σ∆fH products]-[Σ∆fH reactants] (1)
Define mean bond enthalpy. (2)
⇒ enthalpy to break 1 mole of a bond (1)
⇒ averaged over different molecules (1)
What should the states of all substances be when breaking bonds? (1)
⇒ Gaseous form/state (1)
Why does mean bond enthalpy differ from those determined using Hess’s Law? (2)
⇒ They are mean values (1)
⇒ Across a range of different compounds/substances containing that bond. (1)
What is the equation for Bond enthalpy? (1)
⇒ [Σ∆H bonds broken]-[Σ∆H bonds made] (1)
What is the formula for percentage uncertainty? (1)
% 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%
What is the formula for calculating the overall percentage error?
((actual value-acquired value)/actual value)) x100
e.g:
acquired value: -1920
Accurate value: -2422
percantage error: ((-2422-(-1920))/-2422) x 100
= 20.7%
Suggest four improvements that would reduce errors due to heat in a calorimetry experiment? (4)
⇒ 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)
Suggest one source of error in a student’s experiment, without considering: heat loss, apparatus error, or student error (1)
Incomplete combustion
A student concluded that his temperature rise recorded was smaller than it should have been. Suggest a practical reason for this. (1)
⇒ Thermal energy / heat loss
⇒ Incomplete combustion
⇒ Evaporation
Why is it important to stir reaction mixture before recording the temperature? (1)
⇒ ensures that all of the solution is at the same temperature (1)
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)
⇒ 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
State how your answer (-778KJmol-1) is likely to differ from the value quoted in reference sources.
Give one reason for your answer. (2)
⇒ Less negative than the reference (1)
⇒ Heat loss not taken into account (1)
Suggest why a value for the Na-Cl bond enthalpy is not found in any data book. (1)
⇒ Is an ionic bond / not covalent (1)
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)
⇒ Mean bond enthalpies are not exact (1)
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)
⇒ Incomplete combustion (1)
