[3.1.4] Energetics Flashcards
Enthalpy Change, Calorimetry, Applications of Hess's Law and Bond Enthalpies.
What’s the definition of an enthalpy change?
Enthalpy change is the amount of heat energy taken in or given out during any change in a system provided the pressure is constant.
What happens in an exothermic reaction?
What is the ΔH in an exothermic reaction?
Give some examples of exothermic processes.
Draw an energy level diagram to represent this type of reaction.
- In an exothermic reaction energy is transferred from the system (chemicals) to the surroundings.
- The products have less energy than the reactants.
- In an exothermic reaction the ΔH is negative (-ve).
- Exothermic processes include the combustion of fuels and the oxidation of carbohydrates such as glucose in respiration.
What happens in an endothermic reaction?
What is the ΔH in an endothermic reaction?
Give an example of an endothermic process.
Draw an energy level diagram to represent this type of reaction.
- In an endothermic reaction, energy is transferred from the surroundings to the system (chemicals).
- The products have more energy than the reactants.
- In an endothermic reaction the ΔH is positive (+ve).
- An endothermic process could be thermal decomposition.
What’s the definition of standard enthalpy change of formation? What symbol represents it?
- The standard enthalpy change of formation of a compound is the enthalpy change when 1 mole of the compound is formed from its elements under standard conditions (298K & 100 kPa) with all reactants and products being in their standard states.
- Symbol for enthalpy of formation = ΔբH.
What’s the definition of standard enthalpy change of combustion? What symbol represents it?
- The standard enthalpy change of combustion of a substance is the enthalpy change that occurs when **1 mole of a substance ** is combusted completely in oxygen under standard conditions (298K & 100 kPa) with all reactants and products being in their standard states.
- Symbol for enthalpy of formation = Δ꜀H.
What are standard conditions? What symbol is used to show an enthalpy change has been measured at standard conditions?
- Enthalpy changes are normally quoted at standard conditions. These conditions are:
- 100 kPa pressure.
- 298K (room temperature or 25°C)
- Solutions at 1 mol dm⁻³
- All substances should have their normal state at 298K
- When an enthalpy change is measured at standard conditions, the symbol ⦵ is used.
What equation can we use to calculate energy change in a reaction? What are the units for each variable in the equation?
How could you use this equation to calculate the molar enthalpy change for a reaction?
Energy change = mass of substance x specific heat capacity x temperature change
OR q = mcΔT
- Energy change is represented by q and is measured in joules (J).
- Mass of substance is represented by m and is measured in grams (g).
- Specific heat capacity is represented by c and is measured in joule per kelvin per gram ( J g⁻¹K⁻¹)
- Temperature change is represented by ΔT and is measured in kelvins.
Molar enthalpy change (ΔH) = q ÷ moles
- The moles used have to be that of the limiting reagent in the reaction.
What is Hess’s law?
Hess’s law states that total enthalpy change for a reaction is independent of the route by which the chemical change takes place.
What equation could you use to determine enthalpy changes from enthalpy changes of formation?
Draw a hess cycle to represent this too.
ΔH reaction = ΣΔբH products - ΣΔբH reactants
What equation could you use to determine enthalpy changes from enthalpy changes of combustion?
Draw a hess cycle to represent this too.
ΔH reaction = ΣΔ꜀H reactants - ΣΔ꜀H products
Define the term mean bond enthalpy.
Mean bond enthalpy is the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules.
What equation could you use to determine enthalpy changes from mean bond enthalpies?
Draw a hess cycle to represent this too.
Why will ΔH values calculated using this method be less accurate than using formation or combustion data?
IN GENERAL IF ALL SUBSTANCES ARE GASES:
ΔH reaction = Σ bond energies broken in reactants - Σ bond energies made in products
- ΔH will be less accurate because the bond enthalpy values are not exact, they are averaged values of the bond enthalpies from various compounds containing that bond.
