3.1.4 Energetics Flashcards
Bond breaking is…
- Endothermic.
Bond making is…
- Exothermic.
How do we determine whether a reaction is endo or exothermic?
- Sum of energy in vs energy out.
Transition state?
- State at which chemical bonds are partially formed + broken.
Activation energy meaning?
- Minimum energy needed for reactant molecules to have a successful collision + start the reaction.
Do exothermic or endothermic reactions have a higher activation energy?
- Endothermic.
Is enthalpy change +ve or -ve in exothermic?
- -ve.
Is enthalpy change +ve or -ve in endothermic?
- +ve.
Where does the activation energy occur?
- Between reactants + transition state.
Where does enthalpy change occur?
- From reactants to products.
What is enthalpy?
- Total chemical energy inside a substance.
What symbols represent enthalpy change?
- ΔH.
Exothermic reactions are thermodynamically possible because…
- Enthalpy of the reactants is higher than products.
What are 2 examples of exothermic reactions?
- Combustion.
- Neutralisation.
What is an example of an endothermic reaction?
- Thermal decompostion.
What is enthalpy change?
- Energy change of system at constant pressure.
What are standard conditions?
- 1 atm / 100kPa.
- 25℃.
℃ to K?
- ℃ + 273 = K.
What is standard enthalpy of formation?
- ΔH of 1 mole of compound is formed from its constituent elements in standard conditions w/ reactants + products in standard state.
What is standard enthalpy of combustion?
- ΔH of 1 mole of substance is burned completely in oxygen in standard conditions w/ reactants + products in standard states.
What is heat?
- Sum of all particle’s energy ∴ affected by amount of substance.
What is temperature?
- Related to mean kinetic energy of particles in a system ∴ independent of particles present.
How can you calculate enthalpy change from experimental data?
- Q=mcΔT.
How does a flame calorimeter differ from a simple calorimeter?
- Reduces heat loss to surroundings to obtain more accurate results.
- Spiral chimney of copper.
- Enclosed flame.
- Burnt in pure oxygen as opposed to air.
How to measure enthalpy change for reaction occurring in (aq)?
- Expanded polystyrene cup as calorimeter.
- Measure temp change of solution.
- Heat capacity of solution –> 4.18.
- Density –> 1g/cm3.
How can experimental determination of enthalpy change be more accurate?
- Cooling curves.
What is Hess’ law?
- ΔH of reaction is same regardless of route taken.
What is enthalpy of an element?
- 0.
What is bond dissociation enthalpy?
- ΔH required to break a covalent bond w/ all species in gaseous state.
- Differs for same bond in different molecules.
What is mean bond enthalpy?
- Average value for bond dissociation enthalpy of a given bond.
Why may experimental methods for enthalpy determination not be very accurate?
- Heat lost to surroundings.
- Not in standard conditions.
- Reaction may not be complete.
Why is using bond enthalpies not as accurate as using standard enthalpy of combustion/formation?
- Bond enthalpies are a mean across diff molecules.
- ΔH of combustion/formation apply to just that molecule ∴ more accurate.
What is underneath the equation in enthalpy of combustion?
- The oxides of the elements.
In enthalpy of combustion do the arrows point towards or away from the middle?
- Towards.
What is underneath the equation in enthalpy of formation?
- The elements in their standard states.
In enthalpy of formation do the arrows point towards or away from the middle?
- Away.
