3.1.4 Energetics Flashcards
What energy change is breaking bonds associated with?
Endothermic reaction
Energy is taken in to break bonds.
What energy change is making bonds associated with?
Exothermic reaction.
Energy is released to make bonds.
What are some uses of thermochemistry?
- Measuring and comparing the energy values of fuels
- Calculating the energy requirements for industrial processes.
- Working out the theoretical amount of energy released/taken in in a reaction
- Predicting if a reaction will take place or not.
What is an endothermic reaction?
One with an overall positive enthalpy change (+ΔH) -> energy in breaking bonds > energy out making bonds
What is an exothermic reaction?
One with an overall negative enthalpy change (-ΔH) -> energy in breaking bonds < energy out making bonds.
If a reversible reaction is endothermic one way, what type of the reaction is the other way?
Exothermic
Give 2 examples of exothermic reactions.
- Combustion of fuels
- Neutralisation
Give an example of an endothermic reaction.
Thermal decomposition.
Define enthalpy change. What symbol is used to represent it?
Energy change of a system at a constant pressure.
Represented by ΔH.
What are the standard conditions?
100kPa / 1atm pressure
298K / 25°C temperature
What does “in standard state” mean?
The state an element/compound exists at in standard conditions (100kPa, 298K)
Draw an enthalpy change diagram for an endothermic reaction, and one for an exothermic reaction.
Define standard enthalpy of formation.
The enthalpy change when one mole of a compound is formed from its constituent elements in standard conditions (100kPa, 298K), with reactants and products in their standard states.
Give an example of an equation which represents standard enthalpy of formation.
There are many e.g. H₂ (g) + 1/2 O₂ (g) -> H₂O (l)
Define standard enthalpy of combustion.
The enthalpy change when one mole of a substance is burnt completely in oxygen in standard conditions (100kPa, 298K), with reactants and products in their standard states.
Give an example of an equation which represents standard enthalpy of combustion.
E.g. C (s) + O₂ (g) -> CO₂ (g)
What is the difference between heat and temperature?
Heat is the sum of all particles’ energy, therefore it is affected by the amount of substance. Temperature is related to the mean kinetic energy of the particles in a system, so is independent of the number of particles present.
How can you calculate enthalpy change from experimental data?
Use the equation Q = mcΔT
m = mass of the substance being heated
c = specific heat capacity of that substance
ΔT = change in temperature
Draw a simple calorimeter.
How could this calorimeter be made more accurate?
- Add draught screens at the sides.
- Add a lid on top of the beaker
- Add mineral wool around the beaker
All to insulate and reduce heat lost to the surroundings.
What is a flame calorimeter?
How does it differ to a simple calorimeter?
Reduces heat lost to the surrounding to give more accurate results: has a spiral chimney made of copper, an enclosed flame and the fuel is burnt in the pure oxygen, not air.
How would you measure the enthalpy change for a reaction occurring in (aq)?
Use an expanded polystyrene cup as a calorimeter (good insulator -> reduce heat loss).
Heat is generated in the solution. Measure this temperature change. Take heat capacity of solution to be 4.18 and density of solution = 1 g cm⁻³.
What can you use to make experimental determination of enthalpy change of reaction more accurate?
Cooling curves
What is Hess’s Law?
States that the enthalpy change for a reaction is the same regardless of the route taken.
