Enthalpy change: heat energy change at a constant pressure
Examples of exothermic processes:
combustion of fuels
oxidations of carbohydrates
Examples of endothermic processes:
thermal decomposition of calcium carbonate
melting/boiling
Standard conditions:
100kPa pressure
298K temperature
Solutions at 1 mol/dm^3
All substances at their standard state
Calculating the energy change in a reaction:
q = mcΔt
energy change = mass of solution x heat capacity x temperature change
Hess's law: total enthalpy change for a reaction is independent of the route
Mean bond enthalpy: The enthalpy needed to break a covalent bond, averaged over different molecules
Mean bond enthalpy only applies when the substances are in the gaseous state
Enthalpy change: Heat energy change at constant pressure
Standard enthalpy of formation:
The enthalpy change at constant pressure when 1 mole of a compound
Is formed from its constituent elements
With the reactants and products in standard states
Energy is taken in to break bonds → endothermic reaction
Energy is released to make bonds → exothermic reaction
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 reaction is the other way? exothermic
Label these, exothermic or endothermic?
A) Exothermic
B) Endothermic
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.
Draw a simple calorimeter
A) Heatproof mat
B) Spirit Burner
C) Tripod
D) Gauze
E) Thermometer
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 pure oxygen, not air.