1.04 Energetics
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What energy change is breaking bonds associated with ?
Endothermic reactions - as energy is taken in to break the bonds.
What energy change is forming bonds associated with ?
Exothermic reaction - energy is released to form bonds.
What are some uses of thermochemistry ?
• Measuring and comparing the energy values of fuels • Calculating the energy requirements of industrial processes • Working out the theoretical amount of energy taken / released by a reaction • Predicting if a reaction will take place or not.
What is an endothermic reaction ?
• A reaction where the overall enthalpy change is positive (+dH) • Energy in breaking bonds > energy out of making bonds.
What is an exothermic reaction ?
• One where the overall enthalpy change is negative (dH-) • Energy in breaking bonds > energy out of making bonds.
If a reversible reaction is exothermic one way, what is it the other way ?
Endothermic.
Give two 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 ?
• It is the energy change of a system at a constant pressure • It is represented by dH.
What are the standard conditions ?
• 100kPa / 1atm • 298K or 25•C
What does ‘in standard state’ mean ?
The state an element/compound exists at in standard conditions (298k, 100kPa).
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 by its constituent elements in standard conditions (100kPa, 298K), with reactants and products all in their standard states.
Give an example of an equation which represents the enthalpy of formation.
H2 (g) + 1/2O2 (g) -> H2O (l)
Define standard enthalpy of combustion.
The enthalpy change when one mole of a substance is burnt completely in oxygen all in standard conditions (100kPa, 298K), with reactants and products all in their standard states.
Give an example of an equations which represents the enthalpy of combustion.
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 all particles in a system, so it is independent of the amount of substance.
How do you calculate heat change from experimental data ?
• Use the equation Q=mcΔT • m = mass of substance that is being heated (typically water) • c = specific heat capacity of the heated substance (water) • dT is the change of temperature.
Describe a simple calorimeter.
• A spirit burner containing the substance to be tested should be placed below a tripod • Atop of a mesh on a tripod, a beaker containing water should be placed with a thermometer • You can use the change in temperature of water and mass of substance in spirit burner to calculate enthalpy change.
How can calorimeters be made more accurate ?
• Insulate the water beaker as much as possible from all angles but below, eg add a lid and mineral mesh.
Flame
Mark as 5
How would you measure enthalpy change for a reaction occurring in (aq) ?
• Use a polystyrene cup as a beaker (good insulator minimises heat loss) • Heat is generated in the solution during the reaction; calculate the temperature change • Take heat capacity to be waters and density to be 1g/cm3.
What can you use to make experimental determination of enthalpy change of reactions more accurate ?
Cooling curves
What is Hess’ Law ?
Hess’ Law states that the enthalpy change for a reaction is the same regardless of the route taken.