Topic 6 Energetics Flashcards
What is the definition of enthalpy?
The total energy content of a system.
What is the definition of enthalpy change ΔH?
The heat change measured at constant pressure.
What is the sign of ΔH and what are the temperature changes for exothermic reactions?
ΔH is negative.
The reacting substances cool down and the surroundings (beaker, thermometer, water of solution) heat up.
What is the sign of ΔH and what are the temperature changes for endothermic reactions?
ΔH is positive
The reacting substances heat up and the surroundings (beaker, thermometer, water of solution) cool down.
Which unit of energy is used in enthalpy changes?
Kilojoules (kJ)
Which equations are used in calculating enthalpy changes?
ΔH = (no. of moles)/(energy in kJ)
Q = mcΔT
What is the definition of Hess’s law?
The enthalpy change of a reaction is independent of the past taken in converting reactants into products, provided the initial and final conditions are the same in each case.
What is the definition of bond enthalpy?
The enthalpy change when one mole of a bond in the gaseous state is broken.
What is the definition of mean bond enthalpy?
The enthalpy change when one mole of a bond in the gaseous state, averaged over many molecules, is broken.
What is the limitation of using bond enthalpies to calculate standard enthalpy changes of reactions?
Bond enthalpy are measured in the gaseous state while many are compounds are not gaseous in their standard state (such as water).
Define the standard enthalpy change of reaction.
The enthalpy change which occurs when equation quantities of materials react together at standard conditions of 298K and 100kPa
Define the standard enthalpy change of formation.
The slope change when one mole of a substance is formed from its elements in their standard states at standard conditions of 298K and 100kPa
Define the standard enthalpy change of combustion.
The enthalpy change when one mole of a substance is completely burned in oxygen at standard conditions of 298K and 100kPa
Define the standard enthalpy change of neutralisation.
The enthalpy between when one mole of water is produced by the neutralisation of an acid with an alkali at standard conditions of 298K and 100kPa.
Define the standard enthalpy change of atomisation.
The enthalpy change when one mole of gases atoms is formed from an element in its standard state, measured at standard conditions of 298K and 100kPa.
State some common sources of error when measuring standard enthalpy changes.
- Heat classes to the surroundings such as the air or the container.
- Incomplete combustion of the substance.
- Non-standard, such as water vapour being produced, not liquid water.
- If the experiment takes a long time, some of the heat transferred to the water will be lost to the surroundings.
- Uncertainties in the measurements involving apparatus such as measuring cylinders, thermometers, and balances.
- Polystyrene cups are not completely heat insulating.
What do bond enthalpies give an indication about regarding reactions?
Bond was lower enthalpies will break first in a reaction compared to those of the higher enthalpies.
The lower the bond and enthalpies, the more rapidly the reaction will take place at room temperature
