Chapter 9 Flashcards
Enthalpy
Measure of heat energy stroked in a chemical system
Enthalpy change
Difference in enthalpy between reactants and products in a reaction
What is an exothermic reaction
Exothermic reactions give out heat energy
Has a negative enthalpy change because energy needed to make bonds is larger than energy released to break bonds
Energy transferred from system to surroundings
What is an endothermic reaction
Reaction that absorbs heat
Has a positive enthalpy change
Surroundings to system
Activation energy
Ea
Minimum amount of energy needed to break reactant bonds and start a chemical reaction
Standard conditions definition
Standard set conditions for experimental measures that allow comparisons of different sets of data to be made
Standard pressure
100 kPa
Standard temperature
298 K
Standard concentration
1 mol dm-3
Standard state
Physical state of substance
Standard enthalpy change of reaction
Enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation, under sc
Standard Enthalpy change of formation
Enthalpy change when 1 mole of a compound is formed from its elements
Standard enthalpy change of combustion
Enthalpy change when 1 mole of a substance completely reacts with oxygen, under sc
Standard Enthalpy change of neutralisation
Enthalpy change when an acid and an alkaline react together to form 1 mole of water
Enthalpy change of combustion experiment
Pour 150cm3 water into beaker
Record initial temp
Add methanol to spirit burner
Weigh spirit burner
Burn methanol whilst stirring water with thermometer
After 3 mins - record max temperature of water
Re weigh spirit burner
Why the experiment for combustion enthalpy change is not very accurate
- heat lost to surroundings other than water
- incomplete combustion
- evaporation of methanol from wick- must reweigh quick
- non standard conditions
Values will be less exothermic
Experiment for neutralision and reaction
Combine known substances In insulated container Lid on Measure temp change Calculate heat given out using Q=mct
Average bond enthalpy
The energy required to break one mole of a specified type of bond in a gaseous molecule
Calculated from actual bond enthalpies in different chemical environments
Limitations of using average bind enthalpies
- actual bond enthalpies are different
- species need to be gaseous molecules
- non standard enthalpy
Hess law
If a reaction can take place by 2 routes and the start and finish are the same the total enthalpy change is the same for each