Enthalpy (Chapter 9) Flashcards
What is the equation for energy change?
q = mcdeltaT
What is the mass in the equation for energy change?
The total mass of solutions
What is the c in the equation for energy change?
Specific heat capacity - 4.18 (4.2) J/g/K for water
What is the equation for the molar enthalpy change?
DeltaH = -q/n
Exothermic or endothermic with a positive molar enthalpy change?
Endothermic
Exothermic or endothermic with a negative molar enthalpy change?
Exothermic
What is the standard enthalpy change of reaction?
The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with reactants and products in their standard states
What is a standard state?
The physical state of a substance under standard conditions
What is the standard enthalpy change of formation?
The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states
What is the standard enthalpy change of combustion?
The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in their standard states
What is the standard enthalpy change of neutralisation?
The enthalpy change that accompanies the reaction of an acid with a base to form one mole of H2O(l) under standard conditions with all reactants and products in their standard states
What are standard conditions?
A pressure of 100kPa
A stated temperature - usually 298K (25C)
Concentration of 1mol/dm3
What are two examples of an exothermic reaction?
Fuels burning, respiration
What are two examples of an endothermic reaction?
Thermal decomposition of limestone, photosynthesis
What do bond enthalpies tell us?
About the strength of the chemical bond
What is a (average) bond enthalpy?
The energy required to break one mole of a specified type of bond in a gaseous molecule
Is bond breaking exothermic or endothermic?
Endothermic
Why is bond breaking endothermic?
Because it requires energy
Is bond making exothermic or endothermic?
Exothermic
Why is bond making exothermic?
Because it releases energy
What is first needed in a reaction?
Energy to break the bonds (even if bonds are also made after this)
Why might the actual bond enthalpy differ from the average value?
- The bond strength will vary across the different environments in which it is found hence data tables show average bond enthalpies
- The experiment may also have been carried out under non-standard conditions
In terms of bond enthalpies, how do you know if a reaction is exothermic?
- The energy released from making bonds is greater than the energy absorbed breaking bonds
- The bonds broken are weaker than the bonds made
In terms of bond enthalpies, how do you know if a reaction is endothermic?
The energy absorbed breaking bonds is greater than the energy released making bonds
The bonds made are weaker than the bonds broken