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
What is the equation for enthalpy change of reaction, in terms of bond enthalpies?
Sum(bond enthalpies of bonds broken(reactants) - Sum(bond enthalpies of bonds made(products))
What is Hess’s Law?
- If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route
- The enthalpy change that occurs is independent of the route taken
What does Hess’s Law allow us to do?
Indirectly calculate enthalpy changes where we cannot do so directly
Why can’t you always measure enthalpy change directly?
High activation energy
Slow reaction rate
More than one reaction taking place
Whats the equation for Hess’s law when given formation enthalpies? (C to A and B)
DeltaHr = -DeltaH1 + DeltaH2
write out the elements of formation as C
If A goes to C and C goes to B, what is the equation for Hess’s law?
DeltaHr = DeltaH1 + DeltaH2
If A and B both go to C, what is the equation for Hess’s law?
DeltaHr = DeltaH1 - DeltaH2
What is specific heat capacity?
The energy required to raise the temperature of 1g of a substance by 1K
What is activation energy?
The minimum energy input required to break bonds and for a reaction to take place
Why might experimental enthalpy change of combustion values be different from data book values?
Would lead to a less exothermic enthalpy change:
- Heat energy loss to the surroundings
- Incomplete combustion occurring
- Evaporation of water (mass not 100g) then mass is lower or alcohol if using wick
Could lead to a more or less exothermic enthalpy change
- Non standard conditions
What is enthalpy change?
Enthalpy of products - enthalpy of reactants
How to reduce heat loss?
Use a polystyrene cup and lid
What is enthalpy?
A measure of the heat energy in a chemical system
- the energy stored within bonds
What is standard pressure?
100kPa
What is standard temperature (not stated)?
298K
What is standard concentration?
1moldm3
What is 0K equal to?
-273 degrees C
What is different about a neutralisation reaction?
Two solutions react (so use the m of both solutions added together)
When you work out energy change, what are the units of the answer?
KJ
