1.4 Energetics Flashcards
What is an endothermic reaction?
Chemical reactions absorb energy from their surroundings. Positive enthalpy change. Breaking bonds.
What is an exothermic reaction?
Chemical reactions that release energy to its surroundings. Negative enthalpy change. Making bonds.
What are the standard conditions?
100 kPa pressure
298K temperature
What is the standard enthalpy of combustion?
The enthalpy change when 1 mole of a substance is burned completely in the presence of oxygen with the reactants and products in their standard states.
What is the standard enthalpy of formation?
The enthalpy change when 1 mole of a substance is formed from its constituent elements, where all reactants and products are in their standard states.
What is the heat change equation used in calorimetry?
Q = mc△T
What are the units for the heat change equation?
Q = kJ/mol m = mass (g) c = gas constant △T = temperature change (K)
How do you carry out the calorimetry experiment?
- Burn the fuel
- To heat a KNOWN mass of water.
- Then measure the temperature rise of the water.
What is Hess’ Law?
If a reaction can take place by more than one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.
What is the bond dissociation enthalpy?
The enthalpy change to break a covalent bond in its gaseous state.
What is the mean bond enthalpy?
The average value of the bond dissociation enthalpy for a given type of bond is taken from a range of different compounds.
Why does mean bond enthalpy differ from Hess’ Law calculations?
MBE is an average whereas Hess’ law calc is specific to that experiment so the results may differ slightly.
How to find enthalpy change from mean bond enthalpies?
Bonds broken - Bonds made (△H LHS - △H RHS)
