1.4: Energetics Flashcards
(18 cards)
What are standard conditions?
- 100KPa (1atm)
- 198K (room temperature)
Describe an exothermic reaction:
- release energy to surroundings
- more product bonds formed than broken
- negative enthalpy change
- products have less energy than reactants
Describe an endothermic reaction:
- takes energy in from surroundings
- less product bonds are formed than broken
- positive enthalpy change
- products have more energy than reactants
Are respiration and photosynthesis exo or endothermic?
- respiration is exothermic
- photosynthesis is endothermic
What is bond enthalpy?
The energy needed to break a bond
What is mean bond enthalpy?
The average amount of energy it takes break a certain type of bond over a range of compounds
Define standard enthalpy of formation:
The enthalpy Change when 1 mole of a compound is made from its elements in their standard states and under standard conditions.
Define standard enthalpy change of combustion:
The enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all reactants in their standard states.
What is the equation for heat energy transfer?
q = mc/\T
- q = enthalpy change (J)
- m = (g) of solution
- c = specific heat capacity (of water)
- /\T =change in temperature (K)
What is the equation for standard enthalpy change?
Standard /\H (KJ/mol) = q/n
- remember to convert q to KJ
Describe a method to measure the enthalpy change of neutralisation (7):
- add known volume of acid to a beaker
- acclimatise
- measure temperature
- add known volume of alkali and recorded temperature at regular intervals
- stir solution to ensure even heating
- extrapolate to find temperature change
- calculate heat energy given out (q)
What is Hesses Law?
The total enthalpy change for a reaction is independent of the route taken
What does enthalpy change mean when applied to a chemical reaction?
Heat energy change at a constant pressure
Why is the enthalpy value for O=O not a mean value?
O2 is the only substance that has an O=O bond
Describe a results table for the calorimetry practical:
…………Temp *C………………….Mass(g)
Initial Burner before
Final Burner after
/\T Mass burned
State 3 advantages of using a clamped calorimeter
- good conductor
- clamp does not impact heat transfer
- clamp height can be adjusted
What two features could reduce the accuracy of a calorimetry experiment and what can be done to counter them?
- heat loss to surroundings
- lid/ insulate sides of container
- incomplete combustion
How to reduce percentage uncertainty and increase accuracy in an experiment:
- use a biurette or pipette
- reweigh boat after transfer if substance
- increase solute concentration
