Chemistry 5.2 Flashcards
Exothermic reaction definition
Chemical reactions in which heat energy is given out
Endothermic reaction definition
Chemical reactions in which heat energy is taken out
What does a positive + ΔH symbol mean?
Heat energy is absorbed so the reaction is endothermic
What does a negative - ΔH symbol mean?
Heat energy is released so the reaction is exothermic
Is bond-breaking endothermic or exothermic?
Absorbs energy so endothermic
Is bond-making endothermic or exothermic?
Releases energy so exothermic
Would the reactants be higher or lower than the products on an endothermic energy level diagram? Explain why
Heat energy is absorbed and ΔH is positive. This means that the products end up with more energy than the reactants had. Reactants lower than products.
Would the reactants be higher or lower than the products on an exothermic energy level diagram? Explain why
Heat energy is released and ΔH is negative. This means the products end up with less energy than the reactants had. Reactants are higher than the products.
Symbol and unit for Enthalpy change
ΔH kJ/mol
Why do double bonds have higher bond energy?
Stronger bonds require more energy to break and also release more energy when made
How can you calculate ΔH for a reaction using bond energies?
ΔH = bonds broken - bonds made
reactants - products
Formula for heat energy, Q
Q = mcΔT
Q = heat energy j
m = mass of water g
c = specific heat capacity of water
ΔT = temperature change oC
How do you convert heat energy, Q, into ΔH
Turn Q from j into kJ by ÷ 1000
ΔH = -Q ÷ moles
Describe combustion calorimetry
Combustion of methanol to heat water:
Weigh out water and place in a can above burner
Measure initial mass of burner and initial temperature of water
Ignite burner and stir water for 30 seconds.
Measure final mass of burner and final temperature of the water
Find temp difference and mass used of methanol
What are the sources of error with combustion calorimetry?
Heat loss to surroundings
Incomplete combustion - forms black carbon soot
Why is mass of methanol in burner measured quickly in combustion calorimetry?
To ensure mass is recorded before fuel evaporates (it is very volatile)
Why is the water stirred while being heated in combustion calorimetry?
To distribute heat energy evenly in the water
Why is a copper can used in combustion calorimetry?
Copper is a better conductor of heat. More heat energy is absorbed by the water.
