enthalpy reactions + bond enthalpies

Question 1

describe how calorimetry works to calculate enthalpy change

Answer 1

• fuel is burned to raise water temperature by a specific amount
• energy from fuel is transferred to water
• lid prevents heat loss
• windshield prevents draught in flame
• polystyrene cup prevents heat loss

Question 2

what is the equation for energy transfer?

Answer 2

q = mcΔT

Question 3

what is the mass in a calorimetry calculation?

Answer 3

all of the liquids involved

Question 4

how is enthalpy change calculated?

Answer 4

energy change (q) / moles

Question 5

describe an endothermic reaction in terms of breaking/making bonds

Answer 5

more energy is required to break bonds than is released in making bonds

Question 6

describe an exothermic reaction in terms of breaking/making bonds

Answer 6

more energy is released in making bonds than is required to break bonds

Question 7

define average bond enthalpy

Answer 7

the energy required to break one mole of the stated bond in a gaseous state, under standard conditions

Question 8

how is enthalpy change calculated using mean bond enthalpies?

Answer 8

reactants - products

Question 9

what are the limitations of using average bond enthalpies?

Answer 9

• actual bond enthalpy varies depending on the chemical environment
• all species must be gaseous

Question 10

why does enthalpy change become more negative as chain length increases

Answer 10

• number of bonds per mole increases
• so more energy is released per mole