Exam questions Flashcards
(12 cards)
- the enthalpy change of combustion refers to how much energy is released when one mole of pentane is completely burnt in oxygen-
step 1) work out how much pentane was burnt by doing ; 85.6-84.6 = 1.0g
step 2) work out the temperature change by doing; 56-22 = 34 degrees celsius
step 3) work out the enthalpy change of the pentane in the reaction by using the equation ( CONVERT YOUR ANSWER INTO KILOJULES) = 28.24
step 4) work out the enthalpy change of pentane per gram by doing 28.24 divided by 1 = 28.24
step 5) work out the enthalpy change of pentane per mole by doing 28.24 multiplied by 72 because ;
1 mole = 72g
work out the enthalpy change of the reactants
work out the enthalpy change of the products
minus the reactants from the products to find out the enthalpy change of cracking
make sure to add suitable units (Kj/mol) and signs.
give one reason , other then heat loss , why enthalpy change determined in the experiment differs substantially from the data booklet value
give reasons as you state your answers
- poor thermal insulation the beaker , which means that not all the heat energy is transferred to the water
- incomplete combustion - a smaller amount of energy is released then expected
- inaccurate readings from the weighing scale , etc - introduces uncertainty in energy calculations
- heat loss ; energy transfer is underestimated
what is a free radical
- a molecule that contains an unbonded pair of electrons
iii) draw a diagram to show the bonding in the single product of the reaction between BF3 and NF3
- mark for the shape (1)
- mark for correct bond angle (107*) (1)
- maximum separation between pairs of electrons (1)
- lone pair - bond pair repulsions are greater then bond pair - bond pair repulsions which results in a smaller bond angle (1)
- 104.5 degrees
- v shaped
Explanation - there are 2 bond pairs and 2 lone pairs .
- this causes a maximum separation between the pairs of electrons.
- also the lone pair - lone pair bond repels more then the bond pair - bond pair bond , which causes the 104.5 bond angle
state what is meant by the term standard enthalpy change of combustion
7the amount of energy released when one mole of a substance is burnt completely in oxygen under standard conditions (100KPA and 278K)
what are the standard conditions
100KPA , 298K
what is the value span for concordant titres
0.1 cm 3
