FC19: Enthalpy 1

Question 1

Describe the energy profile of an exothermic reaction

Answer 1

energy of reactants is higher than energy of products
energy increases from reactants as reaction progresses, the energy from the reactants and the peak energy is the activation energy, then the energy decreases to the products
the energy difference of reactants to products is the enthalpy change of the reaction.

Question 2

What sign is the enthalpy change of an exothermic reaction

Answer 2

negative

Question 3

Describe the energy profile of an endothermic reaction

Answer 3

energy of reactants is lower than energy of products
energy increases from reactants as reaction progresses, the energy from the reactants and the peak energy is the activation energy, then the energy decreases to the products
the energy difference of reactants to products is the enthalpy change of the reaction

Question 4

What sign is the enthalpy change of an endothermic reaction

Answer 4

positive

Question 5

What is the definition of enthalpy change

Answer 5

heat changes under constant pressure

Question 6

What is the enthalpy change of reaction

Answer 6

enthalpy change when the number of moles of reactants in an equation react to form the products

Question 7

what is the enthalpy change of combustion

Answer 7

enthalpy change when one mole of a substance undergoes complete combustion in excess oxygen

Question 8

what is the enthalpy change of formation

Answer 8

enthalpy change when one mole of a substance is formed from its elements

Question 9

what is standard enthalpy change and state the conditions

Answer 9

enthalpy changes measured at standard conditions :-
25 degrees - 298 K
1 atm - 100kPa
all substances in their standard states

Question 10

what is the enthalpy change of neutralisation

Answer 10

enthalpy change when one mole of water is formed in a neutralisation reaction

Question 11

State 3 assumptions that are made when calculating enthalpy change of combustion from a simple calorimeter

Answer 11

1- all heat from combustion transfers to the water/substance heated
2- no fuel or water evaporates/boils
3- the fuel undergoes complete combustion only

Question 12

what are the formulae for calculating enthalpy change

Answer 12

q=mc/\T
- q/mol = enthalpy change of combustion

Question 13

what are the formulae for calculating enthalpy change in words

Answer 13

quantity of heat = mass of substance heated/cooled x specific heat capacity of substance heated/cooled x temperature change of substance heated/cooled
then you divide that value by the moles of the fuel burnt, and x by -1, as it is an exothermic reaction

Question 14

what are the units for quantity of heat (q) and enthalpy change

Answer 14

q = joules
enthalpy change = J/mol or kJ/mol
(negative if exothermic, positive if endothermic)

Question 15

Describe how you would set up a simple calorimeter when calculating enthalpy change of combustion for a liquid fuel

Answer 15

set up a stand, boss and clamp. Place the spirit burner below the clamp on your desk, containing the liquid fuel you’ll be burning. Put a simple calorimeter in the clamp making sure the clamp is low enough so that the base of the calorimeter is just above the spirit burner. Your calorimeter should have a thermometer, stirrer, and a lid, with a known mass of water. Put two draught shields either side of the apparatus to reduce heat loss to the surrounding air

Question 16

Describe how you would set up a simple calorimeter when calculating enthalpy change of neutralisation/reaction in solution

Answer 16

put an insulating cup in a beaker, insulating cup should have a stirrer and thermometer, and should contain a known volume and concentration of the acid and the base you are using. put lid over entire appartus

Question 17

what do you assume the density of the reaction mixture to be when working out enthalpy change of reaction/neut in solution

Answer 17

to be same as water - 4.18J/K/g

Question 18

what happens to the temperature of the surroundings in an endo/exothermic reaction

Answer 18

endo = temperature of surroundings decreases
exo = temperature of surroundings increases

Question 19

equation for working out a missing bond enthalpy

Answer 19

sum of bonds broken - sum of bonds formed

Question 20

why are bond enthalpies always endothermic?

Answer 20

because the breaking of bonds requires energy, so this is an endothermic reaction and so bond enthalpies are always endothermic

Question 21

what are the limitations of using average bond enthalpies for calculating enthalpy changes?

Answer 21

an average bond enthalpy is calculated using bond enthalpies from different chemical environments, not the actual bond enthalpy of the bond