3.1.4 Energetics

Question 1

Enthalpy Change: definition?

Answer 1

the amount of heat energy taken in or given out during any change in a system provided the pressure is constant

kJmol⁻¹

Question 2

what happens to the energy in an exothermic reaction?

Answer 2

energy is transferred from the system (chemicals) to the surroundings.

the products have less energy that the reactants

Question 3

what happens to the energy in an endothermic reaction?

Answer 3

energy is transferred from the surroundings to the system (chemicals), they require an input of heat energy

the products have more energy that the reactants

Question 4

Standard Enthalpy of Combustion: definition?

Answer 4

the standard enthalpy change when 1 mole of a substance is combusted completely in oxygen under standard conditions, all reactants and products being in their standard states

Question 5

Standard Enthalpy of Formation: definition?

Answer 5

the standard enthalpy change when 1 mole of the compound is formed from its elements under standard conditions , all reactants and products being in their standard states.

Question 6

what are the standard conditions?

Answer 6

-100kPa pressure
-298K (25°C)
-solutions at 1mol dm-3
-all substances should have their normal state at 298K

Question 7

equation for energy change?

Answer 7

Q=mcΔT

Q = joules
m = grams
c = J g⁻¹K⁻¹
ΔT = K

Question 8

equation for enthalpy change?

Answer 8

ΔH = Q/n

ΔH =kJmol⁻¹
Q = joules
n = moles

Question 9

heat capacity of water?

Answer 9

4.18 Jg⁻¹K⁻¹

Question 10

C° —> K

Answer 10

+273

Question 11

what’s the enthalpy of formation of an element?

Answer 11

0 kJmol⁻¹

Question 12

is incomplete combustion more or less exothermic than complete combustion?

Answer 12

less exothermic than complete combustion

Question 13

Mean Bond Energies: definition?

Answer 13

the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules

-> always endothermic

Question 14

in general (if all substances are gases) what is the ΔH?

Answer 14

reactants - products

Question 15

how to balance an equation with enthalpy of formation

Answer 15

always know the products has to be 1 mole

Question 16

how to balance an equation with enthalpy of combustion?

Answer 16

always know the reactants are 1 mole

Question 17

Hess’s Law?

Answer 17

the total enthalpy change for a reaction is independent of the route by which the chemical change takes place

Question 18

how to create a hess cycle when given multiple chemical equations?

Answer 18

combine them by drawing a shape connecting them

like a vector diagram

Question 19

Required Practical: Calorimetry Method

Answer 19

1. wash equipment with solutions to be used
2. put polystyrene cup in a beaker for insulation & support
3. measure desired volumes of solution with a pipette transfer to your insulated cup
4. clamp thermometer into place making sure it’s immersed into solution
5. measure initial temperate of solution, do this every min for 3 mins
6. at 4th minute transfer reagent to cup
   (if solid weight before and after)
7. stir mixture to ensure all solution is at same temperature
8. record temperature every minute after addition for 5 minutes

Question 20

why can it be hard to obtain the exact temperature rise if the reaction is slow?

Answer 20

cooling occurs simultaneously with the reaction

Question 21

errors in method?

Answer 21

-energy transfer from surroundings
-method assumes all solutions have heat capacity of water
-ignores any energy absorbed by apparatus
-reaction / dissolving maybe incomplete or slow
-density of solution is taken to be same as water