energetics

Question 1

what is the definition of standard enthalpy change of formation?

Answer 1

enthalpy change when 1 mole of a substance is formed from its constituent elements with all reactants and products in standard states under standard conditions

Question 2

what is the definition of standard enthalpy change of combustion?

Answer 2

enthalpy change when 1 mole of a substance is completely burnt in oxygen with all reactants and products in standard states under standard conditions

Question 3

what is the definition of standard enthalpy change of neutralisation?

Answer 3

enthalpy change when 1 mole of water is formed in a reaction between an acid and alkali under standard conditions

Question 4

what does each part of the q=mcΔT stand for?

Answer 4

q = energy required
m = mass of liquid heated
c = specific heat capacity of liquid
ΔT= temperature rise of liquid

Question 5

what is the equation for calculating enthalpy change

Answer 5

ΔH=q divide moles in a reaction

Question 6

1) In an experiment, 0.600 g of propane (C3H8) was completely burned in air. The heat evolved raised the temperature of 100 g of water by 64.9°C. Use this data to calculate the enthalpy of combustion of propane (the specific heat capacity of water is 4.18J g-1 K-1

Answer 6

1) q = mcΔT
m = 100
C=4.18
q = 100 x 4.18 x 64.9 = 27130 J (divide 1000 to get KJ)
ΔT = 64.9
ΔH = q / mol
moles of propane = mass / Mr
= 0.600 / 44.0 = 0.01364
ΔН = -27.13 / 0.01364 = -1990 kJ mol-1 (3 sig fig)

Question 7

what is Hess’s law?

Answer 7

the enthalpy change for a reaction is independent of the route taken

Question 8

why is the enthalpy change of formation of an element always 0?

Answer 8

don’t need energy to make an element

Question 9

What is the Hess’s law on formation ?

Answer 9

Question 10

What is the Hess’s law on combustion?

Answer 10

Question 11

how do you calculate the ΔH using bond enthalpies?

Answer 11

ΔH= energy to break all bonds - energy to make all bonds
(reactants-products)

Question 12

how to determine a more accurate value for the enthalpy of reaction

Answer 12

1. insulate the beaker
2. to reduce heat loss
3. record temperature for a suitable time before adding the metal
4. record the temperature values at regular intervals
5. to plot the temperature results against the graph
6. extrapolate the cooling back to the point of addition
7. to establish a theoretical maximum temperature/ temp change