Topic 8: Energetics I

Question 1

Enthalpy change & bond energy

Answer 1

Thermal energy stored in a chemical reaction, shown by change in temp
Bond energy: (E taken in to break bonds) - (E released when bonds are formed)
Specific to each covalent bond

Question 2

Endothermic reactions

Answer 2

Products have more energy than reactants
Enthalpy change is +ve
E.g. photosynthesis, thermal decomposition

Question 3

Exothermic reactions

Answer 3

Products have less energy than reactants (E is given out to surroundings)
Enthalpy change is -ve
E.g: combustion, neutralisation

Question 4

Neutralisation

Answer 4

∆H when one mole of water is formed (acid + base -> water + salt)

Question 5

Combustion

Answer 5

∆H when one mole of substance reacts completely with oxygen

Question 6

Measuring ∆H experimentally

Answer 6

See how much a reaction changes the temperature of another substance (usually water)

Q (J) = m(g) * c (Jg-1°C-1) * ∆T

Question 7

Specific heat capacity (c)

Answer 7

Quantity of energy in Joules needed to change the temperature of 1g of substance by one degree °C
For water & aqueous substances it is 4.18 Jg-1°C-1

Question 8

Hess’ Law

Answer 8

total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place
(If initial & final condition are the same)

Question 9

Hess’ Triangle

Answer 9

step 1: write the equation of the reaction
step 2: add path for alternative route (with known ∆H)
If arrows go on opposite direction: switch sign of ∆H