1.4 Energetics

Question 1

What are the 2 standard conditions for enthalpy change?

Answer 1

100kPa and 298K

Question 2

Define the standard enthalpy of formation.

Answer 2

The enthalpy change when one mole of a substance is formed from its constituent elements under standard conditions, with all reactants and products in their standard states.

Question 3

Define the standard enthalpy of combustion.

Answer 3

The enthalpy change when one mole of a substance is burnt completely in oxygen under standard conditions, with all reactants in their standard states.

Question 4

What is the difference between heat and temperature?

Answer 4

Temperature is the average kinetic energy of each individual particle and is not affected by the number of particles.
Heat is the total energy in a given amount of a substance which means that the number of particles does affect it as each one adds to the heat.

Question 5

What is the equation for energy change in a reaction?

Answer 5

q=mcΔT, energy=mass x specific heat capacity x change in temperature

Question 6

Name 3 ways to improve calorimeter experiments

Answer 6

1. Use a draught screen
2. Burn the substance in pure oxygen
3. Use a copper chimney running through the liquid being heated

Question 7

Why are simple calorimeter experiments fine for comparisons and not for accurate enthalpy change measurements?

Answer 7

Because the same experiment repeated witha different source is likely to have similar errors allowing for a fair comparison.

Question 8

What is Hess’ law?

Answer 8

The enthalpy change for a chemical reaction is the same, whatever route of reactants to products is taken.

Question 9

For the enthalpy of elements, what are the standard states for hydrogen and carbon and why?

Answer 9

H is H₂ as that is normal for 298K and 100kPa

C is graphite represented as C(s) as it is the most stable of all the allotropes of carbon.