Topic 5: Energetics

Question 1

Heat

Answer 1

Heat is a form of energy. Heat always transfers or flows from a higher temperature object to a lower temperature object until both objects are in thermal equilibrium.

Question 2

Absolute temperature (K)

Answer 2

The absolute temperature in Kelvin (K) is directly proportional to the average kinetic energy of the particles in a substance.

Gas at 300K - particles have higher average kinetic energy

Gas at 100K - particles have lower average kinetic energy

Question 3

Heat vs temperature

Answer 3

Heat is a measure of the total energy of a substance and therefore depends on the amount of substance present.

Temperature is a measure of the average kinetic energy of the particles in a substance.

Question 4

Exothermic vs endothermic reactions

Answer 4

Chemical reactions that release heat are exothermic.

Chemical reactions that absorb heat are endothermic.

Question 5

Enthalpy

Answer 5

Enthalpy is the heat absorbed or released in a chemical reaction that takes place at constant pressure.

Enthalpy cannot be measured - only a change in enthalpy can be measured (ΔH)

Question 6

Standard enthalpy change of reaction

Answer 6

The enthalpy change of a reaction carried out under standard conditions (100kPa) with everything in its standard state.

Standard conditions - 100kPa and temperature is usually 298K

Question 7

Exothermic reactions

Answer 7

Heat flows from the system to the surroundings

The temperature of the reactions mixture and the surroundings increases as heat is released

Combustion and neutralisation are both exothermic

Question 8

Endothermic reactions

Answer 8

Heat flows from the surroundings into the system

The temperature of the reaction mixture and the surroundings decreases as heat is absorbed

Photosynthesis and thermal decomposition are endothermic reactions

Question 9

ΔH in exothermic reactions

Answer 9

ΔH is negative

The products have lower enthalpy than reactants

Products are more energetically stable than reactants

Question 10

ΔH in endothermic reactions

Answer 10

ΔH is positive

The reactants have lower enthalpy than the products

Reactants are more energetically stable than the products

Question 11

Calculating ΔH using q=mcΔT

Answer 11

Use q=mcΔT using the values from question

m : the mass of water used in the reaction
c : is specific heat capacity (4.18)
ΔT : temp change of water

this gives J so divide by 1000 to get kJ

next divide this value by moles to get final answer

Question 12

Enthalpy change of formation

Answer 12

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions

ΔH = ΣΔHf (products) — ΣΔHf (reactants)

Question 13

Enthalpy change of combustion

Answer 13

The standard enthalpy change of combustion is the enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions.

ΔH = ΣΔHc (reactants) — ΣΔHc (products)

Question 14

Hess’s Law

Answer 14

The enthalpy change for a chemical reaction is independent of the route by which the chemical reaction occurs.

Question 15

Average bond enthalpy

Answer 15

Average bond enthalpy is the energy required to break one mole of bonds in a gaseous molecule averaged over similar compounds

Bond breaking is endothermic - energy is required to break a bond

Bond formation is exothermic - energy is released when bonds are formed

ΔH = Σ(bonds broken) — Σ (bonds formed )

Question 16

What is wrong with calculating enthalpy changes using bond enthalpies

Answer 16

Average bond enthalpies are calculated by calculating the energy required to break the same bond in similar compounds and then averaging the value - the actual value may be different