Enthalpy

Question 1

Endothermic?

Answer 1

• temperature falls
• reaction absorbs energy so delta H = +
• energy is transferred from the surroundings to the system
• products have more energy than reactants
• activation energy = minimum energy required to break a bond and for a reaction to start

Endothermic reactions

• thermal decomposition
• cracking
• main reaction of photosynthesis

Question 2

Standard conditions?

Answer 2

• temperature : 298 k
• pressure : 100kpa
• solution conc. : 1moldm-3

Question 3

The standard enthalpy change of a reaction?

Answer 3

• the heat energy change at constant pressure when the amounts of reactants react under standard conditions to give products in their standard state

Question 4

Enthalpy of formation?

Answer 4

• the enthalpy change when 1 mole of a compound is formed in its standard state from its constituent elements in their standard States under standard conditions

Question 5

Enthaply of combustion?

Answer 5

• the enthalpy change when one mole of a substance in its standard state reacts completely with O2 under S.C

Question 6

Bond enthalpy?

Answer 6

• the enthalpy change when one mole of a given type of bond is broken by homolytic fission in the gaseous state

Question 7

Exothermic?

Answer 7

• the temperature goes up
• the reaction gives out energy
• energy is transferred from the system to the surrounding
• the products have less energy than the reactants

Exothermic reactions

• oxidation
• combustion of fuel
• oxidation of carbohydrates