3.1 & 3.2 Chemical reactions, energy change

Question 1

define exothermic reactions:

Answer 1

• convert chemical energy to heat (and sometimes light) energy
• release heat to the surroundings
• feel hot as heat is released

Question 2

define endothermic reactions:

Answer 2

• convert heat energy to chemical energy
• absorb heat from the surroundings
• feel cold as heat is absorbed

Question 3

define enthalpy (H)

Answer 3

the amount of chemical energy stored within the bonds of a substance

Question 4

define change in enthalpy (triangleH)

Answer 4

the change in enthalpy for a chemical reaction, measured as the heat absorbed per mole when the reaction occurs at constant pressure

Question 5

explaining enthalpy diagrams

Answer 5

exothermic reaction - if the reactants have more energy than the products, excess energy is released from the system (the reaction) into the environment.

endothermic reaction - if the products have more energy than the reactants, more energy is needed to form them. The extra energy is absorbed from the environment into the system.

Question 6

Explain how exothermic reactions relate to law of conservation, bonds breaking and reforming and heat released by the system to the surroundings.

Answer 6

• Law of conservation of energy states that energy cannot be created or destroyed.
• In exothermic reactions, more energy is released when the bonds are formed in the products than is used to break the bonds in the reactants
• Exothermic reactions are accompanied by an increase in temperature due to the release of heat energy from the system to the surroundings.

Question 7

explain how endothermic reactions relate to law of conservation, bonds breaking and reforming and heat absorbed by the system from the surroundings.

Answer 7

• law of conservation of energy states that energy cannot be created or destroyed.
• In endothermic reactions, more energy is absorbed when the bonds in the reactants are broken than is released when new bonds are formed in the products
• endothermic reaction are accompanied by a decrease in temperature due to the absorption of heat energy by the system from the surroundings

Question 8

breaking bonds between atoms required ____ _____. The stronger the bond, the more energy it takes to break the bond.

forming new bonds always ______ energy. The stronger the bond formed, the ____ energy is released during the bond formation process.

Answer 8

1. adding energy
2. releases
3. more

Question 9

Change in enthalpy positive and negative answers

Answer 9

• endothermic reactions absorb energy so the change in H is positive
• exothermic reactions release energy so their change in H is negative

Question 10

units for enthalpy

Answer 10

• enthalpy (H) is expressed in kJ
• change in enthalpy is expressed in kJ mol -1

Question 11

write thermochemical equations

Answer 11

change in enthalpy = energy required to break bonds in reactants - energy released when bonds made in products)
- ΔH=Σ(bondsbroken) –Σ(bondsformed).

Question 12

What are the bond enthalpy limitations?

Answer 12

• are average values and do not consider that the same bond can have different energies depending on the reaction environment
• do not consider any intermolecular forces that may exist in the reaction system
• were measured using gaseous reactants and products, therefore only really apply to gaseous systems