Chapter 1D - Energy Changes

Question 1

Definition - Thermochemistry

Answer 1

• study of heat changes in chemical reactions
• chemical reactions absorb/release energy generally in the form of heat

Question 2

Chemical energy & enthalpy

Answer 2

• all substances contain chemical energy (energy stored in bonds of compounds)
• total change in chemical energy during a reaction is called enthalpy change

Question 3

Chemical Reactions

Answer 3

• involve rearrangement of atoms
• energy is absorbed to break bonds in reactants
• energy is released as new bonds are formed in products

Question 4

Activation Energy

Answer 4

• minimum energy required to break the bonds
• always a positive value
• increases with increasing bond strength b/w reactant atoms/molecules

Question 5

Change in enthalpy (∆H)

Answer 5

• difference in chemical energy (H) b/w reactants & products
• value can be +ve or -ve
• ∆H = H(products) – H(reactants)

Question 6

Exothermic Reaction

Answer 6

• reaction that releases energy to the environment (commonly as heat)
• energy required to break bonds is less than the energy released when new bonds formed
• decrease in enthalpy (∆H is negative)
• reactants have higher enthalpy than products
• all combustion reactions

Question 7

Endothermic Reactions

Answer 7

• reaction that absorbs energy from the environment
• energy required to break bonds is greater than energy released when new bonds form
• increase in enthalpy (∆H is positive)
• products have higher enthalpy than reactants

Question 8

Reversible reactions

Answer 8

• energy profile can be understood in both directions
• enthalpy change (ΔH) of reverse is same size as (ΔH) of forward reaction, but signs change
• EA will be different for both reactions, with endothermic direction always being larger