Chapter 1D - Energy Changes

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

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

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

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

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

• 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

• 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

• 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