6. endothermic and exothermic reactions

energy

capacity to do work or cause change

law of conservation of energy

energy cannot be created or destroyed, only transformed from one form to another

• total amount of enegy in a system is constant

energy in systems

if energy in a system rises/falls → energy in system’s surroundings will fall/rise

enthalpy

total present energy in the substance

• chemical potential energy (stored in bonds)

• kinetic energy (particle motion)

chemical potential energy in H

not heat → does not increase temperature

kinetic energy in H

• heat exists in the environment (responsible for temperature)

• energy from vibration of particles

chemical reactions

chemical bonds in reactants are broken and new bonds are formed to make the products

• as energy gained/lost → change in how much potential energy is stored in bonds

• as energy must be conserved → change in potential energy must be balanced by an opposite change in kinetic energy (heat)

bond making

potential energy lost in reaction → converted to kinetic energy (heat) → heat released into surroundings → increase in temperature

bond breaking

kinetic energy (heat) absorbed from surroundings → potential energy increase in reaction → decrease in temperature

enthalpy change

chemical reactions that occur at constant pressure (eg: open beaker/test tube) → heat gain/loss of system = ΔH

other name of ΔH

heat of reaction

exothermic reactions

reactions that release heat into surroundings

how exothermic reactions occur

1. chemical potential energy in bonds converts to kinetic energy

2. surplus kinetic energy (heat) is released into surroundings → increasing temperature

3. heat is lost to surroundings → H decreases → ΔH is negative

endothermic reactions

reactions that absorb heat from surroundings

how endothermic reactions occur

1. E_k converts to E_p

2. To replace lost E_k → heat is absorbed from surroundings → decreasing temperature

3. heat is gained from surroundings → H increases → ΔH is positive

bond breaking

• absorption of energy

• raises H

• endothermic

bond breaking in reactions

reactant bonds stronger/more numerous → more energy absorbed to break them than released

reaction that invests more energy into bond breaking than bond forming is endothermic

bond making in reactions

product bonds stronger/more numerous → more energy released to form them than absorbed

reaction that invests more energy into bond forming than bond breaking is exothermic

activation energy

minimum energy required to break bonds in reactants for reaction to occur

energy profile diagrams

include

• ΔH of reaction

• activation energy

x-axis: reaction progress

y: axis: E_p

energy profile diagrams - endothermic

energy in bond breaking in reactants > energy released in bond making in products

energy is absorbed

energy profile diagrams - endothermic

energy in bond breaking in reactants < energy released in bond making in products

energy is released

thermochemical equations

• show ΔH

• show energy (heat) absorbed/released