chapter 6 - chemical reactions: energy, rates, & equilibrium

energy & chem reactions

• chem reactions transfer heat energy by absorbing heat from surroundings or releasing heat to surroundings

• first law of thermodynamics - energy cant be destroyed or created

• breaking a bond requires input of energy, where forming a bond releases energy

  • differences in bond energies is the heat of reaction

• energy measured in joules, calorie

  • 1 cal = 4.184 J

energy diagrams

• heat of reaction is the amount of energy released or absorbed during a chem reaction

  • Hrxn = sum of bond energies of broken bonds - sum of bond energies of bonds formed

  • Hrxn = energy of products - energy of reactants

• energy diagram traces energy pathway from reactants to products

  • difference in potential energy between the reactants & products can be measured

exothermic reaction

• heat is released

• delta H negative

endothermic reaction

• heat is absorbed

• delta H positive

chem equilibrium

• many reactions are reversible

• both forward & reverse reactions occur at same time

le chatelier’s principle

• predicts that a reaction at equilibrium responds to a disturbance to counteract it & reach a new equilibrium

  • disturbances

    • concentration changes

    • temp changes

• more o2, n2, or high temp (adding reagents) reaction moves to the right

• less o2, n2, less temp (remove reagents)     reaction moves to the left