Energy in Chemical Changes

1. Energy in Chemical Changes

2. Chemical change-new substance is formed starting substances  new substances or reactants  products

3. Virtually every chemical reaction is accompanied by absorption or release of energy. • Energy is absorbed to break bonds of reactants and energy is released when bonds form.

4. Heat of Reaction- amount of heat released or absorbed during a reaction (Enthalpy) • ∆H: change in enthalpy (amount of energy absorbed or lost as heat during a reaction) • ∆H = Hproducts – Hreactants • Energy measured in kJ (kilojoules)

5. Endothermic Reactions • Energy is absorbed during a reaction Reactants  Products low energy high energy reactants + energy  products ∆H = heatproducts - heatreactants

6. Endothermic ∆H = Hproducts - H reactants • Heat absorbed • ∆H is positive • Temperature decrease • Reactants have less chemical potential energy than products, difference is absorbed ∆H Energy  activation energy Reactants  Products

7. Exothermic Reactions • Energy is released during a reaction Reactants  Products high energy low energy Reactants  products + energy

8. Exothermic ∆H = Hproducts - H reactants • Heat is given out • ∆H is negative • Temperature increase • Reactants have more chemical potential energy than products, difference is released Activation energy ∆H Energy  Reactants  Products

9. Activation Energy • the energy required to start a reaction • Energy is required to get particles moving fast enough to collide and cause a reaction.

10. 80 60 Energy kJ 40 20 Course of reaction Examples for worksheet ∆H = heatproducts - heat reactants = 35 kJ - 50 kJ = -15 kJ activation energy 80 kJ – 50kJ = 30 kJ Negative sign means that heat is released to the surroundings Energy of products is less than the energy of the reactants Temperature of the surroundings will increase Therefore: this reaction is exothermic

11. 40 30 Energy kJ 20 10 course of the reaction ∆H = heatproducts - heat reactants = 42 kJ - 8 kJ = 34 kJ No activation energy Positive ∆H sign means that heat is absorbed into the reactants from the surroundings Energy of products is greater than the energy of the reactants Temperature of the surroundings will decrease Therefore: this reaction is endothermic