Exothermic and Endothermic Reactions © Teachable . Some rights reserved. http://teachable.net/res.asp?r=1910

Energy and Chemical Reactions Energy is involved in chemical reactions in two ways: to break bonds between atoms in the reactants, or to form new bonds between atoms to make new products

Endothermic and Exothermic Reactions Step 1: Energy must be SUPPLIED to break chemical bonds. Step 2: Energy is RELEASED when new chemical bonds are made. A reaction is EXOTHERMIC if more energy is RELEASED than SUPPLIED. If more energy is SUPPLIED than is RELEASED, the reaction is ENDOTHERMIC © Teachable . Some rights reserved. http://teachable.net/res.asp?r=1910

Two Types of Chemical Reactions We classify chemical reactions based on how the energy of the reactants compares to the energy of the products. Either energy is given off or energy is required.

Whenever chemical reactions occur, energy is transferred to or from the surroundings. An exothermic reaction is one which transfers heat energy to the surroundings An endothermic reaction is one which takes heat energy from the surroundings

heat taken in and temperature drops Measuring Heat reaction Endothermic heat taken in and temperature drops energy is “trapped” in new bonds

Reactants + Energy as Heat  Products

Combustion reactions are exothermic. Exothermic chemical reactions release heat. There is more energy than needed to make the new bonds so the “extra” energy is released as heat. Combustion reactions are exothermic.

Exothermic chemical reactions give off heat and the temperature rises. Heat Released Exothermic chemical reactions give off heat and the temperature rises. reaction

Reactants  Products + Energy as Heat

Exothermic Chemical Reactions Once started, exothermic reactions tend to keep going as each reaction releases more energy to fuel neighboring molecules. What is we wanted to take water apart instead?

Exothermic vs Endothermic: EXOTHERMIC – more energy is given out than is taken in ENDOTHERMIC – energy is taken in but not given out © Teachable . Some rights reserved. http://teachable.net/res.asp?r=1910

EXOTHERMIC ENDOTHERMIC Chemical reactions where HEAT… is transferred from the surroundings into the reactants … indicated by a DROP in temperature Chemical reactions where HEAT… is transferred to the surroundings… indicated by a RISE in temperature

Examples Exothermic Endothermic A candle flame Rusting iron Burning sugar Glow sticks Fire crackers Endothermic photosynthesis cooking food

Activation Energy Activation energy is the energy needed to begin a reaction and break chemical bonds in the reactants.

Activation Energy A flammable material such as gasoline does not burn without a spark. The spark provides the activation energy.