The Law of Mass Conservation & Energy in Reactions

The Law of Mass Conservation states matter can not be created or destroyed during a chemical reaction. This means that all of the atoms present before the reaction must be present after the reaction (they are just “rearranged” when bonds break/form)

Each atom present at the beginning of a reaction will be present at the end of a reaction, just as part of a different substance.

Overall, the total mass of the reactants should equal the total mass of the products Ex/

Open –v- Closed Systems In an open system, matter may escape into the environment, but mass is still conserved Ex/ Burning a log – gas escapes In a closed system, matter can not escape into the environment Ex/ A reaction in a sealed container

Balanced Equations To show mass is conserved during a reaction, a chemical equation must be balanced. In a balanced equation, the total number of atoms of each element on the reactant side of the equation, must be the same as the number of atoms of that element on the product side of the reaction.

Example 2 H2O  2 H2 + O2 Reactants Products H = 4 O = 2 This equation is balanced, and follows the law of mass conservation

Representing balanced chemical reactions using diagrams…

Energy in Reactions When a chemical reaction occurs, the chemical bonds of the reactants break and/or new chemical bonds form. Chemical bonds contain a form of energy called chemical energy The reaction requires energy to break the bonds of the reactants The formation of a chemical bond releases energy to the surroundings

Exothermic Reactions Exothermic reactions release more energy than they absorb. The energy released when the products bond is greater than the energy required to break the bonds You feel this as heat, and the temperature increases

Endothermic Reactions Endothermic reactions absorb more energy than they release. The energy required to break the bonds is greater than the energy released when the products bond. This causes the reaction to feel cold, and the temperature decreases.

What else impacts reactions? The amount of thermal energy absorbed or released by a reactions can be impacted by factors such as the amount of each substance used or the concentration of a given substance.

Homework Complete 4 Square Vocabulary Boxes for the following words: Law of Mass Conservation Endothermic Reaction Exothermic Reaction