Chemical Reactions

Describing Reactions Chemical Equations Balancing Chemical Equations Using Equations to Represent Reactions Conservation of Mass Balancing Chemical Equations

Chemical Equations Reactants- substances that undergo a change Products- New substances that form as a result of the change.

Using Equations to Represent Reactions Reactants Products Equation Analogy

Chemical Equations A chemical equation is a representation of a chemical reaction in which the reactants and products are expressed as formulas. Carbon + Oxygen Carbon Dioxide C + O₂ CO (is the above a “balanced” equation?)

Reading an Equation The reaction of carbon and oxygen yields carbon dioxide.

Conservation of Mass During a chemical reaction the mass of the reactants always equals the mass of the products. Alka-Seltzer reaction C6H8O7(aq) + 3NaHCO3(aq) → 3H2O(l) 3CO2(g) Na3C6H5O7(aq) citric acid sodium bicarbonate water carbon dioxide sodium citrate

Conservation of Mass The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction.

Conservation of Mass

Conservation of Mass

Conservation of Mass

Balancing Equations In order to show that mass is conserved during a reaction, a chemical equation must be balanced. Change coefficients, NEVER change subscripts! Number of atoms on the left must equal the number of atoms on the right.

Balancing Equations

Energy Changes in Reactions Chemical Bonds and Energy Breaking bonds Forming bonds Exothermic and Endothermic Exothermic reactions Endothermic reactions Conservation of Energy

Chemical Bonds and Energy Chemical energy is the energy stored in the chemical bonds of a substance. Energy changes are determined by changes that occur in chemical bonding. Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products.

Breaking Bonds Breaking bonds requires energy.

Forming Bonds The formation of chemical bonds releases energy.

Exothermic and Endothermic Reactions During a chemical reaction, energy is either released or absorbed.

Exothermic Reaction Exothermic Reaction is a chemical reaction that releases energy to its surroundings. The energy released as the products formed is greater than the energy required to break the bonds in the reactants.

Exothermic Reactions

Endothermic Reaction Endothermic reaction is a chemical reaction that absorbs energy form its surroundings. More energy is required to break the bonds in the reactants than is released by the products formed.

Endothermic Reaction

Conservation of Energy Exothermic reactions Chemical energy of reactants is converted into heat plus chemical energy of products. Endothermic reactions Heat plus chemical energy of the reactants is converted into chemical energy of the products.