Chemical Reactions

Physical vs. Chemical What is the difference between a chemical and physical property?

Physical Change Physical change: a change that occurs that does not change the identity of the substance Melting ice Freezing Kool-aid Tearing paper Boiling water

Chemical Changes Chemical change: a change that occurs causing the identity of the substance to change Burning paper Digesting food Electrolysis of water A chemical change is called a chemical reaction

Is it Physical or Chemical? Change Physical Chemical Melting cheese Burning wood Milk souring Wadding up paper Bicycle rusting

Chemical Reactions A chemical reaction produces new substances by changing the way in which atoms are arranged. Bonds between atoms are broken and new bonds form between different atoms The new arrangements of atoms form different substances

Reactants and Products Reactants are the substances present at the beginning of a chemical reaction. Products are the substances formed by a chemical reaction Reactant + Reactant Product The mass of the reactants is always equal to the mass of the products

Detecting a Chemical Reaction Color Change Formation of a Precipitate Formation of a Gas Temperature Change Evolution of Light

Color Change Substance often change color during a chemical reaction. Example: when gray iron rusts, the product that forms is brown.

Formation of a Precipitate Precipitate: A solid product formed when chemicals in two liquids react. Example: Seashells are formed when sea creatures release a liquid that reacts with seawater.

Formation of a Gas Chemical reactions may produce a gas, like that often formed when antacid pills are mixed with excess stomach acid.

Temperature Change Most chemical reactions involve temperature change Example: Concrete gets warmer as it hardens

Evolution of Light Energy can be released in the form of light during a chemical reaction Example—Glow Sticks

Endothermic and Exothermic Reactions Exothermic Reactions: If more energy is released when the products form than is needed to break the bonds in the reactants then energy is released during the reaction Energy is released Endothermic Reactions: If more energy is required to break the bonds in the reactants than is released when the product is formed then energy must be added to the reaction Energy is absorbed

Chemical Equations

Chemical Equation Shorthand form for writing what reactants are used and what products are formed in a chemical reaction Sometimes shows whether energy is produced or absorbed Example: C6H12O6 + 6O2  6CO2 + 6H2O + energy

Components of a Chemical Equation Formulas Chemical Formula  2H2 + O2 2H2O Subscript Coefficient Coefficient Subscripts (Yield) (Products) (Reactants) Sometimes you will see a “yields” sign that looks like this. What do you think it means?

Law of Conservation of Mass Proposed by Antoine Lavoisier In a chemical reaction, atoms are neither created nor destroyed All atoms present in the reactants are also present in the products Chemical equations must account for/show the conservation of mass  balancing equations In its present form, does this chemical equation show a conservation of mass? 2 H2 + O2  2 H2O Reactants Products How would you balance this equation to show the conservation of mass? H  2 O  2 4 2 H  2 O  1 4 2

Hints For Balancing Equations Count the atoms List the number of atoms of each element to see which elements must be balanced Use a coefficient to add atoms to one side of the equation Start with the reactant or product that has the greatest number of different elements Add a coefficient to another reactant or product Make sure that the coefficients in your balanced equation are the smallest whole numbers possible (they should have no common factor other than one)

Tutorial on Balancing Equations http://users.wfu.edu/ylwong/balanceeq/balance q.html