Chemical Reactions + - https://www.youtube.com/watch?v=CgWHbpMVQ1U

Chemical Reactions Objectives: I will be able to: describe what happens to the elements in a chemical reaction. describe how to write a skeleton equation. describe how the rate of a reaction can be changed. review chemical change indicators such as precipitate, evolution of gas, color change, and absorption/release of heat.

Chemical Reaction chemical reaction: the process by which a chemical change occurs Atoms are rearranged, and chemical bonds are broken and reformed. One or more substances change to produce one or more different substances. Types of Reactions Synthesis (creating) Decomposition (separating) Combustion (burning) Single, Displacement/Replacement (switching) Double Displacement/Replacement (switching) What does H2 & O2 gas create? Out of the five types of reactions, which would describe the chemical reaction taking place to the right? H2 O2

Physical changes are not indicative of a chemical reaction physical change: a change in shape, size, color, or state without a change in chemical composition Physical change is a change that is reversible. Examples tearing paper cutting your hair change in state Physical changes are not indicative of a chemical reaction

mixing vinegar & baking soda Chemical Change chemical change: a change in which a substance becomes another substance having different properties and chemical composition a change that is not reversible using ordinary physical means Changes that usually cause production of, heat, sound, light, odor, fizzing/foaming, color changes You usually need more than one of the above characteristics to be considered a chemical change! Examples mixing vinegar & baking soda burning a piece of wood soured milk A chemical analysis is the only 100% way to know a chemical change has occurred.

Chemical Equations chemical equations: 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 Examples: 2H2 + O2  2H2O CH4 + 2O2   CO2  +  2H2O C6H12O6 + 6O2  6CO2 + 6H2O + energy

Chemical Equations chemical formula: a combination of symbols that indicates the chemical composition of a substance coefficient: the number in front of a chemical formula that tells us how many molecules of a given formula are present subscript: a number after an element’s chemical symbol that tells us how many atoms there are within a compound

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

Energy and Chemical Reactions Exothermic Reaction A chemical reaction in which thermal energy is released. The products have greater bond energy than the reactants C6H12O6 + 6O2  6CO2 + 6H2O + energy (respiration) Endothermic Reaction A chemical reaction in which thermal energy is absorbed. The products have lower bond energies than the reactants 6CO2 + 6H2O + energy  C6H12O6 + 6O2 (photosynthesis) Chemistry Comes Alive! Sample Movies TeacherTube - Videos Chemistry Demonstration Videos Can you think of other reactions where energy is gained or released?

Rates of Chemical Reactions The rates at which chemical reactions can take place are based on the interaction (collisions) between the different particles. These rates can be impacted by the following: Temperature: a measure of the average kinetic energy of the particles in a sample of matter Ex. Increasing the temperature when cooking Surface area: amount of material that comes into contact with the reactants Ex. Cutting a potato into smaller pieces when cooking Concentration: amount of substance per volume Ex. Turning the valve on a gas stove to increase the concentration of methane molecules Catalysts (enzymes): organic substances that help speed up chemical reactions, but are not consumed in the reaction

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

Chemical Reactions https://www.youtube.com/watch?v=CgWHbpMVQ1U Objectives: Predict products of chemical reactions such as synthesis, decomposition, replacement, and combustion. Balance chemical equations with adherence to law of conservation of matter. Review chemical change indicators such as precipitate, evolution of gas, color change, and absorption/release of heat.