Unit 4: Chemical Reactions

Think about it! What caused the changes that you see? Did they occur over night? Were they physical changes? Why/why not? What kind of changes were they? Can they be reversed?

Think about it-Continued Flash bulb:Flash bulb What did you observe? How is this similar to the previous picture? How is it different?

Chemical Properties and Changes Chemical Property- property that describes how a substance turns into another substance. –Example: flammability, toxicity, stability (reactivity) Chemical Change- process by which a substance changes into another substance. –Also called chemical reactions –Example: burning, rusting,

Chemical Reactions-Overview Chemical Reaction- process in which the physical and chemical properties of a substance change as new substances with different physical and chemical properties are formed. Examples- burning gasoline, rusting iron, baked bread, flash bulbs, etc… Click here for an explanation by Bill Nye!here

Video Summary Almost everything is made of chemicals Do not confuse the word “chemicals” with something negative! –Your food is made of chemicals! So, what’s not made of chemicals? A chemical reaction always results in the formation of a new substance. A chemical reaction involves a change in energy.

Characteristics of All Chemical Reactions All chemical reactions share certain characteristics. 1.All chemical reactions result in the formation of a new substance. –Example: dark material on a burnt match This is a new material that is not the same substance that was originally on the match. Don’t believe it? Click here to see it in action!here

12/13/13 Warm-up 1.List 3 things that are made of chemicals. 2.List 1 thing that is not made of chemicals. 3.What is 1 thing that all chemical reactions share? 4.Make one observation from the demo!

Characteristics of Chemical Reactions 2.All chemical reactions have reactants and products. Reactants- a substance that enters a chemical reaction Products- a substance that is produced by a chemical reaction So, in a chemical reaction, the reactants change into the products. Click here to see ithere

Back to the flashbulb example A small magnesium metal coil is inside of the bulb with invisible oxygen gas. When set off, magnesium reacts with oxygen. During the reaction, energy is released and magnesium oxide (white powder) is produced. Magnesium+oxygen=magnesium oxide+energy What are the reactants and products in this chemical reaction?

Chemical Equations We can write chemical reactions as equations. Examples: –Sodium+water= sodium hydroxide+hydrogen gas –Na + H 2 O=NaOH + H 2 This provides us a much shorter way to write out chemical reactions! Let’s practice with a partner!

Chemical Equations Reminders Remember: Reactants do the reacting –They go into the reaction –They are always on the left side of the equation Remember: Products are the result of a reaction –They come out of the reaction –They are always on the right side of the equation Reactants change into the products!

Characteristics of Chemical Reactions-Continued 3.All chemical reactions always involve a change in energy. Energy is either absorbed or released during the reaction. Examples: –Energy is absorbed when sugar changes into caramel –Energy is released when gasoline burns Click here for a dramatic example this!here

12/17/13 Warm-up 1.List the 3 characteristics of all chemical reactions that we have discussed. 2.What are reactants? 3.What are products? 4.Provide one example of a chemical reaction.

Energy and Chemical Reactions We can’t see atoms rearranging in chemical reactions, but we can usually detect or observe energy changing form in a chemical reaction. 1.Heat or light are released or absorbed. 2.There is an evident color change. –Can you think of an exception to this? 3.Gas is given off.

Energy and Chemical Reactions Whether energy is absorbed or released will determine how we classify chemical reactions In either instance, energy is neither created nor destroyed. –The energy usually changes position or form

Exothermic Reactions Exothermic Reaction- a chemical reaction in which energy is released –Root “thermic” = heat –Prefix “exo” = to release –So, it means “heat comes out of, or is released from”. –Examples: reactions that involve combustion or burning

Exothermic Reactions Continued Energy that is released was originally stored in the reactants. Since energy is released and products are formed, product energy is less than energy in reactants. Energy of reactants>Energy of products

Add to your warm-up 5.Explain why the demonstrations that you just saw was an example of an exothermic reaction. 6.Why didn’t the dollar burn?

Endothermic Reactions Endothermic Reaction- a reaction in which energy is absorbed. –Root “thermic” = heat –Prefix “endo” = to absorb –So, it means “energy is taken into or absorbed”. –Examples: Photosynthesis, quick ice packs, melting, boiling, evaporation

Endothermic Reactions Continued Energy is absorbed from the environment –Usually in the form of heat or light Since energy is absorbed, the energy of products is usually higher than reactants. Energy that is absorbed is stored in the products. Energy of reactants<Energy of products.

Remember… All reactions involve a change in energy! Exothermic=heat is released Endothermic=heat is absorbed There are a few indicators that we can use to tell if heat is released or absorbed. What are a few of these indicators?

Conservation of Matter No new atoms are created in a chemical reaction. –The total number of atoms is the same before and after the reaction. No atoms are destroyed in a chemical reaction. This means that atoms in reactants must rearrange to make new products.

Conservation of Mass Since no new atoms are created or destroyed in a chemical reaction, mass is also conserved. The mass of reactants=mass of products. Click here for a quick demonstration.here