Chemical Reactions Physical Science

Standards: PS-4.6: Distinguish between chemical changes (including the formation of gas or reactivity with acids) and physical changes (including changes in size, shape, color, and/or phase) PS-4.7: Summarize characteristics of balanced chemical equations (including conservation of mass and changes in energy in the form of heat—that is, exothermic or endothermic reactions) PS-4.8: Summarize evidence (including the evolution of gas; the formation of a precipitate; and/or changes in temperature, color, and/or odor) that a chemical reaction has occurred. PS-4.9: Apply a procedure to balance equations for a simple synthesis or decomposition reaction. PS-4.10: Recognize simple chemical equations (including single replacement and double replacement) as being balanced or not balanced. PS-4.11: Explain the effects of temperature, concentration, surface area, and the presence of a catalyst on reaction rates.

Essential Questions: What is the difference between physical and chemical changes? What are 5 pieces of evidence that indicate a chemical reaction occurred? What are the different parts/symbols of a chemical equation? What is the importance of the Law of Conservation of Mass? What is the difference between a balanced and an unbalanced equation? What are the differences among the 4 main types of equations? What is the difference between the two types of reactions that deal with energy changes? What are four factors that affect the rate of reactions?

Video Clip: “Introduction to Chemical Reactions” (00:49)

Chemical vs. Physical chemical change - change in the arrangement of the atoms involved (so a different substance with different properties is produced) physical change – change in which a new substance is not produced

Classify the following as either a chemical or physical change: cutting a piece of paper in half burning a piece of paper freezing water rusting metal melting ice cream dissolving salt in water dynamite explosion folding a piece of paper sharpening a pencil burning magnesium

When a chemical reaction occurs, there is some observable evidence. Evidence is not proof. It is the combination of evidence that gives validation for a chemical or physical change.

5 Pieces of Evidence: 1. When bubbles form, it may be evidence that a chemical reaction has occurred and that a gas has been formed. 2. When a precipitate forms, it could be evidence that an insoluble solid has formed and fallen out of solution. 3. In all chemical reactions there is an energy change. 4. Color change can be an evidence for a chemical change. 5. An odor being given off is often evidence that a chemical reaction has occurred.

Video Clip: “Chemical Equations” (1:53)

Parts of a Chemical Equation: Reactants → Products reactants = substances that react products = new substances produced

Symbols for Equations: Meaning → produces, forms, yields + plus s solid l liquid g gas aq aqueous (dissolves in water) Coefficients- numbers (normal size) written to the left of symbols and formulas that represent the # of units of each substance in a reaction

Law of Conservation of Mass: In a chemical reaction, matter is not created or destroyed, it is conserved. starting mass equals the final mass of the products Lavoisier’s experiment

coefficients are used to balance equations Balanced Chemical Equation- equation that has the same # of atoms of each element on both sides of the equation the equation shows that the same atoms that existed before the chemical reaction are still there after the reaction. (Mass is conserved.) coefficients are used to balance equations

Example: Unbalanced Mg + O2 → MgO The # of magnesium atoms are balanced, but there are unequal numbers of oxygen atoms on both sides of the equation. Mg = 1 O = 2 O = 1

Example: Balanced 2 Mg + O2 → 2 MgO Mg = 2 O = 2 Now, both the magnesium and oxygen atoms are balanced on both sides of the equation. Mg = 2 O = 2

Let’s Practice Balancing Equations! Website- Balancing Equations Practice

Video Clip: “Four Types of Reactions” (3:12)

4 Types of Reactions: Synthesis Decomposition Single Displacement Double Displacement

1. Synthesis 2 or more substances combine to form another substance (A + B → AB) Ex: 2H2 + O2 → 2H2O

2. Decomposition Ex: 2H2O → 2H2 + O2 1 substance breaks down, or decomposes, into 2 or more substances (AB → A + B) Ex: 2H2O → 2H2 + O2

3. Single Displacement 1 element replaces another element in a compound (A + BC → AC + B) Ex: Zn + 2HCl → ZnCl2 + H2

4. Double Displacement More than 1 (2 elements) switch and replace each other (AB + CD → AC + BD) Ex: CuNO3 + KCl → KNO3 + CuCl

There is always an energy change when a chemical reaction occurs. * 2 types of Energy Changes: Exothermic Reactions Endothermic Reactions

Video Clip: “Exothermic vs. Endothermic” (00:53)

Exothermic Reactions: If heat is given off it is called an exothermic reaction. This type of reaction adds heat to the area around the reaction, so this area will become warmer.

Exothermic Examples: burning paper or wood dynamite explosion Calcium Chloride and H2O Hot Pack

Endothermic Reactions: If heat is absorbed it is called an endothermic reaction. This type of reaction takes heat from the area surrounding it, so the area around the reaction will become cooler.

Endothermic Examples: Ammonium Nitrate and H2O Alka-Seltzer and H2O Cold Pack

Reaction Rates: Chemical reactions occur when reactants collide with sufficient energy to react. Factors that affect reaction rate are as follows: temperature concentration surface area presence of a catalyst

Video Clip: “Rates of Reactions” (1:50)

1. Temperature When the temperature increases, the rate of a chemical reaction increases. The average kinetic energy of the molecules of reactants increases with increased temperatures. There will be more reactant particles with enough energy to react producing more successful collisions and the reaction will proceed faster.

2. Concentration When reactants are more concentrated, the rate of a chemical reaction will increase. There is a greater chance that reactant particles will collide when they are more concentrated. More collisions mean a faster reaction rate.

3. Surface Area When the surface area of reactants increases, the reaction rate increases. Chemical reactions occur when reactants collide at the surface of other reactants. If the particle size is smaller (with the same mass of reactants) there is a greater surface area and there is a greater chance for collisions to occur and the chemical reaction will proceed faster.

4. Catalyst The presence of a catalyst will speed up a chemical reaction. A catalyst lowers the amount of energy needed to start a reaction (activation energy). Since the energy needed for successful collisions is less, there will be more successful collisions and the chemical reaction will proceed faster.

Works Cited: Physical Science Series: Chemical Reactions. United Learning. 1998. unitedstreaming. 19 March 2007 http://www.unitedstreaming.com/ South Carolina Physical Science Support Document https://dev.ed.sc.gov/apps/cso/standards/supdocs_hs.cfm