Chemical Reactions

Properties and Changes in Matter - Observing Chemical Change Properties and Changes in Matter Matter can undergo both physical change and chemical change.

Properties and Changes of Matter - Observing Chemical Change Asking Questions Properties and Changes of Matter Question Answer What are physical properties of matter? Physical properties are characteristics that can be observed without changing one substance into another. What is the evidence for chemical reactions? The evidence for chemical reactions is the formation of new substances and changes in energy.

Chemical Reactions Rearrange atoms Involve changes in energy Exothermic: release energy (heat, fire, fizz, etc) Endothermic: absorb energy (drop in temp.)

Evidence of Chemical Changes Clue Example Description Color change Bread dough baking Changes from white to brown Smell Eggs rotting Smells bad New matter created Wood burning Wood changes to ash Gas created Baking soda added to vinegar Creates carbon dioxide gas Heat or Light created Fireworks Burning chemicals create light and heat Cut along dotted lines and paste into your science notebook.

Properties and Changes in Matter - Observing Chemical Change Properties and Changes in Matter Chemical changes occur when bonds break and new bonds form.

Describing Chemical Reactions Cellular phone messages make use of symbols and abbreviations to express ideas in shorter form. Similarly, chemists often use chemical equations in place of words.

What Are Chemical Equations? - Describing Chemical Reactions What Are Chemical Equations? Chemical equations use chemical formulas and other symbols instead of words to summarize a reaction. The arrow symbol is read YIELDS which means “results in” or “produces”

Chemical Equations ___ CH4 + ___ O2 ___CO2 + ___ H2O methane and oxygen yield carbon dioxide and water ___ CH4 + ___ O2 ___CO2 + ___ H2O Reactants Products The molecules that combine are called REACTANTS and the molecules that result are called PRODUCTS

- Describing Chemical Reactions Chemical Formulas The formula of a compound identifies the elements in the compound and the ratios in which their atoms are present.

- Describing Chemical Reactions Conservation of Mass The principle of conservation of mass states that in a chemical reaction, the total mass of the reactants must equal the total mass of the products.

Balancing Chemical Equations - Describing Chemical Reactions Balancing Chemical Equations To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation.

Balancing Chemical Equations - Describing Chemical Reactions Balancing Chemical Equations Magnesium metal (Mg) reacts with oxygen gas (O2), forming magnesium oxide (MgO). To write a balanced equation for this reaction, first write the equation using the formulas of the reactants and products, then count the number of atoms of each element.

Balancing Chemical Equations - Describing Chemical Reactions Balancing Chemical Equations Balancing Chemical Equations: Balance the equation for the reaction of sodium metal (Na) with oxygen gas (O2), forming sodium oxide (Na2O).

Balancing Chemical Equations - Describing Chemical Reactions Balancing Chemical Equations Balancing Chemical Equations: Balance the equation for the reaction of tin (Sn) with chlorine gas (Cl2), forming tin chloride (SnCl2).

Classifying Chemical Reactions - Describing Chemical Reactions Many chemical reactions can be classified in one of three categories: synthesis, decomposition, or replacement.

A + B → AB AB → A + B AB + C → AC + B AB + CD → AC + BD Chemical Reactions Type of Reaction Definition  Equation Synthesis Decomposition Single Replacement Double Replacement Two or more elements or compounds combine to make a more complex substance A + B → AB AB → A + B Compounds break down into simpler substances Occurs when one element replaces another one in a compound AB + C → AC + B Occurs when different atoms in two different compounds trade places AB + CD → AC + BD A = Red B = Blue C = Green D = Yellow

Identifying Chemical Reactions Use the code below to classify each reaction. S = Synthesis D = Decomposition SR = Single Replacement DR = Double Replacement   ____ P + O2 → P4O10 ____ Mg + O2 → MgO ____ HgO → Hg + O2 ____ Al2O3 → Al + O2 ____ Cl2 + NaBr → NaCl + Br2 ____ CuCl2 + H2S → CuS + HCl

Building Vocabulary - Describing Chemical Reactions Key Terms: Examples: closed system coefficient synthesis decomposition Key Terms: Key Terms: Examples: replacement Examples: chemical equation A chemical equation is a short, easy way to show a chemical reaction. In a closed system, matter is not allowed to enter or leave. Replacement is the process in which one element replaces another in a compound or two elements in different compounds trade places. reactant A coefficient is a number in a chemical equation telling you how many atoms or molecules of a reactant or product take part in the reaction. A substance you have at the beginning of a reaction is a reactant. product A new substance produced in a reaction is a product. conservation of mass Conservation of mass means that during a chemical reaction, matter is not created or destroyed. Synthesis means combining two or more elements or compounds to make a more complex substance. open system Decomposition means breaking down compounds into simpler products. In an open system, matter can enter from or escape to the surroundings.

End of Section: Describing Chemical Reactions