Bell Work What are the three types of chemical reactions? (refer to your notes) List one example for each.

Chemical Changes and Reactions

Properties and Changes of Matter Matter can be described in terms of two kinds of properties: Physical properties Chemical properties

Physical Properties A physical property is a characteristic of a substance that can be observed without changing the substance into another substance. Examples: Ice melting (solid to liquid) Color, hardness, texture, shine, and flexibility

Chemical Properties A chemical property is a characteristic of a substance that describes its ability to change into other substances. Examples: When magnesium burns, it combines with oxygen in the air, forming a new substance called magnesium oxide. Tarnishing and rusting are other examples.

Changes of Matter A physical change is any change that alters the form or appearance of a substance, but does not make the substance into another substance. Other examples include bending, crushing, breaking, cutting, and anything else that changes only the shape or form of matter.

Changes of Matter Sometimes when a change occurs in a substance, the substance itself is changed. A change in matter that produces one or more new substances is a chemical change, or chemical reaction. Burning gasoline in a car’s engine is an example. The new substance formed is the car’s exhaust.

Evidence for Chemical Reactions Chemical reactions involve two main kinds of changes that you can observe: Formation of new substances Changes in energy

Changes in Properties One way to detect chemical reactions is to observe changes in the properties of the materials involved such as: Color change Formation of a precipitate (solid formed from mixing two solutions) Production of gas (bubbles) Not all property changes in matter are chemical, physical changes can produce similar results as well (water boiling produces bubbles).

Changes in Energy As matter changes, it can either absorb or release energy. This change in energy occurs during a chemical reaction. Endothermic reactions are reactions in which energy is absorbed. Exothermic reactions are reactions that release energy in the form of heat.

Rate of Reactions Chemical reactions don’t all occur at the same rate. Some occur very fast, like explosions. Others, like rusting of metal, occur slowly over time. Chemists can control rate of reactions by changing factors such as surface area, temperature, and concentration. They can also use substances called catalysts and inhibitors.

A catalyst is a material that increases the rate of a reaction by lowering the activation energy, or energy needed to start the reaction. Example: enzymes in our bodies. A material used to decrease the rate of a reaction is an inhibitor. They usually work by preventing the reactants from combining together. Example: preservatives added to food to prevent it from becoming stale or spoiling.

Describing Chemical Reactions When you send text messages, you often send abbreviated messages… “Sup?” “NM, U?” “OMG!” “CUL8R!” Scientists also use symbols and abbreviations to describe chemical reactions rather than long sentences.

What are Chemical Equations? Chemical equations use formulas and other symbols instead of words to summarize a reaction. All chemical equations use formulas to represent the substances involved in a reaction. A chemical formula is a combination of symbols that represents the elements in a compound.

Chemical Formulas Example: CO2 is the formula for Carbon Dioxide. This formula tells you that this compound is made up of the elements carbon and oxygen and each molecule has 1 carbon atom and 2 oxygen atoms.

Chemical Formulas More examples include: Water H20 Propane C3H8 Ammonia NH3 Baking Soda NaHCO3

Structure of an Equation All chemical equations have a common structure. A chemical equation tells you the substances you start with and the substances you get at the end. The substances at the beginning are called the reactants. When the reaction is complete, you have new substances called products.

Structure of an Equation Reactant + Reactant  Product + Product The number of reactants and products can vary. Reactant “yields” Products CaCO3  CaO + CO2

Fe + S  FeS Conservation of Mass 14.0 g + 8.0g  22.0 g Iron Sulfur Iron Sulfide If we weighed the iron and sulfur before the reaction takes place, then weigh the product, iron sulfide, we would notice… what? 14.0 g + 8.0g  22.0 g The sum is equal to its parts. This is an example of the Conservation of Mass.

Conservation of Mass This principle, the conservation of mass, states that during a chemical reaction, matter is neither created nor destroyed. This means that all the atoms present at the start of the reaction are still present at the end.

Classifying Chemical Reactions Many chemical reactions can be classified in one of three categories: Synthesis Decomposition Replacement

Synthesis To synthesize is to put things together. When two or more elements or compounds combine to make a more complex substance, the process is called synthesis. Example: Hydrogen and oxygen join together to form water.

Hydrogen peroxide eventually decomposes into water and oxygen. Decomposition Decomposition breaks down compounds into simpler products. 2H2O2  2H2O + O2 Hydrogen peroxide eventually decomposes into water and oxygen.

Replacement 2Cu2O + C  4Cu + CO2 When one element replaces another in a compound, or when two elements in different compounds trade places, this process is called replacement. 2Cu2O + C  4Cu + CO2 Copper metal can be obtained by heating copper oxide with carbon. The carbon replaces the copper.