Chapter 2.1 Observing Chemical Reactions AISG Glenn Jacobsen

Section 2.1 Observing Chemical Reactions OBJECTIVES: State how matter and changes in matter can be decribed.

Section 2.1 Observing Chemical Reactions OBJECTIVES: Explain how you can tell when a chemical reaction occurs.

Chemical Reactions are Everywhere Cooking Respiration

Chemical Reactions are Everywhere Hair Dye Auto Fuel

What is a physical property? A characteristic of a substance that can be observed without changing the substance into another substance Examples: ice melting, color, hardness, texture, shine, etc.

What is a chemical property? A characteristic of a substance that describes its ability to change into other substances Examples: burning magnesium, rusting

What is the difference between a physical and a chemical reaction? A change in matter that produces one or more new substances is a chemical change, or chemical reaction Physical change examples: braiding your hair, squashing a marshmallow Chemical change examples: burning of gasoline, burning a marshmallow

How can you tell when a chemical reaction occurs? Chemical changes occur when bonds break and new bonds form Chemical reactions involve two main kinds of changes that you can observe-formation of new substances and changes in energy

How do you know when a chemical reaction takes place? Color Change Precipitate Formation

How do you know when a chemical reaction takes place? Emission of light or heat

How do you know when a chemical reaction takes place? Gas Formation Odor

How do you know when a chemical reaction takes place? Temperature Change Change in Acidity

How do you categorize changes in energy? Endothermic – A reaction in which energy is absorbed Examples: baking soda and vinegar gets cooler when combined

How do you categorize changes in energy? Exothermic – A reaction that releases energy in the form of heat Examples: burning of airplane fuel

Is the reaction endothermic or exothermic? Explain.

Representing Chemical Reactions Chemists observe chemical reactions and have come up with a way to represent or model what is happening. Making NaCl Solid Sodium combines with Chlorine gas to make solid Sodium Chloride: 2Na (s) + Cl2 (g)  2NaCl

All chemical reactions require two steps: the atoms of the reactants are broken apart b) the atoms of the reactants recombine to form products e.g. magnesium + sulphuric acid ==> magnesium sulphate + hydrogen Mg(s) + H2SO4(aq) ==> MgSO4(aq) + H2(g)