Chapter 6: Chemical reactions

Key concepts: How can matter and changes in matter be described? How can you tell when a chemical reaction occurs? Key terms: Matter, chemistry, physical property, chemical property, physical change, chemical reaction, precipitate, endothermic reaction, exothermic reaction

Observing chemical changes Matter: anything that has mass (REMEMBERRRRRRRRRR???) Chemistry: How matter changes Ohhhhhh puhleeeeease tell me you remember! That would make this all so easy (for you… for me it will be the same)….

Properties and changes in matter Matter can be described in two ways: Physical and chemical properties. Changes in matter can be described in two ways: physical and chemical changes.

Properties of matter Physical property – can be observed WITHOUT CHANGING the substance into another substance Chemical property – characteristic of a substance that describes its ability to change into other substances Ex: tarnishing, rusting,

Changes in matter Physical change – any change that alters the form or appearance of a substance but does not make the substance into another substance Chemical reaction – a change in matter that produces one or more new substances (e.g. burning gasoline in a car’s engine)

Bonding and chemical change Chemical changes occur when bonds break and new bonds form. As a result, new substances are produced. Example: Oxygen gas (O2) in the air consists of molecules made of two oxygen atoms that share electrons. These molecules are broken when oxygen reactions with magnesium. MgO is produced, and energy is released. The new compound (MgO) is unlike oxygen gas OR magnesium.

Evidence for chemical reactions Chemical reactions involved two main kinds of changes that you can observe – formation of new substances and changes in energy Changes in properties result when new substances form. Like what?

Like.. A color change may indicate a new substance. Sometimes, a solid forms when two solutions are mixed. The solid is called a precipitate. A gas may be produced, even from a liquid. Sometimes, these things do NOT mean chemical changes. Sometimes, physical changes will give similar results. Confusing. For example: water boiling creates gas, but it is just a physical change.

Changes in energy As matter changes, it will release or absorb energy. Endothermic (absorb) Exothermic (release) You should remember thissssss from beforrrrrrrrree now your life is eassierrrrrrrrrrrr