Chemical Reactions and Chemical Equations

Quick Review Physical Change A physical change in a substance doesn't change what the substance is. Example: tearing paper, it is still paper

Chemical Change In a chemical change a new substance is formed and energy is either given off or absorbed. Example: burning paper The paper is changed into carbon, water, and heat is given off.

Chemical Change When substances undergo a chemical change, the new substances will have different properties. For example: Burning wood in a campfire.

Evidence of a Chemical Change Color change Light is released A solid (precipitate) is produced An odor is evident Temperature changes A gas is given off Sound is produced

Law of Conservation of Matter Remember that? Matter is NEVER created or destroyed in a chemical change. The particles are just rearranged to form new substances. The same number of particles that existed before the change exist after the change! It’s the law!!!!!

Chemical Equations When writing chemical equations, you MUST follow the law of Conservation of Mass. There must be the same # and kind of atoms on each side of the arrow. This is referred to as balancing an equation

Let’s Try Some H 2 + O 2 H 2 O gas gas liquid Balanced? Let’s balance it: 2H 2 + O 2 2H 2 O Na + Cl NaCl soft, reactive metal poisonous gas white solid Balanced? That is why you see large numbers in front of elements and compounds, to balance the # of atoms on each side of the arrow.

Types of Chemical Equations Synthesis (A + B AB) Na + Cl NaCl Decomposition (AB A + B 2H 2 O 2 2H 2 O + O 2

Types of Chemical Reactions Single Replacement (AB + C AC + B) Mg + HCl MgCl 2 + H 2 (How do I balance this?) Double Replacement (AB + CD AD + BC) Na 2 CO 3 + CaCl 2 NaCl + CaCO 3 (How do I balance this?)