TOPIC 7 CHEMICAL REACTIONS. CHEMICAL CHANGES... What are some ways that you can tell that a chemical reaction has occurred?

TOPIC 7CHEMICAL REACTIONS

CHEMICAL CHANGES...

•What are some ways that you can tell that a chemical reaction has occurred?

IN CHEMICAL REACTIONS…• A NEW SUBSTANCE IS FORMED

• EVIDENCE OF THE REACTION MAY INCLUDE:• COLOR CHANGE

• FORMATION OF AN ODOR

• FORMATION OF A PRECIPITATE

• FORMATION OF A GAS

• RELEASE or ABSORPTION OF HEAT/ENERGY

WHAT ABOUT THESE SCENARIOS?• Smelling a cake in the oven?• Watching ice melt?• Frying up an egg for

breakfast?• Bubbles forming when

mixing baking soda and vinegar?

BREAKING CHEMICAL BONDS

• Energy is stored in chemical bonds• To break bonds, energy must be added

•When bonds form, energy is released

• Reactants - CHEMICALS at the BEGINNING of a reaction.

• Products - NEW MATERIALS produced at the END of a reaction.

Ex.• wood + oxygen carbon dioxide + water + E• What are the + sign and for?

WRITING REACTIONS

WORD equations:

silver + bromine silver bromide

CHEMICAL equations:

Ag (s) + Br2(l ) Ag Br (s)

LAW OF CONSERVATION OF MASS

In all chemical reactions: The total mass of products is always the same as the total mass of reactants.

Reactants mass = Products mass

Matter cannot be created or destroyed.

•Closed System: nothing is allowed to enter or leave the reaction.

•Open System: matter and energy can enter or leave the reaction (so we have to account for this change).

LAW OF CONSERVATION OF MASS• Closed system:

Mg (s) + S(s) MgS(s)

24.3 g 32.1g 56.4g

• Open system:

Alka-Seltzer + Water Product

10g 100g 106 g

• ENERGY can be placed on either side of a chemical reaction depending on whether it’s being released or absorbed.

• Endothermic : energy is a reactant or “absorbed”• Exothermic : energy is a product or “released”

NAME THE REACTANTS AND PRODUCTS IN EACH EQUATION:

1. Ca + Cl2 CaCl2

2. H2O(l) H2(g) + O2(g)

3. 2Mg(s) + O2(g) 2MgO(s) + light

4. aluminum + fluorine aluminum fluoride

5. zinc + hydrogen chloride zinc chloride + hydrogen