Chapter 13.2

Standards: GLE – Distinguish among elements, compounds, and mixtures. GLE – Explain the law of conservation of mass GLE – Interpret the events represented by a chemical equation

Key concepts: Interpret and write simple chemical formulas Identify the reactants and products of a chemical reaction.

Key Vocabulary: Reactant Product Law of conservation of mass

What do you already know? Where do you find reactants? Where do you find products? Why do we use symbols instead of writing out everything? What does conservation mean? Now imagine having to write out names!

Let’s talk compound vs. element: Is it a compound or element? He

Let’s talk compound vs. element: Is it a compound or element? How many atoms? CO 2

Let’s talk compound vs. element: Is it a compound or element? How many atoms? NaClO

Chemical Reaction: Chemical equation uses symbols and formulas to describe a chemical reaction. Parts: – Reactants—starting materials – Products—what you make – Yield—this is the arrow sign This usually means that energy is going in or out Think of this a piece of sheet music... Cheat sheet time! Reactants point to products R  P

Balancing Equations: Conservation means the before is equal to the after…meaning whatever we do to the reactants, we must do to the products. All atoms must be equal Law of conservation of mass states that the mass is neither created nor destroyed in a chemical change. – Reactants = Products How many atoms? -H 2 0 -C 6 H CH 4 R  P

Balancing the Parts: Important parts: – Coefficient: the number that is place in the front of a symbol This is what WE will use to balance the equation – Subscripts Number written below and to the right of the symbol How many atoms? -H 2 0 -C 6 H CH 4

Balancing the Parts: How to balance: 1.Determine the number of atoms for each element 2.Pick an element that is not equal on both sides (start with metals) 3.Add a coefficient in front the formula with that element and adjust your counts 4.Continue to add coefficients to get the same number of atoms of each side Remember to keep track of the atoms! Practice times

Balancing Equations Hydrogen and oxygen are diatomic elements. Their subscripts cannot be changed. The subscripts on water cannot be changed. Hydrogen + oxygen water H 2 + O 2 H 2 O

Balancing Equation Count the atoms on each side. – Reactant side: 2 atoms H and 2 atoms O – Product side: 2 atoms H and 1 atom O H 2 + O 2 H 2 O

Balancing Equations H 2 + O 2 H 2 O If the subscripts cannot be altered, how can the atoms be made equal? Adjust the number of molecules by changing the coefficients.

Balancing Equations Reactants: 2 atoms of H and 2 atoms of O Products: 4 atoms of H and 2 atoms of O H is no longer balanced! H 2 + O 2 2H 2 O

Balancing Equations Reactant side: 4 atoms of H and 2 atoms of O Product side: 4 atoms of H and 2 atoms of O It’s Balanced! 2H 2 + O 2 2H 2 O

Balancing Equations Count atoms. – Reactants: 2 atoms N and 2 atoms H – Products: 1 atom N and 3 atoms of NH 3 N 2 + H 2 NH 3 Nitrogen + hydrogen ammonia

Balancing Equations Nothing is balanced. Balance the nitrogen first by placing a coefficient of 2 in front of the NH 3. N 2 + H 2 2NH 3

Balancing Equations Hydrogen is not balanced. Place a 3 in front of H 2. Reactant side: 2 atoms N, 6 atoms H Product side: 2 atoms N, 6 atoms H N 2 + 3H 2 2NH 3

We need practice! On the back of your notes, let’s practice. Do not write on the hand-outs. I will take these up when you leave. – Balancing Act

Pearls of Wisdom: How do you balance equations? What are some important steps?

Homework: Finish your worksheet.