# D ESCRIBING C HEMICAL R EACTIONS Chapter 6 Section 2.

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D ESCRIBING C HEMICAL R EACTIONS Chapter 6 Section 2

WHAT ARE CHEMICAL EQUATIONS? Is a short, easy way to show a chemical reaction, using symbols instead of words.

Uses chemical formulas with subscripts and coefficients to summarize a reaction

A formula is a combination of symbols that represents elements in the compound

The substances at the left of the equation are called reactants, when the reaction is complete, on the right you have products.

If you have two or more reactants or products they are separated by a + sign Reactant 1 + reactant 2 ----> Product 1 + product 2

C ONVERSION OF MASS During a chemical reaction, matter is neither created nor destroyed, it just changes form.

All the atoms present at the start of the reaction are present at the end

The principle of conservation of mass states that in a chemical reaction, the total mass of the reactants must equal the total mass of the products

O PEN AND CLOSED SYSTEMS Open: matter can enter from or escape to the surroundings. Burning match

Closed: matter is not allowed to enter or leave the system Eg: reaction is sealed in a plastic bag

B ALANCING C HEMICAL E QUATIONS To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation

4 Step process Step 1: Write the equation : Reactants on the Left, products on the right ___H ₂ + ___O ₂ ----> ____H ₂ 0 

Step 2: Count the number of atoms of each element on each side of the equation 2 hydrogen, 2 oxygen on the react side, 1 oxygen and 2 hydrogen on the product side. IF THE ATOMS DO NOT EQUAL GO TO STEP 3

Step 3: Use coefficients to balance atoms A coefficient is a number placed in front of a chemical formula. It is multiplied by the subscript to get the new number of atoms Always check both sides when using a coefficient

Step 4: Check your work

C LASSIFYING C HEMICAL REACTIONS Synthesis: when 2 or more elements or compounds combine to make a more complex substance 2H ₂ + O ₂ ---> 2 H ₂ O

Decomposition: Breaks down compounds into simpler products 2H ₂ O ₂ ----> 2H ₂ O + O ₂

Replacement: When one element replaces another element in a compound or when 2 elements in different compounds trade places 2CuO + C ----> 4 Cu + CO ₂ FeS + 2 HCl ----> FeCl ₂ + H ₂ S

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