CHEMICAL REACTIONS

Chapter 4.1

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Chemical Reactions

Chemical Change– the rearrangement of the atoms in a substance that creates new products with different chemical properties

Chemical Reaction-- the process in which one or more substances undergo a chemical change Change in energy Energy can be released (heat,

light, sound) or absorbed Occur at different rates: a reaction can be slow or fast

(a catalyst is used to speed up reactions)

Demos

Q: How do we know a chemical change has occurred?

Observe:

Demo 1– lead nitrate + potassium iodide

Demo 2– baking soda + citric acid solution

Clues to a Chemical Reaction

Change in colourPrecipitate formsEnergy is released or absorbed Gas is producedDifficult to reverse

REMEMBER: A change in state or dissolving a substance is a physical change-- NOT a chemical change

How can we speed up a chemical reaction?

Increase the temperature

Increase the surface area

Increase reactant concentration

CATALYST: A substance that makes a chemical reaction go faster WITHOUT being consumed.

Chemical Equations

Using words or symbols and formulas to represent a chemical reaction

Starting materials= “reactants”New substances= “products”An arrow is read as “produces”States of matter are in brackets

(s)= solid (l)= liquid (g)= gas (aq)= dissolved in water

REACTANTS PRODUCTS

Word Equations

Words represent the chemical reaction

REMEMBER: any compound that is not molecular, check the criss cross rule first.

Example:

Lithium + Aluminum Chloride Aluminum + Lithium Chloride

Skeleton Equations

An unbalanced equation that represents the chemical formulas of reactants and products

Ex. Na + O2 Na2O

Rules:Write the symbol of a metal NOT in a compoundIf the non-metal is part of hockey stick and puck,

write it as a diatomic molecule (H2, N2, O2, F2, Cl2, Br2, I2)

Write the formula of other compounds using rules you have learned

Law of Conservation of Mass

In a chemical reaction…

The total mass of reactants EQUALS the total mass of products because elements cannot be created nor destroyed in a chemical reaction. They can only be rearranged!

Formula Equations

Formulas represent reactions Include states of matterCoefficients show how the ratios of different

substances in the chemical reaction

Ex. 2 N2 (g) + 5 O2 (g) 2 N2O5 (s)

Balancing Chemical Formulas

Some tips:

Balance polyatomic ions first if the same ion is on both sides of the equations

Balance all other elements other than H and O

Balance HBalance O

Examples

1. FeCl3 + NaOH Fe(OH)3 + NaCl

2. Cu + AgNO3 Cu(NO3)2 + Ag

3. C5H12 + O2 CO2 + H2O

4. C2H6 + O2 CO2 + H2O

Some Helpful Videos

Elephant toothpaste (catalyst example): http://www.youtube.com/watch?v=4N0m95PExHY

Balancing chemical equations: http://www.youtube.com/watch?v=HB6cG7bQew0

Practice balancing chemical equations:http://www.sky-web.net/science/balancing_chemical_equations_examples.htm

Homework

- Balancing chemical equations worksheet- p. 155 # 3, 5