1. Balancing Equations

2. Chemical Changes A change in which new substance is formed
Every chemical change involves a chemical reaction
One or more substances combine or break down to form new substances
Existing atoms are rearranged to form new compounds
Substance is changed forever – look different at the end
The product also has different properties
Energy is absorbed or given off (released)
Breaking of bonds in substances and they look different in the end

3. Describing Chemical Reactions
A chemical reaction is a change in which one or more substances are converted into new substances.
A chemical reaction occurs when you bake a cake.

4. Mechanics of chemical reactions
Two molecules meet with enough energy, bonds break
Simultaneously, new bonds form
Time for breaking/forming bonds is almost immeasurable

5. Chemical reaction Evidences of chemical reaction taking place:
Production of heat or light
Appearance of gas bubbles
Formation of a new solid (precipitate)
Color change

6. Describing Chemical Reactions
The substances that react are called reactants.
The new substances produced are called products.
This relationship can be written as follows:

7. Conservation of Mass
The French chemist Antoine Lavoisier established that the total mass of the products always equals the total mass of the reactants.
For example, the mass of the candles and oxygen before burning is exactly equal to the mass of the remaining candle and gaseous products.

8. The Law of Conservation of Mass (Matter):
mass of the substances is conserved
No mass is lost or gained in a chemical rxn.
Matter can not be created or destroyed in a chemical rxn.
Lavoisier – 1770’s – total mass of the products equal the total mass of the reactants
A closed system must be used to capture the products

9. Writing Equations
Scientists have developed a shorthand method to describe chemical reactions.
A chemical equation is a way to describe a chemical reaction using chemical formulas and other symbols.
Rearrangement of one or more reactants to form one or more products
Bonds are breaking and reforming
Chemical equation is like a recipe – tells ingredients involved and amount needed

10. EXAMPLE
The physical state of each chemical can be indicated by using the symbols
(g) gas (l) liquid (s) solid (cr) crystalline (aq) aqueous = dissolved in water

11. Balancing Equations
Conservation of Atoms – the number of each type of atom on the reactant side must equal the number of each type of atom on the product side
IS THIS EQUATION BALANCED?

12. Coefficients = # of molecules present
Finding out which coefficients to use to balance an equation is often a trial-and-error process.

13. Try Your Balancing Act
Step 1 Write a chemical equation for the reaction using formulas and symbols.
Step 2 Count the atoms in reactants and products.

14. Try Your Balancing Act
Step 3 Choose coefficients that balance the equation.
Remember, never change subscripts of a correct formula to balance an equation.

15. Try Your Balancing Act
Step 4 Recheck the numbers of each atom on each side of the equation and adjust coefficients again if necessary.
Key to chemical equations: New atoms are not created in a rxn; atoms are rearranged in a rxn

16. Diatomic Molecules
Certain elements exist as diatomic molecules, they are H2, F2, N2, Cl2, O2, Br2 At2 I2
Remember most atoms do not like to be alone

17. Classification of Reactions
Combustion
Synthesis Reactions (combination or addition)
Decomposition Reactions
Single Displacement or Replacement Reactions
Double Displacement or Replacement Reactions

18. Combustion Reactions
– an element or compound reacts with oxygen, often producing heat or light
may include hydrocarbons (carbon and hydrogen) – methane (CH4), propane (C3H8), butane (C4H10), octane (C8H18): when this occurs the most probable products are carbon dioxide and water

19. Examples: Fuel (Carbon Compound) + O2  CO2 + H2O
Ex: C6H12O6 + O2  CO H2O
Ex: CH4 + 2O2  CO2 + 2H2O + heat + light
Combustion reaction between an element and oxygen is also an example of a combustion reaction

20. Synthesis Reactions – Combination or Addition
In a synthesis reaction, two or more substances combine to forms one single product. The product must be a compound.
The generalized formula for this reaction type is as follows: A + B  AB.
ex: 2H2 + O2  2H2O
ex: Ca + S  CaS

21. Decomposition Reactions
– a single compound is broken down into two or more products
Most require heat, light or electricity
Hard to predict products unless they are binary
AB  A + B (KEY = one reactant)
ex: 2KClO3  2KCl + 3O2
ex: CaCO3  CaO + CO2
ex: 2H2O  2H2 + O2

22. Single Displacement (or Replacement)
When one element replaces another element in a compound
The metals can replace each other
A + BC  AC + B
ex: Mg + CaCl2  MgCl2 + Ca
ex: Zn + H2SO4  ZnSO4 + H2
The non-metals can replace each other
AC + B  BC + A
Ex: 2KBr + Cl2  2KCl + Br2

23. Double Displacement (or Replacement)
– involve an exchange of positive ions between two compounds
Usually take place between two ionic compounds
AB + CD  AD + CB
ex: NaNO3 + AgCl  NaCl + AgNO3
ex: AgNO3 + KCl  AgCl + KNO3

24. Double Displacement One of the following is always true:
1. one product is slightly soluble and precipitates from the solution
2. one product is a gas that bubbles out of the solution
3. one product is a molecular compound, such as water
A precipitate forms and is an insoluble compound that comes out of solution during this type of reaction

25. Oxidation-Reduction Reactions
One characteristic that is common to many chemical reactions is the tendency of the substances to lose or gain electrons.
Chemists use the term oxidation to describe the loss of electrons and the term reduction to describe the gain of electrons.

26. Oxidation-Reduction Reactions
The substance that gains an electron or electrons obviously becomes more negative, so we say it is reduced.
On the other hand, the substance that loses an electron or electrons then becomes more positive, and we say it is oxidized.

27. How to predict products
Write the chemical formulas for the reactants
Identify the type of reaction
Predict the products and write their chemical formulas
Write and balance the complete equation

28. Rate of Reaction
Collision Theory = substances must come in contact with each other in order to react with each other.
Factors that speed up or slow down a rxns:
Surface Area
Temperature
Concentration
Catalysts – speed up
Inhibitors – slow down

29. What limits a reaction? Limiting reagent
The reagent that is used up first limits the amount of product to be formed
What if you need two eggs to make cupcakes and you only have one? You can only make half a batch of cupcakes
Excess Reagent
The reactant that is not completely used up because the other reactant is used up first

30. Energy in Reactions Exothermic = energy is released by the rxn.
Takes less energy to break the bonds in the reactants than are released by the forming of bonds in the products
Energy is released (gets hotter)
Sometimes hard to detect the temperature change
MRE’s – food wrapped in special sleeve with magnesium when water is added to the sleeve the reaction produces heat which warms the food
exo = out or outer
original substances→ new substances + energy

32. Activation Energy
Activation Energy = energy that is put into an exothermic reaction to start the rxn.

33. Energy of a Rxn (heat change)
Endothermic = energy is put into the rxn, rxn absorbs energy
Takes more energy to break the bonds in the reactants then is released by the forming of bonds in the products
Energy must be put in for the reaction to take place (gets colder)
Instant ice pack – water and ammonium nitrate
endo = inside or into
thermic = heat or energy
original substances + energy → new substances
The energy absorbed can be in the form of light, heat or electricity