Chemical Equations Study Guide
Notes:
Balancing Equations: The # of atoms on both the reactants side & products side of an equation must be equal (Law Of Conservation of Mass / Matter)
1st: Identify # of atoms of each element on both sides of the equation
2nd: Add coefficients to balance the equation
**Remember, you cannot change subscripts or break up molecules to add coefficients – coefficients go in the FRONT of a molecule & apply to all of the atoms within the molecule.
**If you have a word equation, write out the formulas before balancing.
Predicting Products: Rearrange the reactants to show which new products will be formed when a reaction takes place.
Steps: 1. Identify the type of reaction (different rules apply for different rxns)
2. Rearrange the atoms to form new products
3. Identify diatomics (H2, N2, O2, F2, Cl2, Br2, I2) – must be alone to get the 2 subscript?
4. swap and drop oxidation numbers.
**Remember, subscripts DO NOT move from the reactants side to the products side (unless it’s a part of a polyatomic ion).
**the cation (+ ion – metal) always comes first
**Opposites attract
**Use your reference tables!!
5 types of reactions:
1. Synthesis (addition): two atoms/compounds form 1 compound (A + B à AB)
**Don’t forget to swap and drop oxidation numbers after you rearrange elements.
2. Decomposition: 1 compound breaks up into 2 or more atoms/compounds
- AB à A + B
- Reference Table: polyatomic ions: look on the reference table to see what the products will look like. remember “M” is a generic letter that represents the metal (cation).
3. Single Replacement (single displacement): 1 atom + 1 compound à 1 atom + 1 compound. (remember diatomics)
- A + BC à AC + B
- Only occurs if the active element is alone originally (i.e. is the single element higher on the series).
- Reference Table: Have to look at Activity Series on ref. table to see if a rxn will occur
- Atoms that are higher on the series can replace atoms that are lower.
- Metals replace metals, nonmetals replace nonmetals
**Don’t forget to swap and drop oxidation numbers after you rearrange elements.
4. Double Replacement (double displacement): 1 compound + 1 compound à 2 new compounds
- AB + CD à AD + BD
- Reference Table: Have to look at the solubility rules on ref table to determine which compounds are soluble with others.
- (s) means it is a solid & is therefore insoluble
- When a solid is with an (aq) compound, the solid is a precipitate
- (aq) means it is soluble (dissolved in water)
**Don’t forget to swap and drop oxidation numbers after you rearrange elements.
5. Combustion: a hydrocarbon reacts with water to produce CO2 + H2O
- CxHy + O2 à CO2 + H2O
- The only thing that changes is the hydrocarbon – the products are ALWAYS the same.
Indicators of a Chemical Reaction: Must have evidence of a chemical reaction…
1. color changes 3. precipitate is formed
2. there is a heat change 4. a gas is given off.
Factors that speed up the Rate of Reaction: (Collision Theory)
These are things that make the reaction happen faster because they add energy to activate the reaction.
1. Increase collisions – stirring, shaking, adding heat. These make the molecules move faster & hit each other.
2. Increase surface area – the smaller the particle, the more sides there are, so the molecules have more area where collisions can occur
3. Increase the concentration – by adding more solute (substance that does the dissolving) there are more molecules present to collide.
4. Add a catalyst – this lowers the activation energy.
a. Examples: Sunlight, heat, electricity, adding certain other compounds which are not actually part of the reactants – their presence makes the reaction occur faster.
Potential Energy Diagrams
1. Energy of the Reactants – how much energy is stored within the bonds of the reactants.
2. Energy of the Products – how much energy is stored within the bonds of the products
3. Activation Energy – the amount of energy added to the reactants to make all of the bonds break so that new bonds can form to create the products.
4. Activated Complex – the maximum amount of energy during the reaction.
5. Enthalpy (DH) – the difference in energy between the reactants & products (found by subtracting Ep from Er)
6. Endothermic Reactions – energy is absorbed during the reaction (EP > ER)
7. Exothermic Reactions – energy is released during the reaction (EP < ER)
Problems:
1. What does each symbol mean (if there is more than one word, use it? D
a. (aq)______d. (l)______g. à ______
b. à ______e. (s) ______h. (g) ______
Balance the following equations:
2. Mg + HCl à MgCl2 + H2 3. Sb + Cl2 à SbCl3
4. RbCl + O2 à RbClO4 5. Sn + KOH à K2SnO2 + H2
6. Ca(OH)2 à CaO + H2O 7. C4H10 + O2 à CO2 + H2O
For the following, tell the type of reaction in the blank & predict the products.
11. ______Cu(s) + AgNO3(aq) à
12. ______Br2(g) + NaF à
13. ______H2SO4(aq) + KOH(aq) à
15. ______CoO à
16. ______Cs + F2 à
18. ______Ti(OH)4 à
19. ______Ca(NO3)2 + Al(OH)3 à
20. ______C6H12 + O2 à
For the following word equations, (a) write the formulas, (b) predict the products & (c) balance the equations:
21. barium carbonate yields
22. germanium reacts with sulfur to yield
23. Determine if the following are endothermic or exothermic.
A. ______KClO3 + heat à KCl + O2 B. ______DH = -156 kJ
C. ______Energy is absorbed
D. ______the word “heat” or a numerical KJ value can be written on the product side
24. What are three things that can be done to make a reaction happen faster?
25. Use the potential energy diagram to the right to determine the values for the following:
A. Energy of the Reactants (ER)
B. Energy of the Products (ER)
C. Activated Complex
D. Activation Energy
E. Enthalpy (DH)