Balancing Chemical Equations
H2+02 -> H20
2H2+02 -> 2H20
What type of reaction is this: AB → A + B?
Decomposition.
What is a limiting reactant?
The limiting reactant is the substance that runs out first in a chemical reaction, stopping the reaction and determining the maximum amount of product that can be formed.
What is the formula for percent yield?
Percent Yield = (Actual Yield ÷ Theoretical Yield) × 100%
State Boyle's Law in your own words.
At constant temperature, the pressure and volume of a gas are inversely related — if pressure increases, volume decreases, and vice versa (P₁V₁ = P₂V₂).
In the equation H₂ + O₂ → H₂O, which element is unbalanced and why?
Oxygen is unbalanced — there are 2 oxygen atoms on the left but only 1 on the right. The balanced form is 2H₂ + O₂ → 2H₂O.
List the five types of chemical reactions.
Combination (synthesis), decomposition, single displacement (replacement), double displacement (replacement), and combustion.
If you have 10 buns and 8 hot dogs, which is the limiting reactant in making hot dogs? How many can you make?
The hot dogs are the limiting reactant. You can only make 8 hot dogs even though you have 10 buns.
Can percent yield ever be greater than 100%? Why or why not?
Theoretically, no. It would violate conservation of mass. Values above 100% usually indicate impurities in the product or measurement error.
What does Charles's Law describe? What must remain constant?
Charles's Law states that volume and temperature (in Kelvin) of a gas are directly proportional at constant pressure. As temperature increases, volume increases.
Fe+02 -> Fe2O3
4Fe+3O2 -> 2Fe2O3
What is the difference between a single replacement and a double replacement reaction?
In single replacement, one element swaps with another in a compound (A + BC → AC + B). In double replacement, two compounds exchange ions (AB + CD → AD + CB).
What happens to the excess reactant when the limiting reactant is used up?
The excess reactant is left over and unreacted. The reaction simply stops once the limiting reactant is completely consumed.
What is the difference between theoretical yield and actual yield?
Theoretical yield is the maximum amount of product calculated from stoichiometry. Actual yield is the amount actually obtained in the lab, which is typically less due to side reactions or loss during collection.
Why must temperature be in Kelvin when using gas laws?
Celsius can be zero or negative, which would give nonsensical results in calculations. Kelvin is an absolute scale where 0 K is absolute zero — no negative values exist.
True or False: You can change the subscripts in a chemical formula to balance an equation. Explain.
False. Changing subscripts changes the compound entirely. You may only change the coefficients (the numbers in front of each formula).
Why does combustion always require oxygen? What are the typical products?
Combustion is a reaction with oxygen (O₂). Complete combustion of hydrocarbons produces carbon dioxide (CO₂) and water (H₂O).
In a reaction, you need a 2:1 ratio of H₂ to O₂. If you have 6 mol H₂ and 2 mol O₂, which is limiting?
O₂ is the limiting reactant. 2 mol O₂ requires 4 mol H₂, but you have 6 mol H₂ — so O₂ runs out first.
A reaction has a theoretical yield of 50g. If the actual yield is 38g, what is the percent yield?
(38 ÷ 50) × 100% = 76% yield.
What are the four variables in the Ideal Gas Law, and what is the equation?
Pressure (P), Volume (V), moles (n), and Temperature (T). The equation is PV = nRT, where R is the ideal gas constant.
C₃H₈ + O₂ → CO₂ + H₂O
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
A silvery metal is dropped into a blue copper sulfate solution. The solution slowly loses its color and a reddish solid forms at the bottom. Identify the reaction type, name the reactants and products, and explain why this reaction occurred
Single replacement reaction. The metal (likely zinc or iron) displaces copper from copper sulfate solution. The reddish solid is copper metal and the solution fades because copper ions are being removed. This occurs because the added metal is more reactive than copper on the activity series, so it "kicks out" the copper ion and takes its place in the compound.
Why is identifying the limiting reactant important in industrial chemistry?
It determines the maximum product yield and affects cost efficiency. Buying excess of the non-limiting reactant wastes money, so manufacturers optimize reactant ratios.
Name three reasons why actual yield is often less than theoretical yield.
Possible reasons: side reactions produce unwanted products, some product is lost during filtering/transferring, the reaction does not go to completion, or product evaporates.
How does the kinetic molecular theory explain why heating a gas in a sealed container increases pressure?
Higher temperature means gas molecules move faster. Faster-moving molecules hit the container walls more frequently and with greater force, increasing pressure.