3.5 Chemical Reactions, Balancing Equations & Energy
Key Takeaways
- The Law of Conservation of Mass states that matter cannot be created or destroyed in a chemical reaction; the total mass of reactants must equal the total mass of products.
- Balancing a chemical equation requires adjusting coefficients—never subscripts—so that the count of each atom type is identical on both sides of the reaction arrow.
- Chemical reactions are classified into five core types: synthesis, decomposition, single replacement, double replacement, and combustion.
- Exothermic reactions release energy (ΔH < 0) into surroundings and lower product potential energy, while endothermic reactions absorb energy (ΔH > 0) and require activation energy to proceed.
3.5 Chemical Reactions, Balancing Equations & Energy
GED Exam Core Concept: Chemical reactions govern how matter changes form and transforms energy. On the GED test, you must apply the Law of Conservation of Mass, balance chemical equations using coefficients, categorize reaction types, and interpret chemical energy profile diagrams.
The Law of Conservation of Mass
Formulated by Antoine Lavoisier in 1789, the Law of Conservation of Mass states:
In any chemical reaction occurring in a closed system, matter is neither created nor destroyed.
Practical Implications for GED Science:
- Atom Conservation: The total number of atoms of each element present on the reactant side (left side) must exactly equal the total number of atoms of each element on the product side (right side).
- Mass Conservation: The total mass of all reactants consumed equals the total mass of all products produced:
Anatomy of a Chemical Equation
A chemical equation uses chemical symbols and formulas to describe a chemical reaction:
Key Symbols:
- Reactants: Starting materials written on the left side of the reaction arrow ($2\text{H}_2 + \text{O}_2$).
- Products: Final substances produced written on the right side of the reaction arrow ($2\text{H}_2\text{O}$).
- Reaction Arrow ($\rightarrow$): Yields / produces.
- Coefficients (Large Numbers in Front): Represent the relative number of moles or molecules of each substance. Coefficients CAN be changed when balancing equations.
- Subscripts (Small Numbers Below): Indicate the exact atomic composition within a chemical formula (e.g., $\text{H}_2\text{O}$ has 2 Hydrogen atoms for 1 Oxygen atom). Subscripts CANNOT be changed under any circumstances when balancing equations.
- State Symbols: $(s) = \text{solid}$, $(l) = \text{liquid}$, $(g) = \text{gas}$, $(aq) = \text{aqueous (dissolved in water)}$.
Five Core Types of Chemical Reactions
GED Science questions frequently ask you to identify the specific category of a chemical reaction:
| Reaction Type | General Equation | Example Chemical Equation | Key Identifying Feature |
|---|---|---|---|
| 1. Synthesis (Combination) | $A + B \rightarrow AB$ | $2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s)$ | Two or more simple reactants combine into one single product. |
| 2. Decomposition | $AB \rightarrow A + B$ | $2\text{H}_2\text{O}(l) \rightarrow 2\text{H}_2(g) + \text{O}_2(g)$ | One single reactant breaks down into two or more simpler products. |
| 3. Single Replacement | $A + BC \rightarrow AC + B$ | $\text{Zn}(s) + 2\text{HCl}(aq) \rightarrow \text{ZnCl}_2(aq) + \text{H}_2(g)$ | A uncombined element replaces one element in a compound. |
| 4. Double Replacement | $AB + CD \rightarrow AD + CB$ | $\text{AgNO}_3(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_3(aq)$ | Ions in two compounds swap partners, forming a precipitate or water. |
| 5. Combustion | $\text{Hydrocarbon} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}$ | $\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(g)$ | A hydrocarbon reacts rapidly with oxygen ($\text{O}_2$), producing $\text{CO}_2$ and $\text{H}_2\text{O}$. |
Step-by-Step Strategy to Balance Chemical Equations
Follow this universal 4-step algorithm to balance any chemical equation:
- Step 1: Inventory All Atoms. Write down the element symbols and count the number of atoms of each element on both sides of the unbalanced equation.
- Step 2: Balance Elements One by One. Start with elements that appear in only one reactant formula and one product formula. Save Hydrogen and Oxygen for last.
- Step 3: Adjust Coefficients Only. Place whole-number coefficients in front of formulas to balance atom counts. Never change subscripts!
- Step 4: Perform Final Verification. Recount all atoms on both sides to verify that Reactant Count = Product Count.
Chemical Energetics: Exothermic vs. Endothermic Reactions
Chemical bonds store potential chemical energy. Breaking bonds requires energy input, while forming new bonds releases energy.
1. Exothermic Reactions (Energy Released)
- Definition: Reactions that release net thermal energy into their surroundings ($Q < 0, \Delta H < 0$).
- Temperature Effect: The surroundings warm up (temperature rises).
- Potential Energy: The potential energy of the products is lower than the potential energy of the reactants.
- Examples: Combustion of natural gas, neutralization reactions, cellular respiration.
2. Endothermic Reactions (Energy Absorbed)
- Definition: Reactions that absorb net thermal energy from their surroundings ($Q > 0, \Delta H > 0$).
- Temperature Effect: The surroundings cool down (temperature drops).
- Potential Energy: The potential energy of the products is higher than the potential energy of the reactants.
- Examples: Photosynthesis ($6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Sunlight} \rightarrow \text{C}6\text{H}{12}\text{O}_6 + 6\text{O}_2$), chemical cold packs.
Activation Energy & The Function of Catalysts
- Activation Energy ($E_a$): The minimum amount of energy required to initiate a chemical reaction by breaking initial chemical bonds (the energy hill reactants must climb).
- Catalyst: A substance that increases the rate of a chemical reaction without being consumed in the process.
- How it works: A catalyst provides an alternative reaction mechanism with a significantly lower activation energy barrier ($E_a$).
- Important Note: A catalyst does not alter the overall enthalpy change ($\Delta H$) or the total amount of product formed.
Worked Numerical Examples
Worked Example 1: Step-by-Step Equation Balancing
Problem: Balance the chemical equation for the formation of rust: $\text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3$.
Solution:
- Initial Unbalanced Inventory:
- Reactants: $\text{Fe} = 1$, $\text{O} = 2$
- Products: $\text{Fe} = 2$, $\text{O} = 3$
- Balance Oxygen: $\text{O}$ has 2 on the left and 3 on the right. The least common multiple of 2 and 3 is 6. Place a coefficient of 3 in front of $\text{O}_2$ ($3 \times 2 = 6$) and a coefficient of 2 in front of $\text{Fe}_2\text{O}_3$ ($2 \times 3 = 6$):
- Balance Iron: Products now have $2 \times 2 = 4$ iron atoms. Place a coefficient of 4 in front of $\text{Fe}$:
- Final Verification:
- Reactants: $4\text{ Fe}, 6\text{ O}$
- Products: $4\text{ Fe}, 6\text{ O}$
- Equation is perfectly balanced!
Worked Example 2: Conservation of Mass Calculation
Problem: In a closed reaction chamber, $32.0\text{ g}$ of methane gas ($\text{CH}_4$) reacts completely with $128.0\text{ g}$ of oxygen gas ($\text{O}_2$). The reaction produces $88.0\text{ g}$ of carbon dioxide gas ($\text{CO}_2$) and an unknown mass of water vapor ($\text{H}_2\text{O}$). Calculate the mass of water vapor produced.
Solution:
- Apply Law of Conservation of Mass:
- Sum Reactant Mass:
- Set Up Product Mass Equation:
- Solve for Mass of Water:
When the equation Fe + O₂ → Fe₂O₃ is correctly balanced using the lowest whole-number coefficients, what is the coefficient in front of Fe?
In a closed container, a 12.0 gram sample of wood burns completely in the presence of 32.0 grams of oxygen gas. The reaction produces carbon dioxide gas, water vapor, and 1.5 grams of ash. According to the Law of Conservation of Mass, what is the total combined mass of the carbon dioxide and water vapor produced?
How does adding a catalyst affect a chemical reaction?