Chemical Equations and Their Role in Representing Reactions
- Consider you are baking a cake.
- The recipe specifies exact amounts of flour, sugar, eggs, and butter to create the perfect dessert.
- If you use too much or too little of any ingredient, the result might not turn out as expected.
In chemistry, a chemical equation serves as this "recipe" for a chemical reaction. It tells you the precise ratios of reactants (ingredients) and products (results) involved.
Representing Chemical Reactions with Equations
Chemical equation
A chemical equation is a symbolic representation of a chemical reaction. It shows the reactants (starting substances) on the left side, the products (substances formed) on the right, and an arrow indicating the direction of the reaction.
$$C_2H_6 + O_2 \rightarrow CO_2 + H_2O$$
- This equation represents the combustion of ethane ($C_2H_6$) in oxygen ($O_2$) to produce carbon dioxide ($CO_2$) and water ($H_2O$).
- However, this equation is not yet complete: it must be balanced to adhere to the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction.
Balancing Chemical Equations: Why and How?
- When balancing a chemical equation, the goal is to ensure the number of atoms of each element is the same on both sides of the equation.
- This reflects the fact that atoms are simply rearranged during a reaction, not created or destroyed.
Steps to Balance a Chemical Equation:
- Write the unbalanced equation:
- Start by listing the reactants and products.
- For example, $C_2H_6 + O_2 \rightarrow CO_2 + H_2O$
- Count the atoms of each element on both sides of the equation.
- Reactants: 2 carbons, 6 hydrogens, 2 oxygens.
- Products: 1 carbon, 2 hydrogens, 3 oxygens.
- Add coefficients to balance the atoms, starting with the element that appears in the fewest compounds.
- Balance carbon: $C_2H_6 + O_2 \rightarrow 2CO_2 + H_2O$.
- Balance hydrogen: $C_2H_6 + O_2 \rightarrow 2CO_2 + 3H_2O$.
- Balance oxygen: $C_2H_6 + 3.5O_2 \rightarrow 2CO_2 + 3H_2O$.
- Adjust coefficients to whole numbers, if necessary:
- Multiply all coefficients by 2: $2C_2H_6 + 7O_2 \rightarrow 4CO_2 + 6H_2O$.
- Verify the balance:
- Ensure the number of each type of atom is the same on both sides.
- One common mistake is forgetting to adjust all coefficients when converting to whole numbers.
- Always double-check that the equation remains balanced after this step.
State Symbols in Equations
To provide additional information, state symbols indicate the physical state of each substance:
- $(s)$: Solid
- $(l)$: Liquid
- $(g)$: Gas
- $(aq)$: Aqueous (dissolved in water)
The balanced equation for the combustion of ethane with state symbols is:
$$2C_2H_6(g) + 7O_2(g) \rightarrow 4CO_2(g) + 6H_2O(l)$$
Always include state symbols in your chemical equations when known, as they provide important context about the reaction conditions.
Balancing Chemical Equations
Deducing Chemical Equations from Reactants and Products
Sometimes, you are given the reactants and products and need to deduce the balanced equation. Let’s practice this with an example.
Anaerobic Fermentation of Glucose
Write the balanced equation for the anaerobic fermentation of glucose ($C_6H_{12}O_6$) to form ethanol ($C_2H_5OH$) and carbon dioxide ($CO_2$).
Solution
- Write the unbalanced equation:
$C_6H_{12}O_6 \rightarrow C_2H_5OH + CO_2$ - Count atoms for each element:
- Reactants: 6 carbons, 12 hydrogens, 6 oxygens.
- Products: 2 carbons (ethanol) + 1 carbon (carbon dioxide) = 3 carbons; 6 hydrogens (ethanol); 3 oxygens.
- Add coefficients to balance the equation:
- Balance carbon: $C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2$.
- Balance hydrogen: Already balanced with 12 hydrogens on each side.
- Balance oxygen: Reactants have 6 oxygens; products have 4 (ethanol) + 2 (carbon dioxide) = 6 oxygens.
- Verify the balance:
- Reactants: 6C, 12H, 6O.
- Products: 6C, 12H, 6O.
The balanced equation is:
$$C_6H_{12}O_6(aq) \rightarrow 2C_2H_5OH(aq) + 2CO_2(g)$$
In this reaction, glucose is broken down into ethanol and carbon dioxide, a process commonly used in brewing and biofuel production.
- Write the balanced equation for the combustion of propane ($C_3H_8$) in oxygen.
- Deduce the half-equations for the reaction between magnesium and hydrochloric acid, where hydrogen gas is produced.
- How would you explain the importance of balanced equations to someone unfamiliar with chemistry?
