FTCE Chemistry 6-12 common mistakes
FTCE Chemistry 6-12 · Understand the traps and how to avoid them.
When reporting the result of a calculation, the candidate writes down every digit shown on the calculator display instead of rounding to the correct number of significant figures based on the given data and the operation performed.
Candidates often treat the calculator as authoritative and forget that measurement precision limits the validity of the result. They may also be unsure whether addition/subtraction or multiplication/division rules apply when multiple operations are combined.
What to do instead
Before calculating, determine the number of significant figures in each given value. For mixed operations, follow the order of operations and round only at the end, tracking the number of significant figures after each step. Practice with problems that require rounding after addition followed by multiplication, and explicitly state the rule being applied.
The candidate designs an experiment or interprets data without identifying the independent, dependent, and controlled variables, or fails to include a control group when one is needed.
Candidates may focus on the procedure and equipment while overlooking the logical structure of the investigation. They may also confuse correlation with causation when analyzing data tables or graphs.
What to do instead
For every experimental scenario, explicitly list: what is being changed (independent variable), what is being measured (dependent variable), and what is kept constant (controlled variables). Identify the control group if applicable. Practice by reading short experiment descriptions and labeling each component before answering questions.
The candidate classifies a substance as a pure substance, homogeneous mixture, or heterogeneous mixture based solely on its appearance, without considering particle-level composition or whether it can be separated by physical means.
Everyday language uses 'pure' loosely (e.g., pure orange juice), and candidates may rely on visual clarity rather than the definition of a substance with fixed composition. They may also forget that solutions are homogeneous mixtures.
What to do instead
Memorize the decision tree: Can it be separated by physical means? If no, it is a pure substance (element or compound). If yes, is it uniform throughout? If yes, homogeneous mixture; if no, heterogeneous mixture. Apply this to examples like salt water, air, brass, and sand.
When predicting the products of a double-replacement reaction, the candidate fails to check solubility rules and does not identify whether a precipitate, gas, or weak electrolyte forms, leading to an incorrect prediction that no reaction occurs.
Candidates often memorize the general pattern AB + CD → AD + CB but forget that a reaction only proceeds if a driving force exists. They may also misremember solubility exceptions (e.g., group 1 cations and ammonium are always soluble).
What to do instead
For each potential product, consult a solubility table. If both products are soluble strong electrolytes, write 'no reaction.' If one product is insoluble, a gas, or a weak electrolyte, write the balanced equation and include state symbols. Practice with a set of 10 double-replacement scenarios covering precipitates, gas formation, and neutralization.
The candidate calculates enthalpy change using Hess's law or bond energies but makes sign errors, such as reversing the sign of ΔH when reversing a reaction or using bond formation energies with the wrong sign.
Candidates may not consistently apply the convention that exothermic reactions have negative ΔH and that breaking bonds requires energy (positive) while forming bonds releases energy (negative). They may also lose track of coefficients when multiplying equations.
What to do instead
Write each step explicitly: when reversing an equation, change the sign of ΔH; when multiplying an equation by a factor, multiply ΔH by the same factor. For bond energies, use ΔH = Σ(bonds broken) − Σ(bonds formed). Double-check by considering whether the overall process should release or absorb heat.
The candidate draws a Lewis structure that satisfies the octet rule but places formal charges on atoms without minimizing them, or fails to expand the octet for elements in period 3 and beyond when appropriate.
Candidates may apply the octet rule too rigidly and forget that formal charge minimization often leads to multiple bonds or expanded octets. They may also be unfamiliar with the exceptions (e.g., S, P, Cl can have more than eight electrons).
What to do instead
After drawing a trial structure, calculate formal charge for each atom using FC = valence electrons − (nonbonding electrons + ½ bonding electrons). If nonzero formal charges appear, try forming double or triple bonds to reduce them. For period 3 elements, consider expanded octets if it lowers formal charge. Practice with molecules like SO4^2−, PO4^3−, and ClO3−.
The candidate predicts molecular geometry using VSEPR theory but ignores lone pairs on the central atom, or confuses electron-domain geometry with molecular geometry.
Candidates may count only bonded atoms when determining steric number, or they may memorize shapes without understanding that lone pairs occupy space and affect bond angles. They may also forget that multiple bonds count as one domain.
What to do instead
Determine the steric number by counting all electron domains (bonding regions + lone pairs) around the central atom. Use the VSEPR table to find electron-domain geometry, then remove lone pairs to find molecular geometry. Remember that lone pairs repel more than bonding pairs, reducing bond angles. Practice with H2O, NH3, SF4, and XeF4.
The candidate performs a stoichiometry calculation using moles but forgets to convert the given mass to moles, or uses the wrong molar mass, or fails to use the mole ratio from the balanced equation.
Candidates may rush into calculations without a clear plan, or they may incorrectly balance the equation first. They may also confuse the limiting reactant with the excess reactant and use the wrong quantity.
What to do instead
Follow a consistent flowchart: write and balance the equation; convert all given quantities to moles; determine the limiting reactant by comparing mole ratios; use the limiting reactant's moles to calculate the desired quantity; convert back to grams or liters if needed. Practice with problems that include masses of two reactants.
The candidate solves a gas law problem without converting temperature to Kelvin, or uses the wrong gas constant R, or applies the ideal gas law to a situation where the gas is not ideal (e.g., high pressure or low temperature).
Candidates may forget that all gas law calculations require absolute temperature. They may also use R = 0.0821 L·atm/(mol·K) with pressure in kPa or volume in mL, leading to unit errors. They may not recognize when real gas behavior is significant.
What to do instead
Always convert temperature to Kelvin by adding 273.15. Choose R based on the units given: 0.08206 L·atm/(mol·K), 8.314 J/(mol·K), or 62.36 L·torr/(mol·K). For real gas behavior, remember that deviations are greatest at high pressure and low temperature. Practice unit conversions before plugging into equations.
The candidate determines the rate law from experimental data but fails to use the method of initial rates correctly, such as comparing trials where only one reactant concentration changes, or misinterpreting the order with respect to a reactant.
Candidates may not recognize that to isolate the effect of one reactant, the other reactant concentrations must be held constant. They may also confuse zero-, first-, and second-order effects on rate when concentration doubles.
What to do instead
For each reactant, find two trials where only that reactant's concentration changes. Compare the ratio of rates to the ratio of concentrations raised to the unknown order. Use the rule: if doubling concentration doubles rate, order = 1; if quadruples, order = 2; if no change, order = 0. Practice with data tables and write the rate law explicitly.