Chemical Reactions and Equations — Free Chemistry Review Games.

The law of conservation of mass requires equal numbers of each type of atom on both sides of a chemical equation.

Products are the substances formed by a chemical reaction, appearing on the right side of the arrow. Here, water (\(H_2O\)) is the product.

A synthesis reaction combines two or more reactants to form a single, more complex product (\(A + B \to AB\)).

A decomposition reaction breaks a single compound into two or more simpler substances (\(AB \to A + B\)).

The law of conservation of mass states that the total mass of reactants equals the total mass of products.

In a single replacement reaction, a more reactive element displaces a less reactive one from a compound (\(A + BC \to AC + B\)).

Combustion is a rapid reaction with oxygen that produces heat, light, and typically CO2 and H2O.

Endothermic reactions absorb energy from the surroundings, causing the surroundings to feel cooler.

A catalyst lowers the activation energy of a reaction, increasing the rate without being permanently changed.

\(4Fe + 3O_2 \to 2Fe_2O_3\) balances with 4 Fe atoms and 6 O atoms on each side.

Activation energy is the minimum energy barrier that reactants must overcome for a reaction to proceed.

In a double replacement reaction, the cations and anions of two compounds switch partners (\(AB + CD \to AD + CB\)).

Higher temperature increases particle kinetic energy, leading to more frequent and more energetic collisions that exceed activation energy.

In exothermic reactions, products have lower energy than reactants (energy is released). In endothermic reactions, products have higher energy (energy is absorbed).

Redox reactions involve the transfer of electrons: oxidation is electron loss and reduction is electron gain (OIL RIG).

The arrow (\(\to\)) is the standard notation separating reactants from products and shows the direction the reaction proceeds. The choice "An equals sign (\(=\))" is incorrect because equations use a directional arrow, not an equality symbol, since chemical reactions are not reversible balances like math equations. Recognizing the arrow's role helps students correctly identify which substances are starting materials and which are results.

By convention, reactants are always written on the left side of the arrow because they represent the starting substances before the reaction occurs. The choice "The right side of the arrow" is incorrect because that position is reserved for the products formed after the reaction. Knowing this layout is essential for correctly reading and writing any chemical equation.

A coefficient placed before a formula multiplies the entire formula, indicating how many molecules or moles of that substance are involved in the reaction. The choice "The number of atoms within a single molecule" is incorrect because that information is given by subscripts, not coefficients. Distinguishing coefficients from subscripts is critical for correctly balancing equations.

A subscript written after an element's symbol shows how many atoms of that element are bonded within a single molecule of the compound. The choice "The coefficient used for balancing" is incorrect because coefficients are separate numbers placed in front of formulas, and changing subscripts would alter the compound's identity. Students must never change subscripts when balancing, since doing so creates a different substance entirely.

The symbol \((aq)\) stands for "aqueous," meaning the substance is dissolved in water and exists as an aqueous solution. The choice "The substance is a solid" is incorrect because solids are labeled with \((s)\), a distinct state symbol used for undissolved precipitates or reactants. Correctly reading state symbols helps students visualize the physical setup of a reaction.

Oxygen naturally exists as \(O_2\) because two oxygen atoms bond covalently to form a stable diatomic molecule in its elemental state. The choice "\(Na\)" is incorrect because sodium exists as individual metal atoms arranged in a solid lattice, not as paired molecules. Remembering the seven diatomic elements is important for correctly writing balanced equations involving gases.

A precipitate is an insoluble solid product that forms and settles out of solution when two soluble compounds react and produce a new compound that does not dissolve in water. The choice "A gas released during a reaction" is incorrect because gas evolution is a separate type of observable evidence, distinct from solid precipitate formation. Recognizing precipitate formation is a key visual clue that a double replacement reaction has occurred.

Products are the new substances created after the reactants undergo chemical change, and they appear on the right side of the equation's arrow. The choice "The substances that start the reaction" is incorrect because that description defines reactants, which are written on the left side instead. Clearly distinguishing reactants from products is fundamental to interpreting any chemical equation correctly.

The symbol \((g)\) is used to denote that a substance exists in the gaseous state at the conditions of the reaction. The choice "\((l)\)" is incorrect because that symbol specifically represents a liquid, such as water at room temperature, not a gas. Correctly using state symbols allows chemists to communicate the physical form of every substance involved in a reaction.

This is a synthesis reaction because two separate elements, sodium and chlorine, combine to form a single compound, sodium chloride. The choice "Decomposition" is incorrect because decomposition involves one compound breaking apart into multiple simpler substances, which is the opposite of what happens here. Identifying synthesis reactions helps students predict that two or more reactants will always yield exactly one product.

Conservation of mass states that mass is neither created nor destroyed in a chemical reaction, so the combined mass of all reactants must equal the combined mass of all products. The choice "The total number of molecules stays the same" is incorrect because the number of molecules can change even when mass is conserved, since atoms simply rearrange into different combinations. This principle is the foundation for why chemical equations must always be balanced.

The equation \(2H_2 + O_2 \to 2H_2O\) is balanced because there are 4 hydrogen atoms and 2 oxygen atoms on each side, satisfying conservation of mass. The choice "\(H_2 + O_2 \to H_2O\)" is incorrect because it has 2 oxygen atoms on the reactant side but only 1 oxygen atom on the product side. Checking that every element's atom count matches on both sides is the essential test for a balanced equation.

A chemical equation uses chemical formulas and symbols to summarize which substances react and which substances form, along with their relative amounts. The choice "A graph plotting concentration versus time" is incorrect because that describes a kinetics graph, not the symbolic shorthand used to represent a reaction. Understanding this representation allows chemists to communicate reactions concisely and predict quantities using stoichiometry.

Balancing gives \(N_2 + 3H_2 \to 2NH_3\), which provides 2 nitrogen atoms and 6 hydrogen atoms on each side of the equation. The choice "\(2, 3, 1\)" is incorrect because it would leave 4 nitrogen atoms on the reactant side but only 1 on the product side, violating conservation of mass. Balancing nitrogen fixation equations like this is a common application of coefficient adjustment in industrial chemistry.

The fully balanced equation is \(C_3H_8 + 5O_2 \to 3CO_2 + 4H_2O\), which requires 10 oxygen atoms total on each side to balance the 3 carbons and 8 hydrogens converting into \(CO_2\) and \(H_2O\). The choice "\(3\)" is incorrect because it would only supply 6 oxygen atoms, insufficient to form both the required carbon dioxide and water molecules. Combustion reactions of hydrocarbons always require careful oxygen balancing since oxygen appears in both products.

This is a decomposition reaction because a single compound, calcium carbonate, breaks down into two simpler substances, calcium oxide and carbon dioxide. The choice "Synthesis" is incorrect because synthesis reactions combine multiple reactants into one product, which is the reverse pattern shown here. Recognizing decomposition patterns helps students predict that one reactant will split into two or more products.

This is a double replacement reaction because the positive ions of two ionic compounds swap partners, forming a new precipitate, $AgCl$, and a new soluble salt, $NaNO_3$. The choice "Single replacement" is incorrect because that type involves one element displacing another within a compound, not an exchange between two compounds. Double replacement reactions often produce a precipitate, gas, or water as evidence that a reaction has occurred.

Zinc is more reactive than copper according to the activity series, so it displaces copper from the compound, forming $ZnSO_4$ and releasing free copper metal. The choice "No reaction occurs" is incorrect because a reaction does proceed whenever a more reactive metal is placed with the compound of a less reactive metal. Using an activity series correctly predicts whether a single replacement reaction will actually take place.

The total reactant mass is \(10 + 15 = 25\text{ g}\), and since mass must be conserved, the product masses must also total \(25\text{ g}\), so \(D = 25 - 20 = 5\text{ g}\). The choice "\(15\text{ g}\)" is incorrect because it would make the total product mass \(35\text{ g}\), exceeding the mass of the reactants used. This kind of mass-balance calculation directly applies the law of conservation of mass to real numerical problems.

The correctly balanced equation is \(2Al + 3H_2SO_4 \to Al_2(SO_4)_3 + 3H_2\), requiring three sulfate groups to balance the three sulfate groups on the product side. The choice "\(2\)" is incorrect because it would provide only 2 sulfate groups, unable to balance the \(Al_2(SO_4)_3\) product which contains 3 sulfate ions. Treating polyatomic ions like sulfate as single units simplifies balancing equations that contain them.

The equation $Mg + O_2 \to MgO$ is unbalanced because it has 2 oxygen atoms on the reactant side but only 1 oxygen atom on the product side, meaning mass is not conserved as written. The choice "\(2H_2 + O_2 \to 2H_2O\)" is incorrect as an answer here because that equation is already properly balanced with equal atoms of hydrogen and oxygen on both sides. Spotting unbalanced equations by counting atoms of each element is a critical skill for verifying conservation of mass.

The mole ratio between \(H_2\) and \(N_2\) is \(3:1\), so 4 moles of \(N_2\) requires \(4 \times 3 = 12\) moles of \(H_2\) to react completely. The choice "\(4\)" is incorrect because it assumes a \(1:1\) ratio, ignoring the coefficient of 3 in front of \(H_2\) in the balanced equation. Reading mole ratios directly from balanced coefficients is the foundation of all stoichiometric calculations.

A catalyst speeds up a reaction by offering a different mechanism or pathway that requires less activation energy, allowing more particles to react successfully per unit time. The choice "It shifts the position of chemical equilibrium" is incorrect because catalysts affect only the rate at which equilibrium is reached, not the final equilibrium position itself. Understanding that catalysts lower activation energy without being consumed is essential for reaction rate concepts.

Complete combustion occurs when a hydrocarbon reacts with sufficient oxygen to fully oxidize all carbon and hydrogen atoms, producing carbon dioxide and water as the only products. The choice "Carbon monoxide and hydrogen gas" is incorrect because carbon monoxide typically forms only during incomplete combustion, when oxygen supply is limited. Recognizing complete combustion products is important for balancing hydrocarbon combustion equations correctly.

Barium sulfate is insoluble in water, so when barium and sulfate ions meet in solution, they combine to form a solid precipitate that visibly settles out, confirming a double replacement reaction. The choice "Release of gas bubbles" is incorrect because this particular reaction does not produce a gaseous product, only a solid precipitate and a soluble salt. Predicting precipitate formation from solubility rules is a key skill for identifying double replacement reactions.

The balanced equation is \(2C_2H_6 + 7O_2 \to 4CO_2 + 6H_2O\), and adding these coefficients gives \(2+7+4+6 = 19\). The choice "\(15\)" is incorrect because it does not correspond to the smallest whole-number set that balances both carbon and hydrogen atoms while keeping oxygen consistent. Practicing coefficient-sum problems reinforces the full balancing process rather than checking only one element at a time.

Subscripts define the fixed ratio of atoms within a specific compound, so altering them creates an entirely different chemical substance rather than balancing the original one. The choice "Coefficients cannot ever be fractions" is incorrect because fractional coefficients can appear as an intermediate balancing step before being converted to whole numbers. Preserving subscripts while adjusting only coefficients ensures the compounds in the equation remain chemically accurate.

This acid-base neutralization is a double replacement reaction because the hydrogen ion from the acid and the sodium ion from the base exchange partners to form a salt and water. The choice "Synthesis" is incorrect because synthesis reactions combine reactants into a single product, whereas this reaction produces two separate products. Recognizing neutralization as a specific case of double replacement helps connect reaction-type patterns to acid-base chemistry.

On the reactant side, \(O_2\) contributes 2 oxygen atoms, and on the product side, \(2H_2O\) also contributes \(2 \times 1 = 2\) oxygen atoms, keeping the equation balanced. The choice "\(4\)" is incorrect because it overcounts the oxygen atoms by assuming each water molecule contains 2 oxygen atoms instead of 1. Counting individual atom types on each side is the definitive way to confirm an equation obeys conservation of mass.

The balanced equation is \(CH_4 + 2O_2 \to CO_2 + 2H_2O\), which requires 4 oxygen atoms total to form one \(CO_2\) and two \(H_2O\) molecules. The choice "\(1\)" is incorrect because a single \(O_2\) molecule supplies only 2 oxygen atoms, which is not enough to balance both the carbon dioxide and water products. Balancing methane combustion is a common baseline example for practicing hydrocarbon combustion equations.

The mole ratio of \(H_2\) to \(H_2O\) is \(2:2\), which simplifies to \(1:1\), so 5 moles of \(H_2\) produces 5 moles of \(H_2O\). The choice "\(2.5\)" is incorrect because it mistakenly treats the ratio as \(2:1\) instead of the actual equal \(1:1\) ratio shown by the coefficients. Simplifying mole ratios from balanced coefficients before calculating is a useful strategy to avoid stoichiometric errors.

Magnesium reacts with oxygen gas from the surrounding air to form solid magnesium oxide, and since the oxygen mass becomes part of the solid product, the measured solid mass increases even though total system mass is conserved. The choice "Combustion reactions are exceptions to conservation of mass" is incorrect because no chemical reaction violates this law; the apparent gain simply reflects an open system exchanging mass with its surroundings. Recognizing open versus closed systems helps explain apparent mass changes without abandoning conservation of mass.

The balanced equation is \(C_3H_8 + 5O_2 \to 3CO_2 + 4H_2O\), and summing these coefficients gives \(1+5+3+4 = 13\). The choice "\(12\)" is incorrect because it corresponds to an equation that does not fully balance the oxygen atoms required to form both carbon dioxide and water. This type of multi-step balancing and summation tests whether a student can carry out the entire process accurately rather than balancing only part of the equation.

Zinc metal starts with an oxidation state of 0 and ends as \(Zn^{2+}\) in $ZnSO_4$, meaning it loses two electrons and is oxidized during the reaction. The choice "\(Cu\), because it gains a positive charge" is incorrect because copper actually goes from \(+2\) in $CuSO_4$ to \(0\) as free metal, meaning it gains electrons and is reduced, not oxidized. In redox reactions, tracking oxidation state changes for each element is the reliable method for identifying which species is oxidized and which is reduced.

The balanced equation \(4Al + 3O_2 \to 2Al_2O_3\) gives 4 aluminum atoms and 6 oxygen atoms on each side, satisfying conservation of mass. The choice "\(2, 3, 2\)" is incorrect because it would provide only 2 aluminum atoms on the reactant side but 4 aluminum atoms on the product side, an imbalance. Working through multi-atom balancing problems like this reinforces checking every element individually rather than assuming a simple ratio.

By conservation of mass, the total mass of products must equal the 50.0 g of reactant, so the mass of carbon dioxide released is \(50.0 - 28.0 = 22.0\text{ g}\). The choice "\(50.0\text{ g}\)" is incorrect because it ignores the mass of the solid calcium oxide that remained behind rather than escaping as gas. This calculation demonstrates how conservation of mass allows unknown product masses to be determined even when a gas escapes and cannot be directly weighed.

Copper cannot displace iron from $FeSO_4$ because copper is less reactive than iron, and a single replacement reaction only proceeds when the free element is more reactive than the element it is displacing. The choice "$Fe + CuSO_4 \to FeSO_4 + Cu$" is incorrect as a non-proceeding reaction because iron is more reactive than copper, so this displacement does occur spontaneously. Using an activity series to predict reaction feasibility prevents students from assuming every single replacement combination will actually react.

The double replacement between sodium carbonate and hydrochloric acid initially forms carbonic acid, \(H_2CO_3\), which is unstable and rapidly decomposes into water and carbon dioxide gas, explaining the bubbling. The choice "A precipitate always forms whenever a double replacement reaction occurs" is incorrect because double replacement reactions can instead produce a gas or water as evidence of reaction, not only a precipitate. Recognizing that gas evolution can substitute for precipitate formation broadens a student's ability to identify double replacement reactions in varied scenarios.

The balanced equation is \(3Ca(OH)_2 + 2H_3PO_4 \to Ca_3(PO_4)_2 + 6H_2O\), which requires 6 water molecules to balance the 12 hydrogen and 6 oxygen atoms contributed from the hydroxide and phosphoric acid groups. The choice "\(3\)" is incorrect because it would leave excess hydrogen and oxygen atoms unbalanced on the reactant side compared to the product side. Balancing equations involving multiple polyatomic ions requires tracking each polyatomic group as well as the individual atoms freed during the reaction.

In the open container, atmospheric oxygen reacts with the iron in the steel wool and becomes chemically incorporated into the solid iron oxide product, so the solid's mass increases even though total system mass, including the consumed gas, is conserved. The choice "The sealed flask experiment violates the law of conservation of mass" is incorrect because the sealed flask actually demonstrates conservation of mass perfectly, since no matter enters or leaves the closed system. Comparing open and closed systems teaches students that apparent mass changes are due to unaccounted gas exchange, not a true violation of conservation of mass.

Even though a solid precipitate, $PbI_2$, forms while other species remain in solution, every atom of lead, nitrogen, oxygen, potassium, and iodine present in the reactants is still accounted for among the products, satisfying conservation of mass. The choice "Precipitates do not have measurable mass and can be ignored" is incorrect because solid precipitates absolutely have mass, and that mass must be included when verifying that the equation is balanced. This example shows that conservation of mass applies regardless of whether products are dissolved ions or insoluble solids.

By conservation of mass, the total mass of the reactants, \(28\text{ g} + 6\text{ g} = 34\text{ g}\), must equal the total mass of the product formed, so \(34\text{ g}\) of \(NH_3\) is produced. The choice "\(28\text{ g}\)" is incorrect because it accounts for only the mass of nitrogen consumed and ignores the additional 6 g of hydrogen that also became part of the ammonia product. This problem shows how conservation of mass can be used to find product mass directly from reactant masses without needing molar mass calculations.

The balanced equation is $2KClO_3 \to 2KCl + 3O_2$, which gives 2 potassium atoms, 2 chlorine atoms, and 6 oxygen atoms on each side. The choice "\(1, 1, 3\)" is incorrect because it leaves only 3 oxygen atoms on the reactant side while requiring 6 oxygen atoms on the product side, an imbalance. Decomposition reactions involving oxygen-rich compounds like chlorates often require careful attention to even oxygen atom totals when balancing.

Multiplying every coefficient by 2 converts the fractional coefficient \(\frac{7}{2}\) into the whole number 7, giving the fully balanced equation \(2C_2H_6 + 7O_2 \to 4CO_2 + 6H_2O\) with all whole-number coefficients. The choice "Round \(\frac{7}{2}\) down to \(3\) for simplicity" is incorrect because rounding would change the actual atom count and break the balance that was carefully established. Chemical equations must always be expressed with whole-number coefficients, so fractional intermediates are a normal step that requires this final multiplication.

Decomposition reactions tend to be endothermic because the energy needed to break the bonds in the single reactant compound generally exceeds the energy released when the smaller product bonds form, resulting in net energy absorption. The choice "Decomposition reactions never require any activation energy to proceed" is incorrect because every chemical reaction, including decomposition, requires some activation energy to initiate bond breaking, even if the overall process is endothermic or exothermic. Connecting reaction type to energy trends helps students predict whether a reaction will absorb or release energy based on the relative strength of bonds broken versus formed.