Reactant
Definition and meaning of Reactant in chemistry.
A reactant is a starting substance that is consumed during a chemical reaction and converted into one or more products. Reactants are written on the left side of the arrow in a chemical equation. Their bonds break and their atoms rearrange, but no atoms are lost, so the total mass never changes.
In more detail
Chemical equations are read from left to right, like a sentence. Everything on the left of the arrow is a reactant. Everything on the right is a product.
The arrow is not an equals sign; it is read aloud as yields or gives, and it points in the direction the change runs. A plus sign between two formulas just means and. Burning methane in a gas stove is written like this: CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g) Methane and oxygen are the reactants.
Carbon dioxide and water are the products. The letters in parentheses are state symbols, and a complete equation includes them: (s) for solid, (l) for pure liquid, (g) for gas, and (aq) for aqueous, meaning dissolved in water. States matter, because the same chemicals often behave very differently as solids than they do in solution.
Coefficients are not subscripts
This mix-up costs students more points than almost anything else. A coefficient is the full-size number in front of a formula, and it counts whole units. A subscript is the small lowered number inside a formula, and it counts atoms within one unit.
In 2H2O, the coefficient 2 means two water molecules, holding 4 hydrogen atoms and 2 oxygen atoms between them. When you balance an equation you may change coefficients as much as you like. You may never change a subscript, because H2O2 is hydrogen peroxide, a different substance with different chemistry.
Why the mass always balances
Reactants do not vanish. Their bonds break and their atoms get reassembled, so every atom present at the start is still present at the end. Check the methane equation: 1 carbon, 4 hydrogens, and 4 oxygens on the left, and 1 carbon, 4 hydrogens, and 4 oxygens on the right.
Masses balance for the same reason. Heating limestone to about 900 degrees C drives off carbon dioxide and leaves lime behind: CaCO3(s) -> CaO(s) + CO2(g). The reactant has a molar mass of 100.09 g/mol.
The products are calcium oxide at 56.08 g/mol and carbon dioxide at 44.01 g/mol, and 56.08 + 44.01 = 100.09. Decompose 100.09 g of limestone and you recover 56.08 g of solid lime plus 44.01 g of gas. Nothing was destroyed; part of the mass simply left as a gas, which is why open containers appear to lose weight.
Limiting and excess reactants
Reactants are hardly ever mixed in the exact ratio an equation calls for. The one that runs out first is the limiting reactant, and it fixes the maximum amount of product you can make. Whatever remains when the reaction stops is in excess.
Try ammonia synthesis, N2 + 3H2 -> 2NH3, starting with 28.0 g of nitrogen and 4.00 g of hydrogen. In moles that is 28.0 / 28.02 = 1.00 mol N2 and 4.00 / 2.02 = 1.98 mol H2. One mole of nitrogen would need 3.00 mol of hydrogen, and only 1.98 mol is on hand, so hydrogen is limiting.
That hydrogen produces 1.98 x 2/3 = 1.32 mol of ammonia, which is 22.5 g. It consumes 1.98 / 3 = 0.660 mol of nitrogen, or 18.5 g, leaving 9.5 g of nitrogen unreacted. Check the books: 22.5 g of ammonia plus 9.5 g of leftover nitrogen is 32.0 g, exactly what went in.
Things in the mixture that are not reactants
A catalyst may be consumed in one step of a reaction and regenerated in a later one, so its amount is unchanged once the dust settles. That makes it a helper rather than a reactant, and it is normally written above the arrow instead of beside the starting materials.
Spectator ions are the other trap. Mixing silver nitrate and sodium chloride solutions gives AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq). Only the silver and chloride ions actually change, joining to form a solid.
Sodium and nitrate ions drift through the beaker untouched, so they are dropped from the net ionic equation, Ag+(aq) + Cl-(aq) -> AgCl(s). In that stripped-down version the reactants are just those two ions.
Conditions, and reactions that run both ways
Anything written above or below the arrow is a condition rather than an ingredient: a temperature, a pressure, a catalyst formula, a solvent, or the symbol Delta , which simply means heat is applied. Reactant is a role, not a permanent label. A double arrow ( ) marks a reversible reaction in which products turn back into starting materials.
Read that reaction backward and the roles swap: in the reverse of N2 + 3H2 2NH3, ammonia is the reactant. At equilibrium both directions run at the same rate, so each substance is being consumed and made at once. The same swap shows up in multi-step mechanisms, where the product of step one is the reactant of step two.
Key facts
| Field | General Chemistry |
|---|---|
| Chemical role | Starting substance consumed during a reaction |
| Position in an equation | Left of the arrow, separated by plus signs |
| Arrow symbols | -> is read as yields; marks a reversible reaction |
| State symbols | (s) solid, (l) liquid, (g) gas, (aq) dissolved in water |
| Coefficient vs subscript | Coefficients count whole units; subscripts count atoms inside one unit |
| Conservation principle | Total mass of reactants equals total mass of products |
| Limiting reactant | The reactant that runs out first and caps the yield |
| Not reactants | Catalysts (written above the arrow) and spectator ions |
Drop a spoonful of baking soda into vinegar and the foam is proof that a reaction happened. The reactants are sodium bicarbonate, NaHCO3(s), and acetic acid, CH3COOH(aq), which is the active ingredient in vinegar. They sit on the left of the arrow. The products are sodium acetate, water, and carbon dioxide gas. Weigh an open cup before and after and it seems lighter, but only because the carbon dioxide escaped. Seal the container and the mass does not change at all.
Frequently asked questions
What is the difference between a reactant and a product?
Reactants are the substances you start with, and they are used up as the reaction runs. Products are the new substances that form. In a written equation the reactants sit to the left of the arrow and the products to the right, and the arrow itself is read as yields.
How do you identify the reactants in a chemical equation?
Look to the left of the arrow. Every formula before the arrow is a reactant, and plus signs separate one from the next. Anything written above or below the arrow, such as a catalyst, a temperature, or the heat symbol Delta , is a reaction condition rather than a reactant.
What is a limiting reactant and how do you find it?
The limiting reactant is the one that runs out first, so it caps how much product can form. To find it, convert each reactant's mass to moles, then divide each mole amount by that substance's coefficient in the balanced equation. The smallest result marks the limiting reactant, and the others are in excess.
Is a catalyst a reactant?
No. A catalyst speeds a reaction up by offering a lower-energy pathway, and although it can be consumed partway through, it is regenerated before the reaction ends. Its total amount is the same at the start and the finish, so it is written above the arrow instead of on the reactant side.
Do the reactants weigh more than the products?
No. The law of conservation of mass means the total mass of the reactants equals the total mass of the products. Start with 10 g of reactants and you finish with 10 g of products. A reaction in an open container can look like it lost mass, but that only happens when a gas escapes.
What is the difference between a coefficient and a subscript?
A coefficient is the large number in front of a formula, and it multiplies the entire unit. A subscript is the small number inside a formula, and it counts atoms of one element within that unit. You change coefficients to balance an equation. Changing a subscript changes the substance itself.
Are reactants and reagents the same thing?
In everyday lab talk they are used interchangeably, and limiting reagent means the same thing as limiting reactant. Strictly speaking, a reactant is any substance consumed in the reaction, while reagent usually describes a substance added to bring about or test for a reaction, often deliberately in excess.
Can a product turn back into a reactant?
Yes. A double arrow ( ) marks a reversible reaction in which products re-form the starting materials, so the labels depend on which direction you read. The same swap happens in a multi-step mechanism, where an intermediate is the product of one step and the reactant of the next.