Chemical Equations
Definition of chemical equation: A chemical equation is a short and clear way to show a chemical reaction. It uses chemical formulas and symbols to show what substances are reacting and what new substances are made.
Shorthand description: Instead of writing long sentences, chemical equations help scientists quickly describe how substances change in a reaction using symbols.
Function: The main job of a chemical equation is to show what substances you start with (reactants) and what you end up with (products).
Components of a Chemical Equation
Reactants: These are the original substances that take part in a chemical reaction. They are written on the left-hand side of the equation.
Products: These are the new substances that are made during the reaction. They appear on the right-hand side of the equation.
Arrow (→): This arrow acts like an equal sign. It points from the reactants to the products, showing the direction the reaction goes.
Coefficients: These are numbers written in front of chemical formulas. They tell us how many moles of each substance are involved in the reaction.
Balancing example: In the equation 2Mg + O₂ → 2MgO, the number 2 in front of Mg and MgO makes sure we have the same number of each type of atom on both sides of the equation.
Balancing Chemical Equations
Purpose of balancing: We balance chemical equations to follow the law of conservation of mass. This law says atoms cannot be created or destroyed in a chemical reaction.
Balancing method: To balance an equation, we only change the coefficients. We never change the small numbers in the formulas (called subscripts) because that would change the substance.
Carbon and oxygen reaction: The equation C(s) + O₂(g) → CO₂(g) is already balanced, meaning the number of carbon and oxygen atoms are equal on both sides.
Magnesium and oxygen reaction: In 2Mg + O₂ → 2MgO, we use a coefficient of 2 in front of Mg and MgO to balance the number of magnesium and oxygen atoms.
Zinc and HCl reaction: In the reaction Zn + 2HCl → ZnCl₂ + H₂, we use a 2 in front of HCl to balance the atoms of hydrogen and chlorine.
State Symbols
Purpose of state symbols: State symbols are used in chemical equations to show us the physical state of each substance involved in a reaction. They help us understand whether something is a solid, liquid, gas, or dissolved in water at the time of the reaction.
Symbol meanings: The state symbols are:
- (s) means the substance is a solid
- (l) means the substance is a liquid
- (g) means the substance is a gas
- (aq) means the substance is aqueous, which means it is dissolved in water
Combustion example: In the reaction C(s) + O₂(g) → CO₂(g), the state symbols tell us that carbon is in solid form, oxygen is in gas form, and the carbon dioxide produced is also a gas. This helps us picture what is happening during the combustion.
Acid-metal reaction example: In the reaction Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g), we learn that zinc is a solid, hydrochloric acid is dissolved in water (aqueous), zinc chloride is also aqueous, and hydrogen is a gas released in the reaction.
Qualitative and Quantitative Aspects
Qualitative Aspects
Reactants/products identified: A chemical equation shows what substances are reacting (the reactants) and what substances are formed (the products). This helps us know what materials are involved in the reaction.
State information: The state symbols in the equation tell us what physical state each substance is in—whether it’s a solid (s), liquid (l), gas (g), or aqueous (aq).
Reaction conditions: Sometimes extra symbols or words are added to the equation, such as a triangle (∆) to show that heat is needed, or the name of a catalyst (like MnO₂) to show that a helper substance is required for the reaction to happen.
Copper oxide reaction: For example, in CuO(s) + H₂(g) → Cu(s) + H₂O(g), we see that solid copper(II) oxide reacts with hydrogen gas to form solid copper and water in gas form (water vapour). The state symbols help us see which substances are gases and which are solids in this redox reaction.
Quantitative Aspects
Mole ratios: The numbers in front of the chemical formulas (called coefficients) tell us how many moles of each substance react or are made. This is important for calculating the amounts used or produced.
Mass calculations: If we know the molar masses of the substances, we can use the mole ratios to calculate the actual mass (in grams) of each substance needed or formed. This helps in real-life lab work.
Gas volumes: When gases are involved, the mole ratios also tell us how much space (volume) the gases will take up—assuming the temperature and pressure are the same. This is useful in reactions that produce or use gases.
Hydrogen and oxygen example: In the reaction 2H₂(g) + O₂(g) → 2H₂O(g), we learn that 2 volumes of hydrogen gas combine with 1 volume of oxygen gas to make 2 volumes of water vapour. This is based on the mole ratio and works if all gases are at the same temperature and pressure.
Stoichiometry
Definition of stoichiometry: Stoichiometry is the part of chemistry that deals with measuring and calculating how much of each substance is used or produced in a reaction.
Step 1—write equation: First, you need a balanced chemical equation that shows all the substances involved.
Step 2—convert to moles: Next, if you’re given a mass, change it into moles using the molar mass.
Step 3—use mole ratios: Then use the ratio from the balanced equation to figure out how many moles of the other substance you need to find.
Step 4—convert units: Finally, change your answer from moles into the unit asked for—maybe grams, volume, or number of particles.
Magnesium oxide example: In the reaction 2Mg + O₂ → 2MgO, if you start with a certain mass of magnesium, convert it to moles, use the 2:2 ratio to find moles of MgO, and then convert it back to mass.
Application of stoichiometry: Stoichiometry is useful for scientists and industries because it helps them know exactly how much of each material to use or how much product to expect from a reaction.