Chemical Reactions & Balancing Equations

Balanced Chemical Equation

A balanced chemical equation has the same number of atoms of each element on both sides, in accordance with the Law of Conservation of Mass.

Symbol Equation

A symbol equation uses the formulae of the reactants and products to show what happens in a chemical reaction.

Balancing H2O2 Equation

The missing numbers to balance the equation are: 2H2O2 rightwards arrow 2H2O + O2.

Products

Products are the new substances which are on the right-hand side of the arrow in a chemical equation, formed by the reaction of reactants.

Magnesium and Chlorine Reaction

The symbol equation for the reaction of magnesium with chlorine to form magnesium chloride is: Mg + Cl2 rightwards arrow MgCl2.

Balanced Sodium and Chlorine Equation

The balanced symbol equation for the reaction of sodium with chlorine is: 2Na + Cl2 rightwards arrow 2NaCl.

Balancing N2 and H2 Equation

The missing numbers to balance the equation are: N2 + 3H2 ⇌ 2NH3.

State Symbols

State symbols are used to specify the physical state of reactants and products in a chemical reaction.

Changing Formulae in Balancing

When balancing equations, you should not change any of the formulae.

State Symbols Meanings

(s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous / dissolved in water.

Balancing Mg3N2 Equation

The balanced equation is: Mg3N2 (s) + 6H2O (l) → 3Mg(OH)2 (aq) + 2NH3 (aq).

Thermal Decomposition of Calcium Carbonate

The balanced chemical equation for the thermal decomposition of calcium carbonate is: CaCO3 (s) → CaO(s) + CO2(g).

Balancing CO and NO Equation

The balanced equation is: 2CO (g) + 2NO (g) → 2CO2 (g) + N2 (g).

Formation of Hydrogen Chloride Gas

The balanced chemical equation for the formation of hydrogen chloride gas from its component elements is: H2 (g) + Cl2 (g) → 2HCl (g).

Reacting Masses Calculations

When completing reacting masses calculations, the mass unit (e.g. grams, tonnes) does not affect the calculation.

Mass Unit in Calculations

The mass unit only affects the final answer as it must be given in the appropriate units.

Order of Steps in Reacting Mass Calculation

The correct order of steps to complete a reacting mass calculation is: Write a balanced symbol equation, Calculate relevant molar masses, Calculate the moles of the known chemical, Check the molar ratio in the equation, Determine the moles of the target chemical, Calculate the mass of the target chemical.

Copper Carbonate Decomposition

Copper carbonate decomposes to form copper oxide and carbon dioxide.

Molar ratio of copper carbonate to copper oxide

1 : 1

Mass of copper oxide from 12.35 g of copper carbonate

7.95 g

Molar ratio of chlorine to sodium chloride

1 : 2

Mass of sodium required to form 11.7 g of sodium chloride

4.6 g

Balanced equation for reaction

The masses of reactants and products in a reaction can be used to write a balanced equation for the reaction.

Steps to form a balanced chemical equation

Convert all masses to moles, find the molar ratio of all chemicals, simplify the molar ratio, place these values in front of each chemical in the equation.

Molar ratio of A : C in 3A + B rightwards arrow 2C + 2D

3 : 2

Moles, mass, molar mass equation

moles = mass / molar mass

Moles of magnesium from 6.0 g of magnesium

0.25 moles

Molar ratio of Mg : MgO

1 : 1

Maximum mass of aluminium from 51 tonnes of aluminium oxide

27 tonnes

Why balance a chemical equation before calculations

To ensure that the correct molar ratios are used in the calculations.

Equation for molar gas volume

Molar gas volume = volume / moles

Conditions for standard temperature and pressure

0 oC / 273 K and 100 kPa

Calculate moles using volume and molar gas volume

Moles = volume / molar gas volume

Volume occupied by one mole of gas at standard temperature and pressure

22.7 dm3

Molar Gas Volume at STP

At standard temperature and pressure, one mole of gas occupies 22.7 dm3.

Volume Equation

Volume = moles x molar gas volume.

Volume of 1.5 Moles of Gas at STP

The amount of space, in dm3, that 1.5 moles of gas occupies is 34.05 dm3.

Conversion from dm3 to cm3

To convert volume from dm3 to cm3, you multiply by 1000.

Volume of 300 dm3 in cm3

A 300 dm3 cylinder contains 300 000 cm3 of gas.

Avogadro's Law

Avogadro's Law states that equal amounts of gases occupy the same volume of space at the same temperature and pressure.

Moles in 68.1 dm3 at STP

There are 3 moles of gas in 68.1 dm3.

Volume of 2.5 Moles of Gas at STP

2.5 moles of gas will occupy 56.75 dm3 at RTP.

Volume of Ammonia from Hydrogen

The volume of ammonia produced from 600 cm3 of hydrogen is 400 cm3.

Hydrogen to Ammonia Ratio

hydrogen : ammonia = 3:2 ratio.

Volume of Carbon Dioxide from Oxygen

If 250 cm3 of oxygen gas reacted with propane, 150 cm3 of carbon dioxide is formed.

Oxygen to Carbon Dioxide Ratio

oxygen : carbon dioxide = 5:3 ratio.

Volume of Propane Required for Oxygen

100 cm3 of propane was required to react with 500 cm3 oxygen.

Propane to Oxygen Ratio

propane : oxygen = 1:5 ratio.

Change in Moles of Gas

The change in the number of moles of gas is 9 to 4 OR decreases by 5.

Molar Volume Dependence

The molar volume of a gas is the same for all gases at STP (22.7 dm³ mol⁻¹).

Concentration Definition

Concentration refers to the amount of solid / solute there is in a specific volume of the liquid / solvent.

Moles Equation

Moles = concentration x volume.

Moles in 2.0 dm3 of Solution

Moles = 0.15 x 2.0 = 0.3 moles.

Concentration Equation

The equation for concentration using moles and volume is: concentration = moles / volume.

Concentration of 1.0 Mole in 2.0 dm3

What is the concentration of a solution where 1.0 mole of solute is dissolved in 2.0 dm3 of water?

Concentration

The concentration of a solution where 1.0 mole of solute is dissolved in 2.0 dm3 of water is: Concentration = moles / volume Concentration = 1.0 / 2.0 = 0.5 mol dm-3

Volume calculation

The equation to calculate the volume of a solution using moles and concentration is: Volume = moles / concentration

Volumetric analysis

Volumetric analysis is a process that uses the volume and concentration of one chemical reactant (a standard solution) to determine the concentration of another unknown solution.

Titration

Titration is a technique used in volumetric analysis to determine the concentration of an unknown solution using a standard solution of known concentration.

Volumetric pipette

A volumetric pipette is used to measure precise volumes in a titration.

Indicator in titration

In a titration, a few drops of the indicator are added to the solution in the conical flask before adding the solution from the burette.

Concordant results

Concordant results in titrations are multiple trials that yield very close or identical results, typically within 0.1 cm3.

Standard solution

A standard solution is a solution of accurately known concentration, used as a reference in volumetric analysis.

Concentration formula

The formula for calculating concentration in mol dm-3 is: Concentration (mol dm-3) = moles / volume (dm3)

cm3 to dm3 conversion

In concentration calculations, you always need to convert cm3 to dm3 by dividing by 1000.

Back titration

A back titration is a technique used to find the concentration or amount of an unknown substance indirectly by reacting it with an excess of a known reactant and then titrating the excess.

Monoprotic acid

A monoprotic acid is an acid that can donate one proton (H⁺ ion) per molecule in an aqueous solution.

Limiting reactant

A limiting reactant is the reactant that is completely consumed in a chemical reaction and determines the amount of product formed.

Product formation

The amount of product formed in a reaction is determined by the limiting reactant.

Excess reactant

A reactant is in excess when it is present in an amount greater than necessary to react with the limiting reactant.

Limiting reactant determination steps

The steps to determine the limiting reactant are: Write the balanced equation for the reaction. Calculate the moles of each reactant. Compare the moles & deduce the limiting reactant.

Limiting reactant uniqueness

There can only be one limiting reactant in a reaction.

Excess reactants after reaction

After the reaction is complete, the reactants in excess remain unreacted.

Yield and limiting reactant

The presence of a limiting reactant determines the maximum amount of product that can be formed, regardless of the amounts of other reactants present.

Limiting reactant consumption

The limiting reactant is always completely consumed in a reaction.

Limiting Reactant

The limiting reactant is always completely consumed in a reaction.

Determining Limiting Reactant

An easy way to determine the limiting reactant is to find the moles of each substance, divide by the coefficient in the equation, and the lowest resulting number is the limiting reactant.

Theoretical Yield

Theoretical yield is the maximum amount of product that can be produced in a chemical reaction, calculated based on the limiting reactant.

Percentage Yield Comparison

Percentage yield compares the actual yield to the theoretical yield.

Actual Yield

Actual yield is the recorded amount of product obtained from a chemical reaction.

Determining Actual Yield

The actual yield can be determined by experiment only.

Theoretical Yield Definition

Theoretical yield is the amount of product that would be obtained under perfect practical and chemical conditions.

Reacting Mass Calculation

The type of calculation that lets you determine theoretical yield is a reacting mass calculation.

Percentage Yield Equation

The equation for percentage yield is (actual yield divided by theoretical yield) x 100.

High Percentage Yield

A high percentage yield is desirable for economic reasons.

Percentage Yield Calculation

If the actual yield is 1.50 g and theoretical yield is 2.00g, the percentage yield is (1.50 divided by 2.0) x 100 = 75%.

Common Percentage Yield

A percentage yield of 100% is not common in chemical reactions.

Factors Reducing Percentage Yield

Factors that can reduce percentage yield include incomplete reactions, side reactions, loss during processing, and reversible reactions.

Low Percentage Yield Indication

A low percentage yield indicates that the reaction is inefficient, with significant loss of product or incomplete conversion of reactants.

Improving Percentage Yield

Percentage yield can be improved by optimizing reaction conditions, using catalysts, preventing side reactions, and improving separation and purification techniques.

Atom Economy

Atom economy is a measure of the efficiency of a chemical reaction in terms of how well reactants are utilised to produce useful products.

Information from Atom Economy

Atom economy tells us what percentage of the mass of reactants become useful products.

Atom Economy Example Reaction

The atom economy for N2 (g) + 3H2 (g) ⇌ 2NH3 (g) is 100% as there is only one product.

Sustainability and Atom Economy

The higher the atom economy of a process then the more sustainable that process is.

Atom Economy for CaO Production

The atom economy for the production of CaO in the reaction CaCO3 → CaO + CO2 is 56%.

Increasing Atom Economy

The atom economy of a reaction can be increased by selling the by-products of the reaction.

Atom Economy for Ethene Production

The atom economy for the production of ethene is 100%.

Waste Generation and Atom Economy

Higher atom economy means less waste production.

Importance of Waste Reduction

Waste reduction in chemical processes is important because it minimises environmental impact and is more sustainable.