Year 8: Chemical Reactions & Equations

• This booklet for Year 8 Spring term covers the topic of Chemical Reactions.

Representing Chemical Reactions Using Word Equations

• A substance is made of particles and can be changed into different substances or mixed to form new ones, known as chemical reactions.
• The particles composing the substance change during a chemical reaction.
• Chemical reactions differ from physical changes because the particles remain the same in physical changes.
• Equations represent chemical changes, similar to math equations.

Features of a Chemical Equation

• Reactants: Substances before the reaction, shown on the left with "+" between multiple reactants.
• Arrow: Separates reactants from products, indicating the reaction's progression.
• Products: Substances made during the reaction, shown on the right with "+" between multiple products.
• The order of multiple reactants or products does not matter as long as they are on the correct side of the arrow.
• Chemical equations relate to recipes in that reactants are similar to ingredients and products are similar to the finished dish.

Reactions as Rearrangements of Atoms

• During chemical reactions New substances form with new particles that differ from the starting substance, known as Reactants.
• During chemical reactions, the arrangement of atoms changes.
• Atoms chemically bonded together create new combinations.
• Bonds between atoms in reactants break, and new bonds form to create new particles.

Representing Reactions Using Formulae

• A chemical formula is a shorthand notation that shows the types and quantity of atoms in a substance.
• Chemical formulas are useful because they show the different types of elements present, indicate how many atoms of each element are present, used to write balanced equations, are quicker to write and are universally understood.
• A chemical formula uses each element's symbol from the periodic table.
• A small number after an element's symbol indicates the number of atoms of that element (e.g., C2 represents two carbon atoms).
• Brackets around specific groups of atoms indicate that the subscript number applies to everything inside the brackets (e.g., Ca(OH)2 contains one calcium atom, two oxygen atoms, and two hydrogen atoms).

Writing Balanced Equations

• The main benefit of using chemical formulae is that you can write balanced symbol equations which tells how much of something is needed and produced.
• Balanced equations must have the same number of atoms for each element on both sides of the arrow.
• You can use chemical formulas to calculate the number of atoms present on each side of the equation.
• Balance equations by placing a coefficient (large number) which applies to everything in that formula, in front of the chemical formula.
• Only add coefficients in front of formulae to balance equations.

Interpreting Formulae

• C represents one carbon atom.
• O2 represents two oxygen atoms in oxygen gas.
• CO2 represents one carbon atom and two oxygen atoms in carbon dioxide.
• Mg(OH)2 represents one magnesium atom, two oxygen atoms, and two hydrogen atoms found in magnesium hydroxide.

Thermal Decomposition

• Thermal means heat.
• Decomposition means "break down."
• Thermal decomposition is a chemical reaction where a compound breaks down when heated.
• A thermal decomposition reaction has only one reactant but two or more products.
• Metal hydrogen carbonates undergo thermal decomposition to form a metal carbonate, carbon dioxide, and water.
• Metal carbonates undergo thermal decomposition to form a metal oxide.
• Thermal decomposition is an endothermic reaction, one that takes in heat which is required to break bonds in the thermal reactant

Combustion

• Combustion is a scientific term for burning and it involves a fuel reacting with oxygen, releasing heat and light.
• Combustion reactions always have two reactants, including oxygen, and produces oxides of present fuel elements
• Combustion requires heat, fuel and oxygen.
• Complete combustion happens with sufficient oxygen and produces oxides.
• Incomplete combustion happens with too little oxygen and will not react completely since all the fuel elements will not be able to fully react.

Hydrocarbons

• Hydrocarbons contain hydrogen and carbon, with complete combustion producing carbon dioxide and water.
• Incomplete combustion produces carbon monoxide or soot (carbon) and water.
• Carbon monoxide detectors can save lives by detecting carbon monoxide and sounding an alarm.

Dangers of Incomplete Combustion

• Carbon monoxide is a colorless and orderless toxic gas causing dizziness, nausea or death.
• Soot is composed of carbon small particles.
• Breathing in soot particles leads to breathing difficulties.
• Too much soot can cause global dimming in the atmosphere.

Acids, Bases, and Alkalis

• Acids taste sour; examples include lemon juice and vinegar.
• Strong acids like hydrochloric, sulfuric, and nitric acid are available.
• Acids need to be diluted
• Bases neutralize acids and bases that dissolve are called alkalis.
• Alkalis feel soapy.
• Indicators are dyes that change color in acidic or alkaline solutions.
• Litmus paper turns red in acidic and blue in alkaline solution but it can't describe the strength.
• Universal indicator shows how acidic or alkaline a solution is using the pH scale.

pH Scale

• The pH scale measures how acidic or alkaline an aqueous solution is, ranging from 0-14.
• Neutral solutions have a pH of 7.
• Acidic solutions have a pH less than 7.
• Alkaline solutions have a pH greater than 7.

Neutralization Reactions

• Anything that neutralizes an acid is called a base.
• Bases include metal oxides, metal carbonates, and metal hydroxides.
• A base that dissolve in water is called an alkali and are usually hydroxides.
• A neutralization reaction occurs when a base or alkali is added to an acid.
• Salt and water form in these reactions and carbon dioxide is released when metal carbonates neutralize acids.
• Reacting an acid with a metal makes a salt and hydrogen.

Revision Techniques for Chemical Reactions

• Revision is important for better understanding and memory.
• Revision involves reviewing and reinforcing information.
• Effective method of revision: Practice questions simulating real exam conditions.
• Practice questions develop problem-solving skills and confidence.
• Recalling from long-term memory embeds knowledge deeply.

Practice Questions Effectively

1. Review content before attempting questions.
2. Use four pens of different colors.
3. Start under exam conditions (silence, no notes, timed).
4. Complete as many questions as possible within the time limit and skip questions that can't be answered.
5. When time is up: switch pen, complete as many questions as able, switch to the next pen when time is up.
6. Maximize memory recall.

• Use your work booklet to improve the answers you are less confident about.
• Help: Switch to a different pen, use the mark scheme to correct possible.
• Make corrections be as accurate as possible.

Marking Well

• Be formative: write in corrections and improvements to answers.
• Read the mark scheme carefully.
• If an answer is not exactly as written, do not mark it correct.
• Write in the correction, double-check guidance.
• AW/OWTTE/IDEA THAT allows using different wording to communicate the same idea.
• DO NOT ALLOW means if an mentioned, cannot award the mark,