12.1Chapter 12: Chemical Reactions 3 and Rates

Mass and energy in chemical reactions — Alhussan Chemistry - Stage 9 Revision Notes & Topical Questions

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📚 Alhussan Chemistry - Stage 9 Hub / Topical Revision

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12.1 Mass and energy in chemical reactions — Conservation of Mass and Energy — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define conservation of mass and energy in reactions, explain apparent mass changes, and introduce symbol equations and balancing numbers.A…

Mass is conserved
Methane combustion
Balanced equation
Using conservation
Mass increases
Mass decreases

Section 1 of 7Conservation of mass

Atoms

Atoms in a chemical reaction no atoms are created or destroyed, they are rearranged and join together differently to make new substances.

Mass is conserved

Mass is conserved the total mass of the products equals the total mass of the reactants.

Methane combustion

Methane combustion methane reacts with oxygen to make carbon dioxide and water, with 1 carbon, 4 hydrogen and 4 oxygen atoms before and after.
Bacteria make methane from human waste at a sewage works, and it can fuel buses.

Balanced equation

Balanced equation the number of atoms of each element is the same on both sides.
24 g of magnesium reacting with 16 g of oxygen forms 40 g of magnesium oxide.

Using conservation

Using conservation the mass of one product can be found by subtraction, for example 10.0 g of calcium carbonate giving 5.6 g of calcium oxide must give 4.4 g of carbon dioxide.

Section 2 of 7Apparent mass changes

Apparent mass changes

Mass increases

Mass increases when a solid reacts with a gas from the air, such as a metal burning in oxygen, because the oxygen is added to the solid.

Mass decreases

Mass decreases when a gas escapes from an open container, for example carbon dioxide leaving when a carbonate reacts with acid or is heated.
Mass of gas lost = starting mass - final mass.

Closed system

Closed system if the reaction is carried out in a sealed flask, the total mass is unchanged.

Section 3 of 7Experiments that show conservation

Closed flask

Closed flask a test tube of lead nitrate solution inside a flask of potassium iodide solution is stoppered and weighed, then tipped so they mix.
A yellow precipitate of lead iodide forms but the mass stays the same, because nothing has entered or left.

Open container

Open container marble chips with hydrochloric acid lose mass steadily as carbon dioxide escapes.
A graph of mass against time falls and levels off when the reaction finishes, and the total loss equals the mass of gas produced.

Sealed bottle

Sealed bottle the mass stays constant, but the bottle may burst as pressure builds up, which is a safety hazard.

Section 4 of 7Conservation of energy

Energy is conserved

Energy is conserved the total amount of energy does not change in a chemical reaction, whether it is exothermic or endothermic.

Exothermic

Exothermic chemical energy stored in the reactants is transferred to thermal energy in the surroundings.
In the burning of methane, energy is transferred to the bus engine and in the end to the air.

Endothermic

Endothermic thermal energy from the surroundings is transferred to the chemical store of the products.

Spreading out

Spreading out some energy spreads out into the surroundings and becomes less useful.

Section 5 of 7Symbol equations

Word equation

Word equation methane + oxygen → carbon dioxide + water, with reactants on the left, products on the right and an arrow meaning "reacts to make".

Symbol equation

CH₄ + 2O₂ → CO₂ + 2H₂O shows the chemical formula of each substance.
The formulae are CH₄ for methane, O₂ for oxygen, CO₂ for carbon dioxide and H₂O for water.

Balancing numbers

Balancing numbers numbers that show the relative numbers of particles of reactants and products.

They are written to the left of the formula, on the line and the same size as the letters.

Meaning

Meaning one methane molecule reacts with two oxygen molecules to make one carbon dioxide molecule and two water molecules.
One billion methane molecules react with two billion oxygen molecules to make one billion carbon dioxide and two billion water molecules.

Balancing and mass

Balancing and mass conservation of mass is the reason equations must be balanced, as in 2Mg + O₂ → 2MgO.

Section 6 of 7Key terms

Term	Meaning
Conservation of mass	the total mass of reactants equals the total mass of products.
Closed system	a system where no substances can enter or leave.
Energy transfer	the movement of energy from one store to another.
Symbol equation	an equation showing reactants and products using chemical formulae.
Balancing number	a number in front of a formula showing relative numbers of particles.
Rearranged	joined together in a different way.