Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

Chemical Reactions

Chemical Changes — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 11 parts Lesson 2 of 12 Unit quiz

📚 AP CHEMISTRY Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

Chemical Changes — AP CHEMISTRY notes: Distinguish physical from chemical change, represent changes with balanced and ionic equations, and relate processes to bonds and intermolecular forces.A physical change alters appearance or state but the internal composition stays the same.…

  • Molecular equation
  • Complete ionic equation
  • Spectator ions
  • Net ionic equation
  • Coefficient
  • Balanced equation

Section 1 of 11Physical changes

Physical changes
A physical change alters appearance or state but the internal composition stays the same.
Changes of state are physical changes because the particles keep their identity.
Forming or separating a mixture is physical, for example distilling salt water so the water is collected and the salt remains.

Section 2 of 11Chemical changes

A chemical change alters the internal composition; atoms rearrange to form new substances.
Indicators include a colour change, a precipitate, or heat or light production.
Example
  • Example nitric acid with a copper penny gives a brown gas and a blue-green solution.
Burning hydrogen in oxygen gives water, which is chemically different from the reactants.

Section 3 of 11Representing changes

1
Balanced equations show the formulae and state symbols of all species: (s), (l), (g) and (aq).
2
Reactants are the starting substances and products are the substances formed.
3
In a physical change the reactants and products are the same substance in different states.
4
Atoms are not created or destroyed, so mass is conserved and each element has the same number of atoms on both sides.
5
Charge is also conserved in ionic equations.

Section 4 of 11Ionic equations

Molecular equation
Molecular equation complete formulae of all species.
Complete ionic equation
Complete ionic equation soluble ionic compounds written as separate ions.
Spectator ions
Spectator ions ions that appear unchanged on both sides.
Net ionic equation
Net ionic equation only the species that actually react.
Example
  • Pb(NO3)2 with KI gives Pb2+(aq) + 2 I−(aq) → PbI2(s).
Strong electrolytes are written as ions,
  • whereas solids, liquids, gases, weak acids, weak bases and molecular compounds are written as intact formulae.

Section 5 of 11Balancing equations

1
Write correct formulae for the reactants and products.
2
Balance atoms in the most complex substances first.
3
Balance atoms that appear as free elements next by adjusting coefficients.
4
If a coefficient is a fraction, multiply all coefficients by the denominator.
5
Check that every element, and the charge for ionic equations, balances.
Only coefficients change, never subscripts.
Example
  • 2 C4H10 + 13 O2 → 8 CO2 + 10 H2O.

Section 6 of 11Physical processes and intermolecular forces

1
During a physical process the composition does not change but the appearance does.
2
Changes of state involve breaking intermolecular interactions such as hydrogen bonds, dipole-dipole forces and dispersion forces.
3
When water boils, hydrogen bonds break but the polar O–H covalent bonds remain.

Section 7 of 11Chemical processes and bonds

1
Chemical processes break and form bonds, so the products have a different internal structure.
2
Bond breaking absorbs energy and bond formation releases energy.
3
In 2H2 + O2 → 2H2O, two H–H bonds and one O=O bond are replaced by four O–H bonds.

Section 8 of 11Dissolving a salt

Dissolving a salt
Mixing salt and water is a physical process because the components can be separated by distillation.
The ionic lattice is broken and ion-dipole attractions form between Na+ and the oxygen of water and between Cl− and the hydrogens.
Because ionic bonds are broken and new interactions form, it can also be argued to be chemical, so classify it with a justification.

Section 9 of 11More from the specification

Matter
  • Everything you can see and feel; it exists as solid, liquid or gas.
Aqueous
  • Dissolved in water.

Section 10 of 11Key terms

TermMeaning
Physical changeChange in state or appearance without change in composition.
Chemical changeChange in which new substances form.
Spectator ionIon unchanged by a reaction.
Net ionic equationEquation showing only the species that react.
CoefficientNumber in front of a formula in a balanced equation.
Balanced equationEquation with equal atoms of each element on both sides.

Section 11 of 11Exam technique

1
Changing subscripts to balance an equation — only coefficients can change
2
Forgetting to balance charge in ionic equations
3
Calling dissolving chemical only because bonds break — justify the classification by what forms
4
Writing insoluble solids or weak electrolytes as ions

Last updated: