Unit notes and practice questions

Grade 10 Chemistry unit 1: Energy changes and electrochemistry

Everything here is for unit 1 of the Grade 10 Chemistry textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.

5
Cards: 10
Practice questions on unit 1: 2–50
The textbook pages for unit 1

What unit 1 covers

1.Chemical Equations
2.Types of Chemical Reactions
3.Oxidation and Reduction Reactions
4.Molecular and Formula Masses, the Mole
5.Stoichiometry

Short notes for unit 1

The cards filed under this chapter on the Grade 10 Chemistry page. Open one at a time.

Four Basic Types of Chemical ReactionsReference tableCombination · Decomposition · Single displacement · +1

Reaction Type	General Form
Combination	$A + B \rightarrow AB$
Decomposition	$AB \rightarrow A + B$
Single displacement	$A + BC \rightarrow B + AC$
Double displacement	$AB + CD \rightarrow AD + CB$

When you use it

Use when classifying chemical reactions based on how reactants transform into products.

Watch out

In single displacement, a free element only displaces another element if it is more reactive.

Drafted from Grade 10 Chemistry, pages 2-50, then checked twice before it went up

Inverting Oxidizing and Reducing AgentsCommon mistake

When you use it

Use when identifying the substance oxidized, substance reduced, oxidizing agent, and reducing agent in a redox reaction.

Watch out

The substance that loses electrons undergoes oxidation and is the reducing agent. The substance that gains electrons undergoes reduction and is the oxidizing agent.

Drafted from Grade 10 Chemistry, pages 2-50, then checked twice before it went up

Molecular Formula DeterminationFormula

n = \frac{M_{\text{molecular}}}{M_{\text{empirical}}}

n = \frac{M_{\text{molecular}}}{M_{\text{empirical}}}

$n$: Whole number multiplier relating empirical formula to molecular formula $dimensionless$
$M_{\text{molecular}}$: Molar mass of the molecular formula $g\,mol^{-1}$
$M_{\text{empirical}}$: Molar mass of the empirical formula $g\,mol^{-1}$

When you use it

Use when converting an empirical formula to a true molecular formula given the actual molar mass.

Watch out

The multiplier n must be a whole number. Multiply every subscript in the empirical formula by n.

Drafted from Grade 10 Chemistry, pages 2-50, then checked twice before it went up

Percent Composition by MassFormula

\% \text{ composition} = \frac{n \times M_{\text{element}}}{M_{\text{compound}}} \times 100

\% \text{ composition} = \frac{n \times M_{\text{element}}}{M_{\text{compound}}} \times 100

$n$: Number of moles of the element in 1 mole of the compound $dimensionless$
$M_{\text{element}}$: Molar mass of the element $g\,mol^{-1}$
$M_{\text{compound}}$: Molar mass of the compound $g\,mol^{-1}$

When you use it

Use when finding the percentage of a specific element present in a chemical compound.

Watch out

Multiply the molar mass of the element by its subscript subscript n in the formula before dividing.

Drafted from Grade 10 Chemistry, pages 2-50, then checked twice before it went up

Rules for Assigning Oxidation NumbersReference tableUncombined element · Monatomic ion · Group IA, Group IIA · +4

Species or Element	Oxidation Number Rule	Exceptions
Uncombined element	$0$	None
Monatomic ion	Equal to ion charge	None
Group IA, Group IIA	$+1$ for Group IA, $+2$ for Group IIA	None
Oxygen in compounds	$-2$	Peroxides: $-1$ , Superoxides: $-1/2$ , $OF_2$ : $+2$
Hydrogen in compounds	$+1$	Metal hydrides: $-1$
Neutral compound sum	Sum equals $0$	None
Polyatomic ion sum	Sum equals ion charge	None

When you use it

Use when determining the oxidation state of any element in a neutral molecule or polyatomic ion.

Watch out

Do not forget exceptions for oxygen in peroxides like Na2O2 and hydrogen in metal hydrides like NaH.

Drafted from Grade 10 Chemistry, pages 2-50, then checked twice before it went up

Practice questions on unit 1

10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.

Question 1

Which of the following processes is classified as a physical change?

A.Evaporation of liquids
B.Turning of milk into curd
C.Photosynthesis by green plants
D.Rotting of an egg by bacteria

Show the answer and why

Answer: A Evaporation of liquids

Why

Evaporation of liquids is a physical change because no new substance with a new chemical composition is formed. The other three options produce new substances and are chemical changes.

Question 2

A reaction in which two pure substances react directly to form a single substance (represented by the general form A + B → AB) is known as a:

A.Decomposition reaction
B.Combination reaction
C.Single displacement reaction
D.Double displacement reaction

Show the answer and why

Answer: B Combination reaction

Why

A combination reaction (also called synthesis or composition) is a reaction where two elements or compounds combine directly to form a single compound.

Question 3

In terms of electron transfer, how is oxidation defined?

A.The gain of one or more electrons
B.The loss of one or more electrons
C.The sharing of electrons equally
D.No change in electron count

Show the answer and why

Answer: B The loss of one or more electrons

Why

Oxidation is defined as the loss of one or more electrons by an atom. Reduction is defined as the gain of one or more electrons.

Question 4

How many representative particles are contained in exactly one mole of any substance?

A.6.02 × 1022
B.6.02 × 1023
C.1.20 × 1024
D.3.01 × 1023

Show the answer and why

Answer: B 6.02 × 1023

Why

One mole of any substance contains Avogadro's number of particles, which is 6.02 × 10²³ units.

Question 5

A student in Hawassa tests the reactivity of metals by adding dilute hydrochloric acid (HCl) to separate samples of zinc and copper. What will happen?

A.Zinc displaces hydrogen gas, but copper does not react.
B.Copper displaces hydrogen gas, but zinc does not react.
C.Both metals react vigorously with hydrochloric acid.
D.Neither metal reacts with hydrochloric acid.

Show the answer and why

Answer: A Zinc displaces hydrogen gas, but copper does not react.

Why

Zinc is an active metal and displaces hydrogen from dilute acid (Zn + 2HCl → ZnCl₂ + H₂). Copper is less active and does not react with hydrochloric acid.

Question 6

What is the oxidation number of chromium (Cr) in the dichromate ion, Cr₂O₇^2-?

A.+3
B.+5
C.+6
D.+7

Show the answer and why

Answer: C +6

Why

Each oxygen atom has an oxidation state of -2. The total charge is -2: 2(Cr) + 7(-2) = -2. Solving gives 2(Cr) = +12, so Cr = +6.

Question 7

In the chemical reaction Zn + CuSO₄ → ZnSO₄ + Cu, which substance acts as the reducing agent?

A.Cu
B.Zn
C.CuSO4
D.ZnSO4

Show the answer and why

Answer: B Zn

Why

Zinc (Zn) loses electrons and its oxidation number increases from 0 to +2. The substance that is oxidized and supplies electrons is the reducing agent.

Question 8

Which of the following equations represents a double displacement reaction?

A.2Na + Cl2 → 2NaCl
B.2HgO → 2Hg + O2
C.Zn + 2HCl → ZnCl2 + H2
D.AgNO3 + NaCl → AgCl + NaNO3

Show the answer and why

Answer: D AgNO3 + NaCl → AgCl + NaNO3

Why

In AgNO₃ + NaCl → AgCl + NaNO₃, the positive and negative ions of the two reacting compounds exchange partners to form two new compounds.

Question 9

What are the correct stoichiometric coefficients (a, b, c, d) when the equation aPCl₅ + bH₂O → cH₃PO₄ + dHCl is balanced?

A.1, 4, 1, 5
B.1, 2, 1, 3
C.2, 4, 2, 5
D.1, 3, 1, 4

Show the answer and why

Answer: A 1, 4, 1, 5

Why

Balancing phosphorus gives 1 PCl₅ to 1 H₃PO₄. Balancing chlorine requires 5 HCl. Balancing oxygen and hydrogen requires 4 H₂O. Thus, the balanced equation is PCl₅ + 4H₂O → H₃PO₄ + 5HCl.

Question 10

A compound contains 43.6% phosphorus (P) and 56.4% oxygen (O) by mass. What is its empirical formula? (Atomic masses: P = 31.0 g/mol, O = 16.0 g/mol)

A.PO2
B.P2O3
C.P2O5
D.PO3

Show the answer and why

Answer: C P2O5

Why

Moles of P = 43.6 / 31.0 = 1.41 mol. Moles of O = 56.4 / 16.0 = 3.53 mol. Dividing by 1.41 gives 1.00 P and 2.50 O. Multiplying by 2 gives the whole number ratio P₂O₅.

The app holds 12 questions on this unit, marked as you go. These are the first 10.

The textbook pages for unit 1

Unit 1 runs from page 2 to page 50 of the Ministry of Education edition.

Ministry of Education edition, English, 307 pages

Download the PDF

All Grade 10 Chemistry short notes

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