Chemistry Topic 3

What do chemical reactions and phase changes involve?

They involve energy changes, often observable as temperature changes or light emission.

What must a balanced chemical equation include?

Correct chemical formulas, state symbols (s), (l), (g), (aq), and conservation of mass.

Name different types of chemical reactions.

Single displacement, double displacement, acid-base, combustion, combination, decomposition, and redox reactions.

What is heat, and how is it related to temperature?

Heat is a form of energy; temperature measures the average kinetic energy of particles.

How do endothermic and exothermic reactions relate to energy conservation?

They involve breaking and forming bonds while conserving energy.

What is the difference between exothermic and endothermic reactions?

• Exothermic: Releases heat (ΔH is negative)

• Endothermic: Absorbs heat (ΔH is positive)

What is an enthalpy level diagram?

A diagram showing the relative energy of reactants and products for exo- or endothermic reactions.

How are enthalpy level diagrams and equations used to determine ΔH?

They show the relative stability and energy difference (ΔH = H(products) - H(reactants)).

Why are bond enthalpies important in reactions?

They help explain whether a reaction is endothermic or exothermic based on bond energy changes.

What are the limitations of using average bond enthalpies?

They may not accurately reflect specific molecular environments.

What is the formula to calculate heat change (Q)?

Q = mcΔT

• m = mass,

• c = specific heat capacity,

• ΔT = temperature change.

What is the formula to calculate enthalpy change (ΔH)?

ΔH = H(products) – H(reactants)

What kind of data can be used to determine enthalpy change?

Data from combustion, neutralisation, or aqueous reactions involving heat, mass, specific heat, and temperature.

What is a mole?

A defined quantity equal to Avogadro’s number of particles (6.022 × 10²³).

What is the law of conservation of mass?

Mass is neither created nor destroyed in a chemical reaction.

How does the mole concept relate mass, moles, and molar mass?

Moles = Mass (g) / Molar mass (g/mol)

What can the mole concept be used for?

Calculating moles of substances, number of particles, and molar masses of compounds.

How do you calculate percentage composition?

From relative atomic masses or from the mass of each element in the compound.

How is the empirical formula determined?

From percentage or mass composition by simplifying mole ratios.

How is the molecular formula determined?

By comparing the empirical formula mass with the actual molar mass.

What is a limiting reactant?

The reactant that is fully used up first, limiting the amount of product formed.

What is the difference between experimental and theoretical yield?

• Theoretical yield: Maximum possible amount

• Experimental yield: Actual amount obtained

What is the formula for percentage yield?

Percentage yield=(Theoretical yield/Experimental yield​) ×100