Chemistry Lecture – Conservation of Mass & Intro to Energy

Flashcards cover conservation of mass, forms of energy, energy units and conversion, heat capacity, potential-energy diagrams, activation energy, enthalpy, spontaneity, and reaction requirements.

What principle explains why the mass of reactants equals the mass of products in any chemical reaction?

The law of conservation of mass.

If 100 g of wood burns to give 15 g of ash, what is the mass of the other product(s)?

85 g (because 100 g reactants – 15 g ash = 85 g other products).

Burning wood is the reaction of wood with which reactant species?

Oxygen (O₂).

What is the basic definition of energy given in the lecture?

The ability or capacity to do work.

Name the two broad categories of energy based on whether it is stored or in motion.

Potential energy (stored) and kinetic energy (energy of motion).

Which type of energy is stored in chemical bonds?

Potential energy (chemical potential energy).

What common unit of energy is abbreviated "J"?

The joule.

How many joules are in one small calorie (cal)?

4.184 J.

How many small calories equal one food Calorie (Cal)?

1,000 cal (1 Cal = 1 kcal).

Why is the number 4.184 important for water’s specific heat capacity?

Water’s heat capacity is 4.184 J g⁻¹ °C⁻¹, enabling it to moderate Earth’s temperature and support life.

What do we call a fraction that shows two equal magnitudes with different units (e.g., 4.184 J / 1 cal)?

A conversion factor.

On a potential-energy diagram, how is activation energy (Eₐ) calculated?

Eₐ = Potential energy of the activated complex – Potential energy of the reactants.

Write the formula for enthalpy change (ΔH) using a potential-energy diagram.

ΔH = Potential energy of the products – Potential energy of the reactants.

What sign (positive or negative) does ΔH have for an exothermic reaction?

Negative (ΔH < 0).

Describe the relative positions of reactants and products on an energy diagram for an endothermic reaction.

The product line is higher (has more potential energy) than the reactant line.

How does a large activation energy barrier affect reaction rate?

It slows the reaction because more energy must be supplied to reach the activated complex.

What thermodynamic quantity indicates spontaneity, and what sign makes a reaction spontaneous?

Gibbs free energy change (ΔG); a negative ΔG indicates spontaneity.

If ΔG is positive, what does this tell you about the reaction?

The reaction is non-spontaneous under the given conditions.

List the three main conditions needed for a chemical reaction to occur, as mentioned in the lecture.

Sufficient energy, proper molecular orientation, and contact between reactants.

In the example reaction H₂ + F₂ → 2 HF, which bonds must break and which new bonds must form?

Break H–H and F–F bonds; form H–F bonds.