The particulate nature of matter

WELCOME TO another CSEC chemistry series. This year, we hope to include more examination-type questions for discussion. Learn those formulae and practise those equations!

QUICK POINTS:

Matter is made up of particles which are in constant, random motion. Spaces, forces of attraction and repulsion exist between the particles. This forms the basis of the particulate nature of matter.

The particles of matter can be atoms, molecules or ions. Diffusion and osmosis provide evidence that matter exists as particles.

Diffusion is the movement of particles from an area of high concentration to an area of low concentration across a concentration gradient.

When heated, particles gain kinetic energy to break the attraction between them and can then move away from each other faster.

QUESTION 1

A student placed some crystals of salt at the bottom of a beaker of distilled water. She left the contents of the beaker to stand for one hour. The diagram below shows her observations. After one hour, all the salt had disappeared but the solution at point X tasted salty.

Use the kinetic particle theory to explain these observations. How would the time taken for the salt to dissolve change if the liquid in the beaker was heated. Give reasons.

ANSWERS

(a). The kinetic particle theory states that matter is made up of particles which are in constant motion. The salt particles were able to mix with the water particles to form a solution. This explains why the solution tasted salty and provides evidence that the salt dissolved in the liquid. The particles diffused through the liquid until they formed a uniform solution.

COMMENT: A uniform solution is formed as the salt particles are distributed evenly throughout.

(b). If the liquid in the beaker was heated, the salt would take a shorter time to disappear; that is, it would dissolve faster. This happens because heating the solution would cause the salt particles to mix or diffuse faster into the liquid.

COMMENT: Heat increases the temperature and gives particles more energy to move. This helps particles to diffuse faster.

QUESTION 2

Smelling salts are sometimes used by athletes as a stimulant and to help people who feel faint to regain consciousness.

Ammonium carbonate is the main ingredient in some smelling salts. Ammonium carbonate breaks down slowly at room temperature to release ammonia gas.

Reaction: Ammonium carbonate - Ammonia + Carbon dioxide + Water

Write a balanced equation to represent this reaction.

A few crystals of ammonium carbonate were put on a bench at the front of a classroom.

(b) Using ideas about moving particles, suggest what would happen to the smell of ammonia in the room.

(c) Calculate the relative formula mass of ammonium carbonate.

Ammonia is used to make fertilisers such as ammonium sulphate.

(d) Write a balanced equation for this reaction.

(e) Identify the elements present in ammonium sulphate and state why it is useful as a fertiliser?

ANSWERS

Reaction: Ammonium carbonate - Ammonia + Carbon dioxide + Water

COMMENT: First step is to determine the formula for ammonium carbonate. Ions present: NH4+ (ammonium ion) and CO32- (carbonate ion).

Note: the charges on the ions are different and must balance out to give an overall charge of zero.

Formula: NH4 CO3 Charge: +1 -2

Two ammonium ions 2 *(+1) are needed to balance -2 charge on carbonate ion.

Formula: (NH4)2CO3. Equation: (NH4)2CO3 (s) (r) 2NH3 (g) + CO2 (g) + H2O (l)

The smell of ammonia would first be noticed at the front of the room, but over time the particles would diffuse from the front to the back of the room. Students at the back would eventually notice the smell of ammonia.

COMMENT: Ammonia is concentrated at the front and would move from an area of high concentration (front) to an area of low concentration (back).

The relative formula mass (RFF) of ammonium carbonate is calculated by adding up all the atomic masses of the elements that make it up.

RFF = 2*14 + 8*1 + 1*12 + 3*16 = 28 + 8 + 12 + 48 = 96 Relative formula mass of (NH4)2CO3 is 96.

COMMENT: Note that the relative formula mass has no units as it is based on a comparison with the carbon-12 standard.

Ammonia is used to make ammonium sulphate. Equation: 2NH3 (g) + H2SO4 (aq) (r) (NH4)2SO4 (aq) Ammonium sulphate contains the elements N, H, and S.

Ammonium sulphate is a useful fertiliser as it provides the elements nitrogen and sulphur, which are important for plant growth.

Francine Taylor-Campbell is an independent contributor. Send questions and comments to kerry-ann.hepburn@gleanerjm.com.