TOPIC 1 - STOICHIOMETRIC RELATIONSHIPS

What is an element

substances made from one kind of atom

What is a compound

two or more elements chemically combined

Elements chemical reactions

• Elements take part in chemical reactions in which new substances are made

• These processes most often involve an energy change

• In these reactions, atoms combine together in fixed ratios that will give them full outer shells of electrons, producing compounds

What happens to elements / compounds in mixtures

* In a mixture, elements and compounds are mixed with each other, but are **not** chemically combined
* this means that the **components of a mixture** (the original elements / compounds) retain the **same characteristic properties** as when they are in their **pure form**

Homogenous mixture

• A homogeneous mixture has uniform composition and properties throughout

• Therefore, seperate components are not able to be seen

Heterogenous mixture

• A heterogeneous mixture has non-uniform composition, so its properties are not the same throughout

• It is often possible to see the seperate components (e.g bubbles in a soda)

Separating mixtures

• Since the original components of mixtures retain their individual properties - we can often separate them pretty easily

• Image below: mixtures & seperation techniques

Neutralisation reaction

• acid + base → salt + water

• 1 hydrogen from the acid, 1 hydrogen + 1 oxygen from the base to form water

• the other elements form the salt

Complete + incomplete combustion reaction

• Incomplete → substance + O2 → products

• Complete → substance + O2 → CO2 + H2O

What are state changes

• physical changes that are reversible

• these changes do not change the chemical properties or chemical makeup of the substances involved

What does vaporization include

**evaporation** and **boiling**

What does evaporation include

• the change of liquid to gas

• unlike boiling, evaporation occurs only at the surface and takes place at temperatures below the boiling point

Solid → Gas name

sublimation

Gas → Solid name

deposition

Overview of state changes drawing

Draw and explain a graph of the relationship between temperature and energy during state changes

• Between 1 & 2, the particles are vibrating and gaining kinetic energy and the temperature rises

• Between 2 & 3, all the energy goes into breaking bonds – there is no increase in kinetic energy or temperature

• Between 3 & 4, the particles are moving around and gaining in kinetic energy

• Between 4 & 5, the substance is boiling, so bonds are breaking and there is no increase in kinetic energy or temperature

• From 5 & 6, the particles are moving around rapidly and increasing in kinetic energy

Define the avogadro constant

• the number of particles in one mole of a substance

  • applies to atoms, molecules and ions

• the value of the Avogadro constant is 6.02 x 1023 g mol-

What is the relative atomic mass / molar mass

• The relative molecular mass (Mr) is the weighted average mass of a molecule compared to one twelfth the mass of a carbon-12 atom

• Has no units

Draw the moles and particles formula triangle

Draw the moles, mass and molar mass, formula triangle

What is the molecular formula

• the formula that shows the number and type of each atom in a molecule

• e.g: ethanoic acid: C2H4O2

What is the empirical formula

• the simplest whole number ratio of the atoms of each element present in one molecule or formula unit of the compound

• e.g: ethanoic acid: CH2O

How to calculate empirical formula - example: find the empirical formula of a compound that contains 10 g of hydrogen and 80 g of oxygen

How to calculate molecular formula from empirical formula - example: The empirical formula of X is C4H10S and the relative molecular mass of X is 180.42. What is the molecular formula of X? **Relative Atomic Mass**    Carbon: 12.01    Hydrogen: 1.01    Sulfur: 32.07

1. Calculate the total molar mass of the empirical formula
2. Divide the given molecular molar mass of the compound by the molar mass calculated for the empirical formula
3. Multiply each subscript by the whole number that resulted from step 2 - This is now the molecular formula.

\
**ANSWER:** The molecular formula of **X** is **C8H20S2** 

What are the masses of reactants useful for

They are useful to determine exactly how much of the reactants react with each other to prevent waste

Reasons for reactants not forming products

• Other reactions take place simultaneously

• The reaction does not go to completion

• Products are lost during separation and purification

What does the percentage yield show / what is the equation

how much of a particular product you get from the reactants (actual yield) compared to the maximum theoretical amount (theoretical) that you can get

Av

• The actual yield is the number of moles or mass of product obtained experimentally

• The theoretical yield is the number of moles or mass obtained by a reacting mass calculation

Avogadro’s law and gases

• Avogadro’s law allows the mole ratio of reacting gases to be determined from volumes of the gases

• He deduced that equal volumes of gases must contain the same number of molecules

• At standard temperature and pressure(STP) one mole of any gas has a volume of 22.7 dm3

  • The units are normally written as dm3 mol-1(since it is 'per mole')

What are the conditions of STP

• a temperature of 0° C (273 K)

• pressure of 100 kPa

Worked example of gases and avogadro’s law - Example:

\
What is the **total volume of gases remaining** when **70 cm3** of **ammonia** is combusted completely with **50 cm3** of **oxygen** according to the equation shown?

\
**4NH3 (g) + 5O2 (g) → 4NO (g) + 6H2O (l)**

\
**Step 1**: From the equation deduce the molar ratio of the gases, which is NH3 :O2 :NO or 4:5:4 (water is not included as it is in the liquid state)

\
**Step 2**: We can see that oxygen will run out first (the **limiting reactant**) and so 50 cm3 of O2 requires 4/5 x 50 cm3 of NH3 to react = 40 cm3 

\
**Step 3**: Using Avogadro's Law, we can say 40 cm3 of NO will be produced

\
**Step 4**: There will be of 70-40 = 30 cm3 of NH3 left over

\
Therefore **the total remaining volume will be 40 + 30 = 70 cm3 of gases**

Molar gas volume, volume of gas, and moles formula triangle

\

What does the kinetic theory of gases state

that molecules in gases are constantly moving

What are the 5 assumptions that the kinetic theory of gases is making

1. The gas molecules are moving very fast and randomly

2. The molecules hardly have any volume

3. The gas molecules do not attract or repel each other (no intermolecular forces)

4. No kinetic energy is lost when the gas molecules collide with each other (elastic collisions)

5. The temperature of the gas is directly proportional to the average kinetic energy of the molecules - as temperature increases so does kinetic energy

What is the name of gases that follow the kinetic theory

Ideal gases

What does the volume that a gas occupies depend on (2)

• pressure

• temperature

Ideal gas equation

PV = nRT

Why do gases in a container exert a pressure?

as the gas molecules are constantly **colliding** with the **walls** of the container

Describe Boyle’s law

* As volume decreases, pressure increases (at constant temperature)
* this is because it causes the molecules to be **squashed** together which results in more **frequent** **collisions** with the container wall

Describe Charle’s Law

* as volume increases, so does temperature (at constant pressure)
* when a gas is **heated -** the particles gain more **kinetic energy** and undergo more **frequent collisions** with the container walls
* To keep the **pressure constant**, the molecules must get further apart and therefore the **volume increases**

Describe Gay-Lussac’s law

* As temperature increases, so does pressure (at constant volume)
* **Increasing** the **temperature** of the gas causes the molecules to gain more **kinetic energy**
* This means that the particles will move **faster** and **collide** with the container walls more **frequently -** increasing the pressure

What is the equation when you change the conditions of a fixed amount of gas and why?

• For a fixed amount of gas, n and R will be constant, so if you change the conditions of a gas we can ignore n and R in the ideal gas equation

• So the new equation is:

  • (p1v1/t1 = p2v2/t2)

What are standard solutions

solutions which are used for analysis whose concentrations are known precisely

* made as accurately and precisely as possible

Formula triangle for moles, concentration, and volume

Worked example of concentration calculation:

Calculate the volume of hydrochloric acid of concentration 1.0 mol dm-3 that is required to react completely with 2.5 g of calcium carbonate.

\

**Answer:**

\
**Step 1:** Write the balanced equation for the reaction

\
**CaCO3  +  2HCl  →  CaCl2  +  H2O  +  CO2**

\
**Step 2:** Determine the moles of the known substance, calcium carbonate

\
\
\
**Step 3:** Use the balanced equation to deduce the mole ratio of calcium carbonate to hydrochloric acid:

\
1 mol of CaCO3 requires 2 mol of HCl

\
So 0.025 mol of CaCO3 requires 0.050 mol of HCl

\
**Step 4:** Calculate the volume of HCl required

\

What is volumetric analysis (titration)

a process that uses the volume and concentration of one chemical reactant (**a standard solution**) to determine the concentration of another unknown solution

* The technique most commonly used is a **titration**

\

What is the end / equivalence point in a titration

\
The **end point** or **equivalence point** occurs when the two solutions have reacted completely and is shown with the use of an **indicator.**

What are the steps of a titration

• Measure a known volume of solution 1 (with unknown concentration)

• The other solution (solution) (with a known concentration) is placed in the burette

• A few drops of the indicator are added to solution 1

• The tap on the burette is carefully opened and the solution added, portion by portion, to solution 1 until the indicator just changes colour

• Multiple trials are carried out until concordant results are obtained