1. Stoichiometric Relationships Flashcards
What are elements?
Elements are substances made from one kind of atom
What are compounds?
Compounds are made from two or more elements chemically combined
What is a mixture?
In a mixture, elements and compounds are interspersed with each other, but are not chemically combined
What characteristics do components in a mixture have?
This means the components of a mixture retain the same characteristic properties as when they are in their pure form
What is a homogenous mixture?
A homogeneous mixture has uniform composition and properties throughout
What is a heterogeneous mixture?
A heterogeneous mixture has non-uniform composition, so its properties are not the same throughout
How could you differentiate between a homogenous and a heterogenous mixture? (qualitative)
It is often possible to see the separate components in a heterogeneous mixture, but not in a homogeneous mixture
How could you separate air?
fractional distillation - using boiling points
How could you separate sand and salt?
solution and filtration by using solubility in water
How could you separate pigments in food colours?
paper chromatography
How could you separate sulfur and iron?
Use a magnet - magnetism
What happens. to substances in aqueous solutions?
In aqueous solutions, ionic compounds dissociate into their ions
What are spectator ions
The ions that do not take part in the reaction are called spectator ions
What is an ionic equation? What does it show?
An ionic equation shows only the ions or other particles taking part in a reaction, without showing the spectator ions
What types of changes are changes of state?
Changes of state are physical changes that are reversible
Why are state changes not considered chemical changes?
These changes do not change the chemical properties or chemical makeup of the substances involved
WHat 2 state changes fall under the category of vaporisation?
Vaporization includes evaporation and boiling
WHat is the difference between evaporation and boiling?
Evaporation involves the change of liquid to gas, but unlike boiling, evaporation occurs only at the surface and takes place at temperatures below the boiling point
WHen does boiling occur?
Boiling occurs at a specific temperature and takes place when the vapour pressure reaches the external atmospheric pressure
What happens between stages 1&2 on a heating curve?
Between 1 & 2, the particles are vibrating and gaining kinetic energy and the temperature rises
What happens between stages 2&3 on a heating curve?
Between 2 & 3, all the energy goes into breaking bonds – there is no increase in kinetic energy or temperature
What happens between stages 3&4 on a heating curve?
Between 3 & 4, the particles are moving around and gaining in kinetic energy
What happens between stages 4&5 on a heating curve?
Between 4 & 5, the substance is boiling, so bonds are breaking and there is no increase in kinetic energy or temperature
What happens between stages 5&6 on a heating curve?
From 5 & 6, the particles are moving around rapidly and increasing in kinetic energy
What is the relative atomic mass?
The relative atomic mass (Ar) of an element is the weighted average mass of one atom compared to one twelfth the mass of a carbon-12 atom
What is the formula for the relative atomic mass?
Ar = weighted average mass of one atom of an element / 1/12 mass of one atomc of carbon-12
WHat is the relative isotopic mass?
The relative isotopic mass is the mass of a particular atom of an isotope compared to one twelfth the mass of a carbon-12 atom
What is the relative molecular 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
When is the relative formula mass used?
The relative formula mass (Mr) is used for compounds containing ions
It has the same units and is calculated in the same way as the relative molecular mass
What is the molecular formula?
The molecular formula is the formula that shows the number and type of each atom in a molecule
What is the empirical formula?
The empirical formula is the simplest whole number ratio of the atoms of each element present in one molecule or formula unit of the compound
What type of molecules often have different empirical and molecular formulae?
Organic molecules
What type of compound is always an empirical formula?
The formula of an ionic compound is always an empirical formula
what is the limiting reactant?
The reactant which is not in excess is called the limiting reactant
How would you determine which reactant is limiting?
The number of moles of the reactants should be calculated
The ratio of the reactants shown in the equation should be taken into account
What are STP conditions?
a temperature of 0o C (273 K)
pressure of 100 kPa
WHat is the kinetic theory of gases?
The kinetic theory of gases states that molecules in gases are constantly moving
What 5 assumptions does the theory of gases make?
The gas molecules are moving very fast and randomly
The molecules hardly have any volume
The gas molecules do not attract or repel each other (no intermolecular forces)
No kinetic energy is lost when the gas molecules collide with each other (elastic collisions)
The temperature of the gas is directly proportional to the average kinetic energy of the molecules
What does the volume a gas occupies depend on?
Its pressure
Its temperature
What does the ideal gas equation show?
The ideal gas equation shows the relationship between pressure, volume, temperature and number of moles of gas of an ideal gas:
What is the ideal gas equation?
PV = nRT P = pressure (pascals, Pa)
V = volume (m3)
n = number of moles of gas (mol)
R = gas constant (8.31 J K-1 mol-1)
T = temperature (Kelvin, K)
How do gases exert pressure?
Gases in a container exert a pressure as the gas molecules are constantly colliding with the walls of the container
What effect does decreasing the volume have on pressure?
Decreasing the volume (at constant temperature) of the container causes the molecules to be squashed together which results in more frequent collisions with the container wall
The pressure of the gas increases
WHat is the relationship between pressure and volume?
The pressure is therefore inversely proportional to the volume (at constant temperature)
WHat is BOyle’s law - give equation?
Mathematically, we say P ∝ 1/V or PV = a constant
How doesi increasing temperature affect volume?
When a gas is heated (at constant pressure) 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
What is Charle’s law give the equation
The volume is therefore directly proportional to the temperature in Kelvin (at constant pressure)
This is known as Charles’ Law
Mathematically, V ∝ T or V/T = a constant
How does increasing the temperature affect gas pressure?
Increasing the temperature (at constant volume) 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
The pressure of the gas increases
What is the relationship between changing gas pressure and temperature, give equation?
The temperature is therefore directly proportional to the pressure (at constant volume)
Mathematically, we say that P ∝ T or P/T = a constant
What parts of the ideal gas equation can be ignored if there is a fixed amount of gas? What equation is formed?
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
This leads to a very useful expression for problem solving
p1 v1 / t1 = p2 v2 / t2
Where P1, V1 and T1 are the initial conditions of the gas and P2, V2 and T2 are the final conditions
Does the ideal gas equation fit all observations and measurements?
The ideal gas equation does not fit all measurements and observations taken at all conditions with real gases
When does the relationship between pressure, volume and temperature show significant deviation from the ideal gas law?
WHy does this happen?
when the temperature is very low or the pressure is very high
This is because the ideal gas equation is built on the kinetic theory of matter
What assmtpions about volume does the kinetic theory of matter make?
The kinetic theory assumes that the volume the actual gas molecules themselves take up is tiny compared to the volume the gas occupies in a container and can be ignored
How true is the assumption about gases relating to the kinetic theory of matter?
This is broadly true for gases at normal conditions, but becomes increasingly inaccurate at low temperatures and high pressures
At these conditions the gas molecules are very close together, so the fraction of space taken up by the molecules is substantial compared to the total volume
What assumptions does the kinetic theory of matter make about attractive forces?
Another assumption about gases is that when gas molecules are far apart there is very little interaction between the molecules
As the gas molecules become closer to each other intermolecular forces cause attraction between molecules
This reduces the number of collisions with the walls of the container
How true is the assumption made by the kinetic theory of matter relating to the ideal gas equation?
The pressure is less than expected by the ideal gas equation
What are standard solutions?
Chemists routinely prepare solutions needed for analysis, whose concentrations are known precisely
These solutions are termed standard solutions
How are standard solutions made?
They are made as accurately and precisely as possible using three decimal place balances and volumetric flasks to reduce the impact of measurement uncertainties
What is volumetric analysis?
Volumetric analysis is a process that uses the volume and concentration of one chemical reactant (a standard solution) to determine the concentration of another unknown solution
(technique most commonly used is titration)
What apparatus is used during a titration?
The volumes are measured using two precise pieces of equipment, a volumetric or graduated pipette and a burette
How is the end point/ equivalence point shown 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
