KDA Chemistry: Module 2 - Atoms and Reactions

2.1.1-2.1.5

__2.1.1-2__ Define isotope

Atoms of the same element with different number of neutrons

Why does different isotopes of the same element react in the same way?

* Neutrons have no impact on the chemical reactivity
* same number of electrons in outer shell and same electron configuration

Define the term ions

Charged particles that is formed when atoms lose or gain electrons

What is the unit used to measure atomic masses called?

Unified atomic mass unit, u

Define relative atomic mass (Ar)

Weighted mean mass of an atom of an element compared to 1/12th of the mass of an atom of carbon-12

How do you calculate the relative atomic mass from its isotopic abundance?

1. Multiply each relative isotopic mass by its percentage isotopic abundance, and add up the results
2. Divide by 100

Define relative isotopic mass

The mass of an atom of an isotope compared with 1/12th of the mass of an atom of carbon-12

Same as the mass number

What two assumptions are made when calculating mass number?

* contribution of the electron is neglected
* Mass of both proton and neutron is taken as 1.0 u

Define the relative molecular mass

Average mass of a molecule or formula unit, compared to 1/12th of the mass of an atom of carbon-12

What are the uses of mass spectrometry? (3)

* Identify unknown compounds
* Find relative abundance of each isotope of an element
* Determine structural information

How does a mass spectrometer work?

* The sample is made into positive ions
* They pass through the apparatus and are separated according to mass to charge ratio
* A computer analyses the data and produces mass spectrum

A sample of rubidium was analysed in a mass spectrometer to produce the mass spectrum.

Calculate the relative atomic mass of this rubidium sample. Give your answer to three significant figures

* Use the spectra data to find the percentage abundance of each isotope
* The abundance Rb-85 is 72% and for Rb-87 is 28%
* Calculate the total mass for the Rb atoms
* (72 x 85) + (28 x 87) = 8556
* Calculate the average mass of 1 mole of atoms
* 8556/100 = 85.56

How is the group number related to the number of electrons?

The group number is equal to the number of electrons in the outer shell

Which 4 elements don’t tend to form ions and why?

Elements: Beryllium, boron, carbon and silicon

Requires a lot of energy to transfer outer shell electrons

What is the charge of an ammonium ion?

\+1 → NH4 +

What is the charge of a hydroxide ion?

\-1 → OH-

What is the charge of a nitrate ion?

\-1 → NO3-

What is the charge of a carbonate ion?

\-2 → CO3 2-

what is the charge of a sulfate ion?

\-2 → SO4 2-

Define empirical formula

Simplest whole number ratio of atoms of each element present in a compound

A empirical formula of CH2 and a relative molecular mass of 70. What is its molecular formula?

* Find the empirical formula mass of CH2 (C=12, H=1)
* 12+1+1 = 14
* Work out the number of CH2 units in a molecule
* 70/14 = 5
* Work out the molecular formula by multiplying the empirical formula by 5
* 5 x Ch2 = C5H10

Define molecular formula

The actual formula of each type of element in a molecule

What is the molecular formula mass of NaCl?

Atomic mass - Na = 23 and Cl = 35.5

23 + 35.5 = 58.5

__2.1.3 -__ What is the unit used to measure amount of substance?

Mole

How do you find out the number of moles from the number of atoms of molecules

Number of moles = Number of particles you have/ Avogadro’s constant

Calculate the number of barium ions in 1.50g of barium oxide

Give your answer in standard form and to three significant figure

Find moles of BaO:

1\.5/(137.3+16) = 0.01

Use avogadro’s constant to find number of atoms:

0\.01 x 6.02 x 10^23

What does the Avogadro constant represent?

The number of atoms per mole of the carbon-12 isotope

Define molar mass

The relative molecular mass

How do you calculate moles when mass and molar mass are given?

Moles (mol) = mass (g) / molar mass (g mol -1)

How many moles are in 40 grams of water?

* Work out the Mr of water H20
* 1+1+16 =18
* Calculate the moles of moles using the formula moles = mass/Mr
* 40/18 = 2.2 moles

State Avogadro’s law

\

How much volume does a gas occupy at room temperature and pressure?

24 dm3

Define molar gas volume

The volume per mole of gas molecules

Why do different gas particles occupy the same volume?

The gas particles are very spread out, hence individual difference has no effect

How do you calculate moles when gas volume is given?

Moles (mol) = volume (dm3) / 24

20\.0g of calcium nitrate is heated until it fully decomposes. The gas is collected is cooled to room temperature

2Ca(NO3 )2 (s) → 2CaO(s) + 4NO2 (g) + O2 (g)

Find the volume of nitrogen dioxide evolved

* Calculate the moles of calcium nitrate
* 20/164.1 = 0.122
* Using stoichiometry work out the number of moles of nitrogen dioxide
* The molar ratio between calcium and nitrogen dioxide → 2:4
* ∴ number of moles of nitrogen dioxide = 0.122 x 2 = 0.244
* Calculate the volume of nitrogen dioxide
* 24 x 0.244 = 5.85 dm^3

20\.0g of calcium nitrate is heated until it fully decomposes. The gas is collected is cooled to room temperature

2Ca(NO3 )2 (s) → 2CaO(s) + 4NO2 (g) + O2 (g)

Find the volume of oxygen evolved

* Using stoichiometry work out the number of moles of oxygen
* Calcium nitrogen: oxygen → 2:1
* 0.122/2 = 0.061
* Calculate the volume of oxygen
* 24 x 0.061 = 1.464 dm^3
* Find the total volume of gas evolved
* 1.464 + 5.85 = 7.314 dm^3

What are the ideal ways in which gases behave? (5)

* They are in continuous motion
* No intermolecular forces experienced
* Exert pressure when they collide with each other or container
* No kinetic energy is lost in the collisions
* When temperature increases, kinetic energy of gases also increases

Write down the ideal gas equation

pV = nRT

Pressure (Pa) x Volume (m3) = number of moles (mol) x gas constant (8.314 J k-1 mol-1) x Temperature (k)

How many pascals are in a kilopascal

1000

0℃ is equal to how many kelvin?

273 K

At 25 °C and 100 kPa a gas occupies a volume of 20 dm3 . Calculate the new temperature of the gas if The volume is decreased to 10 dm^3 at constant pressure.

* Calculate the number of moles of this gas
* = 100000 x 0.02 / 8.314 x 298

= 0.807 mol
* Rearrange the ideal gas equation to find the temperature and insert values converting between units where necessary
* = 100000 x 0.1 / 0.807 x 8.314

= 149K

At 25 °C and 100 kPa a gas occupies a volume of 20 dm3 . Calculate the new temperature of the gas if the pressure is decreased to 50 kPa at constant volume

* convert 50 kPa into pascals
* 50 x 1000 = 50000 Pa
* Input the values into the rearranged equation
* T = 50000 x 0.02 / 0.807 x 8.314

= 149 K

What does concentration of a solution mean?

Amount of solute dissolved in 1 dm3 of solvent

If a solution is concentrated it will have a large amount of solute per dm3 of solvent

If a solution is dilute it will have a small amount of solute per dm3 of solvent

How do you calculate moles when concentration and volume are given?

Moles (mol) = concentration (mol dm-3) x volume (dm3)

Find the mass of potassium hydroxide required to prepare 200cm^3 of 0.2 moles per dm^3

* Find the amount of KOH in mol, required in the solution
* 0.2 x 0.2 = 0.04
* Convert moles to grams
* Mr of KOH = 39.1+16+1

= 56.1
* 56.1 x 0.04 = 2.244g

What is a standard solution?

A solution of known concentration

Write down the steps to prepare a standard solution

1. Weigh the solute using the weigh by difference methods
2. In a beaker dissolve the solute using the solvent
3. Pour the solution into a volumetric flask
4. Rinse the beaker using the solution and add it to the flask
5. Add solvent to the flask carefully until it reaches the graduation line
6. Mix the solution thoroughly to ensure complete mixing

What is a species in a chemical reaction?

Any particle that takes part in a reaction

What does the percentage yield mean?

The efficiency of which reactants are converted into products

What are the reasons for not obtaining 100% yield? (5)

* Reaction may be at equilibrium
* The reactants may be impure
* Side reactions could happen
* Reactants or products may be left behind while transferring
* Loss of products during separation and purification

How is percentage yield calculated?

Percentage yield = \[actual amount of product (mol) / theoretical amount of product (mol)\] x 100

1\.00 tonne of ammonia is reacted with carbon dioxide to prepare the fertiliser urea, NH2CONH2

2NH3(g) + CO2 → NH2CONH2 (s) + H20 (l)

1\.35 tonnes of urea are formed

Calculate the percentage yield of urea

Show all your working out

Work out the moles of ammonia:

1/17 = 0.059.. moles

Use answer to calculate moles of urea:

0\.059…/2 = 0.029… moles

Find the theoretical yield of urea:

0\.029…/(14+2+12+16+14+2) = 1.765 tonnes

Find the percentage yield:

1\.35/1.765 = 76.5%

what does atom economy tell us about?

The proportion of desired products compared with all the products formed in the reaction

How is atom economy calculated?

Atom economy =

(Molecular mass of desired product/ Sum of the molecular masses of all products) x 100

Does 100% yield mean 100% atom economy?

No, even if all the reactants are converted into products, not all products will be the required products

Which type of reaction has 100% atom economy?

Addition reactions

Why do companies use reactions with high atom economies?

* Environmental and economic benefits
* low atom economy means:
* Lots of waste produced → costs money to separate desired product from waste and to dispose of it safely
* Less sustainable → raw materials limited ∴ sensible to use them efficiently so they last
* Low temperature and pressure are cheaper to run and better for the environment
* Renewable resources better than non-renewable

Define the term anhydrous

A crystalline compound containing no water

Define the term hydrated

a crystalline compound that contains

Define water of crystallisation

Water molecules that form part of the crystalline structure of a compound

11\.25g of hydrated copper sulphate CuSO4 xH2O, is heated until it loses all of its water. Its new mass is found to be 7.19g. What is the value of x?

* Work out the mass of the water lost
* 11.25 - 7.19 = 4.06
* Calculate the moles of the salt and the water
* Mr of CuSO4 = 159.6
* Moles of CuSO4 = 7.19/159.6

= 0.045
* Mr of H2O = 18
* Moles of H2O = 4.06/18

= 0.226
* Divide the smallest number to find the whole number ratio
* CuSO4 = 0.045/0.045

= 1
* H2O = 0.0226/0.045

= 5

X = 5

__2.1.4__ - When an acid is added to water what ion is released into the solution?

Hydrogen ion → H+

Define neutralisation

* Reaction of an acid with a base
* In aqueous solution the ionic equation for the reaction is:

H⁺(aq) + OH⁻(aq) → H₂O(l)

Define acid

Proton donor

State four acids and their molecular formula

* Hydrochloric acid - HCl
* Sulfuric acid - H₂SO₄
* Nitric acid - HNO₃
* Ethanoic acid - CH₃COOH

Describe the dissociation of a strong acid

Full dissociation in aqueous solution

Describe the dissociation of a weak acid

Partial dissociation in aqueous solution

Define base

A substance which readily accepts H+ ions from an acid

State three bases and their molecular formula

* Sodium hydroxide - NaOH
* Potassium hydroxide - KOH
* Ammonia - NH₃

Which base is used to treat acid ingestion?

Magnesium hydroxide

Describe what an alkali is

Releases OH- ions into aqueous solutions

What are amphoteric substances?

Substances that can act as acids and bases

What is formed when acid reacts with carbonate?

Salt, carbon dioxide and water

What is a salt?

|A compound that is formed when H+ of an ion is replaced by a metal ion or positive ion

What is formed when acids react with metal oxide?

Salt and water

What is formed when acid reacts with alkali?

Salt and water

What is formed when acids react with metal?

Salt and hydrogen

Why are the products same when acid reacts with alkali or metal oxides?

Both alkali and metal oxides are types of bases

How are ammonium salts formed?

When acid reacts with aqueous ammonia

What are hydrated crystals?

A crystalline structure containing water

What does anhydrous crystals mean?

Crystalline form that contains no water?

What does a dot formula indicate?

The amount of water present in a crystalline structure

Write down the method to carry out a titration

1. Using a pipette, measure the volume of a solution
2. Add the solution into a conical flask and add an indicator into it
3. Add the other solution into a burette and record the volume
4. Slowly add the solution in the burette into the conical flask
5. Swirl the mixture continuously until the end point is reached
6. Repeat until concordant results are obtained

What is the colour of methyl orange in an acid, base and at end point?

* Acid = red
* Base = yellow
* End point = orange

__2.1.5__ What is an oxidation number?

The number of electrons an atom uses to bond with any other atom

What is the oxidation number of an uncombined element such as C, H, O2 ?

0

What is the oxidation number of combined oxygen such as in H2O?

\-2

What is the oxidation number of oxygen in peroxides?

\-1

What is the oxidation number of combined hydrogen such as in NH3, H2S?

\+1

What is the oxidation number of combined hydrogen in metal hydrides such as LiH?

\-1

What is the oxidation number of a simple ion?

Charge of the ion

Example:

* Na+ → +1
* Cl- → -1

What is the oxidation number of combined fluorine such as in NaF, CaF2?

\-1

When an element has more than one stable oxidation number how is it indicated?

Written as a roman numeral

What is the oxidation number of Fe in iron (III) chloride?

\+3

What are oxyanions?

Negative ions that have an element along with oxygen

What is the oxidation number of S in SO4 2-?

\+6

Define oxidation in terms of electron transfer and oxidation number

Oxidation is:

* Loss of electrons
* An increase in oxidation number

Define reduction in terms of electron transfer and oxidation number

Reduction is

* Gain of electrons
* A decrease in oxidation number

What is the oxidation number of a metal?

0 - because it is an uncombined element