Chemistry OCR - Module 1 Section 1 Atoms and Reactions

What is an atom made up of? What are their relative masses and charges?

1. Proton - 1 - +1

2. Neutron - 1 - 0

3. Electron - 1/2000 - -1

What are ions?

Ions are atoms with a different number of electrons to protons.

Negative ions have more electrons than protons

Positive ions have fewer electrons than protons

What are isotopes? (3)

1. Isotopes are atoms of the same element with different numbers of neutrons.

2. Isotopes have the same chemical properties as it is the electronic configuration that impacts chemical properties.

3. Isotopes of an element do have slightly different physical properties though, such as different densities, rates of diffusion. This is because physics properties tend to depend more on the mass of the atom.

What were the different models of the atom and how did they change? Up to Rutherford

1. Greeks thought matter was made from indivisible particles that were solid spheres.

2. JJ Thomson suggested the Plum Pudding model after measuring the mass and charge of atoms suggested they must contain smaller negatively charged particles called electrons.

3. Rutherford showed that the plum pudding model was wrong by shooting alpha particles as a thin gold sheet, expecting most particles to go straight through, they were deflected more than expect and some bounced back. This suggested that the atom was a tiny, positively charged nucleus at the centre, surrounded by a cloud of electrons and is mostly empty space.

4. These models changed due to new evidence to accept or reject particular models.

What was the Bohr Model?

1. Electrons can only exist in fixed orbits, or shells, and not anywhere in between.

2. Each shell has a fixed energy

3. When an electron moves between shells, electromagnetic radiation is emitted or absorbed.

4. Because the energy of shells is fixed, the radiation will have a fixed frequency.

How are atom models used?

Different models are used as different models best explain different circumstances.

What is relative masses?

1. The relative atomic mass, A_r, is the weighted mean mass of an atom of an element, compared to 1/12th of the mass of an atom of carbon 12.

2. Relative isotopic mass is the mass of an atom of an isotope, compared with 1/12th of the mass of an atom of carbon 12.

What is relative molecular mass?

The relative molecular mass (or relative formula mass), M_r, is the average mass of a molecule or formula unit, compared to 1/12th of the mass of an atom of carbon 12.

What is mass spectrometry?

Mass spectra are produced my mass spectrometers - devices which are used to find out what samples are made up of by measuring the masses of their components.

They can show things such as the relative isotopic masses and abundances of different elements. The y-axis gives the abundance of ions, often as a percentage. For an element, the height of each peak gives the relative isotopic abundance. The x-axis units are given as a ‘mass/charge’ ratio. Since the charge on the ions is mostly +1, you can often assume the x-axis is simply the relative isotopic mass.

How do you calculate relative atomic mass from a graph?

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

2. Divide by the sum of of the sum of isotopic abundances.

What is a mol and what is the equation used to find it?

1. The number of particles in one mole is 6.02×10²³, N_A. The number is the Avogadro constant.

2. Number of moles = (No. of particles)/(No. of particles in a mole)

3. Number of moles = (mass of substance)/(molar mass)

What does amount of substance mean?

The number of particles

What is molar mass?

1. It is the mass of one mole of something - Molar mass is just the same as the relative molecular mass.

2. g mol^-1

What is molar gas volume?

1. The space that one mole of a gas occupies at a certain temperature and pressure is known as the molar gas volume.

2. It has units of dm³ mol^-1

3. At room temperature and pressure, this happens to be 24dm³ mol^-1.

4. Number of moles = (volume in dm³)/( molar gas volume) r.t.p 24

What is the ideal gas equation?

1. pV = nRT

2. p = pressure (Pa), V = volume (m³), n = number of moles, R = 8.314 J K^-1 mol^-1, T = temperature (K)

3. The ideal gas equation lets you find the number of moles in a certain volume at any temperature and pressure.

What is the empirical formula?

The empirical formula gives the smallest whole number ratio of atoms of each element in a compound.

What is the molecular formula?

Gives the actual numbers of atoms of each type of element in a molecule.

What is an ionic equation?

Ionic equations only show the reacting particles.

What is reaction stoichiometry?

The reaction stoichiometry tells you the rations of reactants to products.

When are ions made?

When electrons are transferred from one atom to another.

What are the different ions and their formula that need to be remembered?(7)

1. Nitrate - NO₃^-

2. Carbonate - CO₃^2-

3. Sulfate - SO₄^2-

4. Hydroxide - OH^-

5. Ammonium - NH₄⁺

6. Zinc Ion - Zn²⁺

7. Silver Ion - Ag⁺

What are the properties of salts in ionic compounds in the context of hydrated and anhydrous?

1. All solid salta consist of a lattice of positive and negative ions. In some slats, water molecules are incorporated in the lattice too.

2. The water in a lattice is called water of crystallisation. A solid salt containing water of crystallisation is hydrated. A salt is anhydrous if it doesnt contain water of crystallisation.

What are acids and bases?

1. Acids are proton donors. When mixed with water, all acids release hydrogen ions, these are just protons, but you never get them by themselves in water, they’re always combined with H₂O to form hydroxonium ions.

2. Bases do the opposite, they’re proton acceptors and want to grab H⁺ ions. Bases that are soluble in water are known as alkalis. They release OH^- ions in solution.

What are the names and formulae of the common acids? (4)

1. HCl - Hydrochloric acid

2. H₂SO₄ - Sulfuric acid

3. HNO₃ - Nitric Acid

4. CH₃COOH - Ethanoic Acid

What are the common bases?(3)

1. NaOH - Sodium hydroxide

2. KOH - Potassium hydroxide

3. NH₃ - Ammonia

The reaction between acids and water, and bases and water is …

Reversible, so at any one point in time, both the forwards and backwards reactions will be happening.

What is a strong acid?

e.g. HCl, very little of the reverse reaction happens, so nearly all of the acid will dissociate (or ionise) in water, and nearly all the H⁺ ions are released.

What is a strong base?

e.g. NaOH, the forward reaction is favoured, so nearly all the base dissociate in water and lots of OH^- ions released.

What is a weak acid?

e.g. CH₃COOH, the backwards reaction is favoured, so only a small amount of the acid will dissociate in water and only a few H⁺ ions are released.

What is a weak base?

e.g. NH₃, ionise only slightly in water. The backwards reaction is favoured so only a small amount of the base dissociates and only a few OH^- ions are released.

What is neutralisation?

1. When acids and bases neutralise each other.

2. In this reaction a salt and water are produced.

3. It is the hydrogen ions and the hydroxide ions that combine to form water.

4. You get a salt when the hydrogen ions in the acid are replaced by metal ions or ammonium (NH₄⁺) ions form the alkali.

5. Different acids produce different salts:

   1. Sulfuric acid produces salts called sulfates

   2. hydrochloric acid produces chlorides

   3. nitric acid produces nitrates.

How is ammonia an alkali?

Ammonia is a bit of an exception as it doesn’t directly produce hydroxide ions, but aqueous ammonia is still an alkali. This is because the reaction between ammonia and water produces hydroxide ions. Ammonia accepts a hydrogen ion from water molecules forming an ammonium ion, and a hydroxide ion. In this way ammonia can neutralise acids.

What are the common bases that react with acids and what are their general equations? (4)

1. Metals

• Metal + Acid → Metal Salt + Hydrogen

2. Metal Oxides

• Metal Oxide + Acid → Salt + Water

3. Metal Hydroxide

• Metal Hydroxide + Acid → Salt + Water

4. Metal Carbonate

• Metal Carbonate + Acid → Metal Salt + Carbon Dioxide + Water

When ammonia reacts with acids what does it create?

Ammonia salts

What is the process of carrying out a titration?

1. Measure out some alkali using a pipette and put it in a flask along with some indicator such as phenolphthalein.

2. Do a rough titration to get an idea where the end point is (the point where the alkali is exactly neutralised and the indicator changes colour). To do this, take an initial reading to see how much acid is in the burette to start off with. Then, add the acid to the alkali, giving the flask a regular swirl. Stop when your indicator shows a permanent colour change (end point). Record the fina reading from your burette.

3. Now do an accurate titration. Run the acid in to within 2cm³ of the end point, then add the acid dropwise. If you don’t notice exactly when the solution changed colour you’ve overshot and your result will not be accurate.

4. Work out the amount of acid used to neutralise the alkali. This is just the final reading minus the initial reading. This volume is known as the titre.

5. It’s best to repeat the titration a few times, making sure you get a similar answer each time - your readings should be within 0.1cm³ of each other. Then calculate a mean, ignoring any anomalous results. Remember to wash out the conical flask between each titration to remove any acid or alkali left in in.

6. Take readings from the bottom of the meniscus.

What are the different indicators that can be used for a titration?

Indicators are used that change colour over very small pH range.

1. Methyl orange - turns yellow to red when adding acid to alkali

2. Phenolphthalein - turns pink to colourless when adding acid to alkali.

How is a standard solution created?

1. Using a precise balance, carefully weigh out the required mass of solid onto a watch glass.

2. Transfer this solid to a beaker. Use some water to wash any bits of solid from the watch glass into the beaker.

3. Add water to the beaker to completely dissolve the solid. Use a glass rod to stir the solution to help the solid dissolve.

4. Once the solid has dissolved, transfer the solution into a volumetric flask. You’ll need to use a volumetric flask that’s the same size as the volume of solution you want to make up. Rinse the beaker and glass rod with water, transferring this water into the volumetric flask.

5. Use water to fill the volumetric flask up to the graduation line. Use a pipette to add the final few drops to make sure you don’t add too much water and overshoot the graduation line.

6. Put the lid on the flask and turn the flask over a few times to thoroughly mix the solution.

What are the main equations to find the number of moles from concentration?

1. Number of moles = (concentration x volume cm³)/1000

2. Number of moles = concentration x volume dm³

What is an acid that donates more than one proton called and how does this impact neutralisation?

1. polyprodic acids

2. Compared to monoprotic acid, you’ll need double the number of moles of base to neutralise a diprotic acid. You’ll need to triple the number of moles of base to neutralise a triprotic acid compared to a monoprotic acid.

What is the theoretical and actual yield of a reaction? What is the equation that links these two?

1. Theoretical - The mass of a product that should be made in a reaction if no chemicals are lost in the procedure.

2. Actual - The actual mass of product is always less than the theoretical yield. Some chemicals are always ‘lost’.

3. Percentage yield = ((actual yield)/(theoretical yield)) x 100

What is atom economy?

1. Is a measure of the proportion of reactant atoms that become part of the desired product (rather than by-products) in the balanced chemical equation.

2. % atom economy = ((molecular mass of desired product)/(sum of molecular masses of all products)) x 100

What is the atom economy in an addition reaction and substitution reaction?

1. Addition - The reactants combine to form a single products so 100%

2. Substitution - A substitution reaction is one where some atoms form one reactant are swapped with atoms from another reactant. This type of reaction always results in at least two products - the desired product and at least one by-product.

Why is it important that reactions are sustainable?

1. A low atom economy means there is lots of waste. It costs money to separate the desired product from the waste products and more money to dispose of the waste products safely so they do not harm the environment.

2. Reactant chemicals are usually costly. It is a waste of money if a high proportion of them end up as useless products.

3. Reactions with low atom economies are less sustainable. Many raw materials are in limited supply.

4. Reactions conditions with high energy demands, e.g. high temperatures and high pressures, cost a lot to maintain. However temperatures and pressures are cheaper to run and better for the environment.

5. Raw materials that come from renewable sources, e.g. plants, enzymes, are better for the environment than materials from non-renewable sources, e.g. crude oil, coal.