Download
1 / 45
450 likes | 485 Vues
How many particles?. Scientists often need to know how many particles of a substance they are dealing with. Unfortunately, atoms and molecules are tiny and it is impossible to count them directly. …thirty-four million and one….
E N D
How many particles? Scientists often need to know how many particles of a substance they are dealing with. Unfortunately, atoms and molecules are tiny and it is impossible to count them directly. …thirty-four million and one… Instead of counting, scientists use the relative masses of different atoms and molecules to measure how many are present in a given sample.
How many particles? For example: One carbon atom has a relative atomic mass of 12. The quantity of atoms in 24g of pure graphite can be calculated as 24 = 2 12 One magnesium atom has a relative atomic mass of 24. The quantity of atoms in 24g of pure magnesium can be calculated as 24 = 1 24 So, 24g of carbon contains twice as many atoms as 24g of magnesium, because magnesium atoms are twice as heavy.The unit for these results is called a mole.
What are moles? The term mole is used to describe how many atoms or molecules of a substance are present in a sample. One mole of a substance is equal to its relative atomic mass, or relative formula mass, in grams. relative atomic mass For example, the relative atomic mass of carbon is 12, so one mole of carbon atoms weighs 12g. 24g of pure graphite therefore contains 2 moles of carbon atoms.
How much is in a mole? Scientists have calculated that one mole of any substance contains 602,000,000,000,000, 000,000,000 (6.02 × 1023) particles. One mole of carbon weighs 12g, so 12g of carbon contains 6.02 × 1023 carbon atoms. One mole of sodium weighs 23g, so 23g of sodium contains 6.02 × 1023 carbon atoms. One mole of water weighs 18g, so 18g of water contains 6.02 × 1023 water molecules. 3.01 × 1023 How many carbon atoms are in 6g of carbon?
Avogadro’s number The number of particles in one mole is called Avogadro’s number. It is named after Amedeo Avogadro, an Italian scientist working in the early 19th century. mantle 6.02 × 1023 really is a staggeringly large number of particles. crust marbles If you collected 6.02 × 1023 marbles and spread them over the surface of the Earth, they would form a layer of marbles 50 miles thick!
Calculating moles The number of moles in a sample is calculated using this equation: mass of substance number of moles = molar mass Remember: • mass of substance is measured in grams. • some elements exist as molecules (for example, O2). You will need to use the molecular rather than atomic mass for these. • for compounds, the molar mass is the sum of the relative atomic masses of all the atoms in the formula.
Calculating moles: questions mass of substance number of moles = molar mass 1. How many moles of iron are in 28g of pure iron? 28 Number of moles = = 0.5 moles of iron 56 2. How many moles of CO2 are in 11g of carbon dioxide? Relative molecular mass = 12 + 16 + 16 = 44 11 Number of moles = = 0.25 moles of CO2 44
Using moles in calculations Moles are very useful when reacting substances together. They provide us with a quick way of calculating how many particles of each substance are present. Chemists use moles to calculate: • how much of each reactant they will need • how much of a substance they are likely to make in the end.
Using moles: example 1 If you reacted 56g of iron with excess copper sulfate solution, what mass of copper would you expect to make? Step 1: Write a balanced equation for the reaction: • Fe + CuSO4 Cu + FeSO4 Step 2:Turn the mass of iron into moles of iron: mass 56 moles = = = 1 mole of iron molar mass 56 Step 3: Use the balanced equation to work out how many moles of copper you should make: 1 mole of Fe makes 1 mole of Cu Step 4: Turn moles of copper into mass of copper: mass = moles × RAM = 1 × 63.5 = 63.5g of Cu
Using moles: example 2 What mass of iron oxide would you need to react with excess aluminium to make 1120g of iron? Step 1: Write a balanced equation for the reaction: • Fe2O3 + 2Al Al2O3 + 2Fe Step 2:Turn mass of iron into moles of iron: mass 1,120 moles = = = 20 moles of iron RAM 56 Step 3: Use the balanced equation to work out how many moles of iron oxide you would need: 1 mole of iron oxide makes 2 moles of iron, so 10 molesof iron oxide are needed to make 20 moles of iron. Step 4: Turn moles of iron oxide into mass of iron oxide: mass = moles × RAM = 10 × 160 = 1,600g of Fe2O3
Avogadro’s law Avogadro’s law states that: Equal volumes of gases, at the same temperature and pressure, contain the same number of molecules. This means that one mole of any gas at a given temperature and pressure will always have the same volume. At room temperature and pressure (RTP), one mole of any gas has a volume of 24dm3 (24 litres). What would happen to the volume of a mole of gas if you: • increased the temperature? • increased the pressure?
Gas calculations: example 1 Hydrogen gas reacts with oxygen gas to make water vapour. If 24dm3 of hydrogen gas is burned with excess oxygen, what volume of water vapour is produced? Step 1: Write a balanced equation for the reaction: • 2H2 (g) + O2 (g) 2H2O (g) Step 2: Turn volume of hydrogen into moles of hydrogen: volume 24 moles of H2 = = = 1 mole of H2 24 24 Step 3: Use the balanced equation to work out how many moles of water vapour you should make: 1 mole of hydrogen makes 1 mole of water vapour Step 4: Turn moles of water into volume of water vapour: volume = moles × 24 = 1 × 24 = 24 dm3 of water vapour
Gas calculations: example 2 Nitrogen and hydrogen react together to make ammonia. What is the maximum volume of ammonia that can be made from 2,400cm3 of nitrogen gas? Step 1: Write a balanced equation for the reaction: N2 (g) + 3H2 (g) 2NH3 (g) Step 2: Turn volume of nitrogen into moles of nitrogen: volume moles of N2 = NOTE: To convert cm3 to dm3, divide by 1,000. 24 2.4 = 24 = 0.1 moles of N2
Gas calculations: example 2 continued Step 3: Use the balanced equation to work out how many moles of ammonia you can make: 1 mole of N2 makes 2 moles of NH3, so 0.1 moles of N2 makes 0.2 moles of NH3. Step 4: Turn moles of ammonia into volume of ammonia: volume = moles × 24 = 0.2 × 24 = 4.8dm3 (4,800cm3) of ammonia
Using gases to follow reactions Many chemical reactions produce gases. Scientists can measure how much gas is produced and use it to follow the progress of a reaction. The volume of gas produced can also be converted into moles and used to determine the amount of reactants present at the start of the reaction. Which gas is produced by each of these reactions? Magnesium + hydrochloric acid Calcium carbonate + hydrochloric acid Decomposition of hydrogen peroxide hydrogen carbon dioxide oxygen
Measuring gases in reactions One way of measuring how much gas is produced in a reaction is to use an upturned measuring cylinder filled with water. As the gas bubbles into the measuring cylinder, it displaces the water. This technique works well for gases which are less dense than air and not very soluble in water. What would be the problem with using this method for a gas which is very soluble in water?
Measuring gases in reactions Another method of collecting a gas is to use a gas syringe. This technique works well for all gases. What are the advantages and disadvantages of using a gas syringe rather than a measuring cylinder to collect the gas?
Measuring gases in reactions One other method of measuring how much gas is produced in a reaction is to measure the mass of the reaction mixture. What will happen to the reading on the balance? Why is cotton wool placed in the top of the flask?
Limiting reactants The amount of gas produced in a reaction will depend on thelimiting reactant. This is the reactant that runs out first, bringing the reaction to an end. time (s) volume H2 (cm3) The other reactant(s) are said to be “in excess”. This means that there will be some left after the reaction. 0 0 150 30 300 60 90 450 This table shows hydrogen gas production during a reaction between a small amount of magnesium and excess dilute hydrochloric acid. 120 600 150 700 180 780 210 850 240 895 270 910 After how many seconds was all the magnesium used up? 300 910
Empirical and molecular formulae The molecular formula of a compound tells you how many of each type of atom are present in one molecule. The molecular formula of ethane is C2H6. The empirical formula of a compound gives you the simplest whole number ratio of the types of atoms in one molecule. The empirical formula of ethane is CH3. Ethane: C2H6 Sometimes the empirical and the molecular formulae of a compound are the same, like for water, H2O. Water: H2O
Why are empirical formulae useful? Chemists sometimes make compounds whose identity they are unsure of. Finding the empirical formula of an unknown compound is an important step in identifying it. The empirical formula can often be determined by studying the reactions of the unknown compound. For example, an unknown hydrocarbon can be burned in oxygen to produce carbon dioxide and water vapour. Chemists can measure the volumes of the gases produced by the reaction and use them to work out the empirical formula.
How are empirical formulae found? oxygen out oxygen in unknown compound potassium hydroxide solution calcium chloride The unknown compound is heated in a stream of pure, dry oxygen, reacting to form steam and CO2 gas. The steam is absorbed by a known mass of calcium chloride. The CO2 travels on and reacts with the known mass of potassium hydroxide solution.
How are empirical formulae found? The change in mass of the calcium chloride and the potassium hydroxide can be used to calculate the relative masses of carbon and hydrogen in the original compound. For example: If the calcium chloride increased in mass by 27 grams and the potassium hydroxide solution increased by 66 grams… 27g of H2O was made. H2O = 1 + 1 + 16 = 18. There are 2g of hydrogen in every 18g of water. 2 × 27 = 3g of hydrogen were produced. 18 CO2 = 12 + 16 + 16 = 44. 66g of CO2 was made. There are 12 g of carbon in every 44 g of carbon dioxide. 12 × 66 = 18gof carbon were produced. 44
