Chemistry GCSE (Unit 2) AQA (C2- 3)

Chemistry GCSE (Unit 2) AQA (C2- 3)

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Pre test- How much?

1. What is the mass number of an atom?

2. What do we call atoms of the same element with different numbers of neutrons?

3. Why do we use relative atomic masses?

4. What is a mole?

5. Ho can we find the percentage of an element in a compound?

6. What is the difference between an empirical formula and a molecular formula?

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Pre test- How much? (cont.)

7. What do balanced equations tell us?

8. How much calcium oxide can we make from 10g of calcium carbonate?

9. What is meant by the yield of a reaction?

10. What do we mean by 'atom economy'?

11. How can we recognise a reversible reaction?

12. What is equilibirum?

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Pre test- How much? (cont.)

13. How is ammonia manufactured?

14. Why is the yield of ammonia so low?

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MASS NUMBERS- key points:

1. The mass of a proton is equal to the mass of a neutron.

2. The mass number of an atom is the total number of protons and neutrons in its nucleus.

3. Isotopes are atoms of the same element with different number of neutrons.

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Mass numbers-

  • Protons and neutrons have equal mass.
  • The relative masses of a proton and a neutron are both one unit.
  • The mass of an electron is v. small compared with a proton or neutron.
  • The mass of an atom is made up almost entirely of its protons and neutrons.
  • The total number of protons and neutrons in an atom is called its 'mass number'. 
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Mass numbers- (cont.)

  • Atoms of the same element all have the same atomic number.
  • The number of protons and electrons in an atom must always be the same.
  • BUT  there can be different numbers of neutrons.
  • Atoms of the same element with different numbers of neutrons are called 'isotopes'.
  • The number of neutrons in an atoms is equal to its mass number minus it's atomic number. 

    KEY WORDS: mass number, isotopes. 

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Mass numbers- CHECK YOURSELF QUESITONS

1. What name do we use for the number of protons and neutrons in an atom?

2. What are isotopes?

3. Calculate the number of protons, neutrons and electrons in these atoms: O (mass no.= 16, atomic no.= 8), F (mass no.= 19, atomic no.= 9)

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MASSES OF ATOMS AND MOLES- key points:

1. Relative atomic masses compare the masses of atoms.

2. The relative atomic mass of an element is an average value for the isotopes of an element.

3. One mole of a substance is its relative formula mass in grams.

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Masses of atoms and moles-

  • Atoms are much too small to weigh and so we use 'relative' atomic masses. These are often shown in periodic tables.
  • We use an atom of C (mass no.= 12, atomic no.= 6) as a standard atom and compare the mass of all other atoms with this.
  • The relative atomic mass of an element is an average value that depends on the isotopes the element contains.

  • HOWEVER when rounded to a whole number it is often the same as the mass number of the main isotope of the element.
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Masses of atoms and moles- (cont.)

  • The relative atomic mass of a substance is found by adding up the relative atomic masses of the atoms in its formula.
  • Worked example- Calculate the relative formula mass of CaCl2-
    Solution= Atomic mass of Ca= 40, atomic mass of Cl= 35.5, so- 40 + (35.5 x 2)= 111
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Masses of atoms and moles- (cont.)

  • The relative formula mass of a substance in grams is called 'one mole' of that substance.
  • Using moles of substances allows us to calculate and weight out in grams- masses of substances with the same number of particles.
  • One mole of sodium atoms contains the same number of atoms as one mole of chlorine atoms.
  • Worked example- What is the mass of one mole of NaOH?
    Solution= Relative atomic mass of Na= 23, relative atomic mass of O= 16, relative atomic mass of H= 1
    SO 23g + 16g+ 1g= 40g
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Masses of atoms and moles- (cont.)

KEY WORDS: relative atomic mass (Ar), relative formula mass (Mr), mole

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Masses of atoms and moles- CHECK YOURSELF QUESTION

1. Calculate the relative formula masses (Mr) of:
a) H2 b) CH4 c) MgCl2

(Ar of H= 1, Ar of C= 12, Ar of Mg= 24, Ar of Cl= 35.5) 

2. What is the mass of one mole of water (H2O)?

3. Atoms of which isotope are used as the standard atoms for relative atomic masses?

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PERCENTAGES AND FORMULAE- key points:

1. The percentage mass of an element in a compound can be calculated from its Ar, and the Mr of the compound.

2. The empirical formula of a compound can be calculated from the percentage composition.

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Percentages and formulae-

  • We can calculate the percentage of any of the elements in a compound from the formula of the compound.
  • Divide the relative atomic mass of the element by the relative formula mass of the compound and multiply the answer by 100 to convert it to a percentage.
  • This can be useful when deciding if a compound is suitable for a particular purpose or to identify a compound. 
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Percentages and formulae- (cont.)

  • Worked example- Find the percentage of carbon in carbon dioxide (Ar of C= 12, Ar of O= 16)

    Solution= Mr of CO2= 12 + (16 x 2)= 44

    So the percentage of carbon= (12 / 44) x 100= 27.3 %

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Percentages and formulae- (cont.)

  • The empirical formula  is the simplest ratio of the atoms or ions in the compound.
  • It is the formula used for ionic compounds, BUT for covalent compounds it is not always the same as the molecular formula.
  • E.G. the formula of ethane is C2H6, but its empirical forumla is CH3.

  • We can calculate the empirical formula of a compound from its percentage composition.
     
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Percentages and formulae- (cont.)

  • Divide the mass of each element in 100g of the compound by its Ar to give the ratio of atoms.

  • Then Convert this to the simplest whole number ratio. 
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Percentages and formulae- (cont.)

  • Worked example- Work out the empirical formula of the hydrocarbon that contains 80 % carbon.

    Solution= 
    100g of hydrocarbon contains 80g of C and 20g of H.

    Number of moles of carbon= 80/ 12= 6.67

    Number of moles of hydrogen= 20/1= 20

    Ratio of atoms is 6.67 C: 20 H
    Simplest ratio is 1 C: 3 H 
    So the empirical formula is CH

    KEYWORDS: empirical formula, molecular formula

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Percentages and formulae- CHECK YOURSELF QUESTIONS

1. What is the percentage by mas of calcium in calcium oxide, CaO?

2. What is the empirical formula of propene, C3H6?

3. A compound of iron and chlorine contains 44% iron by mass. What is its empirical formula?

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EQUATIONS AND CALCULATIONS- key points:

1. Balanced chemical equations can be used to calculate masses of reactants and products.

2. In an equation, 2Cl2 can mean 2 molecules of chlorine or two moles of chlorine molecules.

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Equations and calculations-

  • Chemical equations show the reactants and products of a reaction.
  • When they are balances they show the amounts of atoms, molecules or ions in the reaction. 

  • For example- 2Mg + O2 ---> 2MgO
    - shows that two atoms of magnesium react with on molecule of oxygen to form two magnesium ions and two oxide ions. 
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Equations and calculations- (cont.)

  • Working in relative masses this becomes:

    (2 x Ar of Mg) +  (2 x Ar of O) gives (2 x Mr of MgO) or (2 x 24 + 2 x 16= 2 x 40)

  • If we work in moles, the equations tells us that two moles of magnesium atoms react with one mole of oxygen molecules to produce  two moles of magnesium oxide.
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Equations and calculations- (cont.)

  • This means 48g of magnesium react with 32g of oxygen to give 80g of magnesium oxide (Ar of Mg= 24, Ar of O= 16).
  • If we have a known mass of magnesium (Mg), say 5g, we can work out the mass of the magnesium oxide using moles:
    5g= 5/24 x 40g = 8.33g of MgO 

  • We can also do this by calculating the proportion of the amounts in the equation:
    5g Mg will produce 5 x 80/40 g = 8.33g MgO 
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Equations and calculations- CHECK YOURSELF QUESTIO

1. Balance this equation: H2 + Cl2 ---> HCl

2. Calculate the mass of sodium Chloride that can be made from one mole of sodium in the reaction: 2Na + Cl2 ---> 2NaCl

3. Calculate the mass of copper oxide that can be made from 10g of copper in the reaction: 2Cu + O2 ---> 2CuO

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MAKING AS MUCH AS WE WANT- key points

1. The yield of a reaction compares the amount of product actually made with the maximum amount that could be made.

2. Atom economy measures how much of the starting materials become useful products. 

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Making as much as we want-

  • The yield of a chemical process compares how much you actually make with the maximum amount possible.
  • When you carry out chemical reactions it is not possible to collect the amounts calculated from the chemical equations.
  • Reactions may not go to completion and some product may be lost in the process.
  • The yield is often calculated as a percentage:

    percentage yield=  amount of product collected
          -------------------------------          x100
    maximum amount of product possible 

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Making as much as we want- (cont.)

  • It is also important to consider the amount of the starting materials that ends up in useful products.
  • This is called the 'atom economy' of a process.
  • Atom economy is calculated by finding the m*** of all the atoms in the starting materials- and comparing this with the m*** of the atoms in the useful product. 

  • It is also often worked out as a percentage:

    relative formula m*** of useful product
    percentage atom   -------------------------------------------------------   x100
    economy= relative formula m*** of all products

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Making as much as we want- (cont.)

  • Worked example- Zinc is extracted by heating its oxide with carbon. Carbon monoxide is also produced in the reaction.

    ZnO + C ---> Zn + CO

    Work out the atom economy from the equation shown above. (Ar of Zn= 65Ar of C= 12Ar of O= 16)  

    Solution-

    atom economy= relative formula mass of useful product (Zn)
      ---------------------------------------------------------------- x 100
          relative formula mass of all products (Zn + CO) 
           
           =            65           =  65  = 70%
      --------------------    -----
                               (65 + (12 +16)      93

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Making as much as we want- (cont.)

  • To avoid waste both percentage yield and atom economy should be as high as possible.

    KEY WORDS: yield, atom economy 

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Making as much as we want- CHECK YOURSELF QUESTION

1. Why is it not usually possible to collect the maximum yield from a reaction?

2. 5.2g of potassium chloride, KCl, was made from 5.6g of potassium hydroxide, KOH. What was the percentage yield?

3. Quicklime is produced from limestone by the reaction:
CaCO3 ---> CaO + CO2

   -What is the percentage atom economy of this process? 

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REVERSIBLE REACTIONS- key points:

1. Reversible reactions go in both directions.

2. Reversible reactions can reach equilibrium in closed systems.

3. Changing the conditions can change the amounts of reactants and products.

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Reversible reactions-

  • If the products of a chemical can react to produce the reactants, the reaction can go in both directions.
  • This type of reaction is called a reversible reaction.
  • When there are no products the reaction can only go in the forward direction, but as products build up the reverse reaction can happen.
  • In a closed system nothing can escape - the rates of both forward and backward reactions will become equal.

  • When this happens the system is at EQUILIBRIUM. 
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Reversible reactions- (cont.)

  • If the conditions of the system are changed, the amounts of reactants and products may change.
  • Increasing the concentration of a substance, will increase the rate of the reaction away from that substance. 
  • If the system is open and products can escape the forward reaction will continue to completion.
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Making as much as we want- (cont.)

A REVERSIBLE REACTION

1) A + B -----------------> (Reactants only at start of reaction)

2) A + B ---------> C + D (Rate of ---> much greater than <---- at first)
  <-----

3) A + B ---------> C + D (Rate of <---- increases as C + D build up.
                 <------        Rate of ---->  slows down as reactants get used up)

4) A + B ---------> C + D (Eventually the rates of ---> and <--- are the same)
             <--------- 

KEY WORDS: reversible, closed system, equilibrium

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Making as much as we want- CHECK YOURSELF QUESTION

1. What is a reversible reaction?

2. What can happen to a reversible reaction in a closed system?

3. How can the amounts of reactants and products in a reaction mixture in a closed system be changed?

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MAKING AMMONIA (THE HABER PROCESS)- key points:

1. The Haber process produces ammonia from nitrogen and hydrogen.

2. The reaction uses a high temperature, high pressure and a catalyst to produce a reasonable yield of ammonia in a short time.

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Making ammonia- the Haber process-

  • The Haber process is used to manufacture ammonia.
  • Ammonia can be used to make fertilisers and other chemicals.
  • Nitrogen from the air and hydrogen, which is usually obtained from natural gas, are purified and mixed in the correct proportions.
  • The gases are passed over an iron catalyst at a temperature of about 450 degrees, and a pressure of about 200 atmospheres.
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Making ammonia- the Haber process-

  • The reaction is reversible and so some ammonia break downs into nitrogen and hydrogen.
  • The gases are cooled so the ammonia condenses.
  • The liquid ammonia is removed from the unreacted gases and they are recycled.
  • The yield is less than 20%, but the ammonia is produced quickly and no gases are wasted. 
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Making ammonia- the Haber process- CHECK YOURSELF

1. What are the raw materials used to make ammonia?

2. Write a word equation and a balanced equation for the reaction to make ammonia.

3. Why does the reaction not go to completion?

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C2 3 END OF CHAPTER QUESTIONS:

1. Why do we not include electrons in mass numbers?

2. How many protons, neutrons and electrons are there in an atom on 27/13Al?

3. What is the relative formula mass Na2O?

4. What is mass of one mole of CO2?

5. What is the percentage by mass of carbon in methane, CH4?

6. What is the empirical formula of the compound of iron and oxygen that contains 70% iron?

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C2 3 END OF CHAPTER QUESTIONS:

7. Balance this equation: CH4 + O2 ---> O2 + H2O

8. Calculate the mass of lithium chloride you can make from 2.4g of lithium hydroxide.

9. Calculate the percentage yield if 4.1g of zinc was made from 8.1g of zinc oxide in the reaction: ZnO + H2 ---> Zn + H2

10. What is the percentage atom economy of the reaction to produce copper from copper oxford? 2Cu + C ---> 2Cu + CO2

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C2 3 END OF CHAPTER QUESTIONS:

11. The thermal decomposition of ammonium chloride is a reversible reaction: NH4CL ---> NH3 + HCl
                            <--- 

Explain what this means.

12. What sort of system is needed for an equilbrium?

13. What conditions are used for the Haber process?

14. What happens to the unreacted gases in the Haber process?

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