C2 revision topics 5 and 6

Transcription

1 C2 revision topics 5 and 6 To understand heat changes in reactions To be able to describe what controls rates of reactions To be able to explain how to change the rate of a reaction

2 Exothermic Gives out heat Hot ex o ther mic

3 Endothermic Takes in heat Cold end o Ther mick

4 Exothermic Reactions Reactions which generate heat are known as exothermic reaction. These reaction gives out heat to its surroundings. These reactions are hot to feel E.g. fire

5 Endothermic Reactions An endothermic reaction takes in heat. It can therefore be said that an endothermic reaction takes up energy These reactions are cold to feel E.g. Ice

6 Reaction Citric Acid + Limestone Citric Acid + sodium hydrogen carbonate Hydrochloric acid + sodium hydroxide Water + ammonium nitrate Temp before 0 C Temp 0 after C Exothermic or endothermic exo endo exo endo

7 What does rate of reaction mean? The speed of different chemical reactions varies hugely. Some reactions are very fast and others are very slow. The speed of a reaction is called the rate of the reaction. Why are some reactions fast? Why are some reactions slow?

8 How do chemical reactions happen? Reactions take place when particles collide with a certain amount of energy. The minimum amount of energy needed for the particles to react is called the activation energy, and is different for each reaction. The rate of a reaction depends on two things: 1. the frequency of collisions between particles 2. the energy with which particles collide

9 How do chemical reactions happen? Chemical reactions occur when particles of reactant collide with enough energy to react.

10 How do chemical reactions happen? Chemical reactions occur when particles of reactant collide with enough energy to react. C What do you think happens if the particles collide but they are BELOW the activation energy? If particles collide with less energy than the activation energy, they will not react. The particles will just bounce off each other.

11 How can we measure the rate of reaction? ways: 1) Measure how fast the products are formed 2) Measure how fast the reactants are used up

12 How can we measure the rate of reaction? Amount of product formed Slower rate of reaction here due to reactants being used up Fast rate of reaction here Slower reaction Draw and label your graph! Time

13 How can we change the rate of reaction? Anything that increases the number of successful collisions between reactant particles will speed up a reaction. There are 4 ways in which the number of successful collisions can be increased. How many can you think of? 1. Temperature 2. Concentration 3. Surface area C 4. Use of a catalyst

14 1. Temperature At higher temperatures molecules move faster. As a result there are more collisions per second and so a faster reaction occurs. Slow molecules are also less likely to lead to a reaction than fast ones. Fewer collisions per second More collisions per second

15 1. Temperature Food spoils because of chemical reactions that occur. Why does food remain usable for so much longer if it is kept in a freezer? The reactions that cause the food to go off will be slower because there will be fewer and softer collisions between molecules at a reduced temperature.

16 1. Temperature investigation You completed the following reaction: Hydrochloric + sodium sodium + sulphur + sulphur + water acid thiosulphate chloride dioxide

17 What Safety must we follow? What must we do to make sure we stay safe during the practical?? If you burn yourself put it straight under cold water Stand up at all times Make sure you are wearing your safety goggles throughout

18 How to set up the equipment

19 2. Concentration Reactions in solution involve dissolved paticles that must collide before reaction is possible. The more crowded (concentrated) the solution, the faster the reaction. Collisions infrequent Collisions frequent

20 3. Concentration investigation You completed the following reaction: Hydrochloric + magnesium magnesium + hydrogen acid chloride

21 2. Concentration

22 3. Surface area The reactions of solids can clearly only take place at the surface of the solid. If we break a solid into smaller pieces we get more area and a faster reaction. Molecules collide with the surface of the solid Extra surface for molecules to collide with.

23 3. Surface area If we grind up a solid to a powder we massively increase the surface area. We therefore massively increase the rate of any reaction Slow Very fast

24 3. Surface area An indigestion tablet fizzes in water but fizzes much faster if it is crushed. Which glass has the crushed tablet? C A B Use the collision theory to explain your answer B

25 3. Surface area investigation You completed the following reaction: Hydrochloric + calcium calcium + carbon + water acid carbonate chloride dioxide

26 3. Surface area investigation You will be completing the following reaction: Hydrochloric + calcium calcium + carbon + water acid carbonate chloride dioxide

27 Foundation questions

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36 Weighing Atoms All atoms have mass so therefore weigh something Relative Atomic Mass (Ar) = mass of atom Relative formula mass (Mr) = mass of compound Can be used to work out how much stuff is made in a reaction

37 Weighing Atoms The lightest atom is hydrogen. We can use the periodic table to work out its Ar. Hydrogen has a relative atomic mass (A r ) of 1. 1 H 1 Atomic Mass Symbol Atomic Number

38 Weighing Atoms We can use the periodic table to work out its Ar. Magnesium has a relative atomic mass (A r ) of Mg 12 Atomic Mass Symbol Atomic Number

39 The Table shows the mass of various atoms relative to carbon. Calculate their relative atomic mass. C Element Symbol Times as heavy as carbon R.A.M Helium He one third Beryllium Be three quarters Molybdenum Mo eight Krypton Kr seven Oxygen O one and one third Silver Ag nine Calcium Ca three and one third

40 Why isn t r.a.m. always a whole number? Relative atomic mass (r.a.m.) is not always a whole number. For example, the r.a.m. of chlorine is The standard r.a.m. value of each element is actually the average relative atomic mass, which takes all the isotopes of each element into account. Chlorine has two isotopes: chlorine-35 (75%) and chlorine-37 (25%). average r.a.m. of chlorine = (35 x 75%) + (37 x 25%) = (35 x 0.75) + (37 x 0.25) = = 35.5

41 How is Relative Formula Mass calculated (M r ) To find the relative formula mass of a compound, add up the relative atomic masses of all the atoms in its formula. Step 1: Write down the formula of the molecule. Step 2: Find the r.a.m. of each type of atom in the molecule. Step 3: Multiply each r.a.m. by the number of atoms of that element and add these values together.

42 How is Relative Formula Mass calculated (M r ) To find the relative formula mass of a compound, add up the relative atomic masses of all the atoms in its formula. What is the relative formula mass of water? Step 1: formula of water is H 2 O Step 2: r.a.m. values: hydrogen = 1, oxygen = 16 Step 3: relative formula mass = (2 x 1) + (1 x 16) = 18

43 Empirical Formulae When a new compound is discovered we have to deduce its formula. This always involves getting data about the masses of elements that are combined together. What we have to do is work back from this data to calculate the number of atoms of each element and then calculate the ratio. In order to do this we divide the mass of each atom by its atomic mass. The calculation is done in 5 stages:

44 Lets try this one together We found 3.2g of copper reacted with 0.8g of oxygen. What is the formula of the oxide of copper that was formed? (At. Mass Cu=64: O=16) Substance Copper oxide 1. Elements Cu O 2. Mass of each element (g) 3. Mass / Atomic Mass 4. Ratio 5. Formula /64 = /16 =0.05 1:1 CuO

45 Try this We found 5.5g of manganese reacted with 3.2g of oxygen. What is the formula of the oxide of manganese formed? (Atomic. Mass Mn=55: O=16) C Substance Manganese oxide 1. Elements Mn O 2. Mass of each element (g) 3. Mass / Atomic Mass 4. Ratio 5. Formula /55 = /16 =0.20 1:2 MnO 2

46 C A chloride of silicon was found to have the following composition: Silicon 16.5g: Chlorine 83.5g (Atomic. Mass Si=28: Cl=35.5) Substance Silicon Chloride 1. Elements Si Cl 2. Mass of each element (g per 100g) 3. Mass / Atomic Mass /28 = /35.5 = Ratio Divide biggest by smallest Cl Si = ( ) = (3.98) Ratio of Cl:Si =4:1 5. Formula SiCl 4

47 This formulae helps you identify how many of each element is present in a compound. B Empirical Formulae Q) A chloride of titanium has a mass of 5g.It is found that there is 1.26g of titanium in the compound. What is the empirical formula?

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49 Titanium Chlorine Mass 1.26g 5.00g-1.26g = 3.74g Divide RAM Divide by smallest number 1.26 = = = = TiCl4

50 Percentage Composition When making compounds that can combust it is important that the percentage composition is known. % element = number of x A r of element atoms of element M r of compound x 100

51 Lets try this one together What percentage by mass of nitrogen is in ammonia (NH 3 )? (r.a.m.: H = 1, N = 14) Step Work out the relative formula mass (Mr) of NH 3. Number of atoms of nitrogen Work out the relative atomic mass (Ar) of N. % of N in NH 3. Answer Working out 1 nitrogen atom + 3 hydrogen atoms = (1 x 14) + (3 x 1) = x 14 x %

52 Try this C What percentage by mass of oxygen is in potassium nitrate (KNO 3 )? (r.a.m.: K = 39, N = 14, O = 16) Step Work out the relative formula mass (Mr) of KNO 3. Number of atoms of oxygen Work out the relative atomic mass (Ar) of O. % of N in KNO 3 Answer Working out 1 potassium + 1 nitrogen + 3 hydrogen + 3 oxygen = (1 x 39) + (1 x 14) + (3 x 16) = x 16 x %

53 What are the rules? Step 1. Write down the balanced symbol equation. Step 2. Step 3. Step 4. Write down the relative atomic/formula masses of the reactants and products. Use the balanced equation to write down the ratios of reactants and products. Convert to ratio of reacting masses. Step 5. Find the mass of the reactant and apply this to the ratio of reacting masses.

54 Lets try this one together If 28 g of iron reacts with copper sulphate solution, what mass of copper will be made? Step 1. Write down the balanced symbol equation. Fe + CuSO 4 Cu + FeSO 4 Step 2. Write down the relative atomic/formula masses. Fe = 56 Cu = 64 Step 3. Write down the ratio of reactants and products. Fe : Cu = 1 : 1.1 Step 4. Convert to ratio of reacting masses. Fe : Cu = 1 : 1.1 = (1x28)=28g : (1.1x28) = 30.8g Step 5. Find the mass of reactant and apply this to the ratio of reacting masses. mass of Cu made = 30.8 g

55 Lets try this one together In a firework, potassium nitrate (KNO 3 ) is decomposed to potassium nitrate (KNO 2 ) and oxygen (O 2 ). What mass of potassium nitrate is needed in a firework to make 1.6g of oxygen? Step 1. Write down the balanced symbol equation. 2KNO 3 2KNO 2 + O 2 Step 2. Write down the relative atomic/formula masses. 2 x KNO 3 = (16x3) = 202 O 2 = (16x2) = 32 Step 3. Write down the ratio of reactants and products. KNO 3 : O 2 = 6.3 : 1 Step 4. Convert to ratio of reacting masses. KNO 3 : O 2 = 6.3 : 1 = (6.3x1.6g)10.1 g : 1.6 g Step 5. Find the mass of reactant and apply this to the ratio of reacting masses. mass of KNO 3 made = 10.1g

56 Does all of the mass get made? The yield is the amount of product produced by a chemical reaction You have calculated the theoretical yield by working out the reacting masses last lesson and the starter! This mass worked out is only a guide and the ACTUAL yield is usual less than this number. Why?

57 Does all of the mass get made? The yield is the amount of product produced by a chemical reaction This mass worked out is only a guide and the ACTUAL yield is usual less than this number. Why? This is due to: -An incomplete reaction -Losses during preparation -Unwanted reactions

58 What mass of carbon dioxide is formed when 36g of carbon is burnt? For the above problem we can work out: - Theoretical yield - Percentage yield

59 Why bother working out the percentage yield? All manufacturers want reactions to be as efficient as possible. They don t want They don t want to waste resources or energy, and they want to make as much product as possible.

60 How is it calculated? The equation for working out the percentage yield is: percentage yield = actual yield theoretical yield x 100

61 Lets try this one together 1. The theoretical yield for producing water in a reaction of hydrogen and oxygen is 36g. The actual yield is 30g. Work out the percentage yield. C actual yield percentage yield = theoretical yield x 30 percentage yield = 36 x Answer = 83.3%

62 Try these 1. Calculate the percentage yields for: a) Actual yield = 2.0g Theoretical yield = 5.0g C b) Actual yield = 3.2g Theoretical yield = 4.0g c) Actual yield = 19.5g Theoretical yield = 25.0g SHOW ALL WORKING!

63 The Chemical industry state the meaning of waste with respect to chemistry. Describe the problems associated with chemical waste. Name 2 waste products and the processes that create them.

64 Describe the job of an industrial chemist. Why are there conflicts in industrial chemistry? Discuss some of the conflicts that chemists must deal with in commercial chemistry.

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75 Formulae to learn Chromatography calculation Distance compounds travel in liquids: Relative atomic mass: Step 1: Write down the formula of the molecule. Step 2: Find the r.a.m. of each type of atom in the molecule. Step 3: Multiply each r.a.m. by the number of atoms of that element and add these values together.