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Energy Changes Chemistry topic 5.

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Presentation on theme: "Energy Changes Chemistry topic 5."— Presentation transcript:

1 Energy Changes Chemistry topic 5

2 What is a Chemical Reaction?

3 Exothermic and Endothermic Reactions
Learning objectives Be able to understand what exothermic/ endothermic means and apply it to a reaction Give examples of exothermic and endothermic reactions Identify exothermic and endothermic reactions from temperature changes

4 This is what’s going on…
2 Molecules of Hydrogen react with 1 Molecule of Oxygen to produce 2 Molecules of Water 1 Molecules of Hydrogen reacts with 1 Molecule of Chlorine to produce 2 Molecules of Hydrochloric Acid

5 How do we know when a reaction has occurred?

6 It’s all about the Chemistry..
There are different types of reaction, this is two of them: EXOTHERMIC – in these reactions the reaction releases heat energy into it’s surroundings. Exothermic reactions will heat the place up! 2. ENDOTHERMIC – in these reactions the reaction takes heat energy in from it’s surroundings. Endothermic reactions cool the place down! It’s all about the Chemistry..

7 Copy - Examples of Reactions
Exothermic Endothermic Combustion reactions Neutralisation reactions Oxidation Reactions Thermal decomposition Photosynthesis Reactions between citric acid and Sodium hydrogencarbonate Every day uses Self heating cans and hand warmers Sports injury packs

8 Exothermic and Endothermic Reactions
Learning objectives Be able to understand what exothermic/ endothermic means and apply it to a reaction Give examples of exothermic and endothermic reactions Identify exothermic and endothermic reactions from temperature changes

9 Investigation Lesson Learning objective: Use scientific theories and explanations to develop hypothesis Plan experiments to make observations and test hypothesis Evaluate methods to suggest possible improvements and further investigations

10 Exothermic Reactions If heat is given out this energy must have come from chemical energy in the starting materials (reactants). 45o C 25o C Reactants convert chemical energy to heat energy. The temperature rises. The reaction mixture gets hotter. Eventually this heat is lost to the surroundings.

11 Extra energy is needed in order for endothermic reactions to occur.
This comes from the thermal energy of the reaction mixture which consequently gets colder. 25o C 5o C 25o C Reactants convert heat energy into chemical energy as they change into products. The temperature drops.

12 ENERGY LEVEL DIAGRAMS Sunday, 29 October 2017 Learning objectives:
To understand what an energy level diagram is To link exothermic and endothermic reactions to energy level diagrams To be able to interpret energy level diagrams.

13 Collision Theory How do chemical reactions actually happen?
Reactions only happen if the substances are in contact The particles of the reactants must collide to react

14 How does that work then? Reactant particles collide REACTION
Product particles formed

15 A collision but with no effect
Reactant particles collide Product particles not formed as there is not enough energy

16 The Collision Theory Particles are constantly moving
For a chemical reaction to take place the reactant particles must collide first For the collision to be effective the particles must have the right amount of energy (we will come back to this later)

17 Energy Level Diagram Q: What do you think an energy level diagram could be? Discuss with your partner/table. A: Here’s the definition:

18 Energy Level Diagram The graph starts at the energy level of the reactants and finishes at the energy level of the products. These two points are usually joined by a smooth curve.

19 1. Overall Energy Change The overall energy change of a reaction is the difference between the energy of the reactants and the energy of the products. You can find the overall energy change of a reaction from an energy level diagram by looking at the difference in height from where the graph starts to where it finishes.

20 2. Exothermic/Endothermic?
Energy level diagrams show the relative energies of the reactants and the products, so you can use them to work out whether a reaction is exothermic or endothermic. Task - Using the example from the last slide, on a white board draw an energy level diagram for an exothermic reaction and endothermic reaction.

21 2. Exothermic? In an exothermic reaction, the reactants have more energy than the products, because energy is released during the reaction. This means the energy level diagram will start high and finish lower than where it started.

22 2. Endothermic? In an endothermic reaction, the products have more energy than the reactants, because energy is taken in during the reaction. This means the energy level diagram will start low and finish higher than where it started.

23 3. Activation Energy Energy level diagrams don’t normally go straight from the reactants to the products — the graph will curve upwards before it starts to go down again. This is because some energy usually has to be put in to break the bonds in the reactants and get the reaction started.

24 3. Activation Energy The minimum amount of energy needed by reacting particles to break their bonds is called the activation energy. You can find the activation energy of a reaction from its energy level diagram by looking at the difference between where the curve starts and the highest point on the curve.

25 Catalysts Q: Can you remember what a catalyst is? How might they relate to energy level diagrams? A catalyst is a substance that can speed up a chemical reaction without being changed or used up during the reaction. Catalysts work by providing a different pathway for a reaction that has a lower activation energy (so the reaction happens more easily and more quickly). You can see the effect that a catalyst has on the activation energy of a reaction by looking at an energy level diagram.

26 Catalysts The activation energy is lower for the reaction with a catalyst than for the reaction without a catalyst. The overall energy change for the reaction remains the same though.

27 ENERGY IN REACTIONS Learning Objectives
Sunday, 29 October 2017 ENERGY IN REACTIONS Learning Objectives To be able to explain why reactions are exothermic or endothermic in terms of temperature change To be able to explain why reactions are exothermic or endothermic in terms of bond breaking and bond making To be able to calculate energy changes in reactions using bond energies.

28 Exothermic and Endothermic Reactions
Whenever a chemical reaction takes place, energy is transferred between the reaction and the surroundings. This energy can be transferred in either direction — chemical reactions can either absorb energy from the surroundings or release energy into the surroundings. This determines whether the reaction is endothermic or exothermic. Q: Is a reaction that absorbs energy from the surroundings an exothermic or endothermic reaction?

29 Energy transferred to the surroundings
Exothermic Reactions An exothermic reaction is one which gives out energy to the surroundings, usually in the form of heat and usually shown by a rise in temperature. For example: When a fuel burns, lots of heat energy is transferred to the surroundings and the temperature increases — this is an exothermic reaction. Energy transferred to the surroundings

30 Endothermic Reactions
An endothermic reaction is one which takes in energy from the surroundings, usually in the form of heat and usually shown by a fall in temperature. For example: When a substance thermally decomposes, it absorbs heat energy from the surroundings and the temperature drops — this is an endothermic reaction. Energy absorbed from the surroundings

31 Energy and Bonding Energy is transferred in chemical reactions because old bonds are broken and new bonds are formed. Energy must be supplied to break existing bonds — so bond breaking is an endothermic process. Energy is released when new bonds are formed — so bond formation is an exothermic process.

32 Exothermic or Endothermic?
Whether a reaction is exothermic or endothermic depends on what bond breaking and bond making is going on. It all comes down to whether the amount of energy absorbed when the bonds in the reactants are broken is more or less than the amount of energy released when the bonds in the products are made.

33 Energy given out as bonds form between atoms
Making chemical bonds It is reasonable to assume that bond making will be the opposite of bond breaking Energy will be given out in an exothermic process when bonds are formed. Heat given out Energy in chemicals Energy given out Energy given out as bonds form between atoms

34 Changes to chemical bonds- Exothermic Reactions
Again some existing bonds are broken (endothermic) And new bonds are formed (exothermic) Energy in chemicals reactants products Energy taken in as old bonds break Energy given out as new bonds form H Overall exothermic – in this case Where the energy from bond forming exceeds that needed for bond breaking the reaction is exothermic.

35 Breaking chemical bonds
Most chemicals will decompose (break up) if we heat them strongly enough. This indicates that breaking chemical bonds requires energy – is an endothermic process. Heat taken in Energy in chemicals Energy needed Energy needed to overcome the bonding between the atoms

36 Changes to chemical bonds - Endothermic Reactions
In most chemical reactions some existing bonds are broken (endothermic) But new bonds are made (exothermic) Energy in chemicals reactants products Energy given out as new bonds form Energy taken in as old bonds break H Overall endothermic in this case Where the energy for bond breaking exceeds that from bond forming the reaction is endothermic.

37 Exothermic or Endothermic?
Temp. rises In an exothermic reaction: the energy released in bond formation is greater than the energy used in breaking old bonds. The leftover energy is released into the surroundings as heat and the temperature rises. In an endothermic reaction: the energy required to break old bonds is greater than the energy released when new bonds are formed. The extra energy needed is absorbed from the surroundings and the temperature falls. Temp. falls

38 Sodium reacting with water
Sodium reacting with water. This is a very exothermic reaction — it gives off lots of heat. Can do demo of K and water?

39 Bond Energy Not all bonds are the same strength — it requires more energy to break some bonds than others. Every chemical bond has a particular bond energy associated with it. EXAM TIP: Bond energies vary slightly depending on what compound the bond is in. But don’t worry about this — you’ll be given any bond energies that you need in the exam.

40 C C C C C C C C Bond Energy – Example 1
A carbon-carbon (C–C) bond has a bond energy of 348 kJ/mol. This means that it takes 348 kJ of energy to break one mole of C–C bonds. It also means that 348 kJ of energy is released when one mole of C–C bonds is formed. 348kJ of energy C C C C C C C C 348kJ of energy

41 C H C H C H C H Bond Energy – Example 2
A carbon-hydrogen (C–H) bond has a bond energy of 413 kJ/mol. It takes 413 kJ of energy to break one mole of C–H bonds. 413 kJ of energy is released when one mole of C–H bonds are made. 413kJ of energy C H C H C H C H 413kJ of energy

42 Energy change = Energy of bond breaking – Energy of bond making
Overall Energy Change Draw out the displayed formulae of the molecules in the reaction so you can see all the bonds that are being broken and made. Work out the amount of energy used in bond breaking by adding up the bond energies of all the bonds in all the reactants. Work out the amount of energy given out from bond making by adding up the bond energies of all the bonds in all the products. Then, use this formula to work out the overall energy change: Energy change = Energy of bond breaking – Energy of bond making

43 Example Question: Calculate the overall energy change for this reaction: The bond energies you need are:  H–H: 436 kJ/mol; Cl–Cl: 242 kJ/mol; H–Cl: 431 kJ/mol

44 Step 1: If you draw out the displayed formulae of the molecules in this reaction, it looks like this: As you can see, one mole of H–H bonds and one mole of Cl–Cl bonds are being broken and two moles of H–Cl bonds are being formed.

45 The amount of energy used in bond breaking is 436 + 242 = 678 kJ/mol
Step 2: Bonds Breaking 1 H-H bond (436 kJ/mol) 1 Cl – Cl bond (242 kJ/mol) The amount of energy used in bond breaking is = 678 kJ/mol

46 The amount of energy released in bond making is 2 × 431 = 862 kJ/mol
Step 3: Bonds Made 2 H – Cl bond (431 kJ/mol) The amount of energy released in bond making is 2 × 431 = 862 kJ/mol

47 Step 4: So the overall energy change of the reaction is 678 – 862 = –184 kJ/mol

48 ENERGY IN REACTIONS Learning Objectives
Sunday, 29 October 2017 ENERGY IN REACTIONS Learning Objectives To be able to explain why reactions are exothermic or endothermic in terms of temperature change To be able to explain why reactions are exothermic or endothermic in terms of bond breaking and bond making To be able to calculate energy changes in reactions using bond energies.

49 End of Topic 5


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