1 of 39
What does rate of reaction mean? 2 of 39 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. What is the rate of these reactions? rusting baking explosion slow fast very fast
Changing the rate of reactions 3 of 39 Anything that increases the number of successful collisions between reactant particles will speed up a reaction. What factors affect the rate of reactions? increased temperature increased concentration of dissolved reactants, and increased pressure of gaseous reactants increased surface area of solid reactants use of a catalyst.
4 of 39
Effect of temperature on rate 5 of 39 The higher the temperature, the faster the rate of a reaction. In many reactions, a rise in temperature of 10 C causes the rate of reaction to approximately double. Why does increased temperature increase the rate of reaction? At a higher temperature, particles have more energy. This means they move faster and are more likely to collide with other particles. When the particles collide, they do so with more energy, and so the number of successful collisions increases.
6 of 39
Effect of concentration on rate of reaction 7 of 39 The higher the concentration of a dissolved reactant, the faster the rate of a reaction. Why does increased concentration increase the rate of reaction? At a higher concentration, there are more particles in the same amount of space. This means that the particles are more likely to collide and therefore more likely to react. lower concentration higher concentration
Effect of pressure on rate of reaction 8 of 39 Why does increasing the pressure of gaseous reactants increase the rate of reaction? As the pressure increases, the space in which the gas particles are moving becomes smaller. The gas particles become closer together, increasing the frequency of collisions. This means that the particles are more likely to react. lower pressure higher pressure
9 of 39
Effect of surface area on rate of reaction 10 of 39 Any reaction involving a solid can only take place at the surface of the solid. If the solid is split into several pieces, the surface area increases. What effect will this have on rate of reaction? low surface area high surface area This means that there is an increased area for the reactant particles to collide with. The smaller the pieces, the larger the surface area. This means more collisions and a greater chance of reaction.
energy (kj) What are catalysts? 11 of 39 Catalysts are substances that change the rate of a reaction without being used up in the reaction. Catalysts never produce more product they just produce the same amount more quickly. E a without catalyst E a with catalyst Different catalysts work in different ways, but most lower the reaction s activation energy (E a ). reaction (time)
Unit 16: Nuclear Chemistry 12
Radioactivity Emission of subatomic particles or highenergy electromagnetic radiation by nuclei Such atoms/isotopes said to be radioactive 13 of 39 13
Its discovery Discovered in 1896 by Becquerel Called strange, new emission uranic rays Cuz emitted from uranium Marie Curie & hubby discovered two new elements, both of which emitted uranic rays Polonium & Radium Uranic rays became radioactivity 14 of 39 14
Types of radioactivity Rutherford and Curie found that emissions produced by nuclei Different types: Alpha decay Beta decay Gamma ray emission 15 of 39 15
16 of 39 The properties of the different types of radiation The differences between the three types of radiation can be seen by passing them through an electric field.
17 of 39 Characteristics of an alpha particle The alpha particle (a) is deflected to some extent toward the negative plate. This indicated that it is positively (+) charged and has a fairly large mass. Today we know that an a particle is the same as the nucleus of a He atom.
Characteristics of a beta particle 18 of 39 The beta particle (b) is deflected toward the positive plate (+). It is also deflected more than the a particle This indicates that it is negatively (-) charged and has a much smaller mass than the a particle. Today we know that a b particle is the same as an electron.
19 of 39 Characteristics of gamma radiation Gamma radiation (g) is not deflected toward either the positive (+) or negative (-) plate. This indicates that it has no charge. Today we know that gamma rays are a type of electromagnetic radiation made up of photons (packets of energy).
20 of 39 Radiation and Table O The Regents Reference Tables provides us with a summary of the different types of radiation:
Penetrating power of radiation 21 of 39 The ability of radioactive particles to pass through air and other materials is inversely related to their mass. Alpha particles the least penetrating, they travel only a few centimeters through air. They can be stopped by a single sheet of paper. Beta particles more penetrating, they travel several meters through air. They can be stopped by a sheet of Al or plastic. Gamma Rays most penetrating, thick sheets of lead or concrete are required to stop gamma rays.
22 of 39 22
Beta decay 23 of 39 23
24 of 39 Diagram showing penetrating ability www.epa.gov
25 of 39 Where does the radiation come from? Rutherford suggested that the radiation resulted from the breakdown of the nucleus of an atom, resulting in radiation being given off, and the nucleus of the atom changing into a new element. For instance, the fact that U-238 undergoes alpha decay (emits an a particle) can be shown by this reaction:
Why does the atom break up? 26 of 39 Remember that the nucleus of the atom is held together by the strong nuclear force. This force is normally strong enough to hold the protons and neutrons together. However, sometimes the force of repulsion due to the protons having the same charge overcomes the strong nuclear force and the atom breaks apart.
How does beta decay occur? 27 of 39 Sometimes an atom will emit a b particle when it breaks up. In beta decay a neutron apparently spits out an electron (the b particle) and becomes a proton.
Balanced nuclear equations 28 of 39 Nuclear reactions can be represented by equations. These reactions are governed by two laws : The law of conservation of mass number the sum of the mass numbers on the reactant sides must be equal to the sum of the mass numbers on the product side. This law applies to all nuclear equations!
Balanced nuclear equations 29 of 39 The second law is: The law of conservation of charge (atomic #) the sum of the atomic numbers on the reactant sides must be equal to the sum of the atomic numbers on the product side. This law applies to all nuclear equations!
30 of 39 Predicting products in alpha decay The Law of Conservation of Mass Number and the Law of Conservation of Charge allows us to predict products in a nuclear reaction. For instance, suppose we wanted to predict the atom produced when Radon-222 undergoes alpha decay.
31 of 39 Predicting products Based on the two laws we can predict: The mass number of particle X must be 218. The charge (atomic #) of particle X must be 84. The symbol of the element can then be determined from the Periodic Table.
How about beta decay? 32 of 39 It works the same way. Let s look at the beta decay of Strontium-90. Remember the sum of the mass numbers and atomic number on both sides MUST be the same. So atom X must be:
33 of 39 Using balanced nuclear equations to identify the type of radioactivity. Suppose we know that a particular atom undergoes radioactive decay and we are able to identify the atom that is produced. For instance, Iodine-131 is known to form Xenon-131 when it decays. What radioactive particle must it emit? Using the Laws of Conservation of Mass # and Charge, we can identify the type of radiation given off. Particle X must be a b particle:
34 of 39 Another type of radioactive decay Some atoms undergo a decay process that produces a positron. A positron has the same mass as an electron, but is positively charged. Symbols for the positron include: Positrons are a form of anti-matter. Antimatter is made up of particles with the same properties as normal matter, but are opposite in charge.
35 of 39 Positrons are also listed in Table O
Positron Emission 36 of 39 We can use our ability to balance nuclear equations to predict what will be given off when Potassium-37 undergoes positron emission. There s only one atom that will work, and that s Argon-37.
37 of 39 Your turn! Using the Laws of Conservation of Mass # and Charge, write balanced nuclear equations for the following nuclear reactions: 1. Beta decay of Phosphorus-32 2. Alpha decay of U-238 3. Positron decay of Iron-53 4. Decay of Oxygen-17 into Nitrogen-17 5. Decay of Potassium-42 into Calcium-42 6. Decay of Plutonium-239 into Uranium-235
38 of 39 Half Life
Half-lifes 39 of 39 The rate at which a particular radioisotope decays is described by its half-life. The half-life is defined as the time that it takes for one half of a sample of a radioactive element to decay into another element. The half-life of a radioisotope is dependent only on what the radioisotope is.
40 of 39 Table N provides us with a list of various nuclides, their decay modes, and their halflifes. Using Table N, what is the decay mode and half-life for Radium- 226?
Using Table N 41 of 39 Table N indicates that Radium-226 undergoes alpha decay. Based on this we can write a balanced nuclear equation to represent this reaction: This tells us that for every atom of Radium that decays an atom of Radon is produced.
42 of 39 Using Half-life Table N also tells us that Radium-226 has a half-life of 1600 years. Starting with a 100g sample, after 1 halflife (or 1600 years), 50g remain. After another 1600 years, half of the 50g will remain (25g).
Carbon-14 Dating 43 of 39 The age of objects that were once alive can be determined by using the C-14 dating test. In this test, scientists determine how much C-14 is left in a sample and from this determine the age of the object. From Table N we can determine that C-14 undergoes b decay:
44 of 39 Where does the Carbon-14 come from? C-14 is created in the atmosphere by cosmic rays. It becomes part of living things through photosynthesis and the food chain. When the plant or animal dies, the C-14 begins to decay.
Using C-14 to Age Objects 45 of 39 By comparing the amount of C-14 left in a sample to the amount that was present when it was alive, and using the half-life of 5700 years (Table N), one can determine the age of a sample.
Sample Half-life Problem 1 46 of 39 A 10 gram of sample of Iodine-131undergoes b decay, what will be the mass of iodine remaining after 24 days? From Table N, the ½ life of iodine is determined to be approximately 8 days. That means that 24 days is equivalent to 3 half-lifes. The decay of 10 grams of I-131 would produce: 1.25 grams of I-131 would remain after 24 days.
Sample Half-life Problem 2 47 of 39 A sample of a piece of wood is analyzed by C-14 dating. The percent of C-14 is found to be 25% of what the original C-14 concentration was. What is the age of the sample? First, let s analyze how many half-lives have taken place. Two half-lives have gone by while the sample decayed from the original C-14 concentration to 25% of that concentration. Based on Table N, the half-life of C-14 is 5730 years, so