Topic 1 National Chemistry Summary Notes. Rates of Reaction

Transcription

1 Chemical Reactions Topic 1 National Chemistry Summary Notes Rates of Reaction LI 1, 2 and 3 We know a chemical reaction has taken place when we notice one or more of the following things happening: Colour change Gas is produced (also known as effervescence) Energy change i.e. energy is taken in or given out, for example, heat, light etc Solid forming (also known as precipitation) Note: all chemical reactions involve the formation of one or more new substances Effervescence Precipitation 1

2 Everyday Chemical Reactions LI 4 Our lives are full of chemical reactions. Some examples of everyday chemical reactions include: Striking a match Wool growing on a sheep whenever something is produced in nature it is an example of a chemical reaction Baking a cake A car rusting WORD EQUATIONS LI 5 A shorthand way of showing what goes on in a chemical reaction is to write a WORD EQUATION. Substances you start with are called REACTANTS. These appear on the left hand side of the equation. Substances that form are called PRODUCTS. These appear on the right hand side of the equation. In a word equation + means and and the means produces. e.g. magnesium burns in oxygen to produce magnesium oxide. magnesium + oxygen magnesium oxide REACTANTS PRODUCT(S) 2

3 LI6 When is it Not a Chemical Reaction? Sometimes a substance can appear to change appearance drastically or even seem to disappear. These may not be chemical reactions. They could simply be changes of state or mixtures being formed. If the substance can be easily brought back (or separated out) it is not a chemical reaction. Here is a reminder of work you did in first year. Changes of State Melting, freezing, boiling, condensing are not chemical changes but physical changes. They do not produce a new substance, just a new form. Changes in state are not examples of chemical reactions as no new substances are being formed. The four changes of state are: Melting Freezing Evaporating (liquid to gas) Condensing (gas to liquid) 3

4 Separation techniques LI 7, 8 A mixture is formed when two or more substances come together without reacting. As no new substances form in a mixture, it is not an example of a chemical reaction. For example, air is a mixture of gases, mainly nitrogen and oxygen. Mixtures can be separated in different ways depending on their properties some of these ways include filtration, evaporation and chromatography. Filtration Filtration is used to separate a mixture of a solid and a liquid e.g. sand and water. The liquid collected is the filtrate and the solid collected in the filter paper is called the residue. Evaporation Evaporation is used when we want to separate the mixture created when a solid has dissolved in a liquid e.g. salty water. The salty water is heated until all the water evaporates and the salt is left behind in the evaporating basin. 4

5 Chromatography Chromatography is used to separate mixtures of substances into their components. It is particularly useful for separating mixtures of inks into their separate colours. 5

6 LI 9 to 13 Making Solutions To make a solution a solute must dissolve in a solvent. Solute substance being dissolved Solvent substance doing the dissolving Solution solute plus solvent For example, when salt is dissolved in water a salt solution is made. Solute salt Solvent water Solution salt solution Making a solution is not an example of a chemical reaction. This is because the solute and the solvent can be easily brought back. e.g. the salt in salty water can be brought back by evaporation. Note: when the starting substances can be easily brought back a chemical reaction has not taken place. A substance which dissolves in a liquid is said to be soluble; a substance which does not dissolve is said to be insoluble. e.g. chalk is insoluble in water, salt is soluble in water. A saturated solution is one in which no more solute can be dissolved. A dilute solution has less dissolved solute than a concentrated solution e.g. a glass of dilute orange juice has less orange dissolved in it than a glass of concentrated orange juice A solution is diluted by adding more solvent. For example, adding water to concentrated orange juice makes it more dilute and as a result, it tastes less orangey than before. 6

7 LI 14 Factors Affecting the Rate of Reaction We often want to speed up a chemical reaction. There are 4 main ways we can alter the speed (rate) of a chemical reaction. The Effect of Particle Size Solids with a smaller particle size (e.g. powders or small chips) react more quickly than solids with a larger particle size (e.g. lumps). The diagram explains why. acid particle E The perimeter (solid line) of the large chip is 12 units. The acid particles can only collide with the edge of the chip. However, if we break up the large chip into 9 smaller chips: 7

8 then the perimeter around each chip is 4 units, but there are 9 of them so the total perimeter is 4 x 9 = 36 units. Reducing the size of particles increases the rate of a reaction because it increases the surface area available for collisions to take place. This increases the number of collisions. If a solid reactant is broken into small pieces or ground into a powder: its surface area increases more particles are exposed to the other reactant there are more collisions the rate of reaction increases Effect of Temperature and Concentration The rate of a chemical reaction can be increased by raising the temperature. It can also be increased by increasing the concentration of a reactant. Temperature If the temperature is increased: the reactant particles move more quickly they have more energy the particles collide more often, and more of the collisions result in a reaction the rate of reaction increases = particle A = particle B 8

9 Concentration If the concentration of a reactant is increased the reactant particles become more crowded there is a greater chance of the particles colliding the rate of reaction increases Catalysts LI 15 and 16 A catalyst is a substance that speeds up a reaction without being used up itself. Some reactions have catalysts that can speed them up, but for many reactions there is no catalyst that works. Here is an example of a reaction with a catalyst. Hydrogen peroxide decomposes (falls apart) to form water and oxygen gas: hydrogen peroxide > water + oxygen This reaction only occurs very slowly unless we add the compound manganese dioxide that acts as a catalyst for this reaction. When the catalyst is added the reaction speeds up greatly but the manganese dioxide never runs out. 9

10 Note: As a catalyst does not get used up during a reaction, the mass of a catalyst at the start of a reaction will be the same as at the end. Different catalysts catalyse different reactions. The table below summarises some common catalysts used in industry and the reactions they catalyse: Some common catalysts used in industry and the reactions they catalyse Catalyst Reaction Catalysed iron platinum vanadium(v) oxide making ammonia from nitrogen and hydrogen making nitric acid from ammonia making sulphuric acid Transition metals, such as platinum, are used as the catalyst in catalytic converters. Catalytic converters are devices that fit onto the exhausts of cars. They change harmful gases into harmless gases, for example, nitrogen oxide is converted to nitrogen and oxygen. A gauze or honeycomb structure is a better structure for a catalyst than a lump because a gauze or honeycomb structure has a bigger surface area. Gauze 10

11 Measuring the Rate of Reaction LI17 Reactions can be followed by measuring changes in concentration, mass and volume of reactants and products. Note: When the line of a graph levels off, this indicates the reaction has finished i.e. one of the reactants has been completely used up. 11

12 E LI 18 Calculating the Rate of Reaction 10. The average rate of a reaction, or stage in a reaction, can be calculated from initial and final quantities and the time interval. Average Rate = The table shows the volume of hydrogen gas produced over fifty seconds. Time (s) Volume of gas (cm 3 ) Volume of gas (cm 3 ) 12 Time (s)

13 The average rate at which gas is produced can be calculated as shown. = 2 cm 3 /s 13

14 LI 19 to 22 Tests for Gases Many reactions produce gases and it is important to be able to identify them: Oxygen The test for oxygen is that it relights a glowing splint. As air is approximately 20% oxygen, there is not enough oxygen in the air for the test to be positive. Hydrogen The test for hydrogen is that it burns with a pop. 14

15 Carbon Dioxide The test for carbon dioxide is that it turns lime water milky. Summary Gas Carbon dioxide Test Turns lime water milky Hydrogen Burns with a pop Oxygen Relights a glowing splint Note: if a burning splint is placed in a test tube of oxygen it will burn more brightly Nitrogen gas has no chemical test 15

16 Collecting Gases LI23 The best way to collect a gas depends upon the properties of the gas. The following 3 set-ups show the best way to collect a gas. Note: Less dense than air means lighter than air, denser than air means heavier than air. Soluble in water Less dense than air Soluble in water More dense than air Insoluble in water 16

17 LI24 Naming compounds COMPOUNDS When elements join together during a chemical reaction new substances called COMPOUNDS are formed. A COMPOUND is a substance made up of two or more elements joined together. The name of a compound can tell you the name of the elements in it Compounds which end in IDE USUALLY contain only 2 elements. The first element in the name of a compound is usually the furthest left in the periodic table or if in the same group the furthest up in the table. Oxygen is ALWAYS the second element in an oxide compound. A MIXTURE of elements is not a compound. COPY & COMPLETE NAME OF COMPOUND ELEMENTS PRESENT sodium chloride hydrogen and oxygen copper bromide potassium and iodine rubidium fluoride 17

18 EXCEPTIONS TO THE IDE RULE Compounds which end in IDE usually contain only 2 elements but as usual there are always exceptions to any rule. HYDROXIDES and CYANIDES are two classes of compounds which have more than two elements in them. HYDROXIDES contain HYDROGEN and OXYGEN CYANIDES contain CARBON and NITROGEN COPY & COMPLETE NAME OF COMPOUND ELEMENTS PRESENT calcium hydroxide sodium, hydrogen and oxygen hydrogen cyanide barium, carbon and nitrogen 18

19 COMPOUNDS ENDING IN ATE OR ITE Not all compounds end in IDE. Some other compounds names end in ATE or ITE. The ATE or ITE ending tells you that there is a third element in the compound which is always OXYGEN. COPY & COMPLETE NAME OF COMPOUND ELEMENTS PRESENT calcium carbonate magnesium, sulphur and oxygen iron Sulphite lithium, carbon and oxygen sodium nitrate 19

20 Number Learning Intention Success criteria 1. I will find out chemical reactions can be identified by changes in I can identify a chemical reaction by appearance of substance, including colour change, gas evolved, A change of appearance precipitate formed. A colour change A gas evolved A precipitate formed 2. I will find out chemical reactions can be identified by energy changes including temperature change, light given off. 3. I will find out chemical reactions involve the formation of one or more new substances. 4. I will find out about some chemical reactions which occur in our day-today lives. 5. I am going to find out how to write word equations in order to summarise what happens during chemical reactions. 20 I can identify a chemical reaction by energy changes including Temperature increasing or decreasing Light being given off I can state that at least one new substance is produced during a chemical reaction. I can give examples of chemical reactions such as Iron rusting Cooking food Substances burning I can write word equations for chemical reactions including : correctly using the + and symbols recognising reactants and placing them on the left of the recognising products and placing them on the right of the

21 6. 1. I will find out the changes of state melting, freezing, condensing and evaporating are not chemical reactions. 7. I will find out mixtures occur when two or more substances come together without reacting. 8. I will find out mixtures can be separated in different ways depend on their properties i.e. filtration, evaporation, chromatography. 9. I will find out a solution is a mixture formed when a solute dissolves in a solvent. 10. I will find out a substance which dissolves in a liquid is soluble and a substance which does not dissolve is insoluble. I can state that Melting Freezing condensing evaporating are not chemical reactions I can state that mixtures occur when two or more substances come together without reacting. I can state that: filtration is used to separate solids and liquids evaporation is used to obtain the solid which was dissolved in a liquid chromatography can be used to separate different liquids. I can state that a solution is a mixture formed when a solute dissolves in a solvent. I can state that a substance which dissolves in a liquid is soluble and a substance which does not dissolve is insoluble. 11. I will find out a saturated solution is one in which no more substance can be dissolved. 21 I can state that a saturated solution is one in which no more substance can be

22 12. I will find out a dilute solution has a lower concentration of dissolved substance than a concentrated solution. 13. I will find out a solution is diluted by adding more solvent. 14. I will find out that rates of reaction are affected by changes in concentration, particle size and temperature. 15. I will find out catalysts are substances which speed up some reactions and are not used up by the reactions. 16. I will find out about some everyday examples of uses of catalysts. 17. I will find out reactions can be followed by measuring changes in concentration, mass and volume of reactants and products. 18. I will find out the average rate of a reaction, or stage in a reaction, can be calculated from initial and final quantities and the time interval. dissolved. I can state that a dilute solution has a lower concentration of dissolved substance than a concentrated solution I can state that a solution is diluted by adding more solvent. I can state that rates of reaction can be increased by Increasing concentration Increasing temperature Using smaller particles I can state that catalysts are substances which speed up some reactions and are not used up by the reactions I can state that transition metals in car exhaust systems are examples of uses of catalysts. I can state that reactions can be followed by measuring changes in concentration, mass and volume of reactants and products. I can calculate the average rate of reaction using 22

23 19. I will find out the test for oxygen is that it relights a glowing splint. 20. I will find out there is not enough oxygen in the air for the test to be positive. 21. I will find out the test for hydrogen is that it burns with a pop. 22. I will find out the test for carbon dioxide is that it turns lime water milky. 23. I will find out the best way to collect a gas depends upon the properties of the gas. 24. I will find out how to apply the general rules: compounds with the name ending in ide contain the two elements indicated (except hydroxides and cyanides), and that the ending ite or ate indicates the additional element oxygen. I can state that the test for oxygen is that it relights a glowing splint I can state that there is not enough oxygen in the air for the test to be positive I can state that the test for hydrogen is that it burns with a pop I can state that the test for carbon dioxide is that it turns lime water milky. I can state that the best way to collect an insoluble gas is to bubble through water. A soluble gas with a lower density than air is to collect in an upturned test tube A soluble gas with a higher density than air is to collect downwards in a test tube I can state the elements present in a compound given its name. 23