Chemical reactions. Section Describing chemical reactions

Transcription

1 Chemical reactions Section Describing chemical reactions

2 What are everyday examples of chemical reactions?

3 How do you write a chemical equation in words? Chemical equations are like a recipe, telling chemists what is occurring in a reaction Rather than writing out a series of steps, chemists write equations in shorthand, with the reactants on the left hand side, products on the right, separated by an arrow Example - Rust

4 Examples of chemical word equations

5 Writing chemical equations Word equations are long and cumbersome Instead you can use chemical symbols and formulas to write a chemical equation Equations that just show the formulas of the reactants and the products are skeletal equations These do not tell you the amount of the reactants and products You can also use a number of symbols to add an extra level of detail

6 What is a catalyst? A catalyst speeds up the rate of reaction, but is not used up As it is neither a product or a reactant, so it is written above the arrow in a chemical equation

7 What about the number of atoms? Skeletal equations do not provide any information about how many atoms are required They often (but not always) are referred to as unbalanced To balance equations you place coefficients in front of the formulas for the reactants and products to ensure that each side of the equation has the same number of atoms of each element In a balanced equation the two sides of the equations have the same numbers of atoms This relates to dalton s atomic theory; that matter can neither be created or destroyed

8 How do you balance the equation? Write the chemical equation Total the number of atoms for each elements on each side of the equation Use coefficients to balance the equation

9 Summary

10 Key rules for writing and balancing chemical equations

11 Visualising balancing equations When approached with an equation, it can sometimes appear a bit daunting, and you may not know where to begin: The modeling kits can allow you to visualize exactly what it is happening when you balancing a chemical equation We will use the following equation as an example

12 Some additional practice: Balancing equations

13 Types of Chemical Reactions Section 11.2

14 What happens when you burn a candle? Burning a candle requires oxygen; the fuel for a combustion reaction This is one of many types of reaction; if you are able to identify the type of reaction occurring you may be able to predict the products of the reaction

15 What are the different types of chemical reaction? There are five different types of chemical reaction, although not all reactions will generally fit into one category, and sometimes a reaction may fit into two categories Understanding patterns of chemical behavior will allow you to predict the products of many reactions

16 Combination (synthesis) reactions In these reactions, two or more substances react together to form a single new substance For example:

17 But, it is not always that easy - why? If I were to tell you to write a combination reaction for Fe and S, why might you struggle? You do not know if you are dealing with Fe2+ or Fe 3+ When reacting a transition metal and a nonmetal, or two nonmetals together there is often more than one possible product

18 Decomposition reactions This is the reverse of a combination; a single compound will break down to form two or more simpler products There is only ever one reactant, but two or more products Usually, products are difficult to predict, but in the case of the breakdown of a simple binary compound the products are likely to be two elements

19 Decomposition reactions and everyday life We have decomposition reactions to thank for airbags A device that can trigger a decomposition reaction is placed inside an airbag with Sodium Azide pellets (NaN3) When the device is triggered by a collision, the sodium azide pellets decompose, releasing large amount of N2 gas

20 Single Replacement reaction In this reaction, K appears to replace H in the reacting compound A single replacement involves the replacement of one element by another in a compound How can you tell if a replacement reaction is going to place?

21 The Activity series of metals Generally, a more reactive metal will always displace any metal that is less reactive The same is true of halogens; you need to remember that the reactivity of halogens decreases as you move down group 7A Would Bromine react with Sodium Chloride to produce Sodium Bromide?

22 Double replacement Reactions 1) 2) 3) In Double replacement reactions two positive ions change places between two compounds This can sometimes occur when two solutions of ionic compounds are mixed together They commonly take place in aqueous solutions, and often result in one of the following three things happening One the products formed is a precipitate One of the products is a gas One of the products is a molecular compound

23 What does the word combustion mean? A combustion reaction is a chemical change in which an element or compound reactions with oxygen resulting in the loss of light and heat energy Oxygen is always one of the reactants in a combustion reaction Other reactants are commonly hydrocarbons (a compound made of carbon and hydrogen) When a hydrocarbon is involved, carbon dioxide and water are always produced as a product, alongside a large amount of heat energy The following equation is an example of combustion of gasoline in a car engine

24 Reactions in Aqueous Solution Section 11.1

25 Net Ionic Equations Water plays such an important role on our planet; as such it is no surprise that many important chemical reactions take place in water Chemical reactions that take place in water are said to take place in Aqueous solution The following equation is an example of a reaction that take place in aqueous solution, but the balance equation does not provide the full picture as to what is going on; which is why chemists write net ionic equations

26 That equation in more detail Sodium Chloride in water produces Na+ and Cl- ions Silver nitrate in water produces Ag+ and NO3- ions Both of these compounds are said to have dissociated You can use the ions to write a complete ionic equation, which shows all dissolved ions as dissociated free ions, and then you can remove spectator ions to write a net ionic equation

27 Balancing ionic charges When writing net ionic equations, the overall charge must be balanced What is the balanced net ionic equation?

28 Balanced net ionic equation

29 Can you predict if a precipitate will form when you mix two solutions together? Yes, generally it is possible, if the general rules for solubility of ionic compounds are followed: