chemical reactions -...
TRANSCRIPT
Chemical reactionsSection 11.1 - Describing chemical reactions
What are everyday examples of chemical reactions?
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
Examples of chemical word equations
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
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
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
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
Summary
Key rules for writing and balancing chemical equations
Visualising balancing equationsWhen 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
Some additional practice: Balancing equations
Types of Chemical Reactions
Section 11.2
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
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
Combination (synthesis) reactionsIn these reactions, two or more substances react together to form a single new substance
For example:
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
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
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
Single Replacement reactionIn 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?
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?
Double replacement Reactions● 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 happening1) One the products formed is a precipitate2) One of the products is a gas3) One of the products is a molecular compound
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
Reactions in Aqueous Solution
Section 11.1
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
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
Balancing ionic chargesWhen writing net ionic equations, the overall charge must be balanced
What is the balanced net ionic equation?
Balanced net ionic equation
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: