Chemical Reactions Synthesis Reactions The First Class of Reactions

Review: We have learned that chemical reactions are those processes in which chemical connections between the atoms of pure substances are broken and then new connections between the atoms are formed. The result is that the “original” pure substances are changed into new pure substances.

Representing Chemical Reactions The change from original pure substances to new pure substances is a chemical reaction. – The “original” pure substances are the Reactants. – The “new” pure substances are the Products. We represent the reaction with a chemical equation that will have the format: Reactants  Products

Now for the New Stuff ! Chemistry organizes chemical reactions into five different classes. Each of the classes has unique features – this allows us to place a given reaction into its class. We can also use the classification to predict what will happen in the reaction.

Composition Reactions These are the first of the types of reactions. Probably the simplest of the reactions. Also called “Synthesis” reactions (depends on the book).

Details on Composition Reactions Reactants There will be two reactants The reactants will usually both be elements – this is unique to these reactions. These reactions usually need Heat to proceed. Products There will only be ONE PRODUCT – this is unique to this type of reaction. The product will be a compound that is built from the elements that are the reactants. You will write its formula using “SOCCR”

Example of a Composition Reaction “Sodium is reacted with Chlorine” This is a Composition Reaction. We know this because there are only two reactants and they are both elements. That means that there will only be one product – a compound built from the elements.

“Sodium is reacted with Chlorine” Since we said that there will only be one product - a compound built from the reactants in this case – that product must be Sodium Chloride

This gives us the following “word equation” Sodium + Chlorine  Sodium Chloride Now we have to write the correct chemical notations for the reactants and the product. 1.For the Sodium - Na (just the atomic symbol) 2.For the Chlorine - Cl 2 (remember that it is diatomic – go back to the textbook to see the discussion on diatomic elements: H, N, O, F, Cl, Br, I ) 3.For the Sodium Chloride: NaCl (from SOCCR – Na is a +1 and Cl is a -1 before criss-crossing)

Putting all of this together: The equation will start with the symbols and chemical formula from the previous slide. ___Na + ____Cl 2  ____NaCl Now remember from the book exercises that we have to balance the equation. – We have to have the same number of atom of each element on both sides of the equation. That means that we may need to add coefficients in the spaces in front of the symbols and formula.

Finishing this up ___Na + ____Cl 2  ____NaCl Looking at the equation – – There is currently one atom of Na on each side – we say that the Na is balanced. – But, there are two atoms of Cl on the left (subscript!) and only one atom of Cl on the right. – So, we will write in a “2” in the space in front of the NaCl. The coefficient applies to both the Na and the Cl (it distributes like in Math class). That balances the Cl atoms. (next slide)

Almost done… ___Na + ____Cl 2  2 NaCl But there is a problem – in balancing the Cl atoms, we have now “unbalanced” the Na atoms (one on the left and now two on the right). This will happen in these equations – but we can fix it. All that we have to do is now write in a “2” in front of the Na on the left. That will give us two atoms of Na on that side and the new coefficient does not affect the Cl because the + sign blocks it. (next slide)

Finally … 2 Na + 1 Cl 2  2 NaCl Check this out now: – There are two atoms of Na on each side – There are two atoms of Cl on each side – We did not need to write any coefficient in the space in front of the Cl 2, so we can simply write in a “1” (which doesn’t change anything).

Additional Notes In a previous slide, we stated that the Reactants will typically be elements – it is how we can identify this type of reaction. But – there are Composition Reactions in which the reactants will be Compounds. Important – if this is a Composition Reaction, there can only be 1 Product.

Example Calcium oxide + Diphosphorus pentoxide  Calcium phosphate This is still a Composition Reaction even though the reactants are both compounds. How do we know ? There is only one product !!

In Equation Form Calcium oxide + Diphosphorus pentoxide  Calcium phosphate ___ CaO + ____ P 2 O 5  ____ Ca 3 (PO 4 ) 2

Expected Skills Be able to tell us about the reactants and products in Composition Reactions Be able to identify a Composition Reaction from the given reactants (two elements) Be able to identify a Composition reaction as having a single product. Be able to correctly predict the product of a Composition reaction when given the reacting elements.

More… Be able to write the correct representations of the reactants and product in a Composition reaction. Be able to correctly balance the equation for the reaction.