chemistry - key skills

8/6/2019 Chemistry - Key Skills

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Chemistry: Key Skills

Chemical Reaction Types

Decomposition is

the reve rse o fcombination. That is, a single reactant is broken

down into two or more products either elements

or compounds.

EXAMPLE

AB→ A + B2HgO(s) → 2Hg(l) + O2(g)

Decomposition

A reaction between

a n a c i d i c

compound and a basic compound to form a

onic salt and water.

EXAMPLE

Acid + Base→ Salt + WaterHCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)

Neutralisation

C o m b u s t i o n

reactions generally

apply when an organic compound is burned in

air - reacting with the oxygen - producing

carbon dioxide and water.

EXAMPLE

Organic + O2 → CO2 + H2OCH4(g) + 2O2(g) → CO2(g) + 2H2O(l)

Combustion

A d isp lacement

reaction involves

a n e l e m e n treacting with a

compound displacing another element from the

compound.

EXAMPLE

A + BC→ AB + C

Zn(s) + 2HCl(aq) → ZnCl2(s) + H2(g)

Single

Displacement

A d o u b l e

d i s p l a c e m e n t

reaction involves

two compounds

reacting by swapping ions so that each ion hasa new partner.

EXAMPLE

AB + CD→ AC + BD

AgNO3(aq) + NaCl(aq) →

! ! ! ! AgCl(s) + NaNO3(aq)

Double

Displacement

Any reaction in

which two or more

substances combine to form a single product is

a combination reaction.

EXAMPLE

A + B→ AB2Na(s) + Cl2(g) → 2NaCl(s)

Combination



Chemical Equations

The law of conservation of mass

states that matter can neither be

created nor destroyed, so a

chemical equation must have the

same amounts of each particle on

each side of the equation.

States of matter (solid, liquid, gas

and aqueous) must be included for

every reactant and product.

Numbers in subscript after an

element or ion are a part of the

molecule or compound and cannot

be changed.

Coefficients placed in front of a molecule apply to

he whole molecule and must be used to indicate

a balanced ratio of reactants and products.

A balanced equation must show the coefficients

used in the simplest possible whole number ratio.

Rules for wri t ing

chemical e uations

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)

Same amounts of reactants and products.

States of matter shown for each chemical.

Think of the elements and ions in an equation as units . Polyatomic ions that remain

unchanged on both sides of the equation can be counted as a single unit.

Start with an element or ion that is in only one chemical on both sides of the equation.

Work with one element or ion at a time going back and forth between reactants and

products (or vice versa). Once you have balanced an element or ion both sides, balance

any other(s) in the same compound or molecule.

Keep a tally for each unitʼs amount on both sides can make life easier when balancing more

complex chemical equations.

Save balancing any single elements, water molecules, or molecules with only one element

or last.

B a l a n c i n g

chemical equations



Pick a product Al2(SO4)3(aq) + NaOH(aq) → Al(OH)3(aq) + Na2SO4(aq)

Write a balanced half

equation 2Al2(aq) + 6OH(aq) →

2Al(OH)3(aq)

Balance the overallequation. 2Al2(SO4)3(aq) + 6NaOH(aq) → 2Al(OH)3(aq) + 3Na2SO4(aq)

Chemical EquationsThe following two methods are for equations that are too difficult to balance on inspection.

The “odd one out” method is best used to balance composition, decomposition and to a

lesser extent, neutralisation reactions.

The “half and half” method is bettersuited to ionic equations, in particular

double displacement reactions.

The “Odd one out”Method

The “Half and half”

Method

Give the odd one out acoefficient of 2 Ca(OH)2(aq) + H3PO4(aq) → 2Ca3(PO4)2(aq) + H2O(aq)

Balance as necessary 6Ca(OH)2(aq) + 4H3PO4(aq) → 2Ca3(PO4)2(aq) + 12H2O(aq)

Write coefficients insimplest whole number

ratio.

3Ca(OH)2(aq) + 2H3PO4(aq) → Ca3(PO4)2(aq) + 6H2O(aq)

Step 1.! Identify the largest reactant or product with an odd number (except 1) of an

! unbalanced element (it is the odd one out) or ion and give it a coefficient of 2.

Step 2. ! Add coefficients to the !chemicals on the opposite side of the equation to be in

! balance with the !odd one out.

Step 3. ! Balance any remaining unbalanced reactants or products.Step 4. ! If needed, alter the coefficients to be in the simplest whole number ratio.

Step 1.! Pick one of the products of the equation and identify the ions that form it

! from !chemicals on the reactants side.

Step 2. ! Write a half equation using the product you selected and the ions from the

!reactants side including the correct amounts of each.

Step 3. ! Balance the half equation by adding coefficients to the ions and product.

Step 4. ! Transfer the balanced coefficients !from the half equation to the respective

! chemicals in the full equation.

Step 4.! Write a second half equation for the other product and repeat the steps.

chemistry - key skills

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