1 of 35© boardworks ltd 2009. 2 of 35© boardworks ltd 2009

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Balancing equations

An important principle in chemical reactions is that matter cannot be created or destroyed. It is important that symbol equations are balanced.

A balanced equation has the same number of each type of atom on each side of the equation.

Na + Cl2 NaCl1 sodium 2 chlorine 1 sodium 1 chlorine

Balanced:

This shows that two moles of sodium react with one mole of chlorine to make two moles of sodium chloride.

2Na + Cl2 2NaCl2 sodium 2 chlorine 2 sodium 2 chlorine

Unbalanced:


Balancing unfamiliar equations


Balancing ionic equations

Equations containing ions should have the same overall charge on each side in order to be balanced.

This can be achieved by balancing the equation in the normal way:

Ca2+ + Cl- → CaCl22 calcium 1 chloride 2 calcium 2 chloride

+1 charge no charge

2 calcium 2 chloride 2 calcium 2 chloride

Ca2+ + 2Cl- → CaCl2

no charge no charge

Balanced:

Unbalanced:


Balancing ionic equations problems


State symbols

State symbols are letters that are added to a formula to indicate what state each reactant and product is in.

The four state symbols are:

These are added after the formula in brackets and subscript. For example:

2H2(g) + O2(g) 2H2O(g)

s

l

g

aq

solid

liquid

gas

aqueous


Adding state symbols


Reacting masses


Calculating reacting masses

To calculate the mass of a product given the mass of a reactant, use the following steps:

1. Calculate no. moles of reactant:

no. moles = mass / Mr

2. Determine mole ratio of reactant to product:

ensure the equation is balanced

3. Calculate no. moles of product:

use the mole ratio

4. Calculate mass of product:mass = moles × Mr


Reacting masses example

What mass of sodium chloride is produced if 2.30 g of sodium is burnt in excess chlorine?

3. Calculate no. moles of NaCl:

0.100 moles Na = 0.100 moles NaCl

4. Calculate mass of NaCl:

mass = moles × Mr

= 0.100 × 58.5= 5.85 g

1. Calculate no. moles of Na:

no. moles = mass / Mr

= 2.30 / 23.0= 0.100

2. Determine mole ratio of Na to NaCl:

2Na + Cl2 2NaClratio = 2:2

= 1:1


Reacting masses calculations


More reacting masses calculations


What is concentration?

The concentration of a solution is a measure of how much solute is dissolved per unit of solvent.

amount of solute is measured in moles

volume of solvent is measured in dm3

concentration is measured in mol dm-3.

concentration = amount of solute / volume of solvent

Volumes are often expressed in cm3, so a more useful equation includes a conversion from cm3 to dm3.

concentration = (no. moles × 1000) / volume

mol dm3 cm3


Concentration, moles and volume


Concentration calculations


Standard solutions

A standard solution is a solution of known concentration.

Standard solutions are made by dissolving an accurately weighed mass of solid in a known volume of solvent using a volumetric flask.

The volumetric flask has a thin neck, which is marked with a line so it can be filled accurately to the correct capacity.

The standard solution can then be used to find the concentration of a second solution with which it reacts. This is known as volumetric analysis or titration.


Preparing standard solutions


A titration is a procedure used to identify the concentration of a solution by reacting it with a solution of known concentration and measuring the volume required for a complete reaction.

Once the number of moles for the solution is known, the concentration can be easily calculated.

The number of moles in the standard solution is calculated. Using a balanced equation for the reaction, the number of moles in the solution of unknown concentration can also be calculated.

What is a titration?


Carrying out a titration


Titration calculations examples

What is the concentration of an NaOH solution if 25.0 cm3 is neutralized by 23.4 cm3 0.998 mol dm-3 HCl solution?

3. Calculate no. moles of NaOH:

0.0234 moles HCl = 0.0234 moles NaOH

4. Calculate conc. of NaOH: = (0.0234 × 1000) / 25.0

conc. = (moles × 1000) / volume

= 0.936 mol dm-3

1. Calculate no. moles HCl:

moles = (conc. × volume) / 1000

= 0.0234= (0.998 × 23.4) / 1000

2. Determine ratio of NaOH to HCl:

NaOH + HCl NaCl + H2O

ratio NaOH:NaCl = 1:1


Titration calculations


More titration calculations


What are the different types of yield?

The theoretical yield is the maximum mass of product expected from the reaction, calculated

using reacting masses.

To calculate the percentage yield, the theoretical yield and the actual yield must be calculated.

The actual yield is the mass of product that is actually obtained from the real chemical reaction.

The percentage yield of a chemical reaction shows how much product was actually made compared with

the amount of product that was expected.


Calculating yield

The percentage yield of a reaction can be calculated using the following equation:

percentage yield = (actual yield × 100) / theoretical yield

Example: What is the percentage yield of a reaction where the theoretical yield was 75 kg but the actual yield was 68 kg?

percentage yield = (actual yield × 100) / theoretical yield

= 90.7%

= (68 × 100) / 75


What is atom economy?

Atom economy is another measure of the efficiency of a chemical reaction. It is the mass of reactants that end up as the desired product – this is calculated as a percentage.

This concept is useful to chemical industry, because it takes into account the atoms that end up in unwanted waste products as well as the yield of the reaction.

This means a process that produces several worthlessby-products could have a high yield but a low atom economy.

Reactions with a high atom economy tend to be more environmentally friendly as they tend to produce less waste, use fewer raw materials and use less energy.


Calculating atom economy

Example: What is the atom economy of a reaction where the actual yield was 25 000 tonnes but the mass of the reactants was 30 000 tonnes?

= 83.3%

total mass of reactants

mass of desired products × 100atom economy =

mass of desired products × 100total mass of reactants

atom economy =

25 000 × 100

30 000=


Yield and atom economy calculations


Glossary


What’s the keyword?


Multiple-choice quiz

1 of 35© boardworks ltd 2009. 2 of 35© boardworks ltd 2009

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