# Chemistry 5 (C5) - Specification

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## Molar Mass

Molar mass of a substance = relative formula mass in grams

Number of moles = mass/molar mass

The relative atomic mass of an element is the average mass of an atom of the element compared to the mass of 1/12th of an atom of carbon-12.

You can also use ratios to find out the moles:

C      +     O2          CO2

12/12 = 1              32/32 = 1 moles                      44/44 = 1 mole

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## Empirical Formula

The empirical formula gives the simplest whole number ratio of each type of atom in a compound.

1. List all elements in the compound

2. Divide the data for each element by its Ar

3. Select the smallest number from step 2 and divide each answer by that to obtain a ratio

4. The ratio may have to be scaled up to give whole numbers.

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## Volume

The more concentrated a solution the more solute partciles are in a given volume (the more crowded the solute particles).

cm3 into dm3 you divide by 1000.

amount in moles = concentration x volume

concentration = amount in moles/volume

volume = amount in moles/concentration

(It could also be mass of solute in grams rather than moles)

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## GDA

Converting sodium to salt:

What mass of sodium ions are in a serving of 30g of bran?

Salt = 0.4g

1. 0.4g of NaCl = 0.4/58.5 = 0.0068 moles

2. There would be 0.0068 moles of sodium ions.

3. Mass = moles x molecular mass

0.0068 x 23 = 0.16g

The amount may be inaccurate because the sodium ions come from a number of sources.

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## Titrations

You need to take several titre readings for a titration so you get consistent results. The 1st and 2nd may not be accurate. Then take an average of the 2nd 3rd and 4th?

concentration of acid x volume of acid = concentration of alkali x volume of alkali

Single indicators (Litmus) produce a sudden change of colour at the end of a titration - easily shows the end point.

Mixed indicator (universal) produce a continous range of colours, you can estimate the pH by comparing to a chart.

In a titration, a single indicator is better because the sudden change in colour can show the end point. Mixed indicator is harder because of the range of colours.

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## Gas Volumes

Method to measure the volume of gas:

• measure out the reactants
• add the reactants together in a conical flask and start the stopwatch
• record the volume of gas produced at regular time intervals until the volume stops increasing.

Method to measure the mass of gas:

• measure the mass of an empty conical flask
• measure out all the reactants
• record the total mass of the reactants and the flask
• add the reactants together in the flask and start the stopwatch
• record the mass of the flask and reactants at several time intervals, until the mass stops changing.
• the difference in mass, is the gas produced.
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## Gas Volumes 2

The amount of product formed varies with the amount of limiting reactant used. If one of the reactants is used up (limiting reactants), no more product is created.

The limiting reactant is the reactant not in excess that is all used up at the end of the reaction.

The amount of product formed is directly proportional to the amount of limiting reactant used because: the more reactants means more reactant particles so there will be a greater number of collisions. This increases the number of product particles. The number of particle of the limiting reactant determines the maximum number of product particles that can be made.

One mole of any gas occupies a volume of 24dm3 at room temperature.

volume = mole x 24

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## Equilibria

In a reversible reaction at equilibrium:

• the rate of the forward reaction equals the rate of the backward reaction
• the concentrations of the reactants and the products do not change

The position of equilibria is related to the ratio of the concentration of the products to the concentration of the reactants. If lies to the right, concentration of products is greater than concentration of reactants. If lies left, the concentration of the products is less than the reactants.

A change in temperature, pressure or concentration of reactant or product may change the position of equilibrium.

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## Equlibrium

A reversible reaction may reach equilibria if:

• closed system - the conditions are not changed or no substance is added/removed
• rate of forward reaction or backward reaction increases - this happens until they are at the same rate. Eventually, forward equals rate of backwards reaction.

Affecting position of equilibrium:

• removing a product moves it to the right and vice versa
• adding extra reactant moves it to the right and vice versa
• increasing temperature pushes equilibrium to the direction of the endothermic reaction
• increasing the pressure (only if involves gases) moves equlibrium to the side with the least number of moles of gas.
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## Contact Process

Reaction between sulfur dioxide and oxygen is reversible:

• sulfur dioxide + oxygen      sulfur trioxide
• 2SO2      +   O2        2SO3

Condiitons needed in the contact process:

• V2O5 catalyst
• around 450 degrees
• atmospheric pressure
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## Contact Process 2

• Increasing temperature moves the position of equilibrium to the left and increases the rate of reaction so a compromise position is used.
• Addition of catalyst increases reate but does not affect equilibrium
• Even at low pressure, the position of equilibrium is already on the right so expensive high pressure isnt needed.
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## Strong and Weak Acids

An acid ionises in water to produce H+ ions.

A strong acid completely ionises in water and a weak aciid does not fully ionise and forms a equilibrium mixture.

The pH of a weak acid is much higher than the pH of a strong acid of the same concentration because it does not have as high concentration of H+ ions.

• Acid strength = measure of degree of ionisation
• Acid concentration = measure of number of moles of acid in one dm3

Hydrochloric - strong acid (HCl         H+     +   Cl-)

Ethanoic - weak acid      (CH3COOH                   CH3COO-      +   H+)

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## Strong and Weak Acids 2

Ethanoic acid reacts slower than hydrochloric acids in the same concentration:

• ethanoic acids has a lower concentration of hydrogen ions
• in ethanoic acid the hydrogen ions have a lower collision frequency with reactant particles.

The volume of gaseous products of the reactions of acids is determined by the amounts of reactants present, not the acid strength. This is because there will be the same amount of collisions between the reactants, but some acids it will happen faster.

Ethanoic acid is less conductive than hydrochloric acids in the same concentration because there is a lower concentration of hydrogen ions to carry the charge.

Hydrogen is produced during the electrolysis of ethanoic acids and of hydrochloric acid because when electricity is passed through the acid, the hydrogen ions are attracted to the negative electrode to become hydrogen.

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## Ionic Equations and Precipitation

Ionic subtances contain ions which are in fixed positions in the solid but can move in solution.

In a precipitation reaction ions must collide with other ions to react to form a precipitate. Most precipitation reactions are very fast because there is a high chance of collisions between ions in solution.

Barium chloride detects sulfates - white precipitate

Silver nitrate detects halide ions - chlorides are white, bromides are cream and iodides are pale yellow precipitates.

(SEE C4 - SPECIFICATION)

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## Ionic Equations and Precipitation 2

Ionic equation for precipitation reactions:

Ag+     +     Cl-                 AgCl

The Na+ and the NO3- ions stay dissolved in the water and dont do anything. These are called specator ions.

Stages involved in the preparation of a dry sample of an insoluble compound by precipitation:

• mix solutions of the reactants
• filter off the precipitate
• wash the residue in the filter funnel with a little distilled water
• dry the residue (the product) in an oven at 50 degrees C.
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