Chemistry unit 3 - Physical Chemistry

Ch 19 - Energetics, Ch 20 - Rates of reactions Ch 30 - reversible reactions and equilibria pg 207 - 253

What is an exothermic reaction?

A reaction that gives out heat to the surroundings so their temperature increases

Give examples of exothermic reactions

• Reacting metals with acids to form salts (salt preparations)

• Neutralisation reactions

• Displacement reactions

• Combustion reactions

Write the chemical equation of the reaction of a metal with an acid of Zinc and Hydrochloric acid

Zinc + Hydrochloric acid → Zinc chloride + Hydrogen gas

Zn + 2HCl → ZnCl_2​ + H_2​

Write the chemical equation of the neutralisation reaction

NaOH(s) + HCl(aq) → NaCl(s) + H₂O(l)

Sodium Hydroxide + Dilute Hydrochloric acid → Sodium Chloride + Water

Give an example of a displacement reaction

2Al(s) + Fe₂O₃ (s) → Al₂O₃(s) + 2Fe(l)

Aluminium + Iron oxide → aluminium oxide + iron

What is Enthalapy change?

The amount of heat taken in or released to make or break bonds, it is measured in kJ/mol

What does the Enthalapy change of an exothermic reaction look like?

• The Enthalapy change itself is negative because the reactants are losing energy (you always look at it from the perspective of the reactants)

• The energy of the reactants is higher than that of the products

What is an endothermic reaction?

A reaction that absorbs heat from the surroundings so the temperature of the surroundings decreases

What are some examples of endothermic reactions?

• Thermal decomposition

Describe the Enthalapy change in an endothermic reaction

• The energy of the reactants is lower than that of the products

• The Enthalapy change is positive

What is the specific heat capacity?

The amount of heat needed to realise the temperature of 1g of a substance by 1^oC

What’s the formula for specific heat capacity

Q = mcΔT

What is a calorimetry experiment

An experiment used to determine the amount of energy released by a fuel

How can you measure the amount of heat given off when alcohols are burnt

1. Measure 100cm³ of cold water in a measuring cylinder

2. Add the water to a copper can

3. Measure the initial temperature of the water using a thermometer

4. Weigh the spirit burner with a lid on to prevent the alcohol from evaporating use a balance to measure the weight

5. Light the wick and place it under the copper can

6. Add an insulating card to prevent heat loss

7. Use shielding to prevent draught

8. Ensure a constant supply of oxygen is provided to prevent incomplete combustion

9. Leave the water until the temperature rises a reasonable amount, stir and then measure the maximum temperature reached

10. Weigh the spirit burner again with the lid on

11. Substitute into Q = mcΔT

12. Repeat experiment for accurate results

Safety precaution:

• Wear eye protection

• Do not carry a lit spirit burner again

• Do not fill a spirit burner when there is a naked flame nearby

Sources of error:

• Incomplete combustion

• Heat loss

What is the formula for Enthalapy change

ΔH = q/n

Enthalpy change = Heat energy released/ number of moles

Describe how to conduct the Enthalpy change of a displacement reaction

1. Place a polystyrene cup in a glass beaker

2. Measure 50 cm³ of 0.200 mol/dm³ of copper (II) sulfate solution in a measuring cylinder and add it to the calorimeter

3. Measure the initial temperature of the solution

4. Add an excess of zinc

5. Stir the solution

6. Record the maximum temperature reached

7. Sub into the equation Q = mcΔT

Sources of error:

• We assume the solution has the same density and specific heat capacity of pure water

Describe the practical for measuring enthalpy changes when salts dissolve in water

1. Place a polyesterne cup in a glass beaker

2. Add 100cm³ of water in the measuring cylinder and then pour the water into the calorimeter

3. Record the initial temperature of the water using a thermometer

4. Measure the mass of the reagent using a balance and add it to the water

5. Stir the solution vigorously until all the reagent has dissolved

6. Record the minimum temperature of the reaction

How can we determine if a reagent is in excess or is a limiting factor?

If there are 2 reagents:

• The one with more moles reacting is the substance that is in excess

• The reagent with less mole reacting is the substance that is the limiting factor

How does the temperature change of water indicate weather a reaction is endothermic or exothermic

• In an endothermic reaction, the initial temperature of the water is higher than the final temperature (the temp of the water drops)

• In an exothermic reaction, the initial temperature of the water is lower than the final temperature of the water (the temp of the water rises)

Describe the bond energetics in endothermic reactions

Bonds are broken, which is why energy is taken in and the temperature of the surroundings decreases. The internal temperature of the reaction itself increases (energy is taken in)

Describe the bond energetics in an exothermic reaction

Bonds are broken, so energy is given out and the temperature of the surroundings decreases. The internal temperature of the reaction itself decreases (because it gives out some of its energy)

Write the chemical equation of the combustion of methane

Methane + Oxygen → Carbon dioxide + Water

CH_4​ + 2O_2​ → CO_2​+ 2H_2​O

How can you identify a combustion reaction?

• Reactants include oxygen

• Carbondioxide and Water is produced

How can you identify a Neutralisation reaction?

• Reactants include an alkali (an alkali can be spotted when it has OH that’s Hydroxide, all bases contain Hydroxide ions)

• Reactants also include an acid ( an acid can be spotted when it has H in its formula, that’s Hydrogen, all acids contain Hydrogen ions)

• The products are a salt and a anion of the alkaline

How can you identify a displacement reaction?

• A more reactive element takes the place of the less reactive element