# Topic: Energetics

## Energetics (Triple) quiz

1. In a chemical reaction, the overall molar enthalpy is -87 kJ/mol. Is this reaction exothermic or endothermic?

Question 1 of 16

2. Are combustion reactions exothermic or endothermic?

Question 2 of 16

3. State the units of molar enthalpy change.

Question 3 of 16

4. When a solid dissolves, is this process exothermic or endothermic?

Question 4 of 16

5. Use Q=mcΔT and c=4.18J/°C/g. 25cm³ of sulfuric acid is put into a boiling tube. The starting temperature is 21°C. A spatula of iron filings is added. After a while the temperature reaches 33°C. What is the total heat energy change?

Question 5 of 16

6. Explain why experimental values of enthalpy change differ from theoretical values

Question 6 of 16

7. What is meant by the term exothermic?

Question 7 of 16

8. Assuming bond energies in kJ/mol: H-C 412, C-C 348, O-H 463, C-O 360, C=C 612. Calculate the molar enthalpy change for the reaction: ethanol → ethene + water

Question 8 of 16

9. In a calorimetry experiment to investigate the heat energy released by the combustion of ethanol, why should the water in the calorimeter be stirred?

Question 9 of 16

10. What is meant by the term endothermic?

Question 10 of 16

11. In an endothermic reaction, which is greater: the energy taken in when breaking bonds or the energy released when bonds are made?

Question 11 of 16

12. In a combustion calorimetry experiment, 0.78g of ethanol (C₂H₅OH) produced 12,540 J of heat energy. Calculate the molar enthalpy change.

Question 12 of 16

13. What does this diagram represent? Question 13 of 16

14. Is making new bonds exothermic or endothermic?

Question 14 of 16

15. Use Q=mcΔT and c=4.18J/°C/g. A spatula of sodium fluoride is put into a boiling tube with 25cm³ of water. The temperature drops from 22.0°C to 18.6°C. What is the total heat energy change?

Question 15 of 16

16. What does the symbol ΔH mean

Question 16 of 16

2020-02-16T16:59:30+00:00Categories: Uncategorized|Tags: , |

## Flashcards: Energetics

2019-08-28T16:20:23+00:00Categories: Uncategorized||

## Q&A slides – Energetics

2019-11-12T07:39:39+00:00Categories: Uncategorized||

## 3:01 know that chemical reactions in which heat energy is given out are described as exothermic, and those in which heat energy is taken in are described as endothermic

Exothermic: chemical reaction in which heat energy is given out.

Endothermic: chemical reaction in which heat energy is taken in.

(So, in an exothermic reaction the heat exits from the chemicals so temperature rises)

## 3:02 describe simple calorimetry experiments for reactions such as combustion, displacement, dissolving and neutralisation

Calorimetry allows for the measurement of the amount of energy transferred in a chemical reaction to be calculated.

EXPERIMENT1: Displacement, dissolving and neutralisation reactions Example: magnesium displacing copper from copper(II) sulfate Method:

1. 50 cm3 of copper(II) sulfate is measured and transferred into a polystyrene cup.
2. The initial temperature of the copper sulfate solution is measured and recorded.
3. Magnesium is added and the maximum temperature is measured and recorded.
4. The temperature rise is then calculated. For example:
Initial temp. of solution (oC)Maximium temp. of solution (oC)Temperature rise (oC)
24.256.732.5 Note:  mass of 50 cm3 of solution is 50 g

The cup used is polystyrene because:

polystyrene is an insulator which reduces heats loss

EXPERIMENT2: Combustion reactions To measure the amount of energy produced when a fuel is burnt, the fuel is burnt and the flame is used to heat up some water in a copper container

Example: ethanol is burnt in a small spirit burner Method:

1. The initial mass of the ethanol and spirit burner is measured and recorded.
2. 100cm3 of water is transferred into a copper container and the initial temperature is measured and recorded.
3. The burner is placed under of copper container and then lit.
4. The water is stirred constantly with the thermometer until the temperature rises by, say, 30 oC
5. The flame is extinguished and the maximum temperature of the water is measured and recorded.
6. The burner and the remaining ethanol is reweighed. For example:
Mass of water (g)Initial temp of water (oC)Maximum temp of water (oC)Temperature rise (oC)Initial mass of spirit burner + ethanol (g)Final mass of spirit burner + ethanol (g)Mass of ethanol burnt (g)
10024.254.230.034.4633.680.78 The amount of energy produced per gram of ethanol burnt can also be calculated: ## Practical change in temperature – video

2019-02-10T15:12:58+00:00Categories: Uncategorized||

## Exothermic and Endothermic Reactions – video

2022-09-15T14:24:03+00:00Categories: Uncategorized||

## 3:07 (Triple only) use bond energies to calculate the enthalpy change during a chemical reaction

Each type of chemical bond has a particular bond energy. The bond energy can vary slightly depending what compound the bond is in, therefore average bond energies are used to calculate the change in heat (enthalpy change, ΔH) of a reaction.

Example: dehydration of ethanol Note: bond energy tables will always be given in the exam, e.g:

BondAverage bond energy in kJ/mol
H-C412
C-C348
O-H463
C-O360
C=C612

So the enthalpy change in this example can be calculated as follows:

Breaking bondsMaking bonds
BondsEnergy (kJ/mol)BondsEnergy (kJ/mol)
H-C x 5(412 x 5) = 2060C-H x 4(412 x 4) = 1648
C-C348C=C612
C-O360O-H x 2(463 x 2) = 926
O-H463
Energy needed to break all the bonds3231Energy released to make all the new bonds3186

Enthalpy change, ΔH = Energy needed to break all the bonds - Energy released to make all the new bonds

ΔH = 3231 – 3186 = +45 kJ/mol (ΔH is positive so the reaction is endothermic)

## (Triple only) Bond energy calculations – videos

Here are a couple of videos explaining how to do bond energy calculations:

2019-02-10T16:05:23+00:00Categories: Uncategorized||

## 3:08 practical: investigate temperature changes accompanying some of the following types of change: salts dissolving in water, neutralisation reactions, displacement reactions and combustion reactions

Calorimetry allows for the measurement of the amount of energy transferred in a chemical reaction to be calculated.

EXPERIMENT1: Displacement, dissolving and neutralisation reactions Example: magnesium displacing copper from copper(II) sulfate Method:

1. 50 cm3 of copper(II) sulfate is measured and transferred into a polystyrene cup.
2. The initial temperature of the copper sulfate solution is measured and recorded.
3. Magnesium is added and the maximum temperature is measured and recorded.
4. The temperature rise is then calculated. For example:
Initial temp. of solution (oC)Maximium temp. of solution (oC)Temperature rise (oC)
24.256.732.5 Note:  mass of 50 cm3 of solution is 50 g

EXPERIMENT2: Combustion reactions To measure the amount of energy produced when a fuel is burnt, the fuel is burnt and the flame is used to heat up some water in a copper container

Example: ethanol is burnt in a small spirit burner Method:

1. The initial mass of the ethanol and spirit burner is measured and recorded.
2. 100cm3 of water is transferred into a copper container and the initial temperature is measured and recorded.
3. The burner is placed under of copper container and then lit.
4. The water is stirred constantly with the thermometer until the temperature rises by, say, 30 oC
5. The flame is extinguished and the maximum temperature of the water is measured and recorded.
6. The burner and the remaining ethanol is reweighed. For example:
Mass of water (g)Initial temp of water (oC)Maximum temp of water (oC)Temperature rise (oC)Initial mass of spirit burner + ethanol (g)Final mass of spirit burner + ethanol (g)Mass of ethanol burnt (g)
10024.254.230.034.4633.680.78 The amount of energy produced per gram of ethanol burnt can also be calculated: Select a set of flashcards to study:

Terminology

Skills and equipment

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Section 1: Principles of chemistry

a) States of matter

b) Atoms

c) Atomic structure

d) Relative formula masses and molar volumes of gases

e) Chemical formulae and chemical equations

f) Ionic compounds

g) Covalent substances

h) Metallic crystals

i) Electrolysis

Section 2: Chemistry of the elements

a) The Periodic Table

b) Group 1 elements: lithium, sodium and potassium

c) Group 7 elements: chlorine, bromine and iodine

d) Oxygen and oxides

e) Hydrogen and water

f) Reactivity series

g) Tests for ions and gases

Section 3: Organic chemistry

a) Introduction

b) Alkanes

c) Alkenes

d) Ethanol

Section 4: Physical chemistry

a) Acids, alkalis and salts

b) Energetics

c) Rates of reaction

d) Equilibria

Section 5: Chemistry in industry

a) Extraction and uses of metals

b) Crude oil

c) Synthetic polymers

d) The industrial manufacture of chemicals

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