GCSE Chemistry U2.8 Energy Changes in Chemistry

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GCSE Chemistry U2.8 Energy Changes in Chemistry

Transcript Of GCSE Chemistry U2.8 Energy Changes in Chemistry

FACTFILE: GCSE CHEMISTRY: UNIT 2.8
Energy Changes in Chemistry
Learning outcomes
Students should be able to: 2.8.1 demonstrate knowledge and understanding that chemical reactions in which heat is given out are exothermic and that reactions in which heat is taken in are endothermic; 2.8.2 draw and interpret reaction profile diagrams for exothermic and endothermic reactions identifying activation energy; 2.8.3 explain activation energy as the minimum energy needed for a reaction to occur; 2.8.4 recall that bond breaking takes in energy and bond making releases energy, and demonstrate understanding that the overall energy change in a reaction is a balance of the energy taken in when bonds break in the reactants and the energy released when bonds form in the products; 2.8.5 calculate energy changes in a chemical reaction from bond energies by considering bond making and bond breaking energies.
In exothermic reactions, the chemicals give out energy so ∆H (energy change) is negative. The surroundings feel hotter and an increase in temperature is observed. Examples of exothermic reactions include: 1. Combustion – burning fuels for heating and in engines
CH4 + 2O2 CO2 + 2H2O 2. Respiration – oxidation of carbohydrates in living things
C6H12O6 + 6O2 6CO2 + 6H2O
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FACTFILE:GCSEEHCISHTEOMRIYSTORFYA:RUTN/ITAR2C.8HITECTURE

Replaces top figure on page 2 of 2.8 factfile for both DA Chem and GCSE Chemistry Exothermic reactions can be represented on a reaction profile diagram:

activation energy

reaction pathway

Replaces top figure on page 2 of 2.8 factfile for both DA Chem and GCSE Chemistry

reactants

energy change is negative

energy

activation energy

productsreaction pathway

energy

progress of reaction

energy change is

negative

In endothermic rearcetaiocntsa,nthtse chemicals take in energy so ∆H is positive. The surroundings feel cooler and a

decrease in temperature is observed. Diagram below replaces the lower diagram on factfile 2.8 for both DA Chem and GCSE
ChemExamples of endothermic processes include: products

1. Thermal decomposition of calcium carbonate

2. Photosaycnttihveastiison energy

CaCO3 CaO + CO2 progress of reaction
reaction pathway

6CO + 6H O C H O + 6O Diagram below replaces the lower diagram2 on 2factfile6 212.86 for b2 oth DA Chem and GCSE

Chem

products

Endothermic reactions can be represented on a reaction profile diagram:

reactants

energy change is positive

energy

activation energy

reaction pathway

products progress of reaction
reactants

energy change is positive

energy

progress of reaction 2

FACTFILE:GCSEEHCISHTEOMRIYSTORFYA:RUTN/ITAR2C.8HITECTURE

The reaction profile diagrams show the activation energy for each type of reaction. Activation energy is the minimum energy required for a reaction to take place between reacting particles. The activation energy required for endothermic reactions is greater than the activation energy required for exothermic reactions.

Explaining energy changes in a reaction
In all reactions energy is taken in to break the bonds of the reactant molecules and energy is released when new bonds form in the product molecules. The balance of these energy changes determines if a reaction is exothermic or endothermic overall.

REACTANTS bonds broken energy taken in

PRODUCTS bonds formed energy given out

EXOTHERMIC – energy taken in to break bonds in the reactants is less than the energy released when new bonds form in the products.
Ein < Eout
ENDOTHERMIC – energy taken in to break bonds in the reactants is greater than the energy released when new bonds form in the products.
Ein > Eout

Calculation of the energy change of a reaction
The combustion reaction between methane and oxygen is exothermic and using the data provided this can be shown through a calculation. For this calculation bond enthalpy data is required, this will always be provided.
The calculation can be broken down into a number of steps:
Step 1 – write the balanced symbol equation and draw out the molecules showing the bond types.
CH4 + 2O2 CO2 + 2H2O

Step 2 – list the number of each type of bond present for the reactants and the products:

Reactants

4 × C—H 2 × O=O

Products

2 × C=O 4 × O—H

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FACTFILE:GCSEEHCISHTEOMRIYSTORFYA:RUTN/ITAR2C.8HITECTURE Step 3 – calculate the total bond energies for the reactants and products using the data provided:

Reactants

4 × C–H 2 × O=O

4 × 413 = 2 × 497 = TOTAL =

1652 994 2646

Products

2 × C=O 4 × O–H

2 × 805 = 4 × 463 = TOTAL =

1610 1852 3462

Step 4 – use the equation energy change = ∑(bonds broken) – ∑(bonds formed) to calculate the energy change for the reaction where ∑ represents the sum off all the bond energies.

energy change = ∑(bonds broken) – ∑(bonds formed) energy change = 2646 – 3462 energy change = – 816 kJ/mol

The calculation shows a negative energy change confirming that the reaction is exothermic.

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FACTFILE:GCSEEHCISHTEOMRIYSTORFYA:RUTN/ITAR2C.8HITECTURE
REVISION QUESTIONS
1. Methane will burn in air to produce carbon dioxide and water in an exothermic reaction. CH4 + 2O2 CO2 + 2H2O
Draw a energy level diagram to represent this reaction and use the diagram to explain why the reaction is exothermic.
[4] 2. The equation for the reaction of hydrogen with oxygen is:
2H2 + O2 2H2O During the reaction energy is used to break the bonds of the reactants and energy is released when news bonds are formed to make the products. (a) Calculate the energy change for the reaction using the data in the table.
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FACTFILE:GCSEEHCISHTEOMRIYSTORFYA:RUTN/ITAR2C.8HITECTURE
(b) Draw a reaction profile diagram for the reaction and label the activation energy.
[4] 3. The presence of nitrogen dioxide (NO2) in the atmosphere can cause acid rain. It is formed naturally in
the reaction between atmospheric nitrogen and oxygen in an endothermic reaction. (a) Explain in terms of bonds why this reaction is endothermic.
____________________________________________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ ____________________________________________________________________________________ _________________________________________________________________________________ [3] (b) Draw a reaction profile diagram for this reaction.
[4] © CCEA 2017
ReactionEnergyEnergy ChangeReactionsActivation Energy