C7. Energy Changes (Y11 - Autumn 1) Flashcards
🟢 What is the 1st Law Of Thermodynamics?
Energy cannot be created or destroyed
In chemical reactions, energy is coserved. The total amount of energy in universe at the end of a chemical reaction is the same as before the reaction takes place
🟢 What are Exothermic Reactions and How Does Energy Change
An exothermic reaction is one that gives out heat energy to the surroundings so the temperature of the surroundings so the temperature of the surroundings increases.
In an Exothermic reaction more energy is released making new bonds, than is required to break the reactant bonds.
Energy Change: Chemical energy → Heat energy
products have less chemical energy than the reactants, so the temperature is hotter
E.g of an Exothermic is HCl (aq) + Mg (s)
🟢 What Reactions are Exothermic Reactions, and What Exothermic Reactions happen in Everyday Products
What Types of Reactions are Exothermic:
- Neutralisation
- Oxidisation, e.g Respiration
- Displacement Reaction
- Combustion
Everyday Products:
- Hot Cans/Tins (Tinned Soup)
- A Cooker
- Handwarmers
🟢 What are Endothermic Reactions and How Does Energy Change
An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.
In an Endothermic reaction less energy is released making new bonds, than is required to break the reactant bonds.
Energy Change: Heat energy → Chemical energy
products have more chemical energy than the reactants, so the temperature is colder
E.g of an Endothermic reaction is Ba(OH)2 (s) + NH4Cl (s)
🟢 What Reactions are Endothermic Reactions, and What Endothermic Reactions happen in Everyday Products
What Types of Reactions are Exothermic:
- Thermal Decomposition (of a carbonate)
- Electrical Decomposition
- Acid + Hydrogencarbonate
Everyday Products:
- Sherbert Sticks
- Photosynthesis
- Ice Packs (ones that snaps, then releases two chemicals that produce ammonia and water, making it cold)
🟢 What Reactions are Endothermic Reactions, and What Endothermic Reactions happen in Everyday Products
The Y-Axis of thegraph will be Chemical Energy, while the X-Axis will be the time (although the x-axis isn’t always drawn on).
For Exothermic Reactions, there will be a horizontal line on the graph showing the level of chemical energy the reactants have. Once the reaction begins, the activation energy causes the chemcial energy to spike, before the heat energy is transferred to the surroundings, meaning it comes back down lower than the intial line of the reactions, to show the decreased level of chemical emergy in the products.
🟢 How to draw a Graph of an Endothermic Reaction
The Y-Axis of the graph will always be Chemical Energy, while the X-Axis will be the time (although the x-axis isn’t always drawn on).
For Endothermic Reactions, there will be a horizontal line on the graph showing the level of chemical energy the reactants have. Once the reaction begins, the activation energy causes the chemcial energy to spike, before the heat energy is taken in from the surroundings, meaning it comes back higher than the intial line of the reactions, to show the increased level of chemical emergy in the products.
🟢 What is Activation Energy?
(Activation energy = minimum energy particles need to react)
There is a minimum amount of energy needed before colliding particles of reactants have sufficient energy to cause a reaction. This energy needed to start a reaction is called the activation energy. This activation energy is shown on reaction profiles.
If colliding particles of reactants collide with less energy than the activation energy, they will just bounce off each other.
🟢 Which reaction is Exothermic or Endothermic:
- Burning petrol
- Reaction of hydrochloric acid with sodium hydrogencarbonate
- Reaction of sulfuric acid with sodium hydroxide
- Reaction of sulfuric acid with magnesium
- Ammonium chloride dissolving in water in an ice pack
- Thermal decomposition of zinc carbonate
- Electrical decomposition of sodium chloride
Exothermic Reactions:
- Burning petrol
- Reaction of hydrochloric acid with sodium hydrogencarbonate
- Reaction of sulfuric acid with sodium hydroxide
- Reaction of sulfuric acid with magnesium
Endothermic Reactions:
- Ammonium chloride dissolving in water in an ice pack
- Thermal decomposition of zinc carbonate
- Electrical decomposition of sodium chloride
🟢 For this reaction, use the bond energy data to:
- Calculate the energy required to break the bonds
- Calculate the energy released when the bonds in products are made
- Calculate the energy change for the reaction
- State wether the reaction is exothermic or endothermic (+why?)
2 H—H + O===O —> 2 H—O—H
(H-H = 436
O=O = 498
O-H = 463)
Energy To Break The Bonds:
2 x H—H
2 x 436 = 872
1 x O===O
1 x 498 = 498
872 + 498 = 1370
Energy To Make The Bonds:
4 x O—H
4 x 463 = 1852
Energy Change:
1370 - 1852 = - 482kJ/mol
Exothermic or Endothermic:
It’s exothermic because more energy is released making new bonds, than is required to break the reactant bonds.
This is shown through the fact that -482kJ/mol of energy was given out in this reaction
(Exothermic is a negative energy change)
🟢 For this reaction, use the bond energy data to:
- Calculate the energy required to break the bonds
- Calculate the energy released when the bonds in products are made
- Calculate the energy change for the reaction
- State wether the reaction is exothermic or endothermic (+why?)
. H H H H
. | | | |
H—C—C—O—H —> C===C + H—O—H
. | | | |
. H H H H
(C-H = 412 C-O = 360 O-H = 463 C-C = 348 C=C = 612)
Energy To Break The Bonds:
5 x C-H
5 x 412 = 2060
1 x C-O
1 x 360 = 360
1 x C-C
1 x 348 = 348
1 x O-H
1 x 463
2060 + 360 + 348 + 463 = 3231
Energy To Make The Bonds:
1 x C=C
1 x 612 = 612
4 x C-H
4 x 412 = 1648
2 x O-H
2 x 926
612 + 1648 + 926 = 3186
Energy Change:
3231 - 3186 = + 45kJ/mol
Exothermic or Endothermic:
It’s endothermic because less energy is released making new bonds, than is required to break the reactant bonds.
This is shown through the fact that 45kJ/mol of energy was given out in this reaction
(Endothermic is a positive energy change)
🟢 How are Chemical Bonds Made and Broken
+What happens when more/less energy is used to break bonds than what it takes to form
There is always a balance between the energy needed to break bonds and the energy released when new bonds are made in a reaction. This is what decides wether the reaction is endothermic or exothermic.
- In some reactions, the energy released when new bonds afe formed (as the products are made) is more than the energy needed to break the bonds in the reactants. These reactions transfer energy to the surroundings. They are exothermic.
- In other reactions, the energy needed to break the bonds in the reactants is more than the energy released when new bonds in the reactants js more than the energy released when new bonds are formed in the products. These reactions transfer energy from the surroundings to the reacting chemicals. They are endothermic.
🟢 What is Bond Energy?
The energy needed to break the bond between two atoms is called the bond energy for that bond.
Bond energies are measured in kJ/mol. You can used bond energies to work out the energy change for many chemical reactions.
🟢 How Do You Work Out the Energy Change for a Chemical Reaction
To calculate the energy change for a chemical reaction, you need to work out:
- How much energy is needed to break the chemical bonds in the reactants.
- How much energy is released when the new bonds are formed in the products.
🟢 How Does the difference between reactivity of metals affect the voltage?
The greater the difference in reactivity between the two metals used, the higher the volatge produced.
