AQA Topic 5 - Energy Changes 9-1

HideShow resource information
  • Created by: Grace
  • Created on: 06-05-18 21:55
What happens to energy during chemical reactions? Why?
The energy is moved around because chemicals store a certain amount of energy, with diff chemicals storing diff amounts.
1 of 49
What does it mean if the products of a reaction store more energy than the original reactants? What does it mean for the opposite?
That they must have taken in the difference in energy between the products and reactants from the surroundings during the reaction however if they store less then the excess energy was transferred to the surroundings during the reaction
2 of 49
Why does the amount of energy in the universe always stay the same?
Because during reactions the overall energy doesn't change because energy is conserved as it can't be created or destroyed, only moved
3 of 49
Define an exothermic reaction
A reaction which transfers energy to the surroundings, typically by heating and shown by a rise in temp) - EXO = OUT
4 of 49
Give two examples of exothermic reactions
Combustion and neutralisation
5 of 49
Why are many oxidation reactions exothermic?
Because energy is released: for example by adding sodium to water, the reaction releases energy and the sodium moves about on the surface of the water as its oxidised
6 of 49
Describe two examples of everyday uses of exothermic reactions
HAND WARMERS - Use the exothermic oxidation of iron in the air, w/ a salt solution catalyst, to release thermal energy. SELF HEATING CANS - I.e of hot chocolate rely on exothermic reactions between the chemicals in their bases
7 of 49
Define an endothermic reaction
A reaction which takes in energy from the surroundings, this is shown by a fall in temperature - ENDO = IN
8 of 49
Give two examples of endothermic reactions
1.The reaction between citric acid and sodium hydrogencarbonate 2. Thermal decomposition e.g heating calcium carbonate causes it to decompose into calcium oxide and CO2
9 of 49
Describe an everyday use for endothermic reactions
In sports injury packs - chemical reactions allows the pack to becomes instantly cooler without having to put it in the freezer
10 of 49
What is more common: Endothermic or exothermic reactions?
Exothermic
11 of 49
Define reagent
A substance or compound added to a system to cause a chemical reaction
12 of 49
Describe an experiment for measuring energy transfer
Take the temperature of reagents, mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction.
13 of 49
What is the biggest problem with the measuring energy transfer experiment? How is it solved?
The amount of energy lost to surroundings, this is solved by putting the polystyrene cup into a beaker of cotton wool to give more insulation and putting a lid on the cup to reduce energy lost by evaporation
14 of 49
Describe an experiment for testing the effect of acid concentration on the energy released in a neutralisation reaction (1)
Put 25cm3 of 0.25 mol/dm3 of HCl acid and sodium hydroxide in separate beakers. Place the beakers in a water bath set to 25℃ until they are both the same temp. Add the HCl followed by the NaOH to a polystyrene cup w/ a lid and take the temperature of
15 of 49
Describe an experiment for testing the effect of acid concentration on the energy released in a neutralisation reaction (2)
the mixture every 30 seconds and record the highest temp. Repeat using 0.5 and then 1 mol/dm3 of HCl acid
16 of 49
What does a reaction profile show?
Shows the relative energies of the reactants and products in a reaction and how the energy changes over the course of the reaction
17 of 49
Describe the reaction profile for a exothermic reaction
The products are at a lower energy than the reactants
18 of 49
What does the height difference show on a reaction profile?
The difference in height represents the overall energy change in the reaction (energy taken in or given out) per mole
19 of 49
Define activation energy
The activation energy is the minimum amount of energy the reactants need to collide with each other and react so the greater the activation energy = the more energy needed to start the reaction
20 of 49
Describe the reaction profile for an endothermic reaction
The products are at a higher energy than the reactants
21 of 49
In terms of bonds, what happens during a chemical reaction?
Old bonds are broken and news ones are formed
22 of 49
What sort of process is bond breaking and why?
Endothermic because energy must be applied to break existing bonds, therefore energy is taken in
23 of 49
What sort of process is bond formation and why?
Exothermic because energy is released when bonds are formed
24 of 49
Complete the sentences: In ______ reactions the energy released by _____ bonds is greater than the energy used to _____ them
1. Exothermic 2. Forming 3. Break
25 of 49
Complete the sentences: In ____ reactions the energy used to ____ bonds is greater than the energy released by _____ them
1. Endothermic 2. Break 3. Form
26 of 49
What are bond energies? How are they useful?
Every chemical bond has a particular bond energy associated with it, this energy varies slightly depending on the compound the bond occurs in. You can use the bond energies to calculate the overall energy change for a reaction
27 of 49
Refer to page 63 for bond energy equations
N/A
28 of 49
Define an electrochemical cell
A basic system made up of two different electrodes in contact with an electrolyte
29 of 49
Why are the electrodes usually metal?
Because they need to conduct electricity
30 of 49
Explain two electrode factors which affect the voltage of a cell
1. Type of electrodes - Diff metals react differently w/ same electrolyte which causes the charge/voltage different of the cell 2. The reactivity difference of the electrodes - The bigger the difference the bigger the voltage of the cell
31 of 49
Refer to page 64 for example of how to predict voltage of a cell
N/A
32 of 49
Explain two factors which affect the voltage of a cell
1. The electrolyte used - diff ions in solution will react differently with the metal electrodes 2. A battery is made by connected 2+ cells together in series, the voltages of the cells in the battery are combined = there is a bigger voltage overall
33 of 49
What happens in non-rechargeable batteries? (1)
In some cells the reactions at the electrodes are irreversible, over time the reacting particles get used up and turned into the products of the reaction. Once any of the reactants is used up, the reaction ceases so no electricity is produced
34 of 49
What happens in non-rechargeable batteries? (2)
The products can't be turned back into the reactants so the cell can't be recharged. Then they have to be replaced
35 of 49
Why are rechargeable batteries more environmentally friendly?
The reaction inside the cells can be reversed by connecting it to an external electric current so you do not need to replace it
36 of 49
Describe a fuel cell
A fuel cell is an electrical cell thats supplied with a fuel and oxygen/air and uses energy form the react between them to produce electrical energy efficiently
37 of 49
Explain what happens in a fuel cell
Inside a fuel cell, when the fuel enters it becomes oxidised and sets up a p.d within the cell
38 of 49
Describe an environmentally friendly fuel cell
The hydrogen-oxygen fuel cell combines hydrogen and oxygen to produce clean water and release energy = no waste products
39 of 49
What sort of reaction is involved in the hydrogen-oxygen fuel cell?
Redox reaction
40 of 49
What is different about the electrodes in fuel cells?
The anode is negative and the cathode is positive
41 of 49
What is the electrolyte normally used in hydrogen-oxygen fuel cells?
Potassium hydroxide
42 of 49
What are the electrodes normally in a hydrogen-oxygen fuel cell?
Porous carbon with a catalyst
43 of 49
Describe how a fuel cell works (1)
Hydrogen goes into the anode compartment and oxygen in the cathode compartment. At the anode hydrogen is oxidised to produce H+ ions. H+ ions in the electrolyte move towards the cathode. At the cathode oxygen gains electrons and reacts with H+ ions
44 of 49
Describe how a fuel cell works (2)
from the electrolyte to make water (reduction). The electrons flow through an external circuit from the anode to the cathode - the electric current. The overall reaction is hydrogen and oxygen which makes water
45 of 49
Why are conventional fuels bad?
They have a finite supply and are very polluting
46 of 49
Give the advantage of hydrogen fuel cells in vehicles
Do not produce as many pollutants as other fuels, only water and heat
47 of 49
Give two advantages and two disadvantages of battery electric vehicles
Electric don't produce many pollutants either and are rechargeable but the batteries are more polluting to dispose of bc they are made from highly toxic metal compounds and there is a limit to how many times the battery can be recharged (expensive)
48 of 49
Give three disadvantages of using hydrogen fuel cells
Hydrogen is gas so takes up more space to store than a rechargeable battery, hydrogen is explosive when mixed with air so hard to store safely and the process of making the fuel involves electricity which is generated from fossil fuels
49 of 49

Other cards in this set

Card 2

Front

What does it mean if the products of a reaction store more energy than the original reactants? What does it mean for the opposite?

Back

That they must have taken in the difference in energy between the products and reactants from the surroundings during the reaction however if they store less then the excess energy was transferred to the surroundings during the reaction

Card 3

Front

Why does the amount of energy in the universe always stay the same?

Back

Preview of the front of card 3

Card 4

Front

Define an exothermic reaction

Back

Preview of the front of card 4

Card 5

Front

Give two examples of exothermic reactions

Back

Preview of the front of card 5
View more cards

Comments

No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Higher resources »