Energy Changes- Paper 1 Flashcards
What is there always changes in whenever chemical reactions occur?
Energy
Is energy created, moved around or destroyed in chemical reactions?
Moved around
What is an exothermic reaction?
A reaction which transfers energy to the surroundings usually by heating-shown by a rise in temp
Give an example of an exothermic reaction.
Burning fuels (combustion) which gives out a lot of energy. Or a neutralisation (acid+alkali) reaction.
Are oxidisation reactions usually endothermic or exothermic?
Exothermic e.g. adding sodium to water releases energy.
Give everyday examples of exothermic reactions.
Self heating cans of hot drink rely on exothermic reactions between chemicals in their bases.
What is an endothermic reaction?
A reaction that takes in energy from the surroundings- shown by fall in temperature.
Which are more common, exothermic or endothermic reactions?
Exothermic reactions.
Example of endothermic reaction?
Citric acid and sodium hydrogencarbonate
Thermal decomposition e.g. heating calcium carbonate causes it to decompose into calcium oxide and carbon dioxide.
Give some everyday examples of endothermic reactions
Sports injury packs- chemical reaction allows the pack to become instantly cooler without having to put it in the freezer.
What equipment would you need to measure the amount of energy released by a chemical reaction?
Polystyrene cup, lid, thermometer, large beaker, cotton wool and the reaction mixture.
Method for practical?
- ) Put 25cm3 of 0.25mol/dm3 of hydrochloric acid and sodium hydroxide in seprate beakers.
- ) Place the beakers in a water bath set to 25 degrees until they are both at the same temp.
- )Add HCl followed by the NaOH to a polystyrene cup with a lid.
- ) Take the temp of the mixture every 30 seconds and record the highest temp.
- ) Repeat using 0.5 and 1mol/dcm3 of hydrochloric acid
What does an exothermic reaction profile look like?
The curve will start mid graph, then curve up and then down far past the reactants point. That shows how much energy was released.
What does an endothermic reaction profile look like?
The curve starts at the bottom (reactants) and curves up and then when it falls it stops mid way. This shows the energy absorbed.
On a graph where does the activation energy measure?
From the reactants to the highest point of the curve.
How does the graph show it’s endothermic?
Because the products are at a HIGHER energy than the reactants.
How does the graph show it’s exothermic?
Because the products are at a LOWER energy than the reactants.
What is an electrolyte?
A solution that can conduct electricity, e.g. a solution of an ionic compound.
What is true about cells and how long they last?
They can only produce electricity for a certain period of time. Eventually the chemicals in the cells run out.
When do cells produce electricity?
If we use metals with different reactivities.
The size of the potential difference depends on….
the difference in reactivity between the metals.
also the electrolyte
What is a battery?
A battery contains two or more cells connected in series to produce a greater voltage
What is a non-rechargeable battery?
An alkaline battery as once the reactants run out no more electricity can be produced and there is no way to reverse this reaction.
What is a rechargeable battery?
Rechargeable batteries can be recharged as we can reverse the chemical reactions we applying an electrical current.
What do you react in a hydrogen cell?
hydrogen and air or pure oxygen.
What is the only waste product of a hydrogen fuel cell?
Water
What is the equation at the negative electrode?
2h2—> 4h+ +4e-
What is happening to the hydrogen in the fuel cell?
It is becoming oxidised
List the disadvantages of Rechargeable batteries
They become less efficient as they store less electricity the more charging cycles they go through and need replacing eventually.
They require recharging once they’ve run out
List advantages of hydrogen fuel cells
Can be a source of drinking water e.g. on spacecraft
Don’t get less efficient the longer they run
Produce electricity for as long as you are providing hydrogen for them
List advantages of rechargeable batteries
No dangerous fuels are required.
Produce a greater pd than a hydrogen fuel cell.
List disadvantages of hydrogen fuel cells
Hydrogen is an explosive gas so difficult to store safely.
Produce a low pd so several are need together.
Energy is….. when new bonds are formed
Energy is released when new bonds are formed.
Bond formation is
Exothermic
Bond breaking
Endothermic
Energy must be …… to break existing bonds
Supplied.
What are the 3 steps to calculating bond energy?
Find the energy required to break the bonds
Find energy released from the forming of new bonds
Find the overall energy change (CAN BE NEGATIVE)
What IS activation energy?
Minimum amount of energy the reactants need to collide with each other and react.
