Energy of reaction

Question 1

Joules

Answer 1

• Units of energy

* In chemistry to make easy comparisons kJ are often used

Question 2

Endothermic Reaction

Answer 2

• Reaction that overall takes in energy hence will feel colder
•

Question 3

Exothermic Reaction

Answer 3

Reaction that overall gives out energy hence will feel hot.

Question 4

Activation energy

Answer 4

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• Minimum energy required to start a reaction.
• Activation energy is endothermic as bonds are broken.

Question 5

Calorimetry

Answer 5

• Method of measuring energy of a substance that burns by heating water in a glass or metal container.
• Can be used to compare energy in food or fuels

Use equation Q= mass of water x heat capacity x change in Temp

Q = Energy
m = Mass of water  g (NOT FUEL USED)
C= Heat capacity – 4.2 J/C/g (GIVEN)
T = Change in temperature - Degree Centigrade 

May need calculated energy value into kJ per mole or kJ per gram. = Q/grams or Q/moles

Question 6

Energy of a chemical reaction in a solution

Answer 6

Measure temperature change in an insulated cup

• Used for dissolving solids in water or neutralisation
Same equation Q= mCAT is used.

Question 7

What happens in a chemical reaction in terms of energy

Answer 7

• Energy must be supplied to break bonds (endothermic process)
• Energy is released when new bonds are formed (exothermic process)

Question 8

Energy level diagram

exothermic

Answer 8

Graph with Energy on Y-axis and Progress of reaction on x- axis

Reactants HIGHER than products

Hump line from Reactants to products

Activation arrow must start at reactants and end in line with the peak of the activation energy hump.

Overall energy change arrow must start exactly in line with reactants and end exactly in line with products. (DOWN)

Overall energy is negative as the chemicals have lost energy to the surrounding (hence you feel it get hot).

Energy released from forming new bonds is greater than the energy to break existing bonds.

Question 9

Energy level diagram

endothermic

Answer 9

Graph with Energy on Y-axis and Progress of reaction on x- axis

Reactants LOWER than products

Hump line from Reactants to products

Activation arrow must start at reactants and end inline with the peak of the activation energy hump.

Overall energy change arrow must start exactly in line with reactants and end exactly in line with products.

Overall energy is positive as the chemicals have gained energy to the surrounding (hence you feel it get cold).

Question 10

Catalyst

Answer 10

Energy needed to break bonds of the reactants is greater than the energy released by forming new bonds.

Provide an alternative pathway for a chemical reaction that is lower in activation energy.

Activation energy arrow/curve is lower in the energy level diagram.

Hence catalyst speed up the rate of reaction.

Question 11

Hydrogen (H2)

Answer 11

Hydrogen can be burnt to produce energy e.g.

Hydrogen + oxygen → water
4H2 + O2 → 2H2O

Hydrogen can also be use in fuel cells to produce electricity

Hence can be use to power electrical cars.