Prac Test

Question 1

ENERGY

Answer 1

• The capacity to do work or cause change, measured in joules
• Total amount of energy present is constant
• Whenever one form of energy disappears/reduces in quantity then another form of energy will appear or increase in quantity = unchanged total energy
• Energy present in a given SYSTEM falls then there is a corresponding rise in the energy of its SURROUNDINGS so that the total energy in the SYSTEM and SURROUNDINGS is constant (LAW OF CONSERVATION OF ENERGY)

Question 2

ENERGY CHANGES IN CHEMICAL REACTIONS

Answer 2

• During chemical change the system and its surroundings heat up/cool down because of chemical change
• Heating/Cooling effects are results of a change in enthalpy present in products of the reaction compared to enthalpy in the reactants

Question 3

ENTHALPY

Answer 3

• Of a substance is the total energy present in the substance
• Includes the energy stored in chemical bonds; chemical PE and energy due to particle motion; KE of particles
• Particle KE rises with temperature
• Chemical PE of a bond is high for a weak bond and low for a strong bond

Question 4

BONDS IN CHEMICAL REACTIONS

Answer 4

• During chemical change some bonds are broken and new ones are formed; therefore there’s a change (increase/decrease) in amounts of stored chemical PE
• Energy must be conserved, any change in chemical potential energy must be BALANCED by an opposite change in another form of energy; usually particle KE energy (‘heat’)
• Temperature of the reaction system will rise or fall and always flows from hot regions to cool regions
• Flow of heat changes enthalpy of the reacting system
• Chemical changes that happen at CONSTANT PRESSURE, the heat GAIN OR LOSS of the system EQUALS change in enthalpy

Question 5

EXOTHERMIC

Answer 5

• Reactions that lose heat to the surroundings (e.g. combustion)
• Negative value for change in enthalpy, i.e. enthalpy decreases during reaction
• Some of the chemical PE stored in bonds is converted to particle KE resulting in an increase in the SYSTEMS TEMPERATURE
• Energy is conserved with chemical PE becoming ‘heat’ energy
• Change raises the reactions systems temperature, so heat flows out of the hot system into cooler surroundings
• Loss of heat energy from system to surroundings means the systems enthalpy has reduced = negative value

Question 6

ENDOTHERMIC

Answer 6

• Reactions that gain heat from the surroundings
• Positive value for change in enthalpy, i.e. enthalpy increases during reaction
• This will happen is some of the particle KE in the reacting system is converted into chemical PE stored in bonds
• There is no change in enthalpy
• Consequence of the reduced particle KE the reacting systems temperature will fall
• Heat flows from warmer surroundings to the cooler reacting system
• This increases the reacting systems total energy = positive value

Question 7

ENERGY EFFECTS OF BREAKING BONDS

Answer 7

• Bond breaking requires an input of energy, i.e. is endothermic (raises enthalpy)
• Stronger the bonds to be broken, the more energy needed to be absorbed achieve this

Question 8

ENERGY EFFECTS OF BOND MAKING

Answer 8

• Bond making releases energy, i.e. is exothermic (lowers enthalpy)
• Stronger the bonds that are formed the greater is the energy released in forming them