C3.3 Reactions and Energy Flashcards
What is the act of measuring the amount of energy transferred?
Calorimetry
What happens to the mass of reactants in a chemical reaction?
The mass is CONSERVED
How can you calculate energy transferred in a chemical reaction?
Q = MC△T
Q = Heat energy transferred (J/Joules)
M = Mass of liquid pre heating (G / Grams )
C =Specific Heat capacity of liquid (J/g°C)
△T = Change in temperature
What is the specific heat capacity of water?
4.2 J/g°C
What are the different ways energy can be measured in?
- Joules (per gram / mole/ kilo joule)
- Calories
50 cm3 of an acid was added to 50 cm3 of an alkali. The mixture was stirred and the temperature increased from 18°C to 28°C. What was the amount of energy released in J?
Temp change = 10°C
M = 50+50 = 100g
C = 4.2 (always use 4.2 unless told otherwise)
100 x 4.2 x 10 = 4200J
In an experiment, ethanol was burnt from a spirit burner and the energy released was used to heat 50 g of water.
The starting temperature of the water was 19°C but by the end of the reaction, the temperature had risen to 41°C. The mass of fuel in the spirit burner was initially 40.0 g, but this had decreased to 38.5 g by the end of the reaction.
Calculate the energy change in kJ/g of fuel.
First, work out energy transferred to water:
Temp change = 41 - 19 = 22°C
Mass = 50g
C = 4.2
50 x 4.2 x 22 = 4620 J
Then work out the energy transfer rate in the fuel
Mass change = 40 - 38.5 = 1.5
4.62 kj / 1.5 = 3.08 kj/g
What type of reactions have a negative energy change?
EXOTHERMIC
Because energy is released form the reactants not stored in the products
The amount of energy at the end of the reaction is LOWER than that at the beginning
What type of reactions have a positive energy change?
ENDOTHERMIC
Because energy is taken in and gained by the products
What makes exothermic reactions give out energy?
In exothermic reactions, more energy is released by making bonds than is needed when breaking bonds
Do exothermic or endothermic reactions absorb more energy breaking bonds than they release energy through breaking bonds?
Endothermic reactions
What is the activation energy?
The energy needed to break the bonds in the first place and begin the reaction
How is a catalyst used in a reaction to speed it up?
The catalyst can divert the pathway of the reaction and decrease the activation energy needed, leaving more reactants available to react and form the new product
Why is lowering the activation energy beneficial?
Reactions happen more quickly
This is cost effective and industries can produce more of the products using less energy
How can hydrogen be used as an alternative fuel?
- Burning hydrogen directly in the engine with oxygen, the only product of this is water
- In hydrogen fuel cells, hydrogen is reacted with oxygen WITHOUT burning, the released energy produces electricity
