C3.3 Reactions and Energy Flashcards Preview

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Flashcards in C3.3 Reactions and Energy Deck (20):

What is the act of measuring the amount of energy transferred?



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?


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?


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?

1. Burning hydrogen directly in the engine with oxygen, the only product of this is water
2. In hydrogen fuel cells, hydrogen is reacted with oxygen WITHOUT burning, the released energy produces electricity


What are disadvantages of using hydrogen as fuel?

-Most hydrogen is produced from reactions with non renewable sources such as coal or natural gas with steam
- It is not commonly used and so few stations sell hydrogen fuel
- Must be compressed, liquefied and insulated in order to store
- HFCs do not work at low temperatures


How can hydrogen be sourced in a more renewable way?

By using geothermal energy in eg iceland


What are the advantages of using hydrogen as a fuel?

- Does not create Carbon Dioxide when burnt
- HFC are very efficient


How can you calculate the bond energy of a molecule?

Add together the bond energies of the reactants (ENERGY IN)
Add together bond energies of products (Energy out)

Energy in - Energy out = Energy change


Hydrogen and chlorine react to form hydrogen chloride gas:

H−H + Cl−Cl → 2 × (H−Cl)

Bond Bond Energy (kJ/mole)
H−H 436
Cl−Cl 243
H−Cl 432

Work out the energy change, if the reaction is endo or exo thermic

Energy in = 436 + 243 = 679 kJ/mole

Energy out = 2 × 432 = 864 kJ/mole

Energy change = in – out = 679 – 864 = –185 kJ/mole

= exothermic reaction because the energy change is NEGATIVE