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Energy changes for Edexcel A-level Chemistry

Energy changes

This page covers the following topics:

1. Thermal energy
2. Enthalpy change
3. Reaction profile

Calorimetry measures enthalpy changes from combustion using the equipment shown. It can also be used to measure other reactions using a beaker with a cork stopper and thermometer.

Thermal energy

Enthalpy is the overall energy content of substances in a reaction, it is represented by โˆ†H. In order to compare reactions and calculate energy changes accurately โˆ†H is calculated using theoretical standard conditions.

Enthalpy change

Energy change of reaction may need to be calculated to determine whether a reaction is exothermic or endothermic and sketch the reaction profile.

Reaction profile

1

What are the standard conditions?

substances in their standard states, at 298 K (25ยฐC, roughly room temperature), at a pressure of 100 kPa (1 atm) with a 1 mol/L concentration

What are the standard conditions?

2

Is activation energy higher in endothermic or exothermic reactions?

endothermic

Is activation energy higher in endothermic or exothermic reactions?

3

Given that ฮ”H for the breakdown of ammonia is +92.9 kJ/mol, which reaction profile best fits this reaction? Why?

Graph 2, in which case the products have more energy than the reactants and ฮ”H is indicated by an upwards arrow.

Given that ฮ”H for the breakdown of ammonia is +92.9 kJ/mol, which reaction profile best fits this reaction? Why?

4

The ฮ”HOC of methane is -882KJ/mol, what does this mean?

882KJ of energy is released when methane reacts completely with oxygen under standard conditions.

The ฮ”HOC of methane is -882KJ/mol, what does this mean?

5

Sketch an endothermic reaction profile and indicate ฮ”H.

image

Sketch an endothermic reaction profile and indicate ฮ”H.

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