Energy Changes And Reactions Flashcards by Vicky simkin

What is ENTHAPLY?

H, the heat content stored in a chemical system.

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Key things to remember during calculations

1) When writing equations, make sure it’s one mole. 2) Don’t forget the sign. 3) Arrows when using formation ↖️↗️ 4) Arrows when using combustion ↘️↙️

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What is chemical energy?

A form of potential energy within chemical bonds.

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What is an important law about energy?

It is conserved.

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If energy is conserved, what does this mean in terms of heat?

Heat lost in a chemical system = heat gained to surroundings.

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What is the chemical system? What are the surroundings?

Chemical system = the reactants and the products. Surroundings = what is outside the chemical system.

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What is the ENTHAPLY CHANGE?

The heat exchange with the surroundings during a chemical reaction at a constant pressure.

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Equation for ENTHAPLY CHANGE?

ΔH = Hproducts - Hreactants

 (alphabet)

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What two types of heat exchange are there?

EXOthermic (exit) and ENDOthermic (into)

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Define EXOTHERMIC

A reaction in which the ENTHAPLY of the PRODUCTS is SMALLER than the ENTHAPLY of the REACTANTS, resulting in HEAT LOSS to the surroundings.

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Define ENDOTHERMIC.

A reaction in which the ENTHAPLY of the PRODUCTS is GREATER than the ENTHAPLY of the REACTANTS, due to HEAT GAINED from the surroundings.

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What is the sign of the enthalpy change in:

exothermic?
endothermic?

EXO is negative

ENDO is positive

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Diagram of an exothermic enthalpy change

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Diagram of an Endothermic enthalpy change?

Where is the Ea?

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What type of reaction (exothermic or endothermic) is less likely to happen naturally?

Endothermic You have to supply more energy, equal to produced +Ea

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Name 6 exothermic reactions?

▪️respiration

▪️neutralisation

▪️displacements

▪️combustion

▪️hand warmers

▪️CaO + H₂O ➡️ Ca (OH)₂

Name endothermic reactions.

What is a real life use of something exothermic?

Self heating cans / hand warmers

What is a real life use of endothermic?

Ice packs

What happens to the surrounding temperature in:

Exothermic: temperature of surroundings rises as heat exits into surroundings Endothermic: temperature of surroundings lowers as heat is removed from surroundings.

Exothermic?

ENDOTHERMIC?

What is the activation energy? (Ea)

The activation energy is the minimum energy required to start a reaction by the breaking of bonds.

What are the standard conditions that must be met for a reaction to take place?

Pressure 1 atm (100kPa)
Temperature 298K (25°C)
Concentration 1 mol dm^-3

What are standard states?

The physical state of a substance under the standard condition of temp 298K and pressure 1 atm.

What is the standard enthalpy change of reaction? And symbol?

The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states. ΔHr^Ө

What is the standard enthalpy change of combustion? And symbol?

The enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard state. ΔHc^Ө

What must you be extremely careful of when balancing equations?

That the reactant is ONE MOLE.

What is the standard enthalpy of formation?

The enthalpy change that takes place when one mole of compound in its standard state is formed from its constituent elements in their standard states, all reactants and products under standard conditions. ΔHf^Ө

Standard enthalpy change of formation Na + Cl₂→ NaCl Balance

Na + ½Cl₂ → NaCl ## Footnote

What is important to remember when doing diagrams?

The arrowhead in exothermic must be down In endo arrowhead is UP If formation, check it's one mole.

How can we experimentally determine standard enthalpy change?

Measuring the heat exchange to the surroundings. As heat is lost by chemicals, heat is gained by surroundings..

Equation and units for experimentally determining the standard enthalpy change?

3 important things...then what (regarding experimentally determining the standard enthalpy change)

1. Find out the moles that reacted (i.e. 0.02 moles) 2. Divide / multiply to get the heat lost per one mole 3. Remember sign

Standard enthalpy change calcuations using mcΔT steps?

1. Write down mass of what reacted. 2. Put it into mcΔT. 3. Work out the moles that reacted. 4. Get it per one mole.

20cm³of 2 mol dm^-3of nitric acid + 20cm³ of KOH solution. Temperature rise 13.8º. Calculate enthalpy per mole

2 x (20 ÷ 1000) = 0.04 mol 1. Mass = 40 (1cm³= 1g) 2. ΔH = 40 x 4.18 x 13.8 = 230.736 3. n = c x v 4. 230.74 x 25 = 57.7 kjmol^-1