energy changes in chemistry Flashcards
Many chemical reactions …………. energy into the surroundings, but some reactions ………… energy instead. We can explain these changes using ideas about ………and ………..bonds.
Many chemical reactions release energy into the surroundings, but some reactions absorb energy instead. We can explain these changes using ideas about making and breaking bonds.
Reactions that give out energy - usually in the form of heat - are called ……………
reactions. They usually feel hot.
Reactions that give out energy - usually in the form of heat - are called exothermic reactions. They usually feel hot.
Reactions that take in energy from the surroundings - usually in the form of heat - are called endothermic reactions. They usually feel cold.
You can tell if a reaction is exothermic or endothermic by measuring a temperature change. If the temperature increases, the reaction is exothermic
Reactions that take in energy from the surroundings - usually in the form of heat - are called endothermic reactions. They usually feel cold.
Reactions that take in energy from the surroundings - usually in the form of heat - are called endothermic reactions. They usually feel cold.
You can tell if a reaction is exothermic or endothermic by measuring a temperature change. If the temperature increases, the reaction is exothermic
How can you tell if a reaction is exothermic or endothermic ?
You can tell if a reaction is exothermic or endothermic by measuring a temperature change. If the temperature increases, the reaction is exothermic
To start a reaction, energy must be?
To start a reaction, energy must be absorbed by the chemicals in order to break some of the chemical bonds in the reactants.
The amount of energy needed to start a reaction in this way is called the activation energy. This explains why many reactions (even exothermic ones), need?
The amount of energy needed to start a reaction in this way is called the activation energy. This explains why many reactions (even exothermic ones), need a spark or some heat to start them
Breaking the chemical bonds in the reactants always requires energy – it is an ………………. process
Breaking the chemical bonds in the reactants always requires energy – it is an endothermic process
Making new bonds in the products always ………energy – it is an ………………… process.
Making new bonds in the products always releases energy – it is an exothermic process.
If more energy is released when new bonds are made than was absorbed to break the bonds in the reactants, then the reaction is ……………….
If more energy is released when new bonds are made than was absorbed to break the bonds in the reactants, then the reaction is exothermic.
less energy is released when new bonds are made than was absorbed to break the bonds in the reactants, then the reaction is …………………………
less energy is released when new bonds are made than was absorbed to break the bonds in the reactants, then the reaction is endothermic.
what do energy level diagrams show?
Energy changes can be represented by energy level diagrams. These show the energy absorbed when bonds in the reactants are broken, and the energy released when new bonds are made in the products.
What is a bond energy value?
Bond energy calculations – Higher tier
It is possible to calculate the energy change of a reaction using bond energy values.
How do you calculate the energy change when steam is made from hydrogen and oxygen?
This is how to calculate the energy change when steam is made from hydrogen and oxygen:
2H2(g) + O2(g) → 2H2O(g)
The units of bond strength are what?
The units of bond strength are kJ, (which stands for kilojoules).
what are the 4 steps needed to calculate the energy change
1.
Draw out the bonds in each of the reactants and products.
2.
Calculate the energy absorbed when all of the reactant bonds are broken.
3.
Calculate the energy released when all of the new bonds in the products are made.
4.
Subtract the total for the new bonds from the total for the old bonds. A negative final answer means that the reaction is exothermic
