Test 2 Flashcards
What energy change occurs in enthalpy
Kinetic energy increases with increasing temperature
Potential energy decreases with increasing bond strength
What happens to enthalpy in a reaction
During a chemical reaction, bond are broken and new bonds form, so there will be a change in the amount of stored potential energy
What is the enthalpy of a substance
The enthalpy of a substance is the total energy present.
Includes potential and kinetic
What is conversion of energy
During a chemical reaction the total amount if energy must remain constant.
It may be transformed from one form to another.
If potential energy decreases kinetic energy must increase.
Raises temperature and allows heat to flow out of the system.
What are Exothermic reactions
Release heat into the surroundings.
Potential energy in the bonds is lower so potential energy is converted into kinetic and is released from the system
What is endothermic reactions
Absorbs heat
Heat energy flows into the system as kinetic energy is converted into potential energy in the bonds of the substances
What is a heat reaction of enthalpy
ALSO KNOWN AS ENTHALPY CHANGE
The amount of heat released or absorbed during a chemical reaction
This is known as the enthalpy change of the reaction when it is conducted at constant pressure (lab conditions)
What is the symbol for enthalpy change
(Delta) H
How is change in enthalpy measured?
Can be determined by measurement of the temperature change that accompanies a chemical reaction
(Delta)H = enthalpy of product- enthalpy of reaction
How can we tell the Enthalpy of an Exothermic reaction is
(Delta)H negative
H of product
Heat emitted into the surroundings
Temperature of surroundings increase
How can we tell what the Enthalpy of an endothermic reaction is
(Delta)H positive
H of product > H of reactant
Heat absorbed from surroundings
Temperature of surroundings decreases
The (delta)H value of a reaction is shown by
H2(g) + Cl2(g) –> 2HCl(g) ; H= -184 kJ
What are thermochemical equations
balanced stoichiometric chemical equation that includes the enthalpy change, ΔH.
In variable form, a thermochemical equation would look like this:
A + B → C
ΔH = (±) #
Where {A, B, C} are the usual agents of a chemical equation with coefficients and “(±) #” is a positive or negative numerical value, usually with units of kJ.
What are chemical changes
Those in which new substances with different composition and properties are formed
What are physical changes
Occurs without altering the chemical composition of a substance
What happens in a chemical reaction
A chemical change occurs and new substances are formed.
Reactants: substances that are combined together in the chemical reaction
Products: the new substance formed
Atoms of reactant are rearranged to form product
What happens to bonds in chemical reaction
Bonds between atoms are disrupted and new bonds are formed
What are common signs of reaction
Colour change Formation of a solid Production of odour Significant change in temperature Production of gas
What is the Law of conservation of mass
Mass is neither created nor destroyed in a chemical a reaction
Mass of the reactant will be the same as the mass of the product
Balanced equations of Chemical equations
Because atoms are neither created nor destroyed during a chemical reaction the equation must be balanced
Therefore the number of atoms of each type must be the same on both sides of the arrow
How is the state of a reactant or product indicated
(s) - solid
(l) - liquid
(g) - gas
(aq) - in aqueous solution
Ionic equations sometimes written instead of molecular equations for reactions involving ions
Only ions involved in the reaction are shown (others are spectator ions and are not included)
Must be balanced in both atoms and charge
What is a precipitation reaction
Occurs when ions in solution combine to form a new compound that is insoluble in water.
Insoluble substance formed is a precipitate
How are precipitates formed
When two ionic solutions are mixed together, all the ions remain separate unless one of the combinations can form an insoluble substance
We use ionic equations to represent this because the other ions that remain in solution are not actually involved in the reaction
Example of an equation of precipitation
AgNO3(ag) + NaCl(aq) —> AgCl(s) + NaNO3(aq)
Is written as
Ag(aq) + Cl(aq) —> AgCl(s)
What is Colour prediction
The colour of an ionic compound is usually determined by the colour of the ions in which it is composed
Eg. A solid composed of two colourless ions will usually form a white precipitate, a blue ion combined with a colourless ion will usually form a blue precipitate
What are exceptions to colour of a precipitate
Pb2+ (colourless) and I (colourless)
combine to form PbI2 (yellow)
Define relative atomic mass of elements
The weighted average of the relative atomic masses of the isotopes compares to 1/12th the mass of an atom of carbon-12
Define relative molecular mass
Relative molecular mass of a substance is the mass of a molecule of that substance compared to 1/12th the mass of a carbon-12
Calculate the relative molecular mass of sucrose, table sugar, which has a molecular formula of
C12H22O11
= 12 x Ar(C) + 22 x Ar(H) + 11 x Ar(O)
= (12 x 12.01) + (22 x 1.008) + (11 x 16.00)
= 342.3
What is relative formula mass
The relative formula mass of a substance is the weighted average of the masses of the formula units relative to 1/12 of the mass of a carbon-12 atom.
What is a mole
One mole is the amount of substance containing Avogadro’s number of particles. This number is:
6.02 x 10^23
What is molar mass
The mass of one mole of a substance is called its molar mass (M)
This mass is simply, the relative atomic mass (Ar) or relative formula mass (Mr), expressed in grams per mole.
Example of molar mass (Na)
So, molar mass of M(Na) = mass of 1 mol of Na atom
We know that Ar(Na) = 23.0 Therefore M(Na) = 23.0 g mol -1
Example of molar mass = M(SO2)
Ar(S) + 2 x Ar(O)
- 07 + 2 x 16.00
- 07 g mol-1
Calculations - mass to moles
The number of moles of a substance in a given mass can be calculated by
n = m/M
Where:
n is number of moles
m is mass of the substance
M is the molar mass of the substance
Example of mass to mole- calculate the number of moles of sodium chloride in 200g
M(NaCl) = 22.99 + 35.45
= 58.44 g mol-1
n(NaCl) = m/M
= 200/58.44
= 3.42 mol
How is Mole to mass equation written
m = M x n
What are Chemical formulas and moles
The formula of any given substance indicates the number of atoms present in a molecule of a substance
The same concept can be applied to the moles of a substance.
What does 1 mole of H2CO3 cobtains
2 mol of H atoms
1 mol of C atoms
3 mol of O atoms
What is Percentage composition
Percentage composition is the percentage of a formula mass represented by each element.
Percentage composition compares the mass of one part of a substance to the mass of the whole.
How do you find percentage composition
-
Determine the percentage composition by mass for each element in silver nitrate
Mr(AgNO3) = 107.9 + 14.01 + 3 (16.00) = 169.91 %(Ag) = 107.9/169.91 x 100 = 63.50% %(N) = 14.01/169.91 x 100 = 8.24% %(O) = 48/169.91 x 100 = 28.25%
How do you write a balanced equation
1) write word equation
2) unbalance equation showing correct formulae
3) balance the equation
Examples of exothermic reactions
Petrol burns, energy is released
Examples of endothermic reaction
Melting ice, id you place the ice cube in the glass of water, it cools it down
Gaseous molecules can have kinetic energy due to the three different types of motion.
What are these three types
Vibrational
Rotational
Translational
Define Vibrational motion
Atoms move away and towards each other
Define rotational motion
Molecules rotates about its centre
Define transitional motion
Molecules moves from place to place
NOT ALL ATOMS HAVE TRANSITIONAL MOTION
How do you write a balanced equation
1) write word equation
2) unbalance equation showing correct formulae
3) balance the equation
Examples of exothermic reactions
Petrol burns, energy is released
Examples of endothermic reaction
Melting ice, id you place the ice cube in the glass of water, it cools it down
Gaseous molecules can have kinetic energy due to the three different types of motion.
What are these three types
Vibrational
Rotational
Translational
Define Vibrational motion
Atoms move away and towards each other
Define rotational motion
Molecules rotates about its centre
Define transitional motion
Molecules moves from place to place
NOT ALL ATOMS HAVE TRANSITIONAL MOTION
