chemical reactions and balancing equations -...
TRANSCRIPT
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Chemical & Physical Changes
Physical change, the
chemical composition of
the substance remains
constant.
Chemical change, the
chemical composition of
the substance changes; a
chemical reaction occurs.
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Evidence for Chemical Reactions
There are four observations which indicate a
chemical reaction is taking place.
1. Release of gas:
Release of gas can be observed in different
ways; from light fizzing to heavy bubbling.
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Evidence for Chemical Reactions
2. Production of an insoluble solid.
A precipitate, or insoluble solid, forms by adding
two aqueous solutions together.
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Evidence for Chemical Reactions
3. Observation of a permanent color change.
Many chemical reactions involve a permanent
color change.
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Permanent color change as an indication of a chemical reaction
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Evidence for Chemical Reactions
4) A heat energy change is observed.
Exothermic reaction: Heat is released by the
reaction.
Endothermic reaction: Heat is absorbed by the
reaction.
Examples: Observed heat and light given off.
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Writing Chemical Equations
Chemical equation describes a chemical reaction using
formulas and symbols. General formula of a chemical
equation is:
A + B → C + D
A and B are reactants and C and D are products.
Adding a catalyst, reaction speeds up without the catalyst
being consumed. A catalyst is written above the arrow.
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States of Matter
When writing chemical equations, specify the physical
state of the reactants and products.
A(g) + B(l) → C(s) + D(aq)
g: gaseous state,
l: liquid state,
s: solid state,
aq: aqueous state
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Chemical Reactions
HC2H3O2(aq) + NaHCO3(s) → NaC2H3O2(aq) + H2O(l) + CO2(g)
This reaction shows
– Aqueous acetic acid is added to solid sodium
carbonate
to produce
– Aqueous sodium acetate, liquid water, and carbon
dioxide gas.
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Diatomic Molecules
Seven nonmetals occur naturally as diatomic
molecules and these elements are written as diatomic
molecules in the chemical reactions.
Diatomic Molecules:
hydrogen (H2),
nitrogen (N2),
oxygen (O2),
and the halogens, F2, Cl2, Br2, and I2.
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Rules of Balancing Chemical Equations
When balancing a chemical equation:
The number of atoms of each element must be the
same on both sides of the arrow.
To balance place a whole number coefficient in
front of each substance.
A coefficient multiplies all subscripts in a chemical
formula:
– 3 H2O has 6 hydrogen atoms and 3 oxygen atoms
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Balancing Equations
Before placing coefficients in an equation, check that
the formulas are correct.
Never change the subscripts in a chemical formula to
balance a chemical equation.
Balance each element in the equation starting with
the most complex formula.
Balance polyatomic ions as a single unit if it appears
on both sides of the equation.
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Balancing Equations
The coefficients must be whole numbers. If
coefficient is a fraction, multiply the whole equation by
the denominator to get whole numbers:
[H2(g) + ½ O2(g) → H2O(l)]
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2 H2(g) + O2(g) → 2 H2O(l)
After balancing the equation, check, there must be the
same number of atoms of each element (or polyatomic
ion) on both sides of the equation:
4 H, 2 O → 4 H, 2 O
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Balancing Equations
Finally, check that you have the smallest whole
number ratio of coefficients. If all the coefficients can be
divided by a common factor, do so to complete balancing
of the reaction.
[2 H2(g) + 2 Br2(g) → 4 HBr(g)]
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H2(g) + Br2(g) → 2 HBr(g)
2 H; 2 Br → 2 H; 2 Br
Balancing a Chemical Equation
Balance the following chemical equations:
__H2O(l) + __Mg(s) → __H2(g) + __MgO(aq)
__Al2(SO4)3(aq) + __Ba(NO3)2(aq) → __Al(NO3)3(aq) + __BaSO4(aq)
__Al(s) + __CuSO4(aq) → __Cu(s) + __AlSO4(aq) (Check Formulas)
Balancing a Chemical Equation
Balance the following chemical equations:
N2 + H2 → NH3
NaCl + F2 → NaF + Cl2
C8H18 +O2 → CO2 + H2O
AgNO3 + MgCl2 → AgCl + Mg(NO3)2
aluminum + hydrochloric acid → aluminum chloride + hydrogen
calcium hydroxide + phosphoric acid → calcium phosphate + water
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Review
4 ways to understand if a chemical reaction occurred:
1. A gas is detected.
2. A precipitate is formed.
3. A permanent color change is seen.
4. Heat or light is given off.
An exothermic reaction gives off heat and an
endothermic reaction absorbs heat.
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There are 7 elements that exist as diatomic molecules:
– H2, N2, O2, F2, Cl2, Br2, and I2
When we balance a chemical equation, the number of
each type of atom must be the same on both the product
and reactant sides of the equation.
We use coefficients in front of compounds to balance
chemical reactions.
Review